Quote:
Originally Posted by Trance88
Does anyone know the RPM of these discs??
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One rotation has 8 video fields, so it's about 60/8 rotations per second, and thus it would be about 450 RPM. Early machines used an AC synchronous motor that spun the turntable at 1/8th of the 60Hz powerline frequency, thus yielding the 450 RPM. Later on, they used a direct drive scheme that was phase locked looped to spin the disc at the rate needed to get the frequency of either the chroma subcarrier, or the sound subcarrier correct off the disc. The chroma subcarrier was not recorded on the disc at the usual 3.58MHz, but as a "color under" frequency at around 1.53MHz, and there was an upconverter circuit to get it up to 3.58MHz. This would also allow the removal of any wow and flutter off the chroma subcarrier, as the conversion frequency oscillator could be varied to yield a steady 3.58 output subcarrier. Besides, the video bandwidth of the disc was only good to about 3MHz, lower than the 3.58MHz subcarrier. So this was a work-around for that as well.