Keohi HDTV - Expert Tips - Guy Kuo - Avia

[EXPERT TIPS][GUY KUO][AVIA]

Avia Tips from A to Z

AVIA Calibration Tip #1 - Needle Pulses + Steps Video Test Pattern
Needle Pulses + Steps Test Pattern

At first glance, one mistakes this pattern for a traditional Needle Pulse pattern, but there are important changes in the pattern which make it more useful. Most obvious is the addition of a vertical line on the right side of the pattern. The traditional pattern only has a line on t he left side of the pattern. By adding the right side line, AVIA makes it possible to look for closely for high voltage supply problems. Often geometry distortion with high voltage system overload appears worse and earlier along the right edge. Without the right side line, traditional Needle Pulse patterns don't allow you to visualize the change. You might be fooled into thinking a display has perfect geometry stability when in fact it is distorting the image.

Another obvious addition are the gray scale steps in the upper half of the pattern. On CRT displays one looks for both geometry distortion AND blooming in order to find the maximal usable white level setting. This used to require two separate test patterns. By adding the gray scale steps, AVIA makes it possible to look for both changes with a single pattern. No more switching back and forth between patterns.

If you also observe the upper and lower halves of the Needle Pulses pattern closely, you'll notice some bars moving back and forth. The "Black Bars" in the upper half of the pattern are useful for checking the black level (brightness) setting of your display. The "white bars" in the lower half of the pattern are for testing white clipping level of LCD projectors. This means a single pattern allows for testing of three limits of white level and a quick check of black level.

For those who prefer a more traditional pattern, AVIA also includes the Needle Pulses pattern which omits the gray steps but includes the black bars and white bars. I can't really imagine why one would want to omit the test for blooming, but AVIA provides you the choice.

AVIA Calibration Tip #2 - Blue Bars, Red Bars, Green Bars

Color bars are useful for checking the proper function of color decoders in a display. With NTSC displays one can vary the saturation (amount) and hue (phase relationship) of the display. Ideally, the display is adjusted to recreate the same colors as encoded in the signal.

The traditional SMPTE color bar includes not only bars but small patches of colors which are in reverse blue order just below the color bars. By examining the amount of blue (using a filter or better yet by using the display's blue only mode) one can tell if the saturation and hue are correctly adjusted. This works because the gray portions of the pattern are encoded to be an intensity of gray which has exactly the same amount of blue in the blue portions of the pattern. Since gray contains no color content, turning the saturation control up and down does not affect its blue content. This allows the gray to act as a reference point against which to compare the amount of blue is present. Calibrating saturation can thus be done by adjusting blue intensity to match the fixed amount of blue in gray. Hue is observed by comparing the amount of blue in the magenta and cyan portions of the pattern. When hue is correctly adjusted, the blue intensities of magenta and cyan are identical.

Unfortunately, some people find it difficult to accurately tell when the blue intensities are equal. The bars and patches are of unequal size and color separation artifacts can make the color transition zones blurred or of uneven darkness. For these reasons, AVIA's "Blue Bars" add flashing patches within the color bars to aid in finely discerning when the intensities are equal. Human vision is very sensitive to flashing. AVIA takes advantage of this by having users adjust saturation and hue to minimize visible flashing in blue. This allows higher accuracy than comparison of static bars and patches. For those who still prefer more traditional static comparisons, AVIA also provides traditional split color bar patterns.

Several other features are built into the color bars. You may not have noticed that the transitions between bars and patches is closer to the center of the screen than SMPTE bars. This moves the critical comparison area of the pattern further from on screen displays which often appear at the bottom of the screen when televisions are adjusted. The white reference rectangle at the bottom of the pattern includes animated white bars for detecting white clipping. The lower right black portion of the pattern has animated black bars for checking black level. These are positioned where the PLUGE pattern is on SMPTE bars, but animation avoids the optical illusions of aligned rectangle edges that sometimes make it difficult to tell if a PLUGE rectangle is visible. Animation makes visibility obvious. Also the black bars do not rely on a blacker than black component for proper use. However, the black bars in this pattern are only to be used on displays which need a high APL during black level adjustment.

One can also use SMPTE bars in red or green only, but AVIA makes evaluation of green and red primary handling by providing "Red Bars" and "Green Bars." These are used in the same manner as the blue bars, except one views the red bars in red-only and the green bars are to be viewed in green-only. The patches which need to be compared are also moved to be positioned below the bar against which comparison needs to be made. This makes intensity comparisons easier than the wide separations that arise with a blue optimized color bar pattern. AVIA also adds its innovative flashing patches to the red and green bars to enhance viewer accuracy.

Why red and green bars? If the color decoder is perfect, adjusting to blue only accuracy would make the red and green color bar patterns also appear perfect. Unfortunately, it often isn't perfect. We'll visit that in AVIA Calibration Tip --- Color Decoder Check.

AVIA Calibration Tip #3 - Sharpness Pattern

NTSC video carries most of its resolution in the luminance portion of the signal. Onto this is overlaid lower resolution color information to yield the final picture. By doing this, the designers of the NTSC system were able to provide an image which gave much of the perceived effect of having high resolution in both color and luminance but in a smaller amount of bandwidth.

