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16 September 2011

On Spectrograph from Compact Disc (Part I)

Image (top) shows a sample of vinyl record and image (bottom) shows the groove of the record disc.

Compact Disc (CD-ROM) or its relative cousins like DVD and Blu-Ray discs works on same physical operating principle similar to the earlier vinyl records where information is etched on a trench shaped groove in spirals on the disc. A needle from the reader machine (like a gramophone player) called the stylus is then placed on one of those grooves. As the vinyl disc rotates, the stylus plough through the groove and the analogue information stored as features on the trench is then converted to electrical signals which is then electrically amplified to produce sound at output speakers which we perceive as audio recordings.

Image (top) shows logo and a sample of Compact Disc and image (bottom) shows microscopic image of the "broken groove" which composed of many tiny "pits" among a flat "land".

In essence, Compact Discs shares similar working principle with the vinyl record except that the grooves in CDs are much smaller. The reader of CDs have stylus but instead of materials, it is a beam of bright light, i.e. laser. A layer of reflective surface is applied to the top-most layer of the CD. The information stored is being "burned" into many tiny holes called pits on the flat plastic below the reflective disc surface called "land". These on and off pits formed a spiral of broken trench like the vinyl record. When laser light is shone on the bottom surface (where the pits and land is located), the difference in height between the pits and land will produce a reflection of light in different intensity which is detected by the reader. These changes of light intensity is then converted to digital signals, which then being processed to produce audio or video outputs.

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