There was always a desire to know how much recording or playback time remained on a cassette, especially during recording. The time remaining feature first appeared on the Sony SL-2500 as a series of six green bars (illuminated by LEDs) that went dark as the tape was used up. Later models were introduced that, when prompted, would show the time remaining in hours and minutes. No method was very accurate nor did it need to be. Recording was always an overkill affair. If a movie ran 100 minutes then you grabbed a 120 minute cassette just to be sure you got it all. But the way the tape remaining data was computed was very clever. During playback or record a special processor used the rotation of the two drive reels (inside the machine) along with the tapes speed to calculate a measurement. Each reel had circular slots or mirrored surfaces on the bottom that used an optical LED to switch a circuit. The capstan also had a sensor that pulsed a signal as it turned. These three pieces of rotational information were counted and then arrived at a number that was close to the tape still remaining on the supply reel. The number was accurate within about ten minutes either way and the tape had to be recorded at the same speed throughout to maintain accuracy. Mixing speeds on the same tape confused the processor, although it will still could be relatively accurate. No calculation was performed during step-motion, double play, Betascan, BetaSkipscan, or single-frame advance. ©Misterßetamax go to homepage