Channel mapping

Hi there,
I would like to discuss two topics here, both closely related to channel mapping in standard 5.1 and 7.1 multi-channel configurations. If we look at the CEA-861(ancient revision D) channel mapping table below, all 8 channels are numbered and their presence in various configs is labeled with the channel id. Two configs are highlighted - 5.1 and 7.1:



1. Odd-numbered are left channels in digital transmission pairs, even-numbered are right channels. Channel #3 - left in the second stereo pair - is LFE, channel #4 is Centre. Could someone please point out where the expectation/standard of Center channel being transferred in the left channel of the second stereo pair came from? It must be something else than the CEA/CTA standard.

2. Many listeners would like to check the correct channel mapping and there is a number of test files for that available for download or speaker setup utilities with test tones built in many universal players and streamers. Here is an interesting observation. If we play 5.1 test file, for example from this page https://www2.iis.fraunhofer.de/AAC/multichannel.html
and specifically this file
https://www2.iis.fraunhofer.de/AAC/ChID-BLITS-EBU-Narration441-16b.wav
all the channels are mapped as expected. But if we play this 7.1 file
https://www2.iis.fraunhofer.de/AAC/7.1auditionOutLeader%20v2.wav
then the Surround pair seems to be swapped with Surround-Back pair, which I find quite strange. With the CTA-861 as a reference the difference between 5.1 and 7.1 should be just addition of the rear surround pair, certainly not rearrangement of the whole surround part. This has been confirmed with multiple players, via HDMI as well as player's analogue outputs, for which the connector channel labelling would become incorrect with the 7.1 test file.
Could someone with a 5.1/7.1 setup try the files above and report the results back here, please?
Many thanks.

Best Regards,
Pavel

ITU Rec BS.1738 is one source https://www.itu.int/dms_pubrec/itu-r/rec/bs/R-REC-BS.1738-1-201510-I!!PDF-E.pdf

This layout is referred to as SMPTE channel order in the industry, so SMPTE may have designated that channel ordering first. I don't know the specific document though.

Here's a Dolby page mentioning it:
https://professionalsupport.dolby.com/s/article/What-channel-order-should-be-used-for-assigning-bed-audio-to-the-Renderer?language=en_US

Interesting CEA flipped the C/LFE around. I wonder how that was decided.

As far as 7.1 channel order, at least on Windows it can depend on what audio player is being used. I can get the surrounds to flip around just with embedding different channel metadata, and iirc some software players wouldn't respect that metadata. I don't have access to a 7.1 system, otherwise I'd test.

Cheers!

Hi OnaIp,
That explains a lot! It seems that the channel ordering discrepancy between CTA (consumer) and ITU (professional) really does exists. As for the surround channels, my observation was done using a BD player, where you would expect consistent performance. Anyhow, thanks again for sharing your knowledge!
Best Regards,
Pavel

EDIT, removed "NOT"

Dear Pavel: I have a Dolby Atmos test Blu Ray. The Vanity does not appear to obey any channel assignment metadata in a source. Except for the Center/LFE swap option, it is permanently set up to be in this order from the AES outputs:

LF/RF
C/LFE
LS/RS
SBL/SBR

which is SMPTE channel order. I Fortunately it is the standard I'm using in my studio :-)

Once you get into more than 8 channels, the naming gets a bit confusing between the rear pair and the side surrounds. Fortunately, a lot of systems don't seem to obey the speaker allocation portion of an EDID.



Edited 1 time(s). Last edit at 01/11/2024 05:02PM by bobkatz.

Hi Bob,
Correct, the VanityPRO follows the channel assignment as per the CTA standard above, which seems to be fairly common. I am glad the SMPTE has the same mapping :-)
Let's see what happens when we have to go beyond 8 channels with Atmos or some other spatial format, it makes me a bit nervous to be honest :-)

Regards,
Pavel