NBTV Mixamp
Posted: Tue Apr 22, 2014 1:15 pm
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 http://www.all-electric.com/schematic/simp_mix.htm and here http://sound.westhost.com/articles/audio-mixing.htm. 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?
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 http://www.all-electric.com/schematic/simp_mix.htm and here http://sound.westhost.com/articles/audio-mixing.htm. 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?