Re: [linux-audio-dev] surround sound project. looking for suggestions

Dennis Thompson <yknot(a)cipic.ucdavis.edu> wrote: With ALSA it is difficult to do this robustly. ALSA is a very low-level API that directly exposes you to the quirks and differences in your particular audio interface. The effective latency is dependent on the interrelationship between several different parameters you set for each stream. To get an idea of the kind for the kind of code you need to write to use ALSA robustly, browse http://cvs.sourceforge.net/cgi-bin/viewcvs.cgi/jackit/jack/drivers/alsa/als… In my opinion, a better choice for your situation would be to use PortAudio. PortAudio is a higher-level API that can nonetheless give you all the performance the underlying API can support. In addition, your code will be portable to several platforms in case you ever want to run it somewhere else. Unfortunately the currently released version of PortAudio does not support ALSA, but the next version will. The next version will also give you a lot of control over the latency, in a straighforward way (see http://www.portaudio.com/docs/proposals/003-ImproveLatencySpecificationInte… for more information on how this will be implemented). PortAudio is available at www.portaudio.com The primary programming model for PortAudio is different than ALSA: instead of calling a function like snd_pcm_write() that writes data to the interface, you write a callback function that gets called whenever the interface needs more data (and has data for you, if you are capturing). This will lift you of the burden of trying to control the timing, since your callback will automatically be called at the right time. An alternative that would give you the same benefits as PortAudio is JACK, which has the advantage of being available right now. JACK is a sound server whose main goal is the low-latency interconnection of applications. However, it could serve you as an audio interface engine with a simple API that can give you the low-latency performance you desire. It also uses the callback model. JACK is available at jackit.sourceforge.net. All that said, if you still want to use ALSA, here is what you need to do to achieve low-latency. The two main parameters that control latency in ALSA are the period size and the number of periods. Essentially, the hardware buffer is broken up into several periods, where a period is the number of frames the interface processes per interrupt. frames per period * number of periods = buffer size The hardware buffer is a ring buffer, so if your hardware buffer has 4 periods, it will read the first period, then the second, and so on until it reaches the end when it will wrap around to the beginning. The ideal situation for low-latency is when you have two periods, and period sizes as small as will work with your interface. In that case, the audio loop will essentially be: while(1): Your program (host CPU) Sound Interface ----------------------- --------------- 1. Fill period 2 with data Play sound in period 1 2. Wait until write is available Wait until period 1 finished PERIOD 1 finishes playing 3. (still waiting) Generate interrupt 4. Fill period 1 with data Play sound in period 2 5. Wait until write is available Wait until period 2 finished PERIOD 2 finishes playing 6. (still waiting) Generate interrupt In this case, your latency is the time between 1 and 4 (or 4 and 1), since it is the time between when the sound is *generated* and when it is *audible*. And if you follow the "Sound interface" column, you can see that the time it will take to complete 1, 2, and 3 is the amount of time it takes to play the sound in period one (since the interrupt is essentially instantaneous). So the moral of the story: use two periods, the smallest number of frames per period you can reliably sustain, and you will minimize the latency. There's certainly no need to try to slow down your sound generation loop, since the calls you make to write to the interface will block until more space is available to write. Your current strategy of trying to keep the buffer as empty as possible is going about this the wrong way: the correct way is to simply make the buffer smaller (by reducing period size and number of periods). The reason you are getting underruns is probably because your period size was set to be greater than 1024. For example, if it was 2048, ALSA would expect you to have the entire 2048-frame buffer full and ready to go when it hit that period. (To ALSA gurus: please correct any mistakes I may have made!) Josh -- "Only Angels can sing that high, and only dogs can hear it." -- James Evans