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

[Stefan Hajnoczi <stefanha@gmail.com>]

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 <programmingkidx@gmail.com>
> 
> ---
> 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).

Gerd might have an opinion on whether that is okay or not.

Stefan