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NBTV Mixamp

PostPosted: Tue Apr 22, 2014 1:15 pm
by Lawnboy
Now that my Flying Spot Scanner is nearly complete, the next step is to make a video mixing and amplifier board. I never intended to have a sync inserter built into the scanner, but instead have some kind of external mixer and clamp able to handle multiple photocells for better lighting control. I recently found a junked video title maker from the early 90s that should function well as a mixing board after I refit it with the new circuits and a few slide pots.
Here are the preliminary specs:
8 faders - 4 for photocells (just like the original at the BBC) making up the A buss, the 5th wired as a cross fader between the caption scanner photocell and internal waveform/sync generator making up the B buss, a cross fader between the two busses, a master level control, and finally a separate audio fader for an external mic.
A modified version of Karen Orton's Standards Processor (Vol. 38, No. 1), with black level control, sync insertion and gamma adjustment.
+12v outputs to power the photocells
A timing generator with optional missing sync (most likely the club's triple waveform generator)
Aperture correction (a future addition)

First off I would like to get the main mixing circuit worked out. I was looking at a few virtual earth audio mixing circuits here and here Now, since the video signal is not an AC signal, but has only a positive value, would I be able to use a single supply op amp? Would it be as simple as changing the component values of those circuits? Also I was considering using either a stereo 1/8" audio jack to carry both power and video for the photocells on the same cable. Would it be better to try a phantom power approach? Otherwise the mixer might have to deal with a momentary 12v spike when the cable is plugged in. So is all this doable, or am I in over my head again?

Re: NBTV Mixamp

PostPosted: Tue May 03, 2016 9:05 am
by Lawnboy
Hello all. I have recently returned to the Mixamp project, and much has changed since I first started this thread (I don't even have that couch that the title maker is resting on anymore!)
The circuitry is almost finished, and everything is working properly, except for one problem that I can't seem to get past despite weeks of tweaking.
The heart of the circuit is taken from Karen Orton's NBTV Standards Processor in Vol 38 No 1. The modified diagram is shown below. The video input from the photocells does not contain sync pulses, so instead the sync pulse is derived from the club's triple waveform generator, which also acts as the master sync generator.
The problem is that the black level is sitting at 600mv on the output instead of 300mv, and I can't seem to change it. The scope trace shows the 8 level grey-scale pattern, DC coupled from the TWG prior to sync insertion (black at 0v).
Is there a way to half the black level while keeping peak white at 1v? The 91k resistor on the far left has been already been disconnected. Further scope traces are available.