Editing VHS (section)

=== 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.