xiph
Distortion above 120 Hz in LFE channel of 5.1 audio
Encoding with opusenc at even the highest available bitrate apparently does not guarantee a transparent result. The provided sample gets very audibly distorted when being transcoded with opusenc at any bitrate. The FDK AAC encoder has the same issue, even at quality 5. With OGG Vorbis however I can't hear any distortion even at very low quality settings. Here is the original sample and the encodings: 5.1-surround-LFE-only.zip
When I moved the LFE channel to the Right channel and encoded it again the distortion was gone, so this has clearly something to do with the LFE channel being treated differently: 5.1-surround-LFE-only-LFE-on-R.zip
When downmixing the sample to 2.1 stereo the distortion also does not happen: 5.1-surround-LFE-only-2.1.zip
Yikes: 7.1-LFE.zip Opus track again encoded at highest available target bitrate of 510 kbit/s, AAC encoded with FDK AAC quality 5, Vorbis with aoTuV at quality 3.0.
What does the distortion sound like for you, and what are you using for playback? I see some additional peaks in the spectrum at 420, 840, and 1200 Hz, but that's well above the 120 Hz lowpass usually applied to LFE playback. Does that explain what you're hearing?
@rillian Oh. Please excuse my lack of knowledge. With a steep 120 Hz low-pass filter applied the files do indeed sound identical.
I'm gonna close this issue now.
I mean, I think there really is a bug. As you say, It's not a problem with 2.1 channel config. But it perhaps explains why no one noticed before.
@rillian I think the distortion is just a side effect of encoding optimized only for very low frequencies. Why waste bits on preserving audio information that noone is going to hear? I'm sure there is lots of similar distortion above 18 kHz on regular channels.
To me the "bug" seems to rather be that the LFE channel in 2.1 audio is not treated as an LFE channel but encoded like the other channels at full bandwidth.
I was listening with Audacity to the LFE channel, through headphones. All media players I know disregard the LFE channel when down-mixing multi-channel audio to stereo.