Editing VHS (section)

== Recording process ==
[[File:Medion MD8910 - Inject and eject of a VHS cassette.webm|thumb|left|A close-up process of how the magnetic tape in a VHS cassette is being pulled from the cassette shell to the head drum of the VCR]]
[[File:VHS-diagonal-helical-recording.jpg|right|thumb|300px|This illustration demonstrates the helical wrap of the tape around the head drum, and shows the points where the video, audio and control tracks are recorded]]
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 ===
[[File:3-equals-1-waveform-cutting-vhs-decode-2024.png|alt=Adobe Audition spectrograph (Left) ld-analyse with TBC file set loaded (Right) - Munday Demo Tape Public Archive.|thumb|Highlighted selection of a 40msps RAW FM signal captured from a test point on a consumer VCR during playback (left) and its resulting decoded signal image frame or two interlaced fields (right) produced by [https://github.com/oyvindln/vhs-decode/wiki vhs-decode]]]
The erase head is fed by a high-level, high-frequency AC signal that overwrites any previous recording on the tape.<ref>[http://hyperphysics.phy-astr.gsu.edu/hbase/audio/tape.html Tape Recording], Georgia State University</ref> Without this step, the new recording cannot be guaranteed to completely replace any old recording that might have been on the tape.

=== Video recording ===
{{For|the process used to commercially make VHS tapes|Print-through#Video recording}}
[[File:Kopftrommel 2.jpg|thumb|300px|Panasonic Hi-Fi six-head drum VEH0548 installed on G mechanism as an example, demonstrated a typical VHS head drum containing two tape heads. (1) is the upper head, (2) is the tape heads, and (3) is the head amplifier.]]
[[File:Medion MD8910 - VHS Helical scan tape head-8601.jpg|thumb|The upper- and underside of a typical four-head VHS head assembly showing the head chips and rotary transformer]]
[[File:Medion MD8910 - VHS Helical scan tape head-8610.jpg|thumb|Close-up of a head chip]]
[[File:RCA AutoShot VHS Camcorder.jpg|290px|thumb|right|A typical RCA (Model CC-4371) full-size VHS camcorder with a built-in three-inch color LCD screen.]]

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 [[helix|helical]] fashion, assisted by the slanted tape guides.<ref name="poptronics">{{Cite magazine |last=Goldwasser |first=Sam |date=January 2000 |title=VCRs |url=https://search.ebscohost.com/login.aspx?direct=true&db=f6h&AN=2782584&site=eds-live&scope=site |magazine=[[Poptronics]] |volume=1 |issue=1 |pages=77–79 |issn=1526-3681}}</ref> The head rotates constantly at{{efn|The 1800 rpm tape head speed, and corresponding field period time, etc., quoted in this article for NTSC machines are based on the old black and white RS-170 standard. When this was adapted for color under the NTSC standard the actual field time was altered to {{Frac|60000|1001}} of a second, so the actual VHS head rotation speed is accordingly 1798.2 rpm. The pre-color timings are quoted here for simplicity. The corresponding numbers here for PAL are, on the other hand, exact, as PAL's field rate is exactly {{Frac|1|50}} of a second.}} 1798.2 rpm in NTSC machines, exactly 1500 in PAL, each complete rotation corresponding to one frame of video.

Two [[tape head]]s 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 {{fraction|1|5|16}} inches per second corresponds to the heads on the drum moving across the tape at (a writing speed of) 4.86<ref>{{Cite web |title=Betamax PALsite: The Betamax Format |url=https://www.palsite.com/format.html}}</ref><ref name="poptronics" /> or 6.096 meters per second.<ref>{{Cite book |url=https://books.google.com/books?id=BHfNCgAAQBAJ&q=vhs+head+speed+240+inches+per+second&pg=PA127 |title = VCR Troubleshooting and Repair |isbn = 9780080520476|last1 = Brenner|first1 = Robert|last2 = Capelo|first2 = Gregory|date = 26 August 1998|publisher = Elsevier }}</ref>

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.<ref name="poptronics" /> This results, during playback, in destructive interference of the signal from the tracks on either side of the one being played.

Each of the diagonal-angled tracks is a complete TV picture [[interlaced video|field]], lasting {{Frac|1|60}} of a second ({{Frac|1|50}} on PAL) on the display. One [[tape head]] records an entire picture field. The adjacent track, recorded by the second tape head, is another {{Frac|1|60}} or {{Frac|1|50}} of a second TV picture field, and so on. Thus one complete head rotation records an entire NTSC or PAL frame of two fields.

