d88-jwa@nada.kth.se (Jon W{tte) (09/22/89)
In article <REED.89Sep21130447@intelob.intel.com> reed@intelob.intel.com (Robert Reed) writes: >If anyone out there has an on-line document that explains how oversampling >works in CD players, could you please e-mail it to me? I know this subject I thought I'd post this reply as well as mail it, since there's sure to be other people out there wondering the same thing. FOLLOWUPS DIRECTED TO COMP.DSP, which is where these things belong. Oversampling's very simple. To get good sound quality, you have to filter out everything from and above half the sampling frequency. I.e. the waveform out of a CD player looks like this: ____ ! ! ____ ! ! !____ ! ! ! !___! This cases the use of VERY steep filters ( > 100 dB / octave) which causes BAD phase distortion ( ~ 180 degrees :-( ) The solution is to "fake" a higher sample pitch, causing the CD to interpolate the samples between the actual samples. The waveform now looks like this: (4-fold oversampling) _ !_ !_ ____ !_ _! !__ _! !___! !! And requres much less steep filters (typically 30 dB/octave) which means no phase shifting. Good CD players use more bits in the output stage than is recorded on the CD (i.e. 18 or 20 instead of 16) to get better resolution in the oversampling if the variation between each sample is small (i.e. low output) so that they don't have to interpolate 47 -> 48 as 47 47 47 48 48 but could use 47 47.25 47.5 47.75 48 and thus get a smoother, cleaner output. Simple as that ! h+@nada.kth.se -- Have a nice day! -- Trapezoid, n. -- A device for catching zoids.