Television displays provide a sharpness or peaking control whose behavior is much akin to the treble control of an audio receiver. The control should be used to compensate for the attenuation of high frequency video information that blurs images horizontally. Unfortunately, this control is often misused or not designed in a way that accomplishes this goal. Users often keep sharpness set too high and suffer a picture that appears sharper to the naïve eye, but is actually filled with extraneous image artifacts. Sharpness is perhaps the most difficult control to teach people to set properly.

There are often recommendations to simply turn sharpness all the way down, but that can be excessive. It's best to actually use a test pattern which points out how the sharpness control is altering the image. Then you can rationally determine optimum setting on your display. AVIA provides a dedicated Sharpness pattern which combines several tests of parameters important in determining optimal sharpness setting.

A horizontal frequency sweep occupies the top of AVIA's sharpness pattern. This is a constant amplitude sweep that goes from low video frequency to high video frequency. If the video bandwidth of your display is lower in a portion of the video bandwidth, then that section of the sweep appears darker. Ideally, the brightness of the sweep is constant throughout its range. As you adjust sharpness up and down, look to see if any portion of the sweep goes up and down in brightness. That is the set of video frequencies the sharpness control of your display affects. If your display's sharpness control is well designed, all you need do is adjust to make the sweep as evenly bright as possible.

Some people find it difficult to compare different sections of a sweep because the gradation of brightness is continuous from one section to another. The frequency bursts at the bottom of the chart provide discrete patches of video frequencies that can be compared. You'll probably find that the rightmost patch is slightly darker than the rest of the patches even on the best of displays. Just try to equalize the other patches.

There are black vertical lines, diagonal lines, and a circle in the center of the AVIA sharpness pattern. These are needed because many sharpness controls not only alter frequency response but add ringing artifacts. Ringing is overshoot and undershoot of the video signal at abrupt luminance transitions. You see this as false outlines next to the actual black lines. If setting sharpness at the point which equalizes video bandwidth also yields visible ringing, you should decrease sharpness to the point at which the false outlining is just barely visible. Otherwise, you will be adding artifacts rather sharpening actual image detail.

There are also vertical lines set against black and white backgrounds. These are also used to look for ringing, but because the luminance transitions are larger than going from a gray background to black, these serve as especially severe tests for ringing. These are primarily for testing display circuit design quality rather than for actually setting the sharpness control, because these are usually too severe a test for consumer grade displays. Setting sharpness low enough to avoid all outlining of black/white transitions will often yield an excessively blurred image on consumer grade displays.

A vertical frequency sweep occupies the left side of the pattern. This is used in conjunction with the horizontal lines of the pattern to set vertical aperture (vertical sharpness) of video processors which have this control. As with the usual sharpness control, adjust to equalize video bandwidth and avoid false outlining.

Once you have correctly set sharpness, it is very tempting to return to your previous excessively high setting. Don't. Instead, view the picture several days at the new, less artifact inducing setting. You will find that the old overly high setting yields an unnatural picture.

Properly adjusting the sharpness control is only part of getting maximal image detail from your display. Keeping white level below the point of blooming and controlling room lighting should also be done. Once all these are done, you might wish to measure your display's resolution using resolution patterns which I'll cover in another AVIA Calibration Tip.

AVIA Calibration Tip #4 -- Color Decoder Check

NTSC video signals must be separated, decoded, and matrixed to form the final red, green, and blue signals which drive the display. Professional grade displays accurately decode the color signals and render colors correctly. However, consumer grade televisions often break the rules and have non-standard color decoding. This is most often seen as exaggerated reds (red push) and wreaks havoc when one attempts to adjust colors on a consumer display using just color bars.

Color bars are encoded such that the amount of red, green, or blue is 75% in each bar which contains the color. For instance the amount of blue is 75% in the gray, blue, cyan, and magenta portions of color bars. Similarly, the amount of red is 75% in the gray, red, yellow, and magenta portions. Because the amounts of each primary are identical in the various patches, one can compare the intensity of each color to learn how a decoder is functioning. AVIA also includes 50% & 100% color bars for testing of color circuitry linearity but we'll ignore those for now and concentrate on the more commonly used 75% variety of color bars.

75% Gray has zero color difference from gray so adjusting color saturation up and down doesn't alter its appearance. Hence, gray serves as the reference point against which the intensity of color saturation may be compared. Turning saturation up and down alters the intensity of the colored portions of color bars. View the blue portions of color bars in blue-only as you increase saturation. You'll notice that blue increases in intensity with increasing saturation. When saturation is correctly set, the intensity exactly matches that of gray. On a professional display with NTSC accurate color decoding, this same saturation setting also makes the red and green portions of the pattern match gray. Hue is adjusted by comparing portions of the pattern that contain two primaries such as cyan vs magenta.

AVIA has a Color Decoder Check pattern which lets you measure and compensate for non-standard color decoding. The pattern has a gray background against which you compare the brightness of red, green, and blue color patches. The patches range from +25 to -25%. If the color decoder is perfect, then the 0% patches of each color match the gray background (when viewed in only that color). If a display has red push, then a higher (darker encoded) red patch matches the gray background. You can read the percentage push by finding the patch which best matches the gray.

