14.07.2014 18:32, Clemens Eisserer wrote: > Considering the (compared to video) low data rates, I never thought > audio can actually be that expensive. Well, audio in PulseAudio _is_ more expensive than "video in X server". First, the algorithms are more complex. Second, X server doesn't do much. In the most common case (OpenGL video output with direct rendering), all the heavy processing is done by either the application or by the video card. X API simply doesn't have anything more complex than a glorified memset or memcpy, and even that was carefully avoided by giving access to hardware buffers to the application. In simpler terms, X touches each pixel either once or zero times. In contrast, most sound-processing algorithms (even resampling) touch each sample many times, and all the work has to be done in PulseAudio. For concrete numbers, resampling with the quality similar to the default quality of Windows XP requires that each output sample is a weighed sum of ~28 input samples (easily provable by capturing the output of kvm into a wav file). Speex Q1 (default in PulseAudio) needs 16 samples (proof: libspeex/resample.c, search for QualityMapping). So, it would indeed be quite fair to compare the raw video data rate (640x480 pixels * 30 fps * 2 bytes per pixel = 18.4 megabytes per second) and audio processing rate (44100 samples per second * 2 channels * 2 bytes per channel * 16 accesses = 2.8 megabytes per second). Add the fact that X server does just a memcpy in the worst case (and exactly nothing in the best case), and PulseAudio does some arithmetics, and you'll understand why the CPU usage is of the similar order. -- Alexander E. Patrakov
