Editing VHS (section)

== Technical details ==
VHS has been standardized in [[International Electrotechnical Commission|IEC]] 60774–1.<ref>{{Cite web |title=Helical-scan video tape cassette system using 12,65 mm (0,5 in) magnetic tape on type VHS – Part 1: VHS and compact VHS video cassette system |url=https://webstore.iec.ch/preview/info_iec60774-1%7Bed1.0%7Db.img.pdf |website=webstore.ice.ch}}</ref>

=== Cassette and tape design ===

[[File:VHS cassette tape 12.JPG|thumb|Top view of VHS with front casing removed]]
The VHS [[Videocassette|cassette]] is a 187&nbsp;[[Millimetre|mm]] wide, 103&nbsp;mm deep, and 25&nbsp;mm thick (7{{frac|3|8}} × 4{{frac|1|16}}× 1 inch) plastic shell held together with five [[Screw|Phillips-head screws]]. The flip-up cover, which allows players and recorders to access the tape, has a latch on the right side, with a push-in toggle to release it (bottom view image). The cassette has an anti-despooling mechanism, consisting of several plastic parts between the spools, near the front of the cassette (white and black in the top view). The spool latches are released by a push-in lever within a 6.35&nbsp;mm ({{frac|1|4}} inch) hole at the bottom of the cassette, 19&nbsp;mm ({{frac|3|4}} inch) in from the edge label.{{citation needed|date=November 2019}} The tapes are made, pre-recorded, and inserted into the cassettes in [[cleanroom]]s, to ensure quality and to keep [[dust]] from getting embedded in the tape and interfering with recording (both of which could cause signal dropouts)

There is a clear tape leader at both ends of the tape to provide an optical auto-stop for the VCR transport mechanism. In the VCR, a light source is inserted into the cassette through the circular hole in the center of the underside, and two [[photodiode]]s are on the left and right sides of where the tape exits the cassette. When the clear tape reaches one of these, enough light will pass through the tape to the photodiode to trigger the stop function; some VCRs automatically rewind the tape when the trailing end is detected. Early VCRs used an [[incandescent bulb]] as the light source: when the bulb failed, the VCR would act as if a tape were present when the machine was empty, or would detect the blown bulb and completely stop functioning. Later designs use an infrared [[Light-emitting diode|LED]], which has a much longer life.{{citation needed|date=November 2019}}

The recording medium is a Mylar<ref>Noble, Jem. "VHS: A Posthumanist Aesthetics of Recording and Distribution." OxfordHandbooks. Oxford Handbooks, Dec. 2013. Web. September 30, 2015.</ref> [[magnetic tape]], 12.7&nbsp;mm ({{frac|1|2}} inch) wide, coated with metal [[oxide]], and wound on two [[Bobbin|spool]]s.

The tape speed for "Standard Play" mode (see below) is 3.335&nbsp;[[Centimetre|cm]]/[[Second|s]] (1.313 ips) for [[NTSC]], 2.339&nbsp;cm/s (0.921 ips) for [[PAL]]—or just over 2.0 and 1.4 [[metre]]s (6&nbsp;ft 6.7 in and 4&nbsp;ft 7.2 in) per minute respectively. The tape length for a T-120 VHS cassette is 247.5 metres (812&nbsp;ft).<ref>{{cite web |last1=Wallace |first1=Dillon |title=How long is my tape? What is the format? |url=https://southtree.com/blogs/artifact/how-long-is-my-tape-what-is-the-format |website=Southtree |access-date=2 March 2020}}</ref>

