re x oversampling cd player 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 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 heart and the effect is rather spooky it actually reconstructs peaks that weren t there correctly too and fills in the gaps with the properly computed values just as if there had been a real sample taken at that point i use a cpu to do all the math it takes a decent but not unreasonable amount of cpu time to do this you can keep up with things in realtime if you write efficient code 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 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 seriously doubt that the filters cost the same but are better they are built to a price spec and that spec says cheap as possible joel kolstad dave dave mcmahan mcmahan netcom com n w
