Hey Bjorn, Connor, On 12/06/2015 1:27 AM, Bjorn Roche wrote:
The important thing is to do anything that might take an unbounded amount of time outside your callback. For a simple FFT, the rule of thumb might bethat all setup takes place outside the callback. For example, as long as you do all your malloc stuff outside the callback, processing and soon can usually be done in the callback.
All true, but Connor may need to be more careful than that. I.e. make sure that the amount of time taken is guaranteed to be less than the time available in each callback.
An FFT is not exactly a free operation. On a modern 8-core desktop machine it's probably trivial to perform a 2048-point FFT in the audio callback. But on a low-powered device, a single FFT of large enough size may exceed the available time in an audio callback. (Connor mentioned Raspberry Pi on another list). The only way to be sure that the FFT is OK to run in the callback is to: - work out the callback period - work out how long the FFT takes to compute on your device and how many you need to coompute per-callback. - make sure time-to-execute-FFT << callback-period (I'd aim for below 75% of one callback period to execute the entire FFT). This is not something that can be easily amortized across multiple callbacks. The above also assumes that your audio API lets you use 100% of the available CPU time within each callback period. A safer default assumption might be 50%. Remember that that your callback period will be short (64 samples) but your FFT may be large, e.g. 2048 bins. In such cases you have to perform a large FFT in the time of a small audio buffer period. If the goal is to display the results I'd just shovel the audio data into a buffer and FFT it in a different thread. That way if the FFT thread falls behind it can drop frames without upsetting the audio callback. The best discussion I've seen about doing FFTs synchronously is in this paper: "Implementing Real-Time Partitioned Convolution Algorithms on Conventional Operating Systems" Eric Battenberg, Rimas Avizienis DAFx2011 Google says it's available here: http://cnmat.berkeley.edu/system/files/attachments/main.pdf If anyone has other references for real-time FFT scheduling I'd be interested to read them. Cheers, Ross. -- dupswapdrop -- the music-dsp mailing list and website: subscription info, FAQ, source code archive, list archive, book reviews, dsp links http://music.columbia.edu/cmc/music-dsp http://music.columbia.edu/mailman/listinfo/music-dsp