You may find other imbalances with the AVIA Color Decoder Check pattern, but red push is the most important to control. This is because red push is more objectionable to most viewers than under push or green push. A professional calibrator can sometimes correct the color decoder axes to achieve NTSC standard decoding, but for most sets that is not possible. You may want to check the accuracy of color decoding of a display prior to purchase since this problem is often not correctable. The only recourse is to hide (not correct) the error by decreasing saturation to make the measured red push 10 to 20%. This desaturates the overall picture but avoids making flesh tones too orange. Leave hue alone when making this compensation.

There are two other things to remember when considering non-standard color decoding. Don't confuse correcting the color decoder axes with resetting gray scale. This problem cannot be corrected by decreasing red drive because that would alter the underlying gray scale of the picture. The problem is with the way color DIFFERENCES from gray are being interpreted by the display's color decoder, not with the amount of red in the gray scale.

The second thing to remember is that this pattern and color bars are most accurate if one turns off the other two color guns of the display when examining each color. Color filters leak through a bit of the other colors and falsely make the gray background brighter than it really is. This tends to make your observations through filters about 5% lower than if color filtering were perfect. The difference is small, but if you want highest accuracy, turning off or capping the other two color guns is best.

AVIA Calibration Tip #5 -- Resolution Patterns

Look in your local newspaper advertisements and you'll see amazing claims of 800 to higher lines of resolution. Unfortunately, you'll be hard pressed to find a salesman who understand how those numbers were measured or how they relate to the display's actual ability to display fine image detail. One problem is that there is no legal standard for how the number must be derived and reported. A manufacturer could theoretically bypass most of the display's electronics and simply measure the performance of the best amplifier section. In the final analysis, you need to either rely on magazine reviewers to measure true TVL or learn a little about the resolution patterns in AVIA and measure it yourself before you buy a set. The test patterns in AVIA can serve as a powerful means of separating the chaff from the wheat when you're shopping.

TVL (TV Lines) resolution is expressed as the number of vertical black and white lines (counting both black and white) which can be resolved across a width of screen equal to the height of the screen. This differs from computer graphics in which horizontal resolution is measured across the entire screen rather than just a width equal to the height. For a 1.33:1 ratio screen (4:3), this means that TVL resolution is 3/4 the number of discernible lines across the entire screen width. On a 1.78:1 ratio screen (16:9), the TVL resolution is 9/16 of the number of discernible lines across the entire screen.

Exercise: You are writing advertising and press copy for a new projection television. What resolution number would you state, if your engineers tell you the following?

Lenses in the optical system have a maximal theoretical resolution of 1100 lines.

The final video amplifier stage (bypassing most of the electronics) can resolve 800 lines across the entire screen.

The overall electronics & optics resolves 600 lines across the entire screen width

The TVL resolution is 450

Remember, there aren't any legal requirements on how you report the resolution. Would you say that your display only has 450 lines of resolution?

So now we see how "lines of resolution" can mean different things if one doesn't specify how they are being expressed and measured. For actually viewing images, the overall TVL resolution is the most meaningful.

Some people are confused because they also know that NTSC video has a fixed number of scan lines of which about 480 are visible on screen. If the number of scan lines are fixed, then how can lines of resolution vary from set to set? The answer is that TVL resolution is measured along the HORIZONTAL direction, not the vertical direction. It's a totally different performance parameter. As long as the deflection and synchronization circuitry is working correctly, you'll always have the correct number of scan lines (ignoring overscan). The TVL resolution, on the other hand, is critically affected by the quality of the video circuitry and how high frequency video information is preserved. If video bandwidth is limited, then fine image details blur or are completely lost.

I should also mention that TVL resolution refers to luminance resolution in NTSC video. This is because NTSC video carries most of the fine picture information in the luminance channel and very low resolution information for color transitions.

Now it's time to examine AVIA's resolution patterns. There are three resolution patterns supplied in AVIA. They are the WSE Resolution, 100 TVL Resolution, and 200 TVL Resolution patterns. Along with AVIA's multiburst and sweep patterns they are used to measure how well a display resolves fine detail.

The resolution patterns are similar so we'll cover them together.

The pattern has a gray background and markers which indicate percent overscan. The innermost rectangle represent 5% overscan from each edge. In the very center of the pattern is a zone plate pattern consisting of concentric circles of varying spacing. Four resolution wedges are also in the inner portion of the pattern. The outer four circles serve as corner geometry checks and also lines which are spaced at frequencies of special importance.

The Zone Plate portion of the pattern is a test of color separator performance. If you play this pattern through the composite input of a display, the varying angles and spacing of the concentric circles severely test the ability of the display's color separator (often a comb filter) to cleanly separate color and luminance portions of a composite signal. Perfect separation would produce concentric circles without any coloration or disappearance of sections of the concentric circles. You can see how perfect separation appears if you view this pattern via S-video or component connections. Most comb filters produce some cross color (rainbowing). The better the comb filter, the less cross color is present. Some displays cheat and include a notch filter which removes information near 3.58 MHz. This can avoid cross color but dramatically limits image detail.

The following assumes a COMPOSITE connection so you can test the color separator.

The 3.0, 3.58, 4.18, and 6.75 MHz corner circles all contain vertical lines spaced at that frequency. 3.58 MHz is the NTSC color carrier frequency and serves as a very severe test of color separation. 4.18 MHz is near the maximal broadcast resolution, and 6.75 MHz is the maximal resolution possible on DVD video. Many consumer displays don?t have sufficient resolution to display the finely spaced vertical lines in the 6.75 MHz circle. Front projection systems usually have enough TVL resolution to show the individual fine vertical lines.

