Re: DVB Playback Issues

On 13 Dec 2011, [email protected] wrote:

> I successfully got mplayer working with vdpau and playing TV channels,
> however I notice a few issues occasionally.  Sometimes when I tune to
> a channel, I get no audio and the video appears to be laggy. Sometimes
> when this happens I just get blocks near the top (see pic:
> http://i.imgur.com/BvMhT.jpg).

You can try '-afm ffmpeg' instead of using '-ac'.  Sometimes, channels
will change the sound encoding from show to show.  One moment it is 2
channel, the next it is 5.1.  Mplayer is trying to use the 'spdif' hwac
codec.  This passes the audio from the 'air' directly out your audio
output without processing the audio at all.   Anyways, you may have a
card (sound blaster, ICH5, etc) that supports SPDIF but there is no way
to know the cable is connected without configuration.

When you specify '-ac' it will only use that codec.  However, the
particular codec might not be able to play the sound stream of the
moment.  The benefit of '-afm' is that if a suitable codec is not found
in that family, mplayer will still search for others suitable to play
the stream.  You might also try '-ac -hwac3' to prohibit the spif
pass-through 'decoder'.  Also, you might need to install liba52 to get
the stream decoded.  You have these lines,

> Unknown/missing audio format -> no sound
> ADecoder init failed :(
> Requested audio codec family [a52] (afm=liba52) not available.
> Enable it at compilation.
> Opening audio decoder: [hwac3] AC3/DTS pass-through S/PDIF
> hwac3: switched to AC3, 384000 bps, 48000 Hz

> AUDIO: 48000 Hz, 2 ch, ac3be, 384.0 kbit/25.00% (ratio: 48000->192000)
> Selected audio codec: [hwac3] afm: hwac3 (AC3 through S/PDIF)

I think the sound on the channel you are tuning to is switching between
2 channel PCM and 5.1 AC3 encode mp3-type sounds.  You have to decode
this sound in software to downmix to whatever number of speakers you
have.  If you have a TV, you can usually press an 'info' button and get
some information on the stream.  You can also capture the stream when it
works and when it doesn't work and run 'midentify.sh' on it to see what
mplayer is seeing the sound as.

Software decoding of audio is probably not a problem for you as you have
hardware video decoding with vdpau.  If you have hwac3 hooked up, the
signals would pass directly from the cable/antenna through to the video
card and audio out; the CPU would just setup the capture card demux the
stream and forward data to the other devices.  You need an extra step
for the audio do 'decode' the stream before you can forward it to the
audio card.  liba52 has this capability.  I don't know if any other
codec do off the top of my head.

hth,
Bill Pringlemeir.