TAPE PATH
Apart from V8 format which (where a flying erase head is used) has no need for any stationary heads at all, the progress of the tape through the deck is fairly standardised between formats. On emerging from the cassette feed (left side) spool, the tape is leaned on by a spring-loaded feeler (back-tension pole) which governs the friction applied to the feed spool, either by a very simple felt-lined tension band around the turntable (a purely mechanical negative feedback system) or by an electrical feedback system which governs the (reverse) current in the direct-drive feed spool motor. In the latter case the tension-arm position sensor is generally some form of optocoupler. By means of the feeler the back-tension of the tape is regulated to a constant value regardless of the weight of tape on each spool.
Next the tape passes over the full-erase head which during record mode generates a strong a.c. field at about 60 kHz to remove all previously recorded magnetic tracks from the tape. The erase-head’s gap is sufficiently long to accommodate any vertical movement of the tape ribbon. For VHS an impedance-roller is sometimes provided downstream of the erase head to remove any tension fluctuations in
the tape, now approaching the head-wrap and its entry guide, the latter being part of the moving tape-loading system already described. The inclined ‘shelf ’ (rabbet) machined with great precision in the lower drum assembly guides the tape throughout its helical path around the head drum. To bias the tape downwards onto the rabbet a slanted pole is used at each end of the head-wrap.
On leaving the head-wrap, the tape’s run-off angle is governed by the exit guide – for VHS this again is part of the tape-threading system; for some V8 machines the entrance and exit guides are rigidly fixed as parts of the head-casting assembly itself. Next is encountered the fixed head assembly used for audio and control-pulse transfer. The audio head is at the top of the assembly and the control-track head at the bottom; in some VHS designs another impedance roller is present between video and audio/control head assemblies, where it prevents sound flutter by smoothing the progress of the tape ribbon. Finally the tape reaches the capstan which (aided by the heavily sprung pressure roller) has pulled the ribbon through the entire path so far described. The capstan itself is a precision-machined shaft 1–3 mm in diameter, on which, in the very common direct-drive system, is mounted the actual rotor of the drive motor below. The pressure roller is made of rubber and is typically 10 mm in diameter; its mounting is often designed to permit a degree of axis-tilt in order to make it self-aligning against the capstan shaft, whereby the pressure is even over the full width of the tape ribbon.
The slack paid out by the capstan is taken up by the take-up reel on the right-hand side of the cassette. Although the ‘feed-in’ rate is constant, the required take-up spool speed varies tremendously, depending on the amount (hence diameter) of tape on the spool. What is required is a constant torque in a clockwise direction, and it is achieved by a slipping clutch or by a suitably regulated current through the reel motor, which in some videorecorders is a direct- drive type.