Gene Heskett wrote: > The fall time of that computer generated sawtooth is probably 100x the > fundamental frequency of the sawtooth. I suspect what your ears are hearing > is aliasing because some portions of that exceed the sampling frequency, > which would be non-harmonically related, and translated to a tone that is the > difference between the sampling frequency and the upper harmonics of that > 400hz tone. This is why good digitizers will have a truly brick wall analog > filter, cutting off by 60+ db, anything that approaches the 'nyquist > frequency' of the sampler itself.
That makes sense for input, sampling an analogue signal to digital. But for a waveform that's synthesised digitally and output with a DAC, and that listened to with no further sampling in the way, such harmonic aliasing shouldn't be audible. If the high harmonics are producing audible sub-harmonics of the sawtooth fundamental through aliasing, that implies there's a non-harmonically related sampling process somewhere. Most likely the route from his sawtooth generator to the output DAC goes through one or more digital filters to change the sample rate, before getting to the DAC proper. That re-sampling needs to have a very good filter built in to avoid this kind of aliasing (re-sampling introduces other artifacts for which it needs a filter too). Needless to say, re-sampling is very hard to do well for all inputs, and so it isn't, and it's easier not to have the high harmonics in the first place. -- Jamie ------------------------------------------------------------------------- This SF.net email is sponsored by: Splunk Inc. Still grepping through log files to find problems? Stop. Now Search log events and configuration files using AJAX and a browser. Download your FREE copy of Splunk now >> http://get.splunk.com/ _______________________________________________ Alsa-user mailing list Alsa-user@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/alsa-user
