[Sideband]

Back in the day, there was a correct way of setting up playback levels using a calibrated tape recorded with a 1Khz signal. I seem to recall two levels being on the tape, one at 0dB and one at -20dB.

The record levels were set up in a similar way with a calibrated signal generator and a special 'service' tape. Firstly the bias had to be adjusted correctly then signals were fed in at -20dB and 0dB, the idea being that there was minimal difference between 'A' and 'B' (off-tape) monitoring.