On Thursday 11 October 2007, Jonatan Liljedahl wrote: >Jonatan Liljedahl wrote: >> I'm having problems with distorted sound with ALSA. It's not clipping >> distortion but more like a dirty spectra. >> The test I am doing is playing a pure 440Hz sawtooth wave, and the sound >> is very unclean, there's inharmonic partitials that shouldn't be there, >> actually below the fundamental frequency I think. Compared to an analog >> sawtooth oscillator it sounds really awful. > >Here's a picture of the resulting waveform when playing a pure sawtooth >and recording it straight in again on my Terratec Phase 26. As you can >see, there is some kind of ringing happening... > >But, this is interesting, when recording a pure sawtooth from my >analogue modular into my soundcard (through a mixing desk) there was >some ringing too, but it didn't sound at all as dirty as the previous >computer-generated sawtooth. When playing back this recorded analogue >sawtooth, I had a hard time hearing the difference. > >So, I guess it's not the ringing this is about. And now when I listen >again with different frequencies I definitely hear that the polution of >the spectra is subtones, frequencies below the fundamental. >Perhaps the problem is some cyclic jitter? But it seems dependent on the >high overtones of the computer-generated waveforms since I can't hear it >when recording a sawtooth from the outside... > >Any ideas what this is and what I can do about it? 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. And it's both expensive in terms of component count and cost, board real estate and design time to do that without audible group delay effects, which are an entirely different horse, and the filtering _must_ be done before digitization. Aliasing, once in the digital data stream, cannot be removed. Recalculate the sawtooth so that the fall time stays within the systems bandwidth, and I'd bet a bottle of suds most of the effect you are hearing will go away. The fall time should not be less than 1/frequency, answer is in seconds. -- Cheers, Gene "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) Joe Cool always spends the first two weeks at college sailing his frisbee. -- Snoopy ------------------------------------------------------------------------- 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@xxxxxxxxxxxxxxxxxxxxx https://lists.sourceforge.net/lists/listinfo/alsa-user
