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The Science behind Vcrs



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When Sony introduced the first commercially inexpensive VCRs in 1969, the way people watched television was changed forever.  The first video recorder to use reel-to-reel tape was created in 1956 by Ray Dolby and Charles Ginsburg, and prior to this all television broadcasts could only be watched live.  Sony’s 1969 introduction of the Beta Max format eventually gave way to the cheaper VHS format, allowing masses of consumers to have more control over what programming they watched, and when.  Despite their low price, exploring the science behind VCRs reveals an incredibly intricate machine, and a complex feat of engineering.

To understand the science behind VCRs, it is helpful to know how televisions produce an image.  The human brain interprets a series of slightly modified still pictures as a moving image, which in the case of the VCR are displayed in sequence at 29.97 fps (frames per second).  Each image is divided in a series of 525 horizontal scan lines to be displayed on your TV. 

All the video information about which colors the TV should produce, where they should appear, and when they should be displayed is recorded magnetically onto a strip of magnetic tape covered with a layer of oxide.  This tape is pulled past a drum containing record, playback, and erase heads at a speed of 2-3 inches per second. 

Since video signals contain much more information than audio, recording video linearly like an audio recorder would require a prohibitively large amount of tape.  In a VCR, the rotating drum is tilted with respect to the tape, allowing the information to be recorded in a series of diagonal stripes across the tape itself.  This is called helical scanning, and it allows a massive amount of information to be stored on a small cassette. 

In addition to the video information stored through helical scanning, audio tracks are recorded onto the tape as well.  The audio information can be stored linearly, so audio is recorded, erased, and played back using a separate audio head without the helical tilt.  These audio tracks take up far less physical room on a tape, so they are recorded linearly near the edge of the tape.

Since VCRs utilize a variety of playback speeds like SP, LP, and EP, a linear “control track” is recorded parallel to the audio track with playback instructions for the VCR.  This control track ensures that the VCR’s drums keep the tape playing at the correct speed, and are also used to detect the end of a tape spool. 

When using a VCR’s LP or EP settings, the helically recorded lines video information are grouped much closer together on a tape.  This decreases the quality of the recorded video due to magnetic bleeding of neighboring information.  It also means that VCR will require four video heads instead of an SP only VCR’s standard two heads.

During both the record and playback settings, the tape is routed through a complex series of pins and rollers.  These help guide the tape through the system, and ensure that it is properly aligned with the heads and drums of the VCR.  When these fall out of alignment, the delicate machinery of the VCR needs to be adjusted. 

Although VCRs have largely given way to digital formats like DVD, Blu-Ray Disc, and DVR, the VCR was a monumental leap forward for consumer home video.  Many people still have old videotapes and home movies that can only be played with an analog VCR.  Though it may seem antiquated in the 21st century, the science behind VCRs changed the history of video forever. 

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  • InfoBoxCallToAction ActionArrowhttp://electronics.howstuffworks.com/vcr2.htm
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