The original VHS specification had only two video heads. When the EP recording speed was introduced, the thickness of these heads was reduced to accommodate the narrower tracks. However, this subtly reduced the quality of the SP speed, and dramatically lowered the quality of freeze frame and high speed search. Later models implemented both wide and narrow heads, and could use all four during pause and shuttle modes to further improve quality<ref>{{cite book | url=https://books.google.com/books?id=BH8GEAAAQBAJ&dq=vhs+lp+head&pg=PA410 | isbn=978-81-87522-05-8 | title=Audio Video Systems | publisher=Khanna Publishing House }}</ref> although machines later combined both pairs into one.<ref>{{cite book | url=https://books.google.com/books?id=QmaQTIdoWmYC&dq=vhs+lp+head&pg=PA384 | isbn=978-0-7506-4810-3 | title=Newnes Guide to Television and Video Technology | date=2001 | publisher=Newnes }}</ref> In machines supporting VHS HiFi (described later), yet another pair of heads was added to handle the VHS HiFi signal.<ref>{{cite book | url=https://books.google.com/books?id=BHfNCgAAQBAJ&dq=vhs+hi-fi+pair+of+heads&pg=PA294 | isbn=978-0-08-052047-6 | title=VCR Troubleshooting and Repair | date=26 August 1998 | publisher=Elsevier }}</ref> Camcorders using the miniaturized drum required twice<ref>{{cite book | url=https://books.google.com/books?id=0xXCNpEhFEMC&dq=vhs-c+4+head+drum&pg=PA179 | isbn=978-0-08-052051-3 | title=Video and Camcorder Servicing and Technology | date=11 April 2001 | publisher=Elsevier }}</ref> 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.<ref>{{cite web | url=https://archive.org/details/manual_SRW320U_JVC/page/44/mode/2up?q=head | title=Manual: SRW320U JVC }}</ref>

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&nbsp;[[Megahertz|MHz]] of baseband video [[Bandwidth (signal processing)|bandwidth]] and 300&nbsp;kHz of baseband chroma bandwidth.<ref>{{cite book |last=Poynton|first=Charles|author-link=Charles Poynton|title=Digital Video and HDTV Algorithms and Interfaces|edition=1st|publisher=Elsevier Science|year=2003|isbn=1-55860-792-7|pages=350–351, 419}}</ref> The [[Luma (video)|luminance]] (black and white) portion of the video is [[frequency modulation|frequency modulated]] and combined with a down-converted "[[Heterodyne#Analog videotape recording|color under]]" [[Chrominance|chroma]] (color) signal that is encoded using [[quadrature amplitude modulation]].<ref name="poptronics" />  Including [[Sideband|side bands]], the signal on a VHS tape can use up to 10&nbsp;MHz of RF bandwidth.<ref>{{Cite web |title=RF Capture Guide |url=https://github.com/oyvindln/vhs-decode/wiki/RF-Capture-Guide |access-date=2024-01-08 |website=GitHub |language=en}}</ref>

VHS horizontal resolution is 240 [[television lines|TVL]], or about 320 lines across a scan line. The vertical resolution (number of scan lines) is the same as the respective analog TV standard (625 for [[PAL]] or 525 for [[NTSC]]; somewhat fewer scan lines are actually visible due to [[overscan]] and the [[vertical blanking interval|VBI]]). In modern-day digital terminology, NTSC VHS resolution is roughly equivalent to 333×480 pixels for luma and 40×480 pixels for chroma. 333×480=159,840 pixels or 0.16 MP (1/6 of a megapixel).<ref>{{cite book |last1=Taylor|first1=Jim|last2=Johnson|first2=Mark|last3=Crawford|first3=Charles|title=DVD Demystified|publisher=McGraw-Hill Professional|year=2006|edition=3rd|isbn=0-07-142396-6|page=9{{hyphen}}36|url=https://archive.org/details/dvddemystified0000tayl_a1x8|access-date=March 4, 2024}}</ref> 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 frequency modulation of the VHS luminance signal is limited to 3 megahertz, which makes higher resolutions technically impossible even with the highest-quality recording heads and tape materials, 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 [[Original equipment manufacturer|OEM]]s 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 a new format called [[S-VHS|Super VHS]] (often known as S-VHS) which extended the bandwidth to over 5 megahertz, yielding 420 analog horizontal (560 pixels left-to-right). 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.<ref name="Parekh" /> Because of the limited user base, Super VHS was never picked up to any significant degree by manufacturers of pre-recorded tapes, although it was used extensively in the low-end professional market for filming and editing.

=== Audio recording ===

After leaving the head drum, the tape passes over the stationary audio and control head. This records a control track at the bottom edge of the tape, and one or two linear audio tracks along the top edge.<ref name="poptronics" />

==== Original linear audio system ====
In the original VHS specification, 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 linear tape speed. For the VHS SP mode, which already uses a lower tape speed than the compact cassette, this resulted in a mediocre frequency response<ref name="poptronics" /> of roughly 100&nbsp;Hz to 10&nbsp;kHz for NTSC,{{citation needed|date=March 2014}} frequency response for PAL VHS with its lower standard tape speed was somewhat worse of about 80&nbsp;Hz to 8&nbsp;kHz. The [[signal-to-noise ratio]] (SNR) was an acceptable 42&nbsp;dB for NTSC and 41&nbsp;dB for PAL. Both parameters degraded significantly with VHS's longer play modes, with EP/NTSC frequency response peaking at 4&nbsp;kHz. S-VHS tapes can give better audio (and video) quality, because the tapes are designed to have almost twice the bandwidth of VHS at the same speed.