The 3.0, 3.58, and 4.18 MHz circles also have their lower halves filled with diagonal lines at the same horizontal frequency. This is another test of comb filter type. There are several designs of comb filters available and they differ in their ability to avoid cross color. Fairly simple comb filters (1 or 2 line) can avoid cross color on vertical lines (even at the worse case 3.58 MHz), but cannot avoid cross color on diagonal lines. It takes a high grade "3D" comb filter, which uses information from more than one video frame, to avoid cross color in diagonal lines. This is why I included diagonal lines in the lower halves of the corner circles. BTW, the 6.75 MHz circle does not have diagonal lines in the lower in its lower half.

Vertical and horizontal wedges of lines (actually sinusoidals) which converge closer and closer together are used to measure resolution. Use either of the vertically oriented wedges to measure horizontal TVL. Find the point in the wedge at which the lines cease to be separate and blend together. Then read the scale markers to find your display's actual TVL resolution.

You could use a similar pattern (SMPTE Resolution) in Video Essentials. However, that pattern is older and ends at 500 TVL so it cannot test if your display is fully resolving DVD's maximal detail because the pattern never attains the limit. AVIA's resolution patterns reach the full DVD video limit or 540 TVL (6.75 MHz) and allow you to see if your display is capable of showing the full degree of DVD resolution. On a related note, one should not use the VE multiburst or sweep patterns to check DVD player frequency response because both those VE patterns fall off rapidly at high frequencies. In contrast, the AVIA multiburst and sweeps maintain nearly full amplitude out to their limits because they were synthesized directly in the digital domain. They also contain built-in markers for -3 and -6 dB signal attenuation. This is important for setting up video processor and doing equipment reviews, but I digress.

The horizontal wedges are used to measure vertical resolution. Unless you display has markedly poor vertical synch timing, you should see the full 480 scan lines resolved. Some interlace flicker is likely visible in the horizontal wedges.

One final experiment is worth doing while viewing the resolution pattern. Watch the vertical wedges while you turn your display's sharpness control up and down. You'll see the line spacing (frequency) at which the control has its greatest effect go up and down in brightness. When sharpness is adjusted to flatten video frequency response, the vertical wedges have even brightness from top to bottom. However, you still may need to compromise video frequency response to avoid ringing artifacts.

Now for the differences between the three AVIA wedge patterns:

The 100 and 200 TVL patterns differ only in the starting points of their resolution wedges. You can use either, but the 200 TVL pattern was supplied to allow finer measurement of TVL resolution in the range of most consumer displays.

The WSE (widescreen enhanced) resolution pattern is used like the 100 and 200 TVL patterns, but the display must be in 16:9 enhanced mode AND the DVD player must be set to 16:9 shaped video display.

I'll leave you with two study questions: If you can solve these, you understand how TVL resolution is defined. The answers are below, but do try to solve these yourself.

Given that the pixel map in NTSC DVD video is 720 x 480, why is the TVL limit 540 on a 1.33:1 (4:3) screen?

Is the TVL limit the same on a 1.78:1 (16:9) screen?

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Answers:

The TVL is defined over the width which equals the height. That is 720 x 3 / 4 = 540. 2.

On a 1.78 screen, the height is smaller relative to the width, but the full width is still 720 pixels. Hence, TVL resolution max on a 1.78 screen = 720 x 9/16 = 405. This is why the WSE resolution pattern's wedges end at 405 rather than 540 TVL.

AVIA Calibration Tip #6 -- Subwoofer Test Tones and Bass Management

AVIA users have occasionally wondered why nothing comes out of their subwoofers during AVIA's subwoofer setup test tones unless their speaker sizes are set to "small." The test signals are working as they should, but AVIA is probably the first test material you've encountered which starkly reveals the effects of bass management. The subwoofer test tones in AVIA do not DUPLICATE the low bass information from the five main channels (front left, center, front right, surround right, surround left) into the LFE track. We could have duplicated the low bass information into the LFE track. This would have yielded subwoofer output whether one set speaker size to "large" or "small," but would also completely obscure how bass management is actually working. With AVIA, what you get at your subwoofer is only what the bass management system in the receiver is doing, not what we put in the LFE channel. Duplicating the low bass information into the LFE channel would make more systems APPEAR to function as users expected, but wouldn't clue them into what is actually happening in their system's bass management.

You may well discover that setting main speaker sizes to "large" completely stops a receiver's bass management from routing low frequency information in the main channels to the subwoofer. This means a lot of people are finding out that the bass management in their receivers isn't quite doing what they believed. Actually getting low bass information from the main channels to the subwoofer often requires one to set the speakers sizes to "small." Most users don't expect this to be true in their equipment and are shocked to find nothing coming out of their subwoofer when speaker size is set to "large." ----- > If your system behaves this way with the AVIA subwoofer test tones, this is also what it does to low bass in the main channels of a movie sound track. < ----- You may not have realized that you have been relying on only your main speakers to reproduce low bass in the main channels and the subwoofer has ONLY been producing low bass from the LFE channel. AVIA's subwoofer setup tones reveal this facet of bass management.

