Confusion on channels

Oh wow, this is confusing. HDMI devices talk to each other, but what are they actually saying?

As mentioned in the last item, I’ve been working on a roundup of all HD disc players. One of these is the Panasonic DMP-BD10A, now priced at just $1,099. I have been drilling down into what capabilities the various audio decoders of each player have and, in my initial draft, I accused the Panasonic of only decoding Dolby TrueHD to 5.1 channels, even if the source was 7.1 channels. But there were two things that made me hold back. First I had this niggling suspicion I had previously checked this and that it did 7.1 channels okay. The second was that it didn’t make much sense for the Panasonic to have 7.1 channel analogue outputs if it couldn’t decode to 7.1 channels.

So I asked Panasonic and it, in turn, was surprised about my discovery. So I decided to explore further, using the Dolby Laboratories ‘Sound of High Definition’ test Blu-ray disc.

I double checked the settings on the player (Dolby Digital Plus & Dolby TrueHD both set to PCM out). Then I played it. With DD+ the 7.1 tracks seemed to be coming through fine, with my Yamaha RX-V1800B reporting on its signal info screen a format of PCM, sampling of 48kHz and channels of 3/4/0.1. Perfect. But the TrueHD tracks I was playing were reported as PCM, 96kHz and 3/2/0.1 – ie. 5.1 channels.

Then I remembered that there are also ‘Channel Check’ tracks on this Blu-ray disc, so I went to those and played the TrueHD 7.1 trailer and the TrueHD 7.1 channel check. Both of these were reported by the Yamaha as PCM, 48kHz, 3/4/0.1.

So, I thought, I have it nailed down! The Panasonic, I figured, can’t cope with high sampling frequencies and 7.1 channels at the same time, so if the sampling frequency is 96kHz it switches back to 5.1 channel downmixing. But if it’s only 48kHz, then the unit will produce the full 7.1 channels. Or so I thought.

Now I’ve been using a DVDO iScan VP50Pro video processor for the last several months. This does wonderful stuff, such as converting the 1080i output of my Toshiba HD DVD player to 1080p24, and fixing the lousy deinterlacing in some source devices. But it has been a bit buggy (they’re working on it), and in particular, despite featuring HDMI 1.3 connections, it won’t pass through bitstreams of DTS-HD, Dolby TrueHD or Dolby Digital Plus in undecoded format (nor, for that matter, SACD). So having, I thought, resolved that issue I figured I’d try running the Panasonic multichannel PCM output through it to see if it passed through okay.

By golly, not only did it pass through the multichannel PCM at 48kHz in 7.1 channels, suddenly the 96kHz 7.1 channel material started appearing from the Panasonic player!

A new theory: the Panasonic was misunderstanding the capabilities of the Yamaha receiver — or the latter was sending the wrong information down the HDMI cable — causing the Panasonic to downmix 96kHz 7.1 material to 5.1.

That theory didn’t last long either. After the disc had looped through a couple of times, the 96kHz 7.1 track began reverting to 5.1 channels again, just as it had with the direct connection between the Panasonic and the Yamaha. I switched everything off and on again and I was getting 7.1 channels again … for a while, then it went back to 5.1 channels. More switching on and off. This time, 5.1 channels only. Do it all again, and it’s 7.1. It was kind of random. I gave up.

The ways of HDMI are indeed mysterious. I shan’t be able to resolve this until I get ahold of another Blu-ray player with 7.1 channel Dolby TrueHD decoding.

UPDATE (Monday, 21 January 2008, 9:37 am): I’ve just plugged in a Pioneer BDP-LX70A Blu-ray player. Same behaviour on multichannel decoding as with the Panasonic player. That is, the player converts 96kHz 7.1 channel Dolby TrueHD tracks to 5.1 channels.

The Pioneer is capable of delivering all four of the new audio standards as bitstreams, so I can get 7.1 channel TrueHD at 96kHz that way, thanks to the Yamaha’s internal decoding of these bitstreams.

UPDATE 2 (Wednesday, 23 January 2008, 9:32 am): So now I’ve plugged in a Samsung BD-P1400 Blu-ray player. This unit also features full decoding for Dolby Digital Plus and Dolby TrueHD, as well as the ability to deliver all four of the new audio standards as bitstreams.

This player works perfectly with the Yamaha receiver when decoding 96kHz 7.1 channel Dolby TrueHD material. So why won’t the other two?

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