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The Video Home System (better known by its abbreviation VHS ) is a video tape recording standard developed during the 1970s by a Japanese company, JVC. It was released to the public during the latter half of the decade. During the late part of the 1970s and the early 1980s, the home video industry was involved in the VHS vs. Betamax war, which VHS would eventually win. Advantages of VHS include longer playing time, faster rewinding and fast-forwarding, and a less complex tape transport mechanism. The open standard used for VHS technology allowed mass production without licensing costs. VHS would eventually succeed as the dominant home video format, surpassing others by the mid-1980s and into the 90s.
In later years, optical disc formats began to offer better quality than video tape. The earliest of these formats, Laserdisc, was not widely adopted, but the later DVD format eventually did achieve mass acceptance and took over in film studios, then retailers, and finally video rental stores. By 2006, the United States had stopped releasing new movie titles in VHS format, opting for others such as DVD and Blu-ray Disc. On December 31, 2008, the last major United States supplier of pre-recorded VHS tapes, Distribution Video Audio Inc. of Palm Harbor, Florida, shipped its final truckload. Currently, most of the VHS tapes being produced are blank.
Technical details
Video recording
A VHS 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. Other speeds include LP and EP/SLP which double and triple the recording time, for NTSC regions. These speed reductions cause a slight reduction in video quality (from 250 lines to 230 analog lines horizontal); 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.
VHS tapes have approximately 3 MHz of video bandwidth, which is achieved at a relatively low tape speed by the use of helical scan recording of a frequency modulated luminance (black and white) signal, with a down-converted "color under" chroma (color) signal recorded directly at the baseband. Because VHS is an analog system, VHS tapes represent video as a continuous stream of waves, in a manner similar to analog TV broadcasts. The waveform per scan-line can reach about 160 waves at max, and contains 525 scanlines from the top to the bottom of the screen in NTSC (480 visible). PAL variants have 625 scanlines (576 visible). In modern-day digital terminology, VHS is roughly equivalent to 333x480 pixels.
JVC would counter 1985's SuperBeta with VHS HQ, or High Quality. The frequency modulation of the VHS luminance signal is limited to 3 megahertz which makes higher resolutions impossible, but an HQ branded deck includes luminance noise reduction, chroma noise reduction, white clip extension, and improved sharpness circuitry. The effect was to increase the apparent horizontal resolution of a VHS recording from 240 to 250 analog (equivalent to 333 pixels from left-to-right, in digital terminology). The major VHS OEMs resisted HQ due to cost concerns, eventually resulting in JVC reducing the requirements for the HQ brand to white clip extension plus one other improvement .
In 1987 JVC introduced the new format called Super VHS which extended the bandwidth to over 5 megahertz, yielding 420 analog horizontal (560 pixels left-to-right).
Audio recording
Original linear system
In the original VHS format, audio was recorded as baseband in a single linear track, at the upper edge of the tape, similar to how an audio compact cassette operates. The recorded frequency range was dependent on the movement of the tape past the audio head, which for the VHS SP mode, resulted in a mediocre frequency response of roughly 100 Hz to 10 kHz. The signal-to-noise ratio was an acceptable 42 dB. Both parameters degraded significantly with VHS's longer play modes, with EP frequency response peaking at 4 kHz.
More expensive decks offered stereo audio recording and playback. Linear stereo, as it was called, fit two independent channels in the same space as the original mono audiotrack. While this approach preserved acceptable backward compatibility with monoaural audio heads, the splitting of the audio track degraded the signal's SNR to the point that audible tape hiss was objectionable at normal listening volume. To counteract tape hiss, decks applied Dolby B noise reduction for recording and playback. Dolby B dynamically boosts the mid-frequency band of the audio program on the recorded medium, improving its signal strength relative to the tape's background noise floor, then attenuates the mid-band during playback. Dolby B is not a transparent process, and Dolby-encoded program material will exhibit an unnatural mid-range emphasis when played on non-Dolby capable VCRs.
