Difference between revisions 52084 and 52124 on enwiki

The [[worldwide green parties]] are committed to the following '''Four Pillars''':
* [[Non-Violence]]
* [[Grassroots Democracy]]
* [[Ecology]]
* [[Social Justice]]

These were originally defined by European Green Parties and later adopted by the [[U.S. Green Party]].

The four pillars are generally considered interdependent, reinforcing each other in non-obvious ways. They arose from shared concerns of the [[peace movement]] and [[ecology movement]].
⏎
[[Image:Vhs_logo.jpg|150px|left|VHS Logo]]

[[Image:Vhs cassette top.jpg|thumb|200px|right|Top view VHS cassette with US Quarter for scale]]
[[Image:Vhs cassette bottom.jpg|thumb|200px|right|Bottom view of VHS cassette with magnetic tape exposed]]
The '''Video Home System''', better known by its [[acronym]] '''VHS''', is a recording and playing standard for [[video cassette recorder]]s (VCRs), developed by [[JVC]] (ironically, with some of its critical technology under lucrative licensing agreements with Sony) and launched in [[1976]]. VHS officially stands for Video Home System, but it initially stood for ''Vertical Helical Scan'', after the [[Helical scan|relative head/tape scan]] technique. Some early reports claim the name originally stood for [[JVC|Victor]] Helical Scan system.

VHS became a standard format for consumer recording and viewing in the [[1980s]] after competing in a fierce [[format war]] with [[Sony]]'s [[Betamax]] and, to a lesser extent, [[Philips]]' [[Video 2000]]. VHS initially offered a longer playing time than the Betamax system, and it also had the advantage of a far less complex tape transport mechanism.  Early VHS machines could rewind and fast forward the tape considerably faster than a Betamax VCR since they unthreaded the tape from the playback heads before commencing any high-speed winding (most newer VHS machines don't do this any more, as improved engineering has stopped head-tape contact from being an impediment for fast winding). On the other hand, Betamax offers superior picture quality. See [[VCR]] for more details.

A VHS cassette contains a 12.70 mm (approx ½-[[inch]]) wide [[magnetic tape]] wound between two spools, allowing it to be slowly passed over the various playback and recording heads of the [[video cassette recorder]]. The tape speed is 3.335 cm/s for [[NTSC]], 2.339 cm/s for [[PAL]]. A cassette holds a maximum of about 430m 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. Most cassettes have lower recording times because they use thicker tape, which helps avoiding jams; careful users generally avoid the thinnest tapes. More recent machines usually allow the selection of longer recording times by lowering the tape speed even further: LP mode (for PAL) halves the tape speed and doubles the recording time, while EP mode (for NTSC) drops the tape speed to one-third, for triple the recording time. Of course these speed reductions cause corresponding reductions in video quality, also tapes recorded at the lower speed often don't play well on another recoder than the one they were produced on. Because of this, commercial prerecorded tapes were almost always recorded in SP mode. An unofficial LP mode with one of half the standard speed exists on some NTSC machines but is not part of the VHS standard.

VHS tapes have approximately 3 [[MHz]] of video [[bandwidth]], and a horizontal [[display resolution|resolution]] of about 240 discernible lines per scanline [http://www.hometheaterhifi.com/volume_6_3/essay-video-resolution-july-99.html]. The frequency modulation of the luminance signal makes higher resolutions impossible within the VHS standard, no matter how advanced the recoder's technology. The vertical resolution of VHS (and all other analog recording methods) is determined by the TV standard &mdash; a maximum of 486<!-- sic, not 480 --> lines are visible in [[NTSC]] and a maximum of 576 lines in [[PAL]].

The video bandwidth is achieved with a relatively low tape speed by the use of [[helical scan]] recording of a [[frequency modulation|frequency modulated]] [[luminance]] (black and white)  signal, to which a frequency-reduced "[[color under]]" chroma (hue and saturation) signal is added. In the original VHS format, audio was recorded unmodulated in a single (monaural) linear track at the upper edge of the tape, which was limited in frequency response by the tape speed. More recent hi-fi VCRs add higher-quality stereo audio tracks which are read and written by heads located on the same spinning drum that carries the video heads, frequency modulated to the unused frequency range in between the chroma and luma signals. These audio tracks take advantage of ''depth multiplexing'': since they use lower frequencies than the video, their magnetization signals penetrate deeper into the tape. When the video signal is written by the following video head, it erases and overwrites the audio signal at the surface of the tape, but leaves the deeper portion of the signal undisturbed.

Of course, for backward compatibility, hi-fi VCRs still write the linear audio track during recording, and can automatically read it during playback if the hi-fi audio is not present.

Another linear ''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 and to get the rotating heads exactly on their helical tracks rather than having them end up somewhere between two adjacent tracks (a feature called ''tracking''). Since good tracking depends on the exact distance 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 can aditionally hold ''index marks''. These are normally written at the start 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. There was a time when higher-end VCRs provided functions for manually removing and adding these index marks--so that, for example, they coincide with the actual start of the program--but this feature has become hard to find. A sign, perhaps, of the obsolescence of the VHS format.

Several improved versions of VHS exist, most notably [[S-VHS]], an improved analog standard, and [[D-VHS]], which records [[digital video]] onto a VHS form factor tape.  Devices have also been invented which directly connect a [[personal computer]] to VHS tape recorders for use as a [[data backup]] device.  [[W-VHS]] caters for high definition video.

Another variant is VHS-C (''C'' for compact), used in some [[camcorder]]s. Since VHS-C tapes are based on the same magnetic tape as full size tapes, they can be played back in standard VHS players using a mechanical adapter, without the need of any kind of signal conversion. The magnetic tape on VHS-C cassettes is wound on one main spool and used a sort of a gear wheel which moves the tape forward. It can also be moved by hand and so is the spool. This development hampered the sales of the Betamax system somewhat, because the Betamax cassette geometry prevented a similar development.

{| border="0" cellpadding="2" cellspacing="2" align="center"
|-
|valign="top"|[[Image:VHSC.jpg|thumb|250px|right|Bottom and top view of VHS-C, compact video cassette]]
|valign="top"|[[Image:VHSC_Carrier.jpg|thumb|250px|right|Example of a VHS-C adapter]]
|-
|}
<br clear="all">

==External links==
*[http://www.totalrewind.org The 'Total Rewind' VCR museum, covering the history of VHS and other vintage formats]
*[http://news.bbc.co.uk/1/hi/business/4031223.stm BBC News article: Death of video recorder in sight]

[[Category:Video storage]]

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