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95 lines
3.0 KiB
Plaintext
95 lines
3.0 KiB
Plaintext
From: "Benno Senoner" <sbenno@gardena.net>
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To: <music-dsp@shoko.calarts.edu>
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Subject: Re: [music-dsp] coding realtime guitar efx on a "pc"
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Date: Saturday, June 30, 2001 8:19 AM
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Andrè,
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you are solving your problem the wrong way:
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you need to use a single threaded solution which does this:
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- set the audio I/O parameters to fragnum=4 fragsize=128 bytes (=32samples) if
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you use stereo or fragsize=64 bytes (=32 samples) if you use mono.
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(do not forget to activate fulltuplex with using the _TRIGGER_ stuff)
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(you need to frist deactivate audio and then start the trigger after the DAC is
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prefilled (see below))
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This will give you a total input to output latency of 4x32 samples
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= 128 samples which at 44.1kHz correspond to 2.9msec latency.
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now set your process to SCHED_FIFO (see man sched_setscheduler)
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after the initialization your code should do more than less this:
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- write() 4 x 32 samples to the audio fd in order to prefill the DAC.
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Without this you will get dropouts.
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while(1) {
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read() 32 samples from ADC
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perform_dsp_stuff() on the 32 samples
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write() 32 samples to DAC
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}
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If you use a low latency kernel and pay attention to all the stuff above, then
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you will get rock solid 3msec latencies (plus eventual converter latencies but
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these are in the 1-2msec range AFAIK).
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Using multiple threads , pipes etc, only complicates your life and often makes
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it impossible to achieve these low latences.
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Realtime/audio programming is not an easy task , this is why people often
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fail to get the desired results even if their hardware is low-latency capable.
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The problem is that the final latency depends on the hardware you use,
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the application and the operating system.
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cheers,
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Benno.
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http://www.linuxaudiodev.org The Home of Linux Audio Development
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On Sat, 30 Jun 2001, you wrote:
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> On 2001-06-29 21:38 +0200, Benno Senoner wrote:
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>
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> > OSS/Free refuses to use a low # of frags ?
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> >
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> > That's a myth.
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>
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> I hope it is. :-)
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>
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> The fact is that ioctl(SNDCTL_DSP_SETFRAGMENT) succeeds with
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> values as low a 0x10007 (one 128-B fragment) but the latency is
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> still high enough to be clearly noticeable, which suggests that
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> it's *way* above 2/3 ms. This is on an otherwise idle machine
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> equipped with a SB PCI 128.
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>
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> But maybe it's me who's doing something wrong. I've been careful
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> to flush stdio buffers or use unbuffered I/O (write(2)) but I
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> may have let something else through.
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>
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> For example, since the signal processing and the I/O are done by
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> two different vanilla processes communicating via pipes, it may
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> be a scheduling granularity problem (E.G. the kernel giving the
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> I/O process a time slice every 20 ms).
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>
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> --
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> André Majorel <amajorel@teaser.fr>
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> http://www.teaser.fr/~amajorel/
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>
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> dupswapdrop -- the music-dsp mailing list and website: subscription info,
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> FAQ, source code archive, list archive, book reviews, dsp links
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> http://shoko.calarts.edu/musicdsp/
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--
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dupswapdrop -- the music-dsp mailing list and website: subscription info,
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FAQ, source code archive, list archive, book reviews, dsp links
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http://shoko.calarts.edu/musicdsp/
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