=== Tape loading technique ===
[[File:VHS diagram.svg|thumb|200px|VHS M-loading system]]
As with almost all cassette-based videotape systems, VHS machines pull the tape out of the cassette shell and wrap it around the inclined head drum, which rotates at 1,800 rpm in NTSC machines<ref>{{cite web
|url=http://electronics.howstuffworks.com/vcr2.htm
|title=How VCRs Work
|date=February 10, 2011
|access-date=February 10, 2011
|first=Marshall
|last=Brain
|publisher=HowStuffWorks
|page=7
}}</ref> and at 1,500&nbsp;[[Revolutions per minute|rpm]] for [[PAL]], one complete rotation of the head corresponding to one video frame. VHS uses an "M-loading" system, also known as M-lacing, where the tape is drawn out by two threading posts and wrapped around more than 180 degrees of the head drum (and also other [[tape transport]] components) in a shape roughly approximating the letter [[M]].<ref>{{Cite web|url=https://www.newscientist.com/article/mg12316724-000-technology-vhs-video-that-begins-at-the-beginning/|title=Technology: VHS video that begins at the beginning|website=New Scientist|language=en-US|access-date=2020-01-22}}</ref> The heads in the rotating drum get their signal wirelessly using a [[rotary transformer]].

=== Recording capacity===
[[File:VCR load.jpg|thumb|The interior of a late-generation VHS [[Videocassette recorder|VCR]] showing the drum and tape]]
A VHS cassette holds a maximum of about 430&nbsp;m (1,410&nbsp;ft) of tape at the lowest acceptable tape thickness, giving a maximum playing time of about four hours in a T-240/DF480 for [[NTSC]] and five hours in an E-300 for [[PAL]] at "standard play" (SP) quality. More frequently, however, VHS tapes are thicker than the required minimum to avoid complications such as jams or tears in the tape.<ref name="Parekh" /> Other speeds include "long play" (LP), "extended play" (EP) or "super long play" (SLP) (standard on NTSC; rarely found on PAL machines<ref>{{Cite web|url=http://www.crumblenet.co.uk/palep.html|title=List of PAL VHS Video Recorders with SLP/EP}}</ref>). LP and EP/SLP double and triple the recording time respectively, but these speed reductions cause a reduction in horizontal resolution – from the normal equivalent of 250 vertical lines in SP, to the equivalent of 230 in LP and even less in EP/SLP.

Due to the nature of recording diagonally from a spinning drum, the actual write speed of the video heads does not get slower when the tape speed is reduced. Instead, the video tracks become narrower and are packed closer together. This results in noisier playback that can be more difficult to track correctly: The effect of subtle misalignment is magnified by the narrower tracks. The heads for linear audio are not on the spinning drum, so for them, the tape speed from one reel to the other is the same as the speed of the heads across the tape. This speed is quite slow: for SP it is about 2/3s that of an audio cassette, and for EP it is slower than the slowest microcassette speed. This is widely considered inadequate for anything but basic voice playback, and was a major liability for VHS-C camcorders that encouraged the use of the EP speed. Color depth deteriorates significantly at lower speeds in PAL: often, a color image on a PAL tape recorded at low speed is displayed only in monochrome, or with intermittent color, when playback is paused.{{citation needed|date=November 2019}}

=== Tape lengths ===
[[File:VHS tape with time scale.jpg|thumb|VHS cassette with time scale for SP and LP]]
[[File:Quantegy-VHS-cassettes.jpg|550px|thumb|VHS cassettes of different play time labelled both for NTSC and PAL]]
VHS cassettes for [[Multi-standard television|NTSC and PAL/SECAM]] systems are physically identical, although the signals recorded on the tape are incompatible. The tape speeds are different too, so the playing time for any given cassette will vary between the systems. To avoid confusion, manufacturers indicate the playing time in minutes that can be expected for the market the tape is sold in: E-XXX indicates playing time in minutes for PAL or SECAM. T-XXX indicates playing time in minutes for NTSC or PAL-M.

To calculate the playing time for a T-XXX tape in a PAL machine, this formula is used:

: PAL/SECAM recording time = T-XXX in minutes × 1.426

To calculate the playing time for an E-XXX tape in an NTSC machine, this formula is used:

: NTSC recording time = E-XXX in minutes × 0.701

Since the recording/playback time for PAL/SECAM is roughly 1/3 longer than the recording/playback time for NTSC, some tape manufacturers label their cassettes with both T-XXX and E-XXX marks, like T60/E90, T90/E120 and T120/E180.

