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I know that Laserdiscs store video in an analog composite format.

I also know that CDs and DVDs store audio data as a digitally-encoded bitstream with the bits stored as pits and islands in the disc surface, which is simply read by the laser.

How is the analog data physically stored on the laserdisc surface? If it's pits-and-islands, then the data is surely digital and not really "analog", but if it's analog then it must be some kind of surface that has a continuous "reflectivity" which can store multiple channels of data (composite video in some kind of carrier and audio channels) in a single stream, how is this possible? Stereo vinyl discs store stereo information in different dimensions, but you can't do that with a reflective surface.

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Just about everything you'd like to know about LaserVision disks can be found in this Wikipedia page, including the details of how the composite analog video is stored in a series of pits and lands.

Of course it's not strictly analog because it's sampled and later re-integrated, but there is no digitization of the video as we think of it today with Blu-Ray etc.

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    Hi Jim - adding a quick summary, or a quoted section of the key answer points would make this a much more valuable answer. – Dr Mayhem Jan 21 '15 at 11:25
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What is on a laser disc is a 8.1/7.1 MHz FM carrier modulated by the NTSC/PAL video signal.

Since laser can only detects pits-and-lands (as you stated), that signal is encoded using PWM (it encode it by effectively chopping it up into discrete parts).

I found this schema from wikipedia to be very helpful : https://fr.wikipedia.org/wiki/Laserdisc#/media/Fichier:LaserdiscModulation.png

The analog part came from the fact it use variable lengths for pits and lands to encode the signal, while on a digital format like CD-ROM, discrete lengths are used to indicate how many 0's there is between pit-to-land or land-to-pit transitions (which indicates a 1).

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  • Very interesting. So, in a way, laser disk is actually digital video. Crazy. – user606723 May 3 at 5:56
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Well, for a beginning, anything taken like a data stream reflected from a surface such as a laserdisc, dvd, cd or blu ray, is at the very start ANALOG...! because it is only light reflected, but as those light pulses undergo the treatment of several stages, is when some circuits and devices convert those light pulses into digital data. But the matter here is laserdisc is analog or not, it is analog indeed but due to the method choosen to be that way, laserdisc comes from the late 60´s and for those days ther was not such a thing like video compression, so the scientists who developed that system were prone to use an optical signal but considering that the size of a pit or a land, either this was long or short, would equal a voltage value when readed and accounted to ensamble a coherent video signal, so these changing values, would render a series of multiple electrical values derived from the wide of those optical pulses, this is a method called PULSE-WIDE MODULATION, which was the more precise choice to create an optical system for video encoding. Based in continuosly changing reflectivity to be turned into an equivalent continuosly changing voltage signal, which could make an electronic oscilator to generate an RF signal in which a carrier could bring audio and video

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