[mythtv-users] anyone have AC3/DTS passthrough working with revo 1600?

Brian J. Murrell brian at interlinx.bc.ca
Wed Jun 9 13:44:22 UTC 2010


On Wed, 2010-06-09 at 11:35 +1000, Jean-Yves Avenard wrote: 
> Hi

Hey,

> because it only does so for 2 channels.

But the point is that the TV has to be doing AC3 decoding here, isn't
it?  You claimed that there is no AC3/DTS decoder in the TV, but if
that's the case, how is the TV even able to play two channels from an
AC3 stream without decoding it?

> > Further, how can my TV play a video file that mplayer reports as having
> > a digital sound track:
> >
> > Opening audio decoder: [liba52] AC3 decoding with liba52
> > dec_audio: Allocating 3840 bytes for input buffer.
> > dec_audio: Allocating 6144 + 65536 = 71680 bytes for output buffer.
> > Using SSE optimized IMDCT transform
> > AC3: 5.1 (3f+2r+lfe)  48000 Hz  448.0 kbit/s
> > A52 flags before a52_frame: 0x2A
> > A52 flags after a52_frame: 0xA
> > Using MMX optimized resampler
> > AUDIO: 48000 Hz, 2 ch, s16le, 448.0 kbit/29.17% (ratio: 56000->192000)
> > Selected audio codec: [a52] afm: liba52 (AC3-liba52)
> 
> But who does the decoding here ?

To be clear, the above mplayer output was provided merely as
documentation of what the audio stream in the file is.  Mplayer is not
being used to read and play the file.

But to answer the question of who is doing the decoding here...

I would think it's the TV (when I select the audio to go to the TV's
speakers).  To be clear, I am pointing the TV at the above video file on
a DLNA server and the TV reads it from the server (the TV has a network
cable and IP address) and plays it.  If I opt for external audio it
passes the AC3 through to my receiver which displays that it is a DD
stream and it (the receiver) also displays that it's got 5.1 channels in
it.

If I opt for the TV to play the audio directly, the audio comes out on
the TV's speakers.  How can the TV do that without an AC decoder?

> and you're outputting 2 channels audio in your example

I think focusing on the number of channels is a red herring.  The
question was who is decoding AC3 and who is (or isn't) passing it
through to the receiver.

> If you configure mplayer to output passthrough, you'll find yourself
> in the same situation as with myth

Well, the issue is that I can't get anything on the PC (where myth is)
to successfully pass audio through the TV to the receiver.  But to be
clear, I do believe the audio getting to the TV is AC3, and I know that
the TV can pass AC3 to the receiver -- if I believe what the receiver is
telling me about what audio it's receiving.

> use the SPDIF output then from the PC directly instead of going through your TV

The PC (which is a revo 1600) does not have a separate SPDIF, it has an
HDMI only.

I do wish I had something else here, "off the shelf" like a B/R (or
heck, even just a DVD) player that did HDMI out so that I could put to
rest once and for all whether the TV can passthrough the AC3/DTS, as I
really do believe the issue here is at the PC, not the TV.  But really,
I don't want a separate DVD player.  I want Myth to do that too[1], so I
am reluctant to go out and buy one just to do this test.

b.

[1] Although, with all of the recent shenanigans with the media
companies putting out DVDs that violate the standards as some form of
pseudo-copy protection I am becoming less and less convinced that this
is actually an achievable goal.
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