SP is standard play, LP is long play ({{frac|1|2}} speed, equal to recording time in DVHS "HS" mode), EP/SLP is extended/super long play ({{frac|1|3}} speed)<ref name="poptronics" /> which was primarily released into the NTSC market.

{| class="wikitable"
|+ Common tape types, approximate
|-
! rowspan="2" | Label; nominal length <br/>(minutes)
! colspan="2" | Length
! colspan="3" | Recording time, NTSC
! colspan="3" | Recording time, PAL
|-
! (m) !! (ft)
! {{abbr|SP|Standard play}} !! {{abbr|LP|Long play}} !! {{abbr|EP|Extended play}}/{{abbr|SLP|super long play}}
! {{abbr|SP|Standard play}} !! {{abbr|LP|Long play}} !! {{abbr|EP|Extended play}}/{{abbr|SLP|super long play}}
|-
! colspan="10" | ''NTSC market''
|- style="background:#ffe8f0;"
! T-20
| 44 || 145 || 20 min || 40 min || 60 min (1h) || 28.52 min || 57.04 min || 85.56 min (1h 25.56)
|-
! T-30 (typical VHS-C)
| 63 || 207 || 30 min || 60 min (1h) || 90 min (1h 30) || 42.78 min || 85.56 min (1h 25.56) || 128.34 min (2h 8.34)
|- style="background:#ffe8f0;"
! T-45
| 94 || 310 || 45 min || 90 min (1h 30) || 135 min (2h 15) || 64.17 min (1h 04.17) || 128.34 min (2h 8.34) || 192.51 min (3h 12.51)
|-
! T-60
| 126 || 412 || 60 min (1h) || 120 min (2h)||  180 min (3h) || 85.56 min (1h 25.56) || 171.12 min (2h 51.12) || 256.68 min (4h 16.68)
|- style="background:#ffe8f0;"
! T-90
| 186 || 610 || 90 min (1h 30) || 180 min (3h)||  270 min (4h 30) || 128.34 min (2h 8.34) || 256.68 min (4h 16.68) || 385.02 min (6h 25.02)
|-
! T-120 / DF-240
| 247 || 811 || 120 min (2h) || 240 min (4h) || 360 min (6h) || 176 min (2h 56) || 342.24 min (5h 42.24) || 513.36 min (8h 33.36)
|- style="background:#ffe8f0;"
!T-130
|277 || 910 || 130 min (2h 10) || 260 min(4h 20) || 390 min (6h 30) || 185.38 min (3h 5.38) || 370.76 min (6h 10.76) || 556.14 min (9h 16.14)
|-
! T-140
| 287.5 || 943 || 140 min (2h 20) || 280 min (4h 40) || 420 min (7h) || 199.64 min (3h 19.64) || 399.28 min (6h 39.28) || 598.92 min (9h 58.92)
|- style="background:#ffe8f0;"
! T-150 / DF-300
| 316.5 || 1,040 || 150 min (2h 30) || 300 min (5h) || 450 min (7h 30) || 213.9 min (3h 33.9) || 427.8 min (7h 7.8) || 641.7 min (10h 41.7)
|-
! T-160
| 328 || 1,075 || 160 min (2h 40) || 320 min (5h 20) || 480 min (8h) || 228.16 min (3h 48.16) ||  456.32 min (7h 36.32) || 684.48 min (11h 24.48)
|- style="background:#ffe8f0;"
! T-180 / DF-360
