Super 8 fullcoat mag reader

[friolator]

Back in the 1970s, Super8Sound in Cambridge MA was formed by some of the same MIT folks who were responsible for the innovations that made a lot of the portability of 16mm sync sound production possible.

They wanted to do the same with Super 8 film: crystal sync cameras, double system sound, etc. A lot of what they did involved modifying existing hardware, including Uher and Sony portable reel-to-reel decks. They made a crystal sync module you could plug in, and some mechanical mods to accept Super 8 perforated magnetic audio tape.

One of the Uher 4000 decks they modified:

Fullcoat magnetic audio tape:

You’d edit like you would with 16mm or 35mm. There were a handful of flatbed editors out there, but msotly it was sync blocks and viewers, and you could do multi-track synchronous sound this way. Pretty cool setup, but ultimately ill fated because of portable videotape coming along in the early 80s.

Anyway, once you had your final mixes, the tapes tended to be larger than would fit on the portable decks so they also modified some studio decks to take this format. We have one but have been unable to resolve the speed to capture in sync because we were missing external devices for that, and it’s all pretty much undocumented.

Technics RS1500 they modified: note the optical perf reader near the top right pinch wheel, and the little blue lever arm to apply pressure to the tape to keep it in line.

This blue “Sync” port was a mystery.

After many years of trying to find the hardware and hitting dead ends, this past week I invited a local video engineer who was at MIT at the time, and worked on a lot of similar systems to come over and have a look. Ahead of his visit I printed up all the schematics for the deck and built a breakout cable. I was also able to determine which pins on the Sync cable were outputting the pulse from the perf reader.

Pretty close to 24fps, but not quite:

Well, he came over here yesterday morning and we traced everything. It turns out all they’re doing is using the pitch adjustment on the deck to increase or decrease speed, beyond the range of the knob on the front.

A little experimentation and we figure out how to control the speed.

All we need to do is feed about 10-12 volts into one of the pins and the speed will go up and down accordingly.

So the plan here is to read the pulses with an Arduino, then increase or decrease the voltage going to that pin on the Sync connector accordingly. It’s been a while since I’ve done anything with an Arduino, but it only took a few hours today to get it all set up on a bench with a signal generator.

Simulate the incoming pulse:

and figure out the difference:

Next week I’ll get a spare PWM->Voltage board set up to feed back into the deck. Same sort of thing I’ve used on my CNC Lathe to control the spindle - you feed it a PWM signal and the voltage adjusts as you change the duty cycle.

With this, we should be able to run this thing at exactly 24fps (or 18, or 25, since they’ll all be selectable via a knob). The plan is to put it all in a 1U rackmount box and mount it above or below the deck. I’ll post more pictures as I finish it up. Waiting on some more parts to arrive over the coming days.

[cpixip]

@friolator

It turns out all they’re doing is using the pitch adjustment on the deck to increase or decrease speed, beyond the range of the knob on the front.

That was the approach at that time, also with less professionel equipment than the Uher. Mine was portable Cassette-Tapedeck using 4 instead of the usual 2 channels for recording. Two of the channels were used for recording left and right audio channels, the remaining two channels carried sync-channels for modified Super-8 cameras - basically a short beep for every frame taken. The full signal used the full width of the tape, so the compact- cassette could be used only on one side. Playback of the recorded sound onto the magnetic track of the Super-8 film was done via a sound projector sync’ed very much the same as the more advanced setup you are investigating here.

Very interesting journey you are doing here. Keep us updated!

[friolator]

Portable cassette worked in the way Nagras worked: the speed of the motors could fluctuate slightly because the pilotone generated by a fixed reference crystal would get recorded to the tape and effectively map out the motor fluctuations as a tone or pulse. Then on playback, a resolver would correct for those fluctuations on the fly. Some of them did this the way you describe, using a separate track. Some (like Nagra’s neopilot system) recorded the pilotone signal right in with the audio across two tracks, with the pilotone out of phase between the tracks so you can’t hear it, but the resolver can pick it up.

On our deck, there are more pins on the Sync connector than we need for our purposes, but my suspicion is that the other pins were to send a recorded pilotone signal out to the resolver. In the portable decks, you could choose how you wanted it to work - taking a crystal reference from an external box (so the camera and tape deck were completely independent of one another), or recording a pilotone reference signal from a crystal sync camera that was tethered to the tape deck. In the latter case, you’d have to then resolve the recording on playback using that pilotone signal.

I don’t know which one was used more often (probably a mix because while crystal is easier and cleaner, it was also more expensive), but so far the fullcoat we’ve seen from clients has all been on big reels, which suggests they resolved the sound when offloading from the cassette/nagra/fullcoat portable recorder so that the duplicate was locked to 24fps or whatever frame rate they used. From there, we can use the perf detection method and it’ll be in sync.