{"id":315,"date":"2015-08-25T14:35:00","date_gmt":"2015-08-25T14:35:00","guid":{"rendered":"http:\/\/machineryequipmentonline.com\/video-equipment\/uncategorized\/video-disc-technologyspeed-correction-servos\/"},"modified":"2015-08-25T14:35:00","modified_gmt":"2015-08-25T14:35:00","slug":"video-disc-technologyspeed-correction-servos","status":"publish","type":"post","link":"http:\/\/machineryequipmentonline.com\/video-equipment\/video-disc-technologyspeed-correction-servos\/","title":{"rendered":"VIDEO DISC TECHNOLOGY:SPEED-CORRECTION SERVOS."},"content":{"rendered":"<div class=\"gnmcs6a0dc92f8528b\" ><script type=\"text\/javascript\">\n\tatOptions = {\n\t\t'key' : '61e5902552e2353963d8d2f1bd1f4a8f',\n\t\t'format' : 'iframe',\n\t\t'height' : 250,\n\t\t'width' : 300,\n\t\t'params' : {}\n\t};\n<\/script>\n<script type=\"text\/javascript\" src=\"\/\/www.highperformanceformat.com\/61e5902552e2353963d8d2f1bd1f4a8f\/invoke.js\"><\/script><\/div><style type=\"text\/css\">\r\n@media screen and (min-width: 1201px) {\r\n.gnmcs6a0dc92f8528b {\r\ndisplay: block;\r\n}\r\n}\r\n@media screen and (min-width: 993px) and (max-width: 1200px) {\r\n.gnmcs6a0dc92f8528b {\r\ndisplay: block;\r\n}\r\n}\r\n@media screen and (min-width: 769px) and (max-width: 992px) {\r\n.gnmcs6a0dc92f8528b {\r\ndisplay: block;\r\n}\r\n}\r\n@media screen and (min-width: 768px) and (max-width: 768px) {\r\n.gnmcs6a0dc92f8528b {\r\ndisplay: block;\r\n}\r\n}\r\n@media screen and (max-width: 767px) {\r\n.gnmcs6a0dc92f8528b {\r\ndisplay: block;\r\n}\r\n}\r\n<\/style>\r\n<h3 align=\"justify\">SPEED-CORRECTION SERVOS<\/h3>\n<p align=\"justify\">For CAV discs the required motor speed is exactly 1500 r.p.m., as with a video head drum. For CLV discs the speed is variable, falling from an initial 1500 r.p.m. to around one-third that speed at programme end. In both cases the speed of the disc-drive motor must be closely and accurately controlled. Unlike a videocassette recorder the disc system is a replay-only one, so speed control need consist only of ensuring that off-disc line sync pulses come at intervals of 64 \u03bcs precisely. This is easily arranged in a servo identical in principle to those used for control of a videorecorder\u2019s head drum during replay mode, employing speed and phase loops. Here the reference signal will be a stable crystal whose divided-to-<i>f <\/i>h output is phase-compared to off-disc line syncs to produce an error output drive to the disc motor.  <\/p><div class=\"dukbi6a0dc92f85421\" ><script async src=\"https:\/\/pagead2.googlesyndication.com\/pagead\/js\/adsbygoogle.js?client=ca-pub-0778475562755157\"\n     crossorigin=\"anonymous\"><\/script>\n<!-- 300x600 television-and-video -->\n<ins class=\"adsbygoogle\"\n     style=\"display:inline-block;width:300px;height:600px\"\n     data-ad-client=\"ca-pub-0778475562755157\"\n     data-ad-slot=\"6549443290\"><\/ins>\n<script>\n     (adsbygoogle = window.adsbygoogle || []).push({});\n<\/script><\/div><style type=\"text\/css\">\r\n@media screen and (min-width: 1201px) {\r\n.dukbi6a0dc92f85421 {\r\ndisplay: block;\r\n}\r\n}\r\n@media screen and (min-width: 993px) and (max-width: 1200px) {\r\n.dukbi6a0dc92f85421 {\r\ndisplay: block;\r\n}\r\n}\r\n@media screen and (min-width: 769px) and (max-width: 992px) {\r\n.dukbi6a0dc92f85421 {\r\ndisplay: block;\r\n}\r\n}\r\n@media screen and (min-width: 768px) and (max-width: 768px) {\r\n.dukbi6a0dc92f85421 {\r\ndisplay: block;\r\n}\r\n}\r\n@media screen and (max-width: 767px) {\r\n.dukbi6a0dc92f85421 {\r\ndisplay: block;\r\n}\r\n}\r\n<\/style>\r\n<div class=\"nfkvb6a0dc92f85367\" ><script type=\"text\/javascript\">\n\tatOptions = {\n\t\t'key' : '0c1eb4c533eaedb7b996f49a5a4983a9',\n\t\t'format' : 'iframe',\n\t\t'height' : 300,\n\t\t'width' : 160,\n\t\t'params' : {}\n\t};\n<\/script>\n<script type=\"text\/javascript\" src=\"\/\/www.highperformanceformat.com\/0c1eb4c533eaedb7b996f49a5a4983a9\/invoke.js\"><\/script><\/div><style type=\"text\/css\">\r\n@media screen and (min-width: 1201px) {\r\n.nfkvb6a0dc92f85367 {\r\ndisplay: block;\r\n}\r\n}\r\n@media screen and (min-width: 993px) and (max-width: 1200px) {\r\n.nfkvb6a0dc92f85367 {\r\ndisplay: block;\r\n}\r\n}\r\n@media screen and (min-width: 769px) and (max-width: 992px) {\r\n.nfkvb6a0dc92f85367 {\r\ndisplay: block;\r\n}\r\n}\r\n@media screen and (min-width: 768px) and (max-width: 768px) {\r\n.nfkvb6a0dc92f85367 {\r\ndisplay: block;\r\n}\r\n}\r\n@media screen and (max-width: 767px) {\r\n.nfkvb6a0dc92f85367 {\r\ndisplay: block;\r\n}\r\n}\r\n<\/style>\r\n\n<p align=\"justify\">Such a motor-servo system will ensure correct replay timing in the long term, but is unable to handle timing errors having a frequency higher than the 25 Hz disc rotation rate. For the correction of short- term speed errors a <i>timebase corrector <\/i>is used. Short-term variations in disc speed have the effect of bunching up or stretching out in time the off-disc signal to a point beyond the tolerance of a colour system which depends on the phase of the chroma subcarrier signal to convey hue information. To keep the timing error below 8 ns (the limit for good colour reproduction) a variable signal delay system is used, wherein the delay period is varied in equal and opposite characteristic to the off-disc timing variations to effectively iron them out. It can be done with either a CCD delay line (of the type we met in Chapter 7) whose clock frequency is governed by off-disc signals; or by (in later models) a digital \ufb01eld store (see Chapter 12) with a locally generated readout-address clock.<\/p>\n","protected":false},"excerpt":{"rendered":"<p>SPEED-CORRECTION SERVOS For CAV discs the required motor speed is exactly 1500 r.p.m., as with a video head drum. For CLV discs the speed is variable, falling from an initial 1500 r.p.m. to around one-third that speed at programme end. In both cases the speed of the disc-drive motor must be closely and accurately controlled. 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