[alsa-devel] Noisy audio - ALSA Configuration
Does anyone have examples or documentation on how to configure periods, frames and buffers for various sample rates?
I am writing a program to route a mp3 decoder output to ALSA on an ARM processor, and I get noisy audio. For a CD quality stream (S16_LE, 44100 kHz, Stereo, 128 bps) I am using an output buffer of 44100 bytes, a period of 44100/(channels*2). With a buffer this big, I do not get underruns, but all the writes are short writes. I have not been able to find the right configuration to avoid short writes, and more importantly, to get good quality audio.
What do 'short writes' (I am using writei) really mean? Is this a performance issue in the driver, or is mainly because of buffer setup?
I would really appreciate any help with this.
Mag
Iovescu, Magdalena wrote:
Does anyone have examples or documentation on how to configure periods, frames and buffers for various sample rates?
Frames don't need configuration; they are dependent on the sample format only. In your case (S16_LE, 2 channels), one frame = four bytes.
The buffer size determines latency. Low latency is only important when the audio data is generated from user input in real time; when you can read (and decode) ahead, you should better use a rather large buffer to reduce the possibility of underruns.
Periods aren't that important. A period is the interval between two interrupts, so it might be better to have more small periods than fewer large ones (unless the interrupt overhead gets too high) so that your application can get woken up more often, but there isn't much difference in practice.
[...] With a buffer this big, I do not get underruns, but all the writes are short writes. I have not been able to find the right configuration to avoid short writes,
Short writes happen where there is less free space in the buffer than you are trying to write. This is the normal case, there's nothing wrong with that. Is there a reason why you want to avoid them?
Regards, Clemens
Clemens,
Thanks. I wanted to avoid short writes as I thought that they may be the cause of the noise I have in my audio.
What may be other reasons I get bad quality audio?
Magdalena
-----Original Message----- From: Clemens Ladisch [mailto:cladisch@fastmail.net] Sent: Thursday, August 16, 2007 2:06 AM To: Iovescu, Magdalena; alsa-devel@alsa-project.org Subject: Re: [alsa-devel] Noisy audio - ALSA Configuration
Iovescu, Magdalena wrote:
Does anyone have examples or documentation on how to configure
periods,
frames and buffers for various sample rates?
Frames don't need configuration; they are dependent on the sample format only. In your case (S16_LE, 2 channels), one frame = four bytes.
The buffer size determines latency. Low latency is only important when the audio data is generated from user input in real time; when you can read (and decode) ahead, you should better use a rather large buffer to reduce the possibility of underruns.
Periods aren't that important. A period is the interval between two interrupts, so it might be better to have more small periods than fewer large ones (unless the interrupt overhead gets too high) so that your application can get woken up more often, but there isn't much difference in practice.
[...] With a buffer this big, I do not get underruns, but all the writes are short writes. I have not been able to find the right configuration to avoid short writes,
Short writes happen where there is less free space in the buffer than you are trying to write. This is the normal case, there's nothing wrong with that. Is there a reason why you want to avoid them?
Regards, Clemens
participants (2)
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Clemens Ladisch -
Iovescu, Magdalena