Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!uwm.edu!gem.mps.ohio-state.edu!ginosko!uunet!microsoft!brianw From: brianw@microsoft.UUCP (Brian Willoughby) Newsgroups: comp.dsp Subject: Re: how oversampling works Summary: Also: how digital mixing works Message-ID: <7900@microsoft.UUCP> Date: 29 Sep 89 22:33:34 GMT References:<1737@draken.nada.kth.se> <2757@phred.UUCP> Reply-To: brianw@microsoft.UUCP (Brian Willoughby) Organization: Microsoft Corp., Redmond WA Lines: 42 In article <2757@phred.UUCP> jefft@phred.UUCP (Jeff Taylor) writes: > >Linear Interpolation introduces disortion (changes the spectrum). >[which can be componsated for in a digital filter]. Adding zeros does not. >(plus the calculations are so simple). This explains how digital mixing works. My first musings on digital mixing involved using addition to mimic analog op-amp mixers. This method has the same drawbacks as analog mixing, because you must now be concerned with signal overload due to wave peaks coinciding. In the analog world the voltage sum reaches the supply limits, and in the digital realm the values overflow the arithmetic unit. But the first digital sampler circuitry I examined utilized time multiplexing of the signals. My first reaction was that this was a cheap solution full of distortion from unwanted high frequencies. But reviewing your post, I see that time multiplexing N signals is equivalent to oversampling each channel by N times and then adding them (where adding zero is a no-op). The only difference is that each channel is shifted in time. This method of digital mixing of multiple sample channels obviously requires that you have a constant sampling rate for all channels, and that you follow the combined output with an appropriate digital filter. I hope I wasn't too far off on that, since I don't practice DSP math. >What follows is a hand waving (no math) justification on why this is logical >(although it defies common sense). (posted about 3 years ago to rec.audio) Thanks for a very informative (and surprisingly understandable) post. Your explaination might defy common sense, but I still can't help but examine the results of zero-filling intuitively. I.e. following the zero-filled, over-sampled data with a digital filtering algorithm tends to smooth out the data so that ideally you have a wave which looks very similar to a linearly interpolated wave (to the eye), but has less distortion from errors in approximation. Brian Willoughby UUCP: ...!{tikal, sun, uunet, elwood}!microsoft!brianw InterNet: microsoft!brianw@uunet.UU.NET or: microsoft!brianw@Sun.COM Bitnet brianw@microsoft.UUCP