re x oversampling cd player mcmahan netcom com dave mc mahan writes in a previous article kolstad cae wisc edu joel kolstad writes in article hcbc un l dd netcom com hcb netcom com h c bowman writes i just bought a new portable cd player for the office and i notice that it proudly proclaims times oversampling on the box now while i think i understand what oversampling is the rate of discrete samples exceeds the highest frequency component of interest by some factor i don t understand this times business it seems to me that when i bought my first cd player was it really years ago the specs said times could someone please tell me whether i m getting senile if i m not then what good does it do for the player to take samples at a higher rate the data is only ever read once barring mistracks and such of course and eventually gets turned into khz bit two channel data oversampling takes two discrete data points and interpolates n points between them for n times oversampling when i asked people said that the interpolation was not simply linear interpolation but significantly more complicated you are quite correct in your understanding the filtering is not interpolation as that would distort the frequency content of the signal they do interpolate it s not linear interpolation though you are correct that the frequency content is not altered more specifically the baseband spectrum is preserved and so is every nth image spectrum the other n in n image spectra are removed where n is the oversampling rate you are listening to generally these players run the samples thru an all pass filter network i have done this for ecg waveforms from a person s ideally the filter has a gain of from hz to hz and a gain of from to the new nyquist frequency in practice a finite transition band is required and there is also a certain amount of pass band ripple and stop band leakage with a high order eg taps digital filter a very good approximation can be easily achieved bit deleted about observations of interpolation filter at work in case you care the filtering method uses an fir finite impulse response filter i d guess that cd makers use the same kind of method anybody out there know the real answer i d say that they use a tapped delay line with resistor op amp weighting to accomplish the filtering this strikes me as the most cost effective method for volume production runs no they actually use a digital fir just like yours but built in hardware on custom vlsi chips for mainly economic reasons anyway then the purpose of oversampling is to move the effective sampling rate up to n times khz in order to use higher frequency antialiasing filters for the same quality filter higher oversampling lets you build cheaper filters whereas for the same price filter higher oversamplings lets you build better filters so assuming the quality of all other components in a cd player remained the same oversampling should allow a manufacturer to produce slightly better sound due to anti alias filtering actually i think the only reason they do this is so that they can say that they have a marketting gimic i would guess that it is acutally cheaper to filter an oversampled signal than not you can use sloppier components and give the filter a roll off that isn t so sharp it s too bad that they charge more for something that i think is actually less costly to build i guess that the answer is somewhere between the two as an interesting aside some of you out there may be aware of a scheme used by pioneer and wadia which is called by pioneer at least legato link i haven t heard one of these players but by reading advertising blurb and a couple of pseudo technical articles i deduced that all they do is build a lousy digital oversampling filter and let a load of the first image ie aliassed spectrum through talking to bob stuart of boothroyd stuart aka meridian confirmed my suspicion he said that it sounded awful but then he would wouldn t he christopher christopher hicks paradise is a linear gaussian world cmh uk ac cam eng also reported to taste hot and sweaty
