No sound with AC3 passthrough

The passthrough (in this mode) does not work with all receivers. It is not fully following the spec because there is no choice. The brief "pop" is a result of AC3 audio being sent over PCM format, and other receivers can detect this, and appropriately decode. This cannot be corrected until Apple fixes rdar 5435791. So, go to bugreporter.apple.com and demand they resolve it. Then, and not before, can I consider doing passthrough in the correct manner which should work with all receivers.

If you try it with other receivers, it should work. My dad's made by Pioneer does exactly the same thing. I have had good luck with Sony. I know that isn't helpful, but unfortunately, it is true.

So my guess wasn't far off. Thanks for your time and quick reply. I will definitely get on Apple's case since it's bad enough in this day and age that Quicktime can't already do AC3 passthrough properly, something that ought to be dead simple (no decoding - duh!) and has existed in just about any other media player for years and years. But it's even worse if they make it difficult for others like you to implement it without hacks.

Good luck with your efforts!

I had a friend at Apple look at the rdar bug. The status is "On Hold", meaning no one is working on it, and there are 3 duplicates of the bug.

I was told that having more unique filers for the same bug will help prioritize it.

So given that - what should the bug report say exactly that Apple will add it as a duplicate to rdar 5435791?

Thanks

I'm seeing the same thing on a Sony Reciever using a M-Audio Transit, I went to see if I could see this bug on apple to add to it but it would only let me create a new one..

any chance of getting this to work?

I too have an M-Audio Transit USB and a Sony Receiver, and although the Transit has a properly qualified SPDIF port that is recognizable as a Digital Out in DVD Player, EyeTV, VLC etc for encoded passthrough, from all the testing I have done I have found the A52 codec does not work with this device no matter what I try!

For the Transit, (perhaps others are similar, I'm not sure), you have to enable it's AC3 passthrough capability via it's pref pane. This is necessary for it's SPDIF port to gain the "digital out" status (aka Encoded Digital Output in AMS), which happens to also prevent you from changing it's sample rate from 48khz. If this mode is not chosen, the output becomes a standard PCM output via SPDIF with it's sample rate adjustable.

What I did not realize is the A52codec is not looking for nor taking advantage of this Encoded Digital Output... it *needs* the output to be a standard PCM out via SPDIF so it can pass the AC3 data through unadulterated. What I have then realised is the M-Audio Transit WILL NOT PERFORM THIS regardless of whether the AC3 passthru on the hardware is enabled or disabled. All that comes through is jagged pink noisey static when the A52codec passthru is enabled, and stereo when it is disabled.

I confirmed this by connecting a USB SoundBlaster? SB0270 MP3+ sound card which has SPDIF out, is natively supported in OS X *except* it's SPDIF out is deliberately not granted "Digital Out" status as Apple don't like cheap USB sound cards with optical outputs as they give heralded digital sound to old Mac's (which means you don't buy a new one). It's basically sat around doing nothing for ages and ages as it won't output AC3 from anything normally, and forced me to buy the M-Audio Transit. So it's output is a bog boring, PCM output via SPDIF, whose sample rate is adjustable up to 48khz.

Plugged this in, played Thelma and Louise from within QT, and smack me sideways, up pops the 5.1 speaker config on my Sony Amp and Dolby Digital 3/2.1 scrolled across the display. Tested in Front Row, same result. Played a non AC3 file, PCM 48khz stereo output. Perfect! From a hardware device that is not allowed to output AC3 in the normal Apple way using the normal "encoded digital ouput" passthrough. How ridiculous!! So I have to plug in the M-Audio to get AC3 from DVD player, EyeTV (which I use exclusively for DVB-T HDTV viewing, therefore a lot!) and then change to the theoretically unsupported Sound Blaster to get AC3 from Front Row/Quicktime.

So summarizing:

- M-Audio Transit USB does not pass thru correctly, it is still arriving at decoder (amp) as PCM regardless of hardware config. Sounds like jagged pink noisey static. Possibly because the Transit hardware encoder is forcing its output format status to PCM regardless of whats being fed into it. M-Audio Transit USB does have a SPDIF Digital Out that is recognised by OS X as passthru capable but as the A52codec is not formally using this, the hardware falls back to PCM output which is being forced as PCM and the decoder is not able to detect the AC3 audio.

- Sound Blaster SB0270 MP3+ however works as the AC3 audio fed from the A52codec is being fed through to the decoder amp untouched. This is, I suspect, the same as the sound cards in the Apple TV and intel mac's as long as the sample rate is set as 48khz. The SoundBlaster? SPDIF output however is not recognised as a passthrough capable output but as the A52codec does not formally use this it is not necessary.

Hope this may help those who can't figure out why it's working for some people and not others. It all comes down to whether your sound card is forcing its output as PCM.

Note that for my Sony Amp I had to set it's format decode to Auto rather than PCM, which may mean a half second delay on any audio arriving to it.

Edit for above - half a second delay *before* audio is heard - not meant to imply there was a consistent delay! Sorry if confussing.