From Encyclopedia PRO
VHS (Video Home System) is a consumer-level video standard developed by JVC and launched in 1976. Originally VHS was an acronym for Vertical Helical Scan (a reference to the recording system used) but was later changed to the more consumer-friendly Video Home System.
VHS format history
The Video Home System, better known by its abbreviation VHS, is a recording and playing standard for video cassette recorders (VCRs), developed by JVC (with some of its critical technology under lucrative licensing agreements with Sony) and launched in September 1976. There is a relatively popular belief that VHS officially stands for Video Home System. In fact it initially stood for Vertical Helical Scan, after the relative head/tape scan technique (making the popular version a backronym).Some early reports claim the name originally stood for Victor Helical Scan System. In the absence of an authoritative source for the claim that Video Home System is the official meaning, such claims for about this new meaning should be viewed skeptically. The JVC VHS trademark Web site makes no such claim.
VHS became a standard format for consumer recording and viewing in the 1980s and 1990s after competing in a fierce format war with Sony‘s Betamax and, to a lesser extent, Philips‘ Video 2000. VHS initially offered a longer playing time than the Betamax system, and it also had the advantage of a far less complex tape transport mechanism. Early VHS machines could rewind and fast forward the tape considerably faster than a Betamax VCR since they unthreaded the tape from the playback heads before commencing any high-speed winding. Most newer VHS machines do not perform this unthreading step, as head-tape contact is no longer an impediment to fast winding, due to improved engineering.
A VHS cassette contains a ½ inch (12.7 mm) wide magnetic tape wound between two spools, allowing it to be slowly passed over the various playback and recording heads of the video cassette recorder. The tape speed is 3.335 cm/s for NTSC, 2.339 cm/s for PAL. A cassette holds a maximum of about 430 m of tape at the lowest acceptable tape thickness, giving a maximum playing time of about 3.5 hours for NTSC and 5 hours for PAL at “standard” (SP) quality. Most cassettes have lower recording times because they use thicker tape, which helps avoid jams; careful users generally avoid the thinnest tapes. More recent machines usually allow the selection of longer recording times by lowering the tape speed: LP mode (for PAL and some NTSC machines) halves the tape speed and doubles the recording time, while EP mode (for NTSC and some newer PAL machines, aka SLP mode) drops the tape speed to one-third, for triple the recording time. Of course, these speed reductions cause corresponding reductions in video quality; also, tapes recorded at the lower speed often exhibit poor playback performance on recorders other than the one they were produced on. Because of this, commercial prerecorded tapes were almost always recorded in SP mode. The only exceptions were “discount” tapes, usually containing children’s cartoons or older shows, usually recorded at SLP speed but sometimes including Hi-Fi audio to help enhance sound quality. An unofficial LP mode with half the standard speed exists on some NTSC machines, but is not part of the VHS standard.
As with almost all cassette-based videotape systems, VHS machines pull the tape from the cassette shell and wrap it around the head drum. VHS machines, in contrast to Betamax and Beta’s predecessor U-matic, use an M-loading system where the tape is drawn out by two threading posts and wrapped around the head drum (and other tape transport components) in a shape roughly approximating the letter M. (Betamax and U-Matic use a rotating loading disk which more closely approximates the shape of the letter U, hence the name U-Matic.) VHS tapes have approximately 3 MHz of video bandwidth, and a horizontal resolution of about 240 discernible lines per scanline . The frequency modulation of the luminance signal makes higher resolutions impossible within the VHS standard, no matter how advanced the recorder’s technology. Signal-to-Noise ratio of the image signal is around 43 dB. The vertical resolution of VHS (and all other analog recording methods) is determined by the TV standard — a maximum of 486 lines are visible in NTSC and a maximum of 576 lines in PAL.