High-end consumer recorders took advantage of the linear nature of the audio track, as the audio track could be erased and recorded without disturbing the video portion of the recorded signal. Hence, "audio dubbing" and "video dubbing", where either the audio or video are re-recorded on tape (without disturbing the other), were supported features on prosumer editing-decks. Without dubbing capability, an audio or video edit could not be done in-place on master cassette, and requires the editing output be captured to another tape, incurring generational loss.
Studio film releases began to emerge with linear stereo audiotracks in 1982. From that point onward nearly every home video releases by Hollywood featured a Dolby-encoded linear stereo audiotrack. However, linear stereo was never popular with equipment makers or consumers.
Hi-Fi audio system
Around 1985, JVC added Hi-Fi audio to VHS (in response to Betamax's introduction of Beta Hi-Fi.) Both VHS Hi-Fi and Betamax Hi-Fi delivered flat full-range frequency response (20 Hz to 20 kHz), excellent 70 dB signal-to-noise ratio (in consumer space, second only to the compact disc), dynamic range of 90 dB, and studio-grade channel separation (more than 70dB). VHS Hi-Fi audio is achieved by using audio frequency modulation (AFM), recording each of the 2 stereo channels (L, R) on a frequency-modulated carrier and embedding the modulated audio signal pair into the video signal. To avoid crosstalk and interference from the primary video carrier, VHS's implementation of AFM relied on a form of magnetic recording called depth multiplexing . The modulated audio carrier pair was placed under the luminance carrier (below 1.6 MHz), and recorded first. Subsequently, the video head erases and re-records the video signal over the same tape surface, but video signal's higher center frequency results in a shallower magnetization of the tape, allowing both the video and residual AFM audio signal to coexist on tape. (PAL versions of Beta Hi-Fi use this same technique). During playback, VHS Hi-Fi recovers the depth-recorded AFM signal by subtracting the audio head's signal (which contains the AFM signal contaminated by a weak image of the video signal) from the video head's signal (which contains only the video signal), then demodulates the left and right audio channels from their respective frequency carriers. The end result of the complex process was audio of outstanding fidelity, which was uniformly solid across all tape-speeds (EP or SP.) Since JVC had gone through the complexity of ensuring Hi-Fi's backward compatibility with non-Hi-Fi VCRs, virtually all studio home video releases contained Hi-Fi audiotracks (in addition to the linear audio track). Under normal circumstances, all Hi-Fi VHS VCRs will record Hi-Fi and linear audio simultaneously to ensure compatibility with VCRs without Hi-Fi playback, though only early, high-end Hi-Fi machines provided linear stereo compatibility.
Due to the path followed by the video and Hi-Fi audio heads being striped and discontinuous- unlike that of the linear audio track- head-switching is required to provide a continuous audio signal. Misalignments may lead to imperfect joining of the signal, resulting in low-pitched buzzing.
The sound quality of Hi-Fi VHS stereo is comparable to the quality of CD audio, particularly when recordings were made on high-end or professional VHS machines that have a manual audio recording level control. This high quality compared to other consumer audio recording formats such as compact cassette attracted the attention of amateur and hobbyist recording artists. Home recording enthusiasts occasionally recorded high quality stereo mixdowns and master recordings from multitrack audio tape onto consumer-level Hi-Fi VCRs. However, because the VHS Hi-Fi recording process is intertwined with the VCR's video-recording function, advanced editing functions such as audio-only or video-only dubbing are impossible. Some VHS decks also had a "simulcast" switch, allowing users to record an external audio input along with off-air pictures. Some televised concerts offered a stereo simulcast soundtrack on FM radio and as such, events like Live Aid were recorded by thousands of people with a full stereo soundtrack despite the fact that stereo TV broadcasts were some years off (especially in regions that adopted NICAM).
The considerable complexity and additional hardware limited VHS Hi-Fi to high-end decks fo
VHS - Wikipedia, the free encyclopedia
The Video Home System [1] (better known by its abbreviation VHS [2]) is a video tape recording standard developed during the 1970s by a Japanese company, JVC.
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