Transcode for multichannel audio (EAC3, TrueHD etc) are transcoded to 2 ch Opus instead of 5.1 AC3 when using optical.
In settings pass through is set to optical.
AC3 set as supported, DTS set to not supported.
AC3 movies AC3 is passed through.
All other audio (multichannel) are converted to opus. Plex server say it transcode to opus.
I have no problem when reverting to 8.23.1.28030. Then everything is transcoded to AC3 or passed through as AC3. Plex server say it transcodes to AC3.
Later version itās broken.
Can confirm. Same issue for me, running Plex App on an Sony Android TV. 28030 and older play E-AC3 just fine, on later builds it gets transcoded to Opus 2ch due to E-AC3 not being supported.
This change in transcoding has now hit the stable Android release.
From my conversation with one Plex employee, itās unlikely that this will be fixed. I was told that they arenāt many people using passthrough and that Plex needs to do whatās best for the greatest number of users (so not us)
This will be fixed, Iām not sure where youāve got that impression from. Passthrough support is important to us, for most cases when EAC3 is not supported, then OPUS would be a preferred target over AC3 as it allows the quality to remain. However in this case we have neglected to take into account that OPUS with Optical passthrough would only provide 2 channels which is obviously a regression.
As I said early, I asked how to get AC3 transcoding to work over optical passthrough following this latest update. I got a lot of talk about how Opus is better, and why would I even want to use AC3, and Opus can be sent to my receiver instead of AC3. I was never once told the issue will be passed on to the developers, or that this would be fixed. It wasnāt until I reposted the problem here in the forum citing that earlier conversation, that a Plex employee finally responded with a luke warm āwe can look into thatā.
I have seen Plex retire plenty of features. Iām happy to hear this will not be one.
Were not retiring optical passthrough, this was a regression that was overlooked due to the inclusion of Opus and other codecs as transcode targets - which as I mentioned above does improve things except when using optical.
Will be fixed in the new beta, should be available later today.
I know youāre fixing the regression, and Iām not affected by this issue. So please donāt take this as āpiling onā or criticizing.
