Re: Sound lag over HDMI

On 12.07.2013 23:54, Ulrich Spörlein wrote:
> I'm trying to setup XBMC on a -CURRENT box with an IvyBridge CPU and
> GPU. While testing playback via mplayer on a LG TV over HDMI, I noticed
> that sound is lagging video by about 100-200ms or so. When I switch to
> using the jack outputs powered by some Realtek chip, audio is perfectly
> fine.
>
> Is HDMI lag a known problem? Can this be fixed?
>
> root_at_coyote:~# dmesg | egrep vgapci\|pcm
> vgapci0: <VGA-compatible display> port 0x3000-0x303f mem 0xe0000000-0xe03fffff,0xc0000000-0xdfffffff irq 16 at device 2.0 on pci0
> agp0: <IvyBridge mobile GT2 IG> on vgapci0
> pcm0: <Realtek ALC888 (Rear Analog)> at nid 20 and 24 on hdaa0
> pcm1: <Realtek ALC888 (Rear Digital)> at nid 30 and 31 on hdaa0
> pcm2: <Intel Panther Point (HDMI/DP 8ch)> at nid 6 on hdaa1
> pcm3: <Intel Panther Point (HDMI/DP 8ch)> at nid 7 on hdaa1
> drmn0: <Intel IvyBridge (M)> on vgapci0

I don't know what to say. I am now using HDMI audio from NVIDIA card to 
quite old external 5.1 receiver with XBMC every day, and I haven't 
noticed lags. Before that I've also successfully used SPDIF connection 
for the long time. Though I've never specially tested it somehow other 
then watching movies. :) If you have some good testing methodology -- 
please, welcome to share. By the HDA driver HDMI is handled exactly the 
same way as analog output from the point of data buffering, so I would 
not expect there major differences.

You may try to experiment with hw.snd.latency sysctl to tune buffering 
in kernel to see whether it affect the result.

Also you may compare delays when doing AC3/DTS pass-through with case of 
software decoding and discrete (multichannel) PCM playback.

The only potentially related effect I have noticed is that my receiver 
eats first second or about that of playback stream. It makes short 
sounds like GUI event notifications inaudible sometimes. I guess that 
could be made to restore audio sync after some unavoidable startup 
delay, but that is only my guess.

-- 
Alexander Motin