Whereas DIN sync uses two different signals, the clock pulse signal and the run/stop signal, tape sync combines these into one signal, to fit on a single channel of a multitrack tape recorder. The stunningly simple way it does this is by completely ignoring the clock pulses when run/stop is low, and faithfully recording them when run/stop is high, starting with the next pulse. In other words, it logically ands the two signals together.
If you're wondering why it doesn't cause problems that the sequencer doesn't know which tempo it's playing at until after it's started playing, bear in mind that it doesn't ever know which tempo it's playing at. Only the clock is concerned with the timing of the pulses. The sequencer merely counts them.
The next part of making the signal tape friendly, now that it fits on a single channel, is to guarantee it's in an audible frequency that audiotape can be reasonably expected to record and play back, regardless of the tempo. This simply involves modulating it with a square wave, which is a lot easier to do than it sounds. When the clock pulse is high, a high frequency square wave is played; when the clock pulse is low, a comparatively low frequency square wave is played.
So tape sync consists of a 1.3 kHz carrier signal, modulated to 2.1 kHz whenever there's a clock pulse. Every time you count 24 such switches to the higher tone and back again, a quarter note has passed.
(Technically, modulating the frequency like this is a simple form of frequency modulation. Modulating and demodulating data this way is what a modem does, and indeed modem stands for modulator/demodulator. Computers communicated over tape the same way they communicated over landlines: by turning data into sounds and back again.)
...a means of synchronizing tracks on tape. First I used pulse recording, but that would feed through to other tracks. It sounded like a buzzsaw. Then I read about these FSK modem chips with phase locked loops, and decided to use them — much better!
— Ralph Dyck, 2010
- "MC-8 manual" Roland, 1979
- "MC-4 service notes" Roland, Jan 1982
- "Exclusive Interview With Ralph Dyck, Godfather of the MC-8" Peahix, Jan 2010