| 369 || 1,210 || 180 min (3h) || 360 min (6h)||  540 min (9h) || 256.68 min (4h 16.68) ||  513.36 min (8h 33.36) || 770.04 min (12h 50.04)
|-
! T-200
| 410 || 1,345 || 200 min (3h 20) || 400 min (6h 40)||  600 min (10h) || 285.2 min (4h 45.2) || 570.4 min (9h 30.4) || 855.6 min (14h 15.6)
|- style="background:#ffe8f0;"
! T-210 / DF-420
| 433 || 1,420 || 210 min (3h 30) || 420 min (7h) || 630 min (10h 30) || 299.46 min (4h 59.46) || 598.92 min (9h 58.92) || 898.38 min (14h 58.38)
|-
! T-240 / DF-480
| 500 || 1,640 || 240 min (4h) || 480 min (8h) || 720 min (12h) || 342.24 min (5h 42.24) || 684.48 min (11h 24) || 1026.72 min (17h 6.72)
|-
! colspan="9" | ''PAL market''
|- style="background:#e8f0ff;"
! E-30 (typical VHS-C)
| 45 || 148 || 22.5 min || 45 min || 68 min (1h 08) || 32 min || 64 min (1h 04) || 96 min (1h 36)
|-
! E-60
| 88 || 290 || 44 min || 88 min (1h 28) || 133 min (2h 13) || 63 min (1h 03) || 126 min (2h 06) || 189 min (3h 09)
|- style="background:#e8f0ff;"
! E-90
| 131 || 429 || 65 min (1h 05) || 131 min (2h 11) || 196 min (3h 16) || 93 min (1h 33) || 186 min (3h 06) || 279 min (4h 39)
|-
! E-120
| 174 || 570 || 87 min (1h 27) || 174 min (2h 54) || 260 min (4h 20) || 124 min (2h 04) || 248 min (4h 08) || 372 min (6h 12)
|- style="background:#e8f0ff;"
! E-150 
| 216 || 609 || 108 min (1h 49) || 227 min (3h 37) || 324 min (5h 24) || 154 min (2h 34) || 308 min (5h 08) || 462 min (7h 42)
|-
! E-180
| 259 || 849 || 129 min (2h 09) || 259 min (4h 18) || 388 min (6h 28) || 184 min (3h 04) || 369 min (6h 09) || 552 min (9h 12)
|- style="background:#e8f0ff;"
! E-195
| 279 || 915 || 139 min (2h 19) || 279 min (4h 39) || 418 min (6h 58) || 199 min (3h 19) || 397 min (6h 37) || 597 min (9h 57)
|-
! E-200 
| 289 || 935 || 144 min (2h 24) || 284 min (4h 44) || 428 min (7h 08) || 204 min (3h 24) || 405 min (6h 45) || 612 min (10h 21)
|- style="background:#e8f0ff;"
! E-210
| 304 || 998 || 152 min (2h 32) || 304 min (5h 04) || 456 min (7h 36) || 217 min (3h 37) || 433 min (7h 13) || 651 min (10h 51)
|-
! E-240
| 348 || 1,142 || 174 min (2h 54) || 348 min (5h 48) || 522 min (8h 42) || 248 min (4h 08) || 496 min (8h 16) || 744 min (12h 24)
|- style="background:#e8f0ff;"
! E-270 
| 392 || 1,295 || 196 min (3h 16) || 392 min (6h 32) || 589 min (9h 49) || 279 min (4h 39) || 559 min (9h 19) || 837 min (13h 57)
|- 
! E-300
| 435 || 1,427 || 217 min (3h 37) || 435 min (7h 15) || 652 min (10h 52) || 310 min (5h 10) || 620 min (10h 20) || 930 min (15h 30)
|}