Many receiver and speaker systems are actually more appropriately set to "small" size despite their having "full range" speakers. This allows the subwoofer to receive and reproduce low bass from all the channels rather than only the LFE. The "large" setting implies that the speakers are capable of reproducing the pounding LF effects one usually gets from a subwoofer in a home theater setup. At least try it both ways to see which yields better results in your system.

Sometimes bass management limitations make it objectionable to set speakers to "small." Perhaps one has full range main speakers, a high crossover frequency in the receiver, and a sub which does not integrate well in the low-mid bass. The resultant gap in coverage is difficult to fix. A more flexible audio processor or some creative feeding to the sub of bass from main speakers (set to large) + bass from the LFE track may be needed. Two channel listening of music also comes to mind as a special situation.

AVIA's subwoofer setup tones reveal how your equipment's bass management system actually deals with low bass. This gives you a chance to understand and optimize bass management on your equipment.

Black and White Level Adjustment

Let's first examine what the black level (brightness) and white level (contrast) controls actually do inside the set and the performance limitations that set the bounds within which you should keep the controls.

Within the television, there are video amplifier stages. Somewhere near the end of the video signal processing chain, just before the signal reaches the final amplifiers, the black level and white level controls operate. At the amplifier stage where these two controls operate, a small video signal undergoes amplification before being passed to the next circuitry stage. The higher the signal, the higher the associated beam current and hence more light output. (I'm ignoring the inverted polarity of the signal to simplify this discussion.)

Essentially, the black level control sets the baseline (bias) level of the amplifier stage. Think of it as setting what the lowest point (black) upon which the rest of video signal rides. The white level control sets how much amplification is performed on the signal. A higher white level setting means that video signal excursions are larger.

The combination of black level and white level controls allows you to control the amplitude and baseline of the video signal. If the display were capable of unlimited light output as video signal amplitude increases, one would only have to worry about light output and the blackness of black when setting these controls. Real life displays have limitations and exceeding these limits can damage or shorten a display's life.

I'll concentrate on CRT displays for now. The video signal is processed and eventually delivered to the red, green, and blue electron guns. A higher signal makes the gun emit more electrons and produce a brighter spot on the screen. CRT's are limited in how much beam current can be safely used. If too high an output is attempted, the phosphor at the front of the screen can be physically damaged by the electron beam. It isn't practical for you to measure the beam current so we use the proxy of beam defocusing to estimate when too much beam current is being used. As beam current increases, it becomes more and more difficult to confine it to a sharply focused beam. For most CRT's the point at which beam focus worsens is below the point at which phosphor damage can occur. By staying below this point of "blooming," you avoid immediate phosphor damage.

There are other limitations of usable beam current. Heat is produced by the current flow. Direct view CRT's use a metal mask to help direct the red, green, and blue beams to the appropriate color phosphors. A high beam current can heat up the metal mask and warp it. This is seen as a sudden shift in color of the screen and could indicate the danger of permanent mask warping. Obviously you want to keep your controls below this point.

Projection CRT's are driven at much higher beam currents, so high that liquid cooling of the phosphors is virtually required. This means that projection sets are even closer to the physical limits of the phosphors and particular attention must be paid to never overdriving the tubes.

Over time, phosphors age and solarize. The more light emission they are forced to produce, particularly if near maximal output, the faster their light output drops. This is why bright, fixed images can permanently burn themselves into a screen. By limiting beam current to reasonable levels, you slow this process and reduce the risk to your screen.

The electrons are emitted by a heated filament, the cathode, of a CRT display. The cathode is coated with special rare earth elements to improve electron emission. Over time, this coating loses its effectiveness and you notice this as a blurring of the electron beam even at low light output. The aging process causes a larger portion (other than the tip) to become involved with emitting electrons. Since the beam spot is essentially an image of the active region of the cathode, this increase in active region appears as enlarged (blurred) electron beam spot size. Higher beam currents accelerate this aging, but not to the same degree that it damages the phosphor.

Now we've gone over some reasons to keep white level down in order to protect the display. There are also imaging quality reasons. We've already mentioned the defocusing of the beam when current is too high. This blurs the image and reduces resolution. Also, running too high causes the relationship of input signal to output light to be altered. If this runs outside the "linear" range of the CRT, the relative brightness of signal levels from black to white are distorted. You perceive this as something being unrealistic or wrong with the contrast of a picture.

The high voltage supply used to produce the electron beam is often derived from the horizontal deflection circuitry of the television. If beam current demands are too high, the demand can drag down the horizontal deflection circuit. You see this as a horizontal geometry distortion. Hence the visible bending of the left and right vertical lines on either side of a Needle Pulses pattern in AVIA. Although not something that will damage a set, this type of geometric distortion degrades the image. By keeping white level down, you also avoid this problem. Some displays are designed such that this effect doesn't occur. In these displays, you never see the vertical lines bend, however you will still see blooming.

In short, white level should be set to avoid increasing the risk of permanent damage to the display, slow aging of the phosphors & cathode, and improve image resolution, gamma response, and geometric accuracy.

Black level, which determines the baseline upon which the video signal rides sets the appearance of black, not how much overall light is output by the display. This needs to be set at the lighting condition which is to be used for viewing because the correct setting varies with ambient light. Too high a black level washes out the picture. Too low a black level causes shadow details to be clipped and displayed as black.

