Re: [Qemu-devel] [PATCH] hw/usb/dev-audio.c: make USB audio card sound perfect

[Programmingkid]

On Oct 26, 2015, at 7:00 AM, Stefan Hajnoczi wrote:

> On Fri, Oct 16, 2015 at 09:54:12AM -0400, Programmingkid wrote:
>> 
>> On Oct 16, 2015, at 8:15 AM, Stefan Hajnoczi wrote:
>> 
>>> On Tue, Sep 22, 2015 at 07:32:01PM -0400, Programmingkid wrote:
>>>> The USB audio card would not play audio well because its buffer was too 
>>>> small. 
>>>> Increasing it made it play perfectly. All the crackling and dropouts are 
>>>> gone.  
>>>> 
>>>> Signed-off-by: John Arbuckle <address@hidden>
>>>> 
>>>> ---
>>>> This patch was tested on qemu-system-ppc running Linux and qemu-system-i386
>>>> running Windows XP. Windows XP sound output thru the USB audio card sounded
>>>> perfect. Linux did improve in sound quality, but it still wasn't perfect. I
>>>> think there are problems with the hcd-ohci.c file. The Mac OS 10.2 guest 
>>>> in 
>>>> qemu-system-ppc did not play sound due to USB issues unrelated to this 
>>>> patch. 
>>>> 
>>>> hw/usb/dev-audio.c |    3 ++-
>>>> 1 files changed, 2 insertions(+), 1 deletions(-)
>>>> 
>>>> diff --git a/hw/usb/dev-audio.c b/hw/usb/dev-audio.c
>>>> index f092bb8..e4e4989 100644
>>>> --- a/hw/usb/dev-audio.c
>>>> +++ b/hw/usb/dev-audio.c
>>>> @@ -88,6 +88,7 @@ static const USBDescStrings usb_audio_stringtable = {
>>>> #define USBAUDIO_PACKET_SIZE     192
>>>> #define USBAUDIO_SAMPLE_RATE     48000
>>>> #define USBAUDIO_PACKET_INTERVAL 1
>>>> +#define BUFFER_MULTIPLIER        32
>>>> 
>>>> static const USBDescIface desc_iface[] = {
>>>>    {
>>>> @@ -664,7 +665,7 @@ static const VMStateDescription vmstate_usb_audio = {
>>>> static Property usb_audio_properties[] = {
>>>>    DEFINE_PROP_UINT32("debug", USBAudioState, debug, 0),
>>>>    DEFINE_PROP_UINT32("buffer", USBAudioState, buffer,
>>>> -                       8 * USBAUDIO_PACKET_SIZE),
>>>> +                       BUFFER_MULTIPLIER * USBAUDIO_PACKET_SIZE),
>>> 
>>> I'm not familiar with the code but I guess this also increases audio
>>> latency by a factor of 4 (8 -> 32).
>> 
>> Didn't hear any problems. When I tried it out. 
> 
> 8 buffers * 192 bytes / 3 byte (24-bit) mono samples = 512 samples
> 32 buffers * 192 bytes / 3 byte (24-bit) mono samples = 2048 samples
> 
> At 48 kHz sample rate that is 10.6 milliseconds vs 42.6 milliseconds
> latency.
> 
> I have never tried real-time audio processing under QEMU but when I use
> Linux for guitar effects it becomes noticable when latency is above
> around 12 milliseconds.  ~5 milliseconds latency with USB audio is
> achievable on bare metal.
> 
> So this change would make real-time audio feel laggy.  Unless Gerd has a
> strong feeling that it's an improvement for QEMU, I wouldn't merge this
> change.

Have you done any testing using this patch yet? 

I have played several MP3 files and a video file. The audio plays very well.
For the video file the audio was in-sync with the audio. I played a pinball
game and the audio did match what was going on with the action of the
game. With a powerful enough computer someone could play Guitar Hero
in QEMU and not have any problems.