I attached the schematics of both the Electrohome VNB and my design.
The VNB's most attractive property is that it can be fully driven only with 0.7V input signal and its design inherently allows super bandwidth, so basically not much preamplification is needed. My design is based exactly on this advantage.
A detailed description about the VNB you can find in the service manual section of this site, but in a few keywords:
-The VNB has a spot kill feature +5V kills the beam current (internal pull up resistor also included)
-The VNB has internal brightness controll, with +/-2.5V controll range, also this is paired with a clamping circuit, to minimalize circuit /input level drift.
-Has a proportional current sens output and an over current detector.
-Needed supply lines: +/-85V (for end stages), +/-15V (for auxiliary circuits), +/-5V (for low level circuits including high freq. circuits)
Let's my design:
It still uses the good old AD835, but in an unusual application. The whole thing is centralized around the fact that the AD835 has an output function of W=X*Y+Z.
The video input is directly tied to the input Y, the contrast signal is fed to input X, at input X there is an analog switch, that can toggle between the contrast voltage (normal operation) and the ground (blanking the input). Then there is the Z input where again there is an analog switch, that is toggled between the OSD/testpattern signal source and the ground.
-During normal video, the contrast voltage is fed to X and Z is grounded. so the OSD/testpattern is bypassed. The analog switch might have a crosstalk, but even then it is conducted down to the ground through its Rds(on), so in fact a quite good rejection is achieved this way. The output therefore is: W=Y*X+0
-When OSD/test pattern mode is on the X input is grounded (aka. muted video input) but the Z (OSD) signal is let through ie. W=Y*0+Z.
-During blanking the Marquee switches back to OSD source and injects blacker than black level through the Z input (OSD), this however may not be the case with other projectors, so an optional blanking circuit was also added to the design "just for sure".
The AD835 when properly used has about -60dB rejection to the input signal when it is multiplied with 0, on the other hand the dynamic range of each color is only 48dB (as it has 8bit resolution)
However in this way the OSD/test pattern is not affected by the contrast setting, but I don't think this would be a real problem.
So all needed feature was done only by playing around the AD835, but nothing was put in the video path, only the AD835, but you can't omit that, since some sort of contrast controlling is needed anyway...
The AD835 is slew rate limited by the first hand, that means high output voltage cost a lot of bandwidth, on the original 02p VIM therefore the output was not even properly terminated for the 75Ohm transmission line, still about 25% signal loss occuring on the coax cable through the VNB. By putting this small board right at the VNB you gain this 25% signal level (that translates to bandwidth overhead), and the input level requirement is still the same.
With this circuit I measured around 1,5ns rise/fall times at 0.7V output (1 pixel at 1080p 72Hz is 5ns wide) which is still plenty fast.
There is a separate low noise +/-5V power supply for each board that also feed the adjacent VNB's +/-5V line too.
The gamma correction is also "interesting" for the blue. With these multipliers used in CRT applications it is often overlooked that all inputs are actually high bandwidth capable, So unlike to the VIM the gamma is not added to the output signal as a serial element.
he gamma is what gave me the most headache... As I saw from calibration curves is that the blue drops the high end with linear drive. So I aimed adding more gain dynamically based on the input signal. The first version was a simple comparator so under the threshold level the gain was A over the threshold the gain was B, but as turned out this is not really what was needed, the second trial was to linearize the comparator so it would give a proportional output to the input signal (the higher the input the more it „push”), this was far better but then it turned out that at high frequencies like 1080P 72Hz the comparator introduced quite some lag in the signal, so the input signal was at it s peak with a single pixel, but the gamma generator output was lagged by several nanoseconds, so it was actually modulated the gain in the wrong way, with slower signals it tracked fine at least. There were other problems though. The correction waveform and the contrast singnal was summed at an emitter folower’s base, being that way the summing was carired out by changing the ratio of the resistors on the base, this was double bad on one hand the resistance needed to be as low as possible, but still needed to be around 1k from the corrector circuit, the other bad was the gamma rate was seriously interacted with the actual contrast setting. You set the gamma rate and your contrast went away as well and vice versa.
To overcome these problems active summation was made thereafter, knowing the input of the multiplier is X=(X1-X2). Unfortunately the contrast and the gamma signal have to be added not subtracted. Fortunately the multiplier knows elementary math well: Gamma-(-Contrast)=Gamma+Contrast. Obviously I chosed to invert the contrast signal which is a quasi DC signal anyway, rather than the precious gamma signal. With the newest solution the gamma curve is derived by a simple voltage divider from the input signal (no lag), since the impedace was kept at quite low level we don’t have to care about frequency response either (I tested a 200 Ohm SMD trimmer before that did not affect the bandwidth at any setting in an 50 Ohm terminated system even without added peaking) and here the resistance even smaller by an order. This way gamma and contrast signals are not interacting.
After all it turned out this gamma concept is essentially the same as what was made in the Barco 909 -which I did not understand for a long time how/why is it working… It is a popular trick to disable the gamma on those barco boards but there the input derived signal was switched by conventional multiplexers, with only 4 fixed settings and even the resistance wasn’t kept at a very low levels so they had to peak those dividers, also in the barco the gamma is made in a separate AD835, that made a difference in signal chain integrity between green and the other colours as well.
Attachments:VNB_DB_V3b.pdf (58.15 KB)
VNB.PDF (73.5 KB)