Most consumer displays complicate setting of black level because they do not hold black level constant as overall picture level changes. That is, black is displayed differently depending on how bright the rest of the image is. This is also called imperfect DC restoration or clamping. The solution in this case is to bias the display with a moderate picture level image while setting black level. This allows you to arrive at a compromise level which works for most images.

Test patterns traditionally used a blacker-than black signal to help indicate when black level was correctly set. After all, you can't actually make the display any blacker than black so if "black" on the display is brighter than it should be the even darker (signal wise) BTB signal would appear be visible as a dark feature. When the BTB and black just appear identical, the display is correctly set to make black appear black.

Many DVD players and some video processors don't pass the BTB signal so this method of detecting when black level is correctly set doesn't always work. Also, the traditional patterns unfortunately aligned edges of pattern features with each other leading to optical illusions which confused viewers whether or not the BTB bar is visible. For these reasons AVIA uses "Black Bar" patterns to indicate correct black level. These are a pair of animated bars which move back and forth on screen. The motion makes it easy to see if a bar is visible and helps avoid the ambiguity of optical illusions of aligned, fixed lines. One bar is very near black, the other slightly brighter. When black level is correct, the darker black bar is just very barely visible. If black level is too low, one or both bars disappear. This allows one to find correct black level whether or not equipment passes blacker than black.

The situation is different on LCD's because the usual phenomena of geometry distortion and electron beam defocusing (blooming) don't occur with LCD's. Another limiting factor comes into play. LCD control circuitry in LCD projectors have a relatively abrupt point above which video signals become displayed as white. We call this "white clipping" on AVIA. electron guns.

Basically, if you set white level too high on an LCD projector you will find that near white details turn into white instead of something that is darker than white. Highlight details are hidden and the image looks solarized. Finding the white level setting which avoids white clipping is the key to maximizing LCD projector light output without degrading image quality.

As an aside, digital domain video displays can also exhibit a similar clipping effect when the video signal cannot be represented within the bit range of the system. You can sometimes see this on computer monitors displaying video from a DVD-ROM.

AVIA has new moving "white bars" in its main pattern for adjusting white level, the Needle Pulses + Log Steps pattern. You'll find a pair of near white bars which move back and forth. If you set white level too high on a LCD projector you'll see one or both of these white bars become white ( and disappear since you can't see white on a white background). The maximal white level setting is found by adjusting your LCD projector to just below the point at which the rightmost (brighter) white bar becomes white. Once that point is found, you know the max usable white setting for your LCD projector.

The Needle Pulses + Log Steps pattern in AVIA serves as a unified tool for both CRT and LCD display white level adjustment by combining tests for geometry distortion, blooming, gray scale linearity, and white level clipping into a single pattern.

Once the limits for white level are found for your display. The next step is to drop down to a white level setting which is below the max and still produces a white which appears white rather than gray.

Black level is set the same way with AVIA on both LCD and CRT projectors.

Black Level Clamping (aka DC Restoration)

Why does the appearance of black vary with different scenes?

The appearance of "black" often varies depending on the remainder of the image on consumer grade video displays. Another more technical term for how stable black remains is "DC Restoration."

Let's review how the video signal is represented as an electronic signal. (Yes, I'm ignoring phase inversions in the amplifier stages for this discussion because it would confuse the central issue) The image information is an AC signal whose voltage level corresponds to how bright the image should be at that moment of the image scanning process. In American NTSC video, the luminance runs from 7.5 (black) to 100 IRE (maximal white). Synchronization pulses are negative to -40 IRE. Notice that black is represented by the voltage level being at 7.5 IRE rather than 0 IRE. In Japanese NTSC, black is represented by 0 IRE. The reasons and ramifications of this difference would be an entire other topic.

As the signal passes through the video system, it requires several stages of amplification. Each stage is coupled to the next usually via a capacitor. The capacitor passes AC signals but not the DC offset from relative to ground to the next stage. This means that the zero level of the signal is not actually transferred from stage to stage. You get a signal which varies up and down in voltage but trying to tell by looking at the final signal, you can't tell where black should be because you don't have an absolute reference of where on the waveform is black. The appearance of black to drift up and down with the average signal amplitude.

The above would be entirely unacceptable so the video circuits are actually designed with various means of "clamping" the black level at a known level. This process of "DC Restoration" is usually imperfect in consumer sets. Hence, the appearance of black darkens and lightens depending on the average picture level. This means usually means that black is a bit lighter during dark scenes than bright scenes. Professional level monitors and projectors include circuits which produce nearly perfect black level clamping.

It has been argued whether or not perfect DC restoration is desirable. Some viewers like black to be slightly lighter during darker scenes so they can better see shadow details in difficult night scenes. Others believe this compromises the intent of the scene.

Most of use have displays which have imperfect DC restoration. How should one deal with the calibration of a signal who appearance varies? The solution used in both AVIA and VE is to use calibrate black while displaying a test pattern whose average picture level is moderate. For instance AVIA uses Black Bars with a Half Gray pattern for setting black level. This means that black level is calibrated to a compromise which satisfies most viewing situations.

It is sometimes desirable to calibrate black level intentionally with a higher or lower APL test pattern. For instance, you may have a projector which has nearly perfect black level clamping but has some light scatter. A high APL image on such a display would tend to obscure the appearance of black bars. In this instance you may prefer to use a low APL black bar pattern. On the other hand, one could argue that a higher APL pattern would compensate for the light scatter.

