[alsa-devel] Noisy audio - ALSA Configuration

Eliot Blennerhassett linux at audioscience.com
Thu Aug 16 00:01:17 CEST 2007

> Eliot,
> Thank you very much. Here is the message. If Hotmail is the problem I can
> subscribe using my work e-mail; I wanted to avoid that in the first place.
> Magdalena

The Hotmail message is all HTML, with no plaintext version.  I'm not sure if 
this list accpets html mails. OR  The text of the mail is all encapsulated in 
a number of HTML <DIV> blocks, maybe confused html to text converter...

Here is the original question:

> 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

Be careful about the distinction between bytes and frames (i.e. a frame of 
stereo S16 is 4 bytes)

Start with the period 
1024 frames =  4096 bytes
then make your buffer at least 2x this:
8192 bytes

(I'm not an app developer, so others may jump in with a better explanation)

Did you look at the code in alsa-lib/test?

> a buffer this big, I do not get underruns, but I do get all 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.
> Magdalena

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