My question is about bitstream multichannel signals - do these suffer the same fate? For example, if a Dolby 5.1 multi-tone signal is sent to the AVR with only two channels enabled on it, is the performance similarly compromised as the PCM case?
I do not know how one would even go about creating a Dolby encoded multi-channel test signal (single- or multi-tone).
If (and that is a big if)
@amirm has the time now or in the future to run a quick test that will answer some of the most asked questions and narrow down the problems, here are some test files that can be used. You can test if
encoded multi-channel show this problem or only unencoded PCM files. You can also test if this is a HDMI only problem or also in Optical TOSLINK by playing the attached encoded file through those ports from the PC. They are about 6 seconds long each. Don't know if this is sufficient to test. It can be extended if necessary.
The test tone for L and R is from the RME Audio
https://archiv.rme-audio.de/old/english/download/audtest.htm using their 0_16 wav file which as they document is
44,1 kHz, 16 bit, stereo, 1 kHz sine at 0 dBFS (full level), phase right -180∞, dither.
This clean sine is free of distortion, but has a limited signal to noise ratio. Measuring
distortion with a hi-class measurement system will show a THD+N at 0,001%.
Peak level meter show exactly 0 dBFS, in no case over must be shown. The corresponding
RMS level is exactly -3 dB (or dBFS). As this value is normally scaled + 3 dB for better
handling, the RMS meter will often show the same as the peak level meter (0 dBFS).
I created the remaining files using the above and Audacity and ffmpeg for Dolby encoding. The empty channels will remain empty in all of this processing.
The zip file attached contains:
0_16_original_rme_stereo.wav - The 0-16 PCM file from RME in stereo as-is. Playing this will set the base measure for measured L and R performance that will not have any degradation from the problem as it is a 2-channel stereo file. The absolute noise or distortion in this file is not so important as whether changing the speaker setting later creates degradation.
0_16_2chOn_6chSilent.wav - This is the above channels with 6 empty channels added. L and R from above are in channel 1 and channel 2 which will be interpreted correctly by any AVR to send it to L and R. This file can be played to see if the problem as tested with the AP can be reproduced. First test with 7.1 speaker setting and then with 2.0. If there is degradation (absolute values don't matter, just the relative) then we have reproduced what happened with the test signal coming from the AP and the rest of the testing can proceed. If it doesn't then there is a problem in the test set up.
The above files can be played by any audio player - foobar, Windows Media Player, etc. Since it is straight PCM, no processing is done by the players. Just need to make sure that one is using a direct mode (Wasapi, ASIO) to the device or have selected 16bit 44.1 hz for the device in the Windows Sound Device Manager if playing in shared mode so that it is not resampled.
0_16_2chOn_4chSilent.wav - This is the same as above but in 5.1 configuration with 4 channels empty. Test again with 2.0 and 7.1 speaker settings (and optionally 5.1 setting). This test will show whether the degradation also exists with 5.1 input and if it is to the same level as the 7.1. If not, then the issue is proportional to how many channels are being down-mixed. If it is the same level of degradation as 7.1, then it is not how many channels are being down-mixed but if ANY are being down-mixed. If there is no degradation with 5.1, then that would be interesting as it indicates a different handling with the 2-additional channels. If there is no degradation for 5.1 setting but exists for 2.0, then we know it is the down-mixing causing problems.
If the 5.1 PCM file shows degradation for 2.0, the you can try the last file
0_16_2chOn_6chSilent.ac3 - This is the same as above (5.1) but encoded with Dolby Digital (AC3) . To use this you will need a way to have the player pass it through without decoding (bit streaming). Easier to do with video players (vlc, Kodi, MPC, etc) than typical audio players. All the video players have pass-through settings for specific encodings like AC3 and will take care of bit streaming over optical.
If you play this through the HDMI (the AVR should show Dolby Digital on its panel if it is really pass-through) with 2.0 setting, then a degradation would imply it occurs for encoded signals as well. not otherwise. You can also have the player pass-through this file over an optical connection to test if the problem occurs with TOSLINK also or just HDMI.