HD Audio downsampled to 2.0

I have a Spiderman 3 Blu-Ray with two Dolby True HD 5.1 Audio tracks (English and German). Playback is set to SPDIF which works fine. Audio streams of trailers and extras are fed to the receiver and accepted as Dolby Digital 5.1 but when I start the movie both HD Audio tracks are sent as 2.0 and the receiver shows Dolby Prologic II Movie. Is this a known limitation because of the protected audiopath? I though downsampling to 5.1 or 7.1 at 94 or 48 kHz is allowed? I am running TMT 2.1.6.131. Thanks.

Make sure you have the mixer option ticked in the TMT options. It will then downconvert as needed.

Yes, I enabled the downmixing (192 kBit). The HD format is 5.1 and unfortunately the only one available. But I still do not get a downsampled 5.1 signal that should not exceed the SPDIF bandwidth.

But after some further testing I have the impression that either my Onkyo TX-DS 797 is just too outdated (8 years!) to properly handle the bitstream. Or Windows 7 x86 - which added support for Dolby Digital Plus - acts odd. Because I have a similar problem in MCE. I have several videos with AC3 which play fine using Zoomplayer on a XP machine. But when played back in MCE7 I also get 2.0 only (Dolby ProLogic II). Same is with live TV and AC3 language streams, which I can select. Maybe the bitstream is sent over as DD+ instead of DD 5.1, which would explain the fallback to 2.0 because of the same bandwidth limitation using SPDIF. Maybe Microsoft can implement a "force SPDIF output to DD 5.1 for backward compatibility"-switch for older AV receivers. I try to borrow an actual receiver to see if it detects the bitstream correct.

DVDs however with DD and DTS play fine, same is with Blu-Rays that have uncompressed 5.1 PCM. With these I get a correct 5.1 signal over SPDIF.

I have TMT2 (see 1st post). So you are saying that TMT3 has more advanced decoding features over TMT2?

Edit: I also found an explanation for my MCE 2.0 instead of 5.1 output. The DD+ standard defines following:
For a stereo presentation, the decoder plays a two-channel downmix.
For 5.1, the decoder rematrixes the two-channel downmix with a second 3.1-channel substream. For 7.1, the decoder rematrixes the reconstructed 5.1-channel program with the third two-channel substream.

That means with Windows 7 and its DD+ implementation we get a 2.0 output with a 3.1 substream for 5.1 formats. And I am almost sure that this 3.1 substream is not recognized by my fairly old receiver. I borrowed a newer model and will report back my test results later today.


Edited by Dr.DOS - 04 Nov 2009 at 7:39am

OK. I dug deeper into the mixing options available with Dolby True HD and Dolby Digital Plus according to the standard. (interresting read on the Dolby website ->  http://www.dolby.com/uploadedFiles/zz-_Shared_Assets/English_PDFs/Professional/DPlus_TrueHD_whitepaper.pdf)

And I also verified the output with another receiver. It is two-channel PCM. Which is all I will get over SPDIF. Same behaviour has been seen with Spiderman 3 and PowerDVD 9 for example. In the above document it is explained why (see page 3 and 9 respectively):

1. Lossless Dolby True HD is built of: 2 ch core mix + 3.1 ch extension A + 2 ch extension B
2. The backward compatible output over SPDIF is 2 channel PCM (the core mix).

A receiver with HDMI 1.3 compliant input is needed (aside a HDMI soundcard). Switching over to TMT3 will not make a difference here - since it does not incorporate Dolby Digital Life to perform a real-time reencode to DD 5.1 (correct me if I am wrong). That would be the only option to get a 5.1 signal pushed through the bandwith limited SPDIF interface.