LONG-PLAY VARIANTS
As new formats were introduced over the years, each capable of achieving a higher density of signal information on tape than its predecessors, attention was turned to the possibility of increasing the playing time of the established formats. Thinner tapes were introduced to pack four hours’ recording time into a standard VHS cassette (E240 package) and then five hours in the E300 variant. These thin tapes led to mechanical problems with some (especially older) deck mechanisms, and an alternate approach was adopted: reducing the forward speed of the tape and narrowing the video tracks.
In VHS-LP mode the rate of forward progress of the tape is halved to 1.17 cm/s and special narrow heads on the drum write video tracks of half width: 24.5 microns. The track layout for this system differs only in this respect from that given in Fig. 13.3(a). The picture qual- ity suffers surprisingly little, though the slow forward tape speed limits longitudinally recorded audio response to about 6 kHz. To overcome the degradation of vision S/N ratio on VHS-LP special noise- reduction systems are used in the luminance signal-processing circuits. To satisfy the one-field-per-head sweep requirement, the head-drum rotation speed remains at 1500 r.p.m. The narrow LP video heads are usually incorporated on the same ferrite chips as the standard- play heads, and have the same azimuth-offset characteristics.