AVIA supplies Black Bar patterns with black background or a half white background, color bars, and in the Needle Pulses patterns. You can follow the recommendation for most situations, but you also have other choices. Just think about your goals in selecting a different APL black bar pattern.

So in short, the varying appearance of black on your display is probably normal for your display and not an indication of improper function.

Should I Set my DVD Player to Enhanced or Normal Black?

NTSC video signal levels are commonly measured in IRE units which can be converted to voltage levels. The conversion isn't important to this discussion. IRE units are easier to use so we all tend to refer to signal levels in IRE rather than volts.

100 IRE represents the brightest white. 7.5 IRE, rather than 0 IRE, is black due to historical limitation in equipment. This is true in North American video equipment. In Japan, 0 IRE is black. The offset from 0 to 7.5 IRE is referred to as "setup." Your DVD player adds the setup to the output signal to make "black" come out at 7.5 IRE if you use normal settings.

Your player also gives you the option of making black come out of the player at 0 IRE. This is the "enhanced" black setting. Why might you want to do that? Well, normal NTSC video has 100-7.5 IRE or 92.5 IRE of dynamic range from black to white. By selecting "enhanced" blacks the dynamic range is 100 IRE, a little bit larger of a voltage swing. Is this a good thing? It depends.

Your television must be told what level IRE is black. That's essentially what you are doing when you adjust the black level control (brightness). The AVIA test patterns include portions which are black and very near black so you can tell what you are doing with a known target. As long as you set the television to display black (whether it is 7.5 or 0 IRE) truly as black you're fine. This would seem to indicate that it's good to use the enhanced black setting since black gets correctly displayed. You get a larger signal dynamic range to avoid degradation, but there is a catch. The standard level for black for all your other sources is 7.5 IRE, not 0 IRE.

If your display allows you to independently set black level for each video source, then it's no problem to set the DVD video input settings to display 0 IRE as black. You'd then calibrate the other video input settings to show 7.5 IRE as black. Unfortunately, some displays don't let you independently set black level for each input. Getting one right makes the others wrong.

If the display is calibrated to display 7.5 IRE as black and you view an "enhanced" black signal, all the shadow details which between 0 and 7.5 IRE will be lost as pure black.

If the display is calibrated to display 0 IRE as black and you view standard American NTSC material which has black at 7.5 IRE, the picture will be somewhat washed out because the blackest black will actually be a dark gray.

The answer depends on whether or not your display remembers separate black level settings for each video input. If it can, then you could go ahead and use the enhanced black setting. You'll still have to calibrate your other inputs for 7.5 IRE black. For practical purposes, the increased dynamic range won't really make a difference because a no matter the dynamic range, the calibrated display still displays black as the same black and white as the same white.

Another pitfall may occur if you use video processors. Their inputs must also be configured to recognize black at the setup level you choose at the player. If you decide to use enhanced blacks, you need to think about how that affects the rest of your system. Selecting normal, 7.5 IRE black means less to worry about.

Setting Speaker Level with AVIA

Let's go back and examine what is happening when setting a system to reference level. You are attempting to set sound reproduction level such that for any given sound you produce the same loudness as was heard in a mixing studio whose system is also set to reference level. Maximum SPL level is 105 dB, but a reference tone recorded at maximal SPL would be difficult to use so reference tones are usually recorded at least 20 dB softer. In the case of AVIA's level setting tests, the encoded sound should produce a measured level of 85 db SPL when the amplifier and speaker are set to reference levels. In a larger room, the amount of energy needed from the amplifier would be larger but the same SPL level of 85 dB would need to be attained. With VE, the test tone is recorded yet another 10 dB softer thus the target SPL measurement is also 10 dB softer, 75 db SPL when the system is at reference level. Each channel needs to be adjusted to attain that signal level for the entire system to be at reference level.

Now for the oddities. Unless your receiver is THX certified, the built in test tones are not guaranteed to be the right level to set reference level. In higher end processors, the built-in tones are set to the correct level such that they can be used to set reference level, usually targeting 75 dB SPL. You have to check with the manufacturer or manual to see what target SPL to aim for when using built-in tones. For all receivers with built-in tones, you can use the built-in tones to balance the channels relative to each other, but not necessarily to set absolute level to reference.

There are complications. We don't all have identical sounding speakers and room acoustics. Some systems have large speakers and satellites. The room may absorb some frequencies more from some speakers than others. For this reason, test tones for setting channel level are usually not pink or white noise. Instead, a shaped noise whose spectral distribution is centered at a frequency which most speakers can reproduce is used. That way the effect of non-identical speakers can be partially compensated. Room effects are more difficult to avoid. This is important when one goes from one set of test tones to another. Although both sets are encoded at the correct levels in a perfect system, real world speaker and acoustics can cause the measured SPL's to slightly differ. The frequency energy distribution would have to be identical between the tones to achieve exactly the same result. This is why your calibration can be a bit different between built-in tones and those in AVIA or VE. Nothing is wrong with your equipment, it is an effect of acoustics.

Where the master volume control ends up when you are at "reference" level differs from model to model of receiver. Some processors let you set the master to 0 dB (or it automatically sets master to 0 dB) and then you adjust the individual channel levels until they each produce reference level SPL's. In such a system, 0 dB on the master volume is reference level. On many receivers, one cannot place the master volume at the 0 dB point and still have enough range in the individual channel adjustments to achieve reference level. On such receivers, the master volume level for reference is whatever setting you had it at when you set your individual channels to reference. If that setting was -12 dB, then reference level on that receiver would correspond to -12 dB on the master volume.

