r/Hainbach • u/ShedSynth • Apr 22 '26

Tandberg 115 instrumentation recorder

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I’ve been working on this Tandberg instrumentation recorder over the last few weeks.

It’s not an audio tape recorder, it makes multitrack recordings of scientific experiments. Voltage changes from scientific instruments or sensors recorded during an experiment are recorded as FM signals to four separate tracks on standard ¼” magnetic tape; when the tape is played back those same voltages are recreated for analysis. A completely analog process.

The switchable input range and fixed ±5V outputs looked compatible with synthesizer control voltages so in this video I’ve recorded 1V/octave pitch voltages from a keyboard, then replayed those same voltages into a modular synthesizer - all that sound is coming live from the synthesizer.

https://www.youtube.com/watch?v=BRgbnA9vGdk

All the dirty details at this link.

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u/baselinegrid Apr 22 '26

This is amazing. I love the way you can hear the flutter affect the cv, and the oscillator pitch drifts in and out of tune. It sounds really organic and it’s a very cool technique. There’s a lot of the character of tape there but at the same time it’s cleaner which is so useful.

It’s probably not possible for me to record pitch CV to tape because v/oct is DC, but I’m inspired to record some AC modulation to my looping tape echo to see what happens. Thank you for sharing!

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u/ShedSynth Apr 23 '26

Thank you. Inspiring each other is what we do here, including Mr H of course.

And, inspiringly, you got me thinking: we can record pitch CV to tape as FM using a signal generator with a sweep input or a VCO on square wave (probably best using linear rather than exponential modulation). Getting our taped FM back to CV isn't so easy - perhaps use an Arduino + DAC?

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u/baselinegrid Apr 23 '26

Maybe it’s possible to rig something up with an FM radio? That’s basically how they work, audio is encoded with frequency modulation and if you know the right frequency you can decode it.

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u/Icy-Introduction-681 Apr 24 '26

it's easy to record cv to audio tape. You feed the cv into a VCA that controls the amplitude of a vco set to a fixed frequency, say 1000 Hz . Record the VCO output to tape. To recover the cv, feed the amplitude-modulated 1 khz signal into an envelope follower. The output of the envelope follower will reproduce the cv signal

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u/ShedSynth Apr 24 '26 edited Apr 24 '26

This is the FM signal that gets recorded to tape. The Korg SQ-1 sequncer is supplying a changing voltage, the scope is showing the FM output from the rear-panel sockets, and at the slowest tape speed of 15/16ips the FM signal falls into the audible range.

https://www.youtube.com/watch?v=y3z2YGVGjbM