=== Copy protection ===

As VHS was designed to facilitate recording from various sources, including television broadcasts or other VCR units, content producers quickly found that home users were able to use the devices to copy videos from one tape to another. Despite [[generation loss]] in quality when a tape was copied,<ref>{{cite book|url=https://books.google.com/books?id=dqP8FCEiMGgC&pg=PA267|title=Bioimaging: Current Concepts in Light and Electron Microscopy|last1=Chandler|first1=Douglas E.|last2=Roberson|first2=Robert W.|publisher=[[Jones and Bartlett Publishers]]|date=2009|access-date=2022-04-09|pages=267–268|isbn=978-0-7637-3874-7}}</ref> this practice was regarded as a widespread problem, which members of the [[Motion Picture Association of America]] (MPAA) claimed caused them great financial losses.<ref>{{cite book|url=https://books.google.com/books?id=dTHxh9SAF-AC&pg=PA314|title=Time Strategies, Innovation and Environmental Policy|series=Advances in Ecological Economics|last1=Sartorius|first1=Christian|last2=Zundel|first2=Stefan|publisher=[[Edward Elgar Publishing]]|date=2005|access-date=2022-04-09|pages=314–318|isbn=1-84542-090-X}}</ref><ref>{{cite book|url=https://books.google.com/books?id=RQHFDwAAQBAJ&pg=PA209|title=Copyrighting Culture: The Political Economy of Intellectual Property|series=Critical Studies in Communication and in the Cultural Industries|last=Bettig|first=Ronald V.|publisher=[[Routledge]]|date=2018|access-date=2022-04-09|pages=209–210|isbn=978-0-8133-3304-5}}</ref> In response, several companies developed technologies to protect copyrighted VHS tapes from casual duplication by home users. The most popular method was [[Analog Protection System]], better known simply as [[Macrovision]], produced by a company of the same name.<ref>{{Cite web|url=https://hackaday.com/2018/05/27/rolling-old-school-with-copy-protection-from-the-1980s/|title=Rolling Old School With Copy Protection From The 1980s|date=May 28, 2018}}</ref> According to Macrovision: <blockquote>The technology is applied to over 550 million videocassettes annually and is used by every MPAA movie studio on some or all of their videocassette releases. Over 220 commercial duplication facilities around the world are equipped to supply Macrovision videocassette copy protection to rights owners...The study found that over 30% of VCR households admit to having unauthorized copies, and that the total annual revenue loss due to copying is estimated at $370,000,000 annually.<ref>{{Cite web|url=https://electronics.howstuffworks.com/question313.htm|title=How does copy protection on a video tape work?|date=April 1, 2000|website=HowStuffWorks}}</ref></blockquote>

The system was first used in copyrighted movies beginning with the 1984 film ''[[The Cotton Club (film)|The Cotton Club]]''.<ref name="deatley19850907">{{cite news | url=https://news.google.com/newspapers?id=lc8vAAAAIBAJ&pg=5630%2C870934 | title=VCRs put entertainment industry into fast-forward frenzy | work=The Free Lance-Star | date=September 7, 1985 | agency=Associated Press | access-date=2015-01-25 | author=De Atley, Richard | pages=12–TV}}</ref>

Macrovision copy protection saw refinement throughout its years, but has always worked by essentially introducing deliberate errors into a protected VHS tape's output video stream. These errors in the output video stream are ignored by most televisions, but will interfere with re-recording of programming by a second VCR. The first version of Macrovision introduces high signal levels during the [[vertical blanking interval]], which occurs between the video fields. These high levels confuse the [[automatic gain control]] circuit in most VHS VCRs, leading to varying brightness levels in an output video, but are ignored by the TV as they are out of the frame-display period. "Level II" Macrovision uses a process called "colorstriping", which inverts the analog signal's colorburst period and causes off-color bands to appear in the picture. Level III protection added additional colorstriping techniques to further degrade the image.<ref name="anarchivism-rip-vhs">{{cite web |url=http://anarchivism.org/w/How_to_Rip_VHS |title=How to Rip VHS |work=Anarchivism |date=December 14, 2012 }}</ref>

These protection methods worked well to defeat analog-to-analog copying by VCRs of the time. Consumer products capable of digital video recording are mandated by law to include features which detect Macrovision encoding of input analog streams, and disrupt copying of the video.{{citation needed|date=June 2018}} Both intentional and false-positive detection of Macrovision protection has [[Criticism of copyright|frustrated archivists]] who wish to copy now-fragile VHS tapes to a digital format for preservation. As of the 2020s, modern software decoding<ref>{{Citation |last=oyvindln |title=oyvindln/vhs-decode |date=2024-01-08 |url=https://github.com/oyvindln/vhs-decode |access-date=2024-01-08}}</ref> ignores Macrovision as software is not limited to the fixed standards that Macrovision was intended to disrupt in hardware based systems.