A feature built into AVIA's tutorial probably adds some confusion. Listening at reference level is very loud. In most home settings, a master volume of about 10 dB below reference is more comfortable for listening. You'll note that the beginner's tutorial in AVIA has you initially balance your channels at 75 dB SPL rather than 85 dB SPL. This sets the system 10 dB below reference and automatically places you in an appropriate level for home listening. It isn't until the next section that you learn to target 85 dB SPL to actually be at reference level. I think it's even easier to simply calibrate all channels to reach 85 dB SPL, then turn down the master volume to comfortable listening level, and finally recheck channel balance. Most receivers track the channel levels correctly with each other as one alters master volume, but you may wish to recheck individual channel SPL levels at your usual master volume setting to double check that your receiver tracks all the channels together as master volume is changed. If you find the individual channels are not tracking together, you may wish to readjust the channels to make them match each other at your normal listening level.

On occasion, you may be told that your subwoofer should be calibrated to produce an SPL which is 10 dB higher than the other channels. There is no need to do this with AVIA's subwoofer test tones because they are recorded 10 dB softer to automatically produce a 10 dB higher subwoofer setting when you target the same 85 dB SPL. If one wishes to be technically more correct, the 10 dB offset actually is only true if one is using an RTA to measure sound levels. We're using an SPL which would read about 3 dB lower for the same subwoofer setting. If you want to be completely correct, place sub level 3 dB lower at 82 dB instead of 85 dB SPL.

Some receivers have built-in test tones and force you to listen to them during adjustments rather than using those from a test disc. In such receivers, one can do an initial setup with the built-in tones, then re-measure with tones from the calibration DVD. For each channel, one would then note the amount of error in dB. Next go back and adjust each channel the amount needed to correct the error. It may take a few rounds to get everything right if the receiver doesn't let you use external tones.

The LFE channel level is normally already set in the receiver. On a few units you can adjust the relative LFE level. If I recall correctly, DD has a suggested LFE offset of 0 dB, and DTS suggests a 0 dB offset for music and +10 dB for movie sound tracks.

Using AVIA Phase Tests to Fine Tune Speaker Distance and Delay

AVIA's speaker phase testing signals are also useful for very accurate adjustment of speaker delays and distances. You'll need an analog RS SPL meter set to fast response in order to take advantage of this tidbit. This may seem a bizarre way to check delays and speaker distances but it is surprisingly accurate.

The phasing tests work by playing noise in the two channels being tested in phase and 180 degree out of phase intermittently. If the speaker distances and delays are both set correctly, then the in phase sounds from both speakers reinforce each other at the prime listening positioning. During the out of phase (diffuse) portion of the test, the sounds cancel. An SPL meter set to fast response can readily show the magnitude of the cancellation/reinforcement.

Start by playing the Phase left front/right front signal. Move your SPL meter slowly left and right at your listening position. If you have set distance and delays correctly the maximal SPL delta will occur in the middle of your sitting position. I get about a 6 dB needle bounce on my system. If it happens right of center, then your right speaker is either too farther away than the left speaker or delayed more than the left speaker. Conversely, if the peak SPL delta occurs left of your prime listening spot, the left speaker is too far or excessively delayed.

Once you have the front left and right speaker distanced and delayed exactly right, the SPL meter position at peak delta will be in the middle of your prime listening position. Note that position carefully. You'll need to be able to refer to that point within half an inch during the next step.

Now comes the trickery that gets the center speaker also precisely phased and delayed. The AVIA disc also has a Phase Left Front/Center test. We can take advantage of it to bring all three front speakers into very tight phase alignment. From the previous step we already know where the two front main speakers are in phase. Leave the left and right delays and speaker positions alone now. We'll next adjust the center speaker to be in phase with the left front. This places all three into phase.

Play the Phase Left Front/Center test and once more move the SPL meter left and right to find the maximal SPL delta point. Compare this new position to the one for the front mains. If all is perfect, they exactly coincide. If the left/center maximal SPL delta point is left of the left/right point, then the center speaker is either too close or insufficiently delayed. If the left/center max delta point is right of the left/right max delta, then the center speaker is too far. Move or adjust CENTER channel delay as needed to get the left/center max SPL delta to occur at the exact same place as for the left/right channels.

Your left, center, right speakers are now in phase. You'll probably note that a 1 msec adjustment in channel delay makes for a considerable shift in max SPL delta position. After all, that is about a 1 foot speaker distance equivalent. Use very small speaker movements to fine tune the center speaker into phase alignment.

Put your head at the center of the max SPL delta position and listen to some stereo and 5 channel material. You will be pleased with what has happened to sound imaging in your system.

Moving your speakers to achieve exact phase match isn't the entire story. One must also position the speakers with relation to room acoustics to smooth frequency response. Sometimes, moving speakers into exact phase also moves one or more of them into positions that yield uneven frequency response. In such cases, some compromise is needed to address both imaging and frequency response concerns. Happily, the home theater sound processor does have delays and these can sometimes help bring speakers into phase, while still keeping them closer to best tonal balance position.