Sound cannot be recorded on a VHS tape without recording a video signal because the video signal is used to generate the control track pulses which effectively regulate the tape speed on playback. Even in the audio dubbing mode, a valid video recording (control track signal) must be present on the tape for audio to be correctly recorded. If there is no video signal to the VCR input during recording, most later VCRs will record black video and generate a control track while the sound is being recorded. Some early VCRs record audio without a control track signal; this is of little use, because the absence of a signal from the control track means that the linear tape speed is irregular during playback.<ref name="poptronics" />

More sophisticated VCRs offer stereo audio recording and playback. Linear stereo fits two independent channels in the same space as the original mono audiotrack. While this approach preserves acceptable backward compatibility with monoaural audio heads, the splitting of the audio track degrades the audio's signal-to-noise ratio, causing objectionable tape hiss at normal listening volume. To counteract the hiss, linear stereo VHS VCRs use [[Dolby noise-reduction system|Dolby B noise reduction]] for recording and playback. This dynamically boosts the high frequencies of the audio program on the recorded medium, improving its signal strength relative to the tape's background noise floor, then attenuates the high frequencies during playback. Dolby-encoded program material exhibits a high-frequency emphasis when played on non-Hi-Fi VCRs that are not equipped with the matching Dolby Noise Reduction decoder, although this may actually improve the sound quality of non-Hi-Fi VCRs, especially at the slower recording speeds.

High-end consumer recorders take 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 is re-recorded on tape (without disturbing the other), were supported features on [[wikt:prosumer#Etymology 2|prosumer]] [[linear video 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, nearly every home video release by Hollywood featured a Dolby-encoded linear stereo audiotrack. However, linear stereo was never popular with equipment makers or consumers.

==== Tracking adjustment and index marking ====
Another linear [[control track|''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, so that the high speed rotating heads remained exactly on their helical tracks rather than somewhere between two adjacent tracks (known as "[[video tape tracking|tracking]]"). Since good tracking depends on precise distances 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 track is also used to hold ''index marks'', which were 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 ''n''th specified index mark, and resume playback from there. At times, higher-end VCRs provided functions for the user to manually add and remove these marks.<ref>{{cite web|url=http://www.crutchfield.com/S-PFiOFC1Dt8s/learn/learningcenter/home/vcr_glossary.html|title=VCRs Glossary|author=Loren Barstow|work=Crutchfield|access-date=2014-07-30|archive-date=2014-08-12|archive-url=https://web.archive.org/web/20140812193000/http://www.crutchfield.com/S-PFiOFC1Dt8s/learn/learningcenter/home/vcr_glossary.html|url-status=dead}}</ref><ref>[http://www.retrevo.com/support/JVC-HR-S7300U-VCRs-manual/id/318ag718/t/2/ JVC HR-S7300 manual] {{Webarchive|url=https://web.archive.org/web/20140810142541/http://www.retrevo.com/support/JVC-HR-S7300U-VCRs-manual/id/318ag718/t/2/ |date=2014-08-10 }}: features list: ''"..., Index Search, Manual Index Mark/Erase ..."''</ref>

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).<ref>[https://archive.today/20120729233622/http://www.elektroda.pl/rtvforum/instrukcjeobslugi?id=2176 Panasonic Video Cassette Recorder NV-HS960 Series Operating Instructions], VQT8880, Matsushita Electric Industrial Co., Ltd.</ref>

==== 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&nbsp;Hz to 20&nbsp;kHz), excellent 70&nbsp;dB [[signal-to-noise ratio]] (in consumer space, second only to the [[compact disc]]), [[dynamic range]] of 90&nbsp;dB, and [[professional audio]]-grade channel separation (more than 70&nbsp;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 [[Sound recording and reproduction|audio]] carrier pair was placed in the hitherto-unused frequency range between the luminance and the color carrier (below 1.6&nbsp;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 [[Compact disc|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 [[Audio mixing (recorded music)|mixdown]]s and [[master recordings]] from [[multitrack recording|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 [[Digital Audio Tape|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.<ref>{{Cite web |title=Helical-scan video tape cassette system using 12,65 mm (0,5 in) magnetic tape on type VHS – Part 2: FM audio recording |url=https://webstore.iec.ch/preview/info_iec60774-2%7Bed1.0%7Db.pdf |website=webstore.iec.ch}}</ref>

===== 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.<ref name="stason">{{cite web|url=http://stason.org/TULARC/entertainment/audio/general/14-18-Is-VHS-Hi-Fi-sound-perfect-Is-Beta-Hi-Fi-sound-perfec.html14.18|title=14.18 Is VHS Hi-Fi sound perfect? Is Beta Hi-Fi sound perfect?|work=stason.org | access-date=August 6, 2019}}</ref> 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.<ref name="stason"/>