The video bandwidth is achieved with a relatively low tape speed by the use of helical scan recording of a frequency modulated luminance (black and white) signal, to which a frequency-reduced “color under” chroma (hue and saturation) signal is added. In the original VHS format, audio was recorded unmodulated in a single (monaural) linear track at the upper edge of the tape, which was limited in frequency response by the tape speed (about 100Hz-8Khz with 42dB S/N ratio at SP). More recent hi-fi VCRs add higher-quality stereo audio tracks (20Hz-20Khz with more than 70dB S/N ratio at SP) which are read and written by heads located on the same spinning drum that carries the video heads, frequency modulated to the unused frequency range in between the chroma and luma signals. These audio tracks take advantage of depth multiplexing: since they use lower frequencies than the video, their magnetization signals penetrate deeper into the tape. When the video signal is written by the following video head, it erases and overwrites the audio signal at the surface of the tape, but leaves the deeper portion of the signal undisturbed. The excellent sound quality of hi-fi VHS has gained it some popularity as an audio format in certain applications; in particular, ordinary home hi-fi VCRs are sometimes used by home recording enthusiasts as a handy and inexpensive medium for making high-quality stereo mixdowns and master recordings from multitrack audio tape.
Of course, for backward compatibility, hi-fi VCRs still write the linear audio track during recording and can automatically read it during playback if the hi-fi audio is not present.
A peculiarity of VHS machines is a jittering dot at the bottom of the screen, corresponding to the point at which the VCR’s electronics switch from one head to the other as the rotating head drum completes reading a stripe of video. The “switching point” used to be obscured in older TV sets which tended to overscan more than newer sets.
Some higher-end VHS and S-VHS VCRs once offered “audio dubbing” and “video dubbing” functions. These would move the tape past the heads and keep the video unchanged while recording new linear audio or keep the linear audio unchanged while recording new video, respectively. This was useful, for example, for laying a song over a previously edited-together montage of short video clips that were the same total duration as that song. Without the dubbing features, this task would have required the tape to be copied to another tape which would cause generational loss. Due to the different ways in which linear and HiFi audio are recorded, these kinds of dubbing were not possible with the HiFi tracks. Another high-end feature was manual audio level control, which made the VHS HiFi format much more useful for high-quality audio-only recording purposes as discussed above. Some higher end machines, particularly S-VHS VCRs made by JVC, still offer audio and video dub features, though most modern VCRs do not.
Another linear control track, at the tape’s lower edge, holds pulses that mark the beginning of every frame of video; these are used to fine-tune the tape speed during playback and to get the rotating heads exactly on their helical tracks rather than having them end up somewhere between two adjacent tracks (a feature called tracking). Since good tracking depends on the exact distance between the rotating drum and the fixed control/audio head reading the linear tracks, which usually varies by a couple of micrometers between machines due to manufacturing tolerances, most VCRs offer tracking adjustment, either manual or automatic, to correct such mismatches.
The control can additionally hold index marks. These are normally written at the beginning of each recording session, and can be found using the VCR’s index search function: this will fast-wind forward or backward to the nth specified index mark, and resume playback from there. There was a time when higher-end VCRs provided functions for manually removing and adding these index marks — so that, for example, they coincide with the actual start of the program — but this feature has become hard to find in recent models.
VHS format variations
Several improved versions of VHS exist, most notably S-VHS, an improved analogue standard, and D-VHS, which records high definition digital video onto a VHS form factor tape. Devices have also been invented which directly connect a personal computer to VHS tape recorders for use as a data backup device. W-VHS caters for analog high definition video.
Another variant is VHS-C (C for compact), used in some camcorders. Since VHS-C tapes are based on the same magnetic tape as full size tapes, they can be played back in standard VHS players using a mechanical adapter, without the need of any kind of signal conversion. The magnetic tape on VHS-C cassettes is wound on one main spool and uses a gear wheel to advance the tape; the wheel and spool can also be moved by hand. This development hampered the sales of the Betamax system somewhat, because the Betamax cassette geometry prevented a similar development.
There is also a JVC-designed component digital professional production format known as Digital-S or (officially) D9 that uses a VHS form factor tape and essentially the same mechanical tape handling techniques as an S-VHS recorder. This format is the least expensive format to support a pre-read edit. This format is most notably used by Fox for some of its cable networks.