There is always a desire to know how much recording or playback time is left 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, 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 an L-500 (120-minute) cassette just to be sure you got it all. The way the tape remaining data is computed is very clever. During playback or record a special processor users the rotation of the two drive reels (inside the machine) along with the tape's speed to calculate a measurement. Each of these reels has circular slots or mirrored surfaces on the bottom that uses an optical LED to switch a circuit. Some designs used the reels magnetic switching as it passed by special pickups. The capstan also had a sensor that sent a signal as it turned. These three pieces of rotational information are counted, and a number is decided that is close to the tape still remaining on the supply reel. This is accurate within about ten minutes either way, and the tape has to be recorded at the same speed throughout to maintain accuracy. Mixing speeds on the same tape confuses the processor, although it will still be fairly accurate. No calculations are performed during pause, fast play, Step-motion, Betascan, BetaSkipscan,, or single-frame advance. ©Misterßetamax go to homepage