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Re: [Qemu-devel] wav output sounds good - what does this mean?

From: Programmingkid
Subject: Re: [Qemu-devel] wav output sounds good - what does this mean?
Date: Wed, 9 Sep 2015 10:37:03 -0400

On Sep 9, 2015, at 5:17 AM, Stefan Hajnoczi wrote:

> On Sun, Sep 06, 2015 at 04:53:29PM -0400, Programmingkid wrote:
>> I have been working on making the sound output from the USB sound card 
>> actually sound good. When the audio is sent to CoreAudio, the sound is not 
>> very good. But when I use the wav file output option, the sound in the wave 
>> file sounds perfect. What does this mean? 

What I meant by doesn't sound very good is that the audio has a lot of static 
in it. It kind of sounds like a vinyl record being played back. There is also 
this deformed audio sound. Hard to describe in words. I could send you a sample 
of the audio, but I think it might be easier for you to try it yourself. If you 
are on Linux, use this command to build QEMU:
./configure --target-list=i386-softmmu --audio-drv-list=alsa --enable-sdl && 

To test the wav output option, use this command before starting QEMU:
export QEMU_AUDIO_DRV=wav

To test out live playback, use this command before starting QEMU:
export QEMU_AUDIO_DRV=alsa

Then to use the usb audio card, just add this to the command options you send 
to QEMU:
-usb -device usb-audio
"device_add usb-audio" in QEMU's monitor

I use qemu-system-ppc, but this problem is also present on qemu-system-i386. I 
have used Windows XP, Debian Linux, and Mac OS 10.2 as a guest, and they all 
have problems playing back audio using the usb sound card. So anyone of these 
operating systems can be used in the guest for testing. Using a Linux guest 
would probably yield better results. 

My attempts to 'make' QEMU on a Linux host haven't been very good lately, so I 
can't test it out right now. I probably need to reinstall Linux. It would be 
interesting to see how the usb-audio card sounds for you on Linux. If you do 
try, could you let us know how it sounds?

> You didn't describe or post an example of the "not very good" sound via
> CoreAudio.
> Perhaps QEMU is not filling playback buffers quickly enough, causing
> underruns?  These are drop-outs in the audio and are caused by missing
> the real-time deadlines for filling playback audio buffers.

I was thinking it was a buffer issue also. :) Maybe increasing the size of the 
would help.

> The wav output wouldn't have that problem since data is appended while
> QEMU is running but you only listen to it later.  So the timing doesn't
> matter and it will sound fine.
> Another possibility is that QEMU isn't writing audio samples in the
> format that CoreAudio expects, so you hear weird artifacts. 

I have tried every other format I could find. By that I mean these:

The best sounding one is the one that is currently being used: AUD_FMT_S16.

> More info is needed...
> Stefan

On second thought, I will send you audio samples of both live playback and wav 
output in a second email.

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