Date: Fri, 13 Aug 2004 12:03:05 +0100 From: Peter Wood <peter@alastria.net> To: freebsd-chat@freebsd.org Subject: FreeBSD pcm(4) latency (From write() to audible output) Message-ID: <411C9FE9.3000905@alastria.net>
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Good Morning, I'm currently in the middle of writing an automated radio playout system for a northern student radio station in the UK (post SBN liquidation). I have the requirement of it running on a *nix system. I'm a large BSD advocate between our friends, so would prefer not to use Linux. The system design consists of a daemon for each soundcard used in the studio. I'm currently in the process of writing this daemon. I've found that there is about a 800ms delay between the output data being written to /dev/dsp and being able to hear the output from the soundcard on FreeBSD. I'm working to a 200ms deadline. I'm opening the soundcard with: audio_fd = open((char *) device, O_WRONLY | O_FSYNC | O_DIRECT); I've used O_DIRECT and O_FSYNC to try and get rid of this delay, but alas that didn't work, device is cast as it's coming from a void * (thanks to pthread_create ;). The daemon outputs blocks of 0 when there is no actual audio to output so the sound card is always fed. While this isn't a problem for mpg123, it is for this application. I've tried to find where the delay is, however I haven't had any luck. Could anyone knowledgable let me know if there's any hope for getting rid of it (hacking the kernel is fine ;) or if I should "give up" and go to linux? I suppose Linux does have ALSA which claims 2.6ms latency but I was hoping to just use OSS. Oh well. For reference the test machine is a 2.2Ghz machine with a AC97 chip (yeah crap I know, studio playout machines will probably have a Creative card) running 4.10-STABLE (Yeasturday's cvsup). My appologies if this isn't the right place, I'm happy to go pester else where. Heh. Many thanks, Pete. -- Peter Wood BSc (Hons) :: <peter@alastria.net> :: Tel +44 7974 799440
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