The recording process in VHS consists of the following steps, in this order: • The tape is pulled from the supply reel by a capstan and pinch roller, similar to those used in audio tape recorders. • The tape passes across the erase head, which wipes any existing recording from the tape. • The tape is wrapped around the head drum, using a little more than 180 degrees of the drum. • One of the heads on the spinning drum records one field of video onto the tape, in one diagonally oriented track. • The tape passes across the audio and control head, which records the control track and the linear audio tracks. • The tape is wound onto the take-up reel due to torque applied to the reel by the machine.
Erase head ] The erase head is fed by a high-level, high-frequency AC signal that overwrites any previous recording on the tape. Without this step, the new recording cannot be guaranteed to completely replace any old recording that might have been on the tape.
Video recording The tape path then carries the tape around the spinning video-head drum, wrapping it around a little more than 180 degrees (called the
omega transport system) in a
helical fashion, assisted by the slanted tape guides. The head rotates constantly at 1798.2 rpm in NTSC machines, exactly 1500 in PAL, each complete rotation corresponding to one frame of video. Two
tape heads are mounted on the cylindrical surface of the drum, 180 degrees apart from each other, so that the two heads "take turns" in recording. The rotation of the inclined head drum, combined with the relatively slow movement of the tape, results in each head recording a track oriented at a diagonal with respect to the length of the tape, with the heads moving across the tape at speeds higher than what would otherwise be possible. This is referred to as
helical scan recording. A tape speed of inches per second corresponds to the heads on the drum moving across the tape at (a writing speed of) 4.86 To maximize the use of the tape, the video tracks are recorded very close together. To reduce
crosstalk between adjacent tracks on playback, an
azimuth recording method is used: The gaps of the two heads are not aligned exactly with the track path. Instead, one head is angled at plus six degrees from the track, and the other at minus six degrees. although machines later combined both pairs into one. In machines supporting VHS HiFi (described later), yet another pair of heads was added to handle the VHS HiFi signal. Camcorders using the miniaturized drum required twice as many heads to complete any given task. This almost always meant four heads on the miniaturized drum with performance similar to a two head VCR with a full sized drum. No attempt was made to record Hi-Fi audio with such devices, as this would require an additional four heads to work. W-VHS decks could have up to 12 heads in the head drum, of which 11 were active including a flying erase head for erasing individual video fields, and one was a dummy used for balancing the head drum. The high tape-to-head speed created by the rotating head results in a far higher bandwidth than could be practically achieved with a stationary head. VHS machines record up to 3
MHz of baseband video
bandwidth and 300 kHz of baseband chroma bandwidth. The
luminance (black and white) portion of the video is
frequency modulated and combined with a down-converted "
color under"
chroma (color) signal that is encoded using
quadrature amplitude modulation. VHS horizontal resolution is 240
Television Lines or TVL, about 320 lines across a scan line. The vertical resolution (number of scan lines) is the same as the respective analog TV standard (
625 or
525 lines; somewhat fewer scan lines are actually visible due to
overscan and the
VBI). In modern-day digital terminology,
NTSC VHS resolution is roughly equivalent to ~333×480 (~159,840 pixels or ~0.16 MP) for luma and ~40×480 pixels for chroma.
PAL VHS resolution is roughly ~333×576 pixels for luma and ~40×576 pixels for chroma (although when decoded PAL and
SECAM half the vertical color resolution). JVC countered 1985's SuperBeta with VHS HQ, or High Quality. The goal was to increase the apparent horizontal resolution by recording white signals at a higher level, and reduce tape background noise. The bandwidth remained unchanged to preserve compatibility with older machines. In 1987, JVC introduced a new format called
Super VHS (often known as S-VHS) which extended the bandwidth to over 5 megahertz, yielding 420 TVL (equivalent to 560 pixels left-to-right, in digital terminology). Most Super VHS recorders can play back standard VHS tapes, but not vice versa. S-VHS was designed for higher resolution, but failed to gain popularity outside Japan because of the high costs of the machines and tapes. By the late 1990s, some high-end VCRs offered more sophisticated indexing. For example, Panasonic's Tape Library system assigned an ID number to each cassette, and logged recording information (channel, date, time and optional program title entered by the user) both on the cassette and in the VCR's memory for up to 900 recordings (600 with titles).
Hi-Fi audio system Around 1984, JVC added
Hi-Fi audio to VHS (model HR-D725U, 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
professional audio-grade channel separation (more than 70 dB). VHS Hi-Fi audio is achieved by using audio frequency modulation (AFM), modulating the two stereo channels (L, R) on two different frequency-modulated carriers and embedding the combined 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 in the hitherto-unused frequency range between the luminance and the color carrier (below 1.6 MHz), and recorded first. Subsequently, the video head erases and re-records the video signal (combined luminance and color signal) over the same tape surface, but the 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 result of the complex process was audio of high fidelity, which was uniformly solid across all tape-speeds (EP, LP 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 produced after this time contained Hi-Fi audio tracks, 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. The sound quality of Hi-Fi VHS stereo is comparable to some extent 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. A short-lived alternative to the HiFi feature for recording mixdowns of hobbyist audio-only projects was a
PCM adaptor so that high-bandwidth digital video could use a grid of black-and-white dots on an analog video carrier to give pro-grade digital sounds though
DAT tapes made this obsolete. 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). Other examples of this included network television shows such as
Friday Night Videos and
MTV for its first few years in existence. Likewise, some countries, most notably
South Africa, provided alternate language audio tracks for TV programming through an FM radio simulcast. The considerable complexity and additional hardware limited VHS Hi-Fi to high-end decks for many years. While linear stereo all but disappeared from home VHS decks, it was not until the 1990s that Hi-Fi became a more common feature on VHS decks. Even then, most customers were unaware of its significance and merely enjoyed the better audio performance of the newer decks. VHS Hi-Fi audio has been standardized in IEC 60774-2.
Issues with Hi-Fi audio 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. While the video signal can easily hide the head-switching point in the invisible vertical retrace section of the signal, so that the exact switching point is not very important, the same is obviously not possible with a continuous audio signal that has no inaudible sections. Hi-Fi audio is thus dependent on a much more exact alignment of the head switching point than is required for non-HiFi VHS machines. Misalignments may lead to imperfect joining of the signal, resulting in low-pitched buzzing. The problem is known as "head chatter", and tends to increase as the audio heads wear down. Another issue that made VHS Hi-Fi imperfect for music is the inaccurate reproduction of levels (softer and louder) which are not re-created as the original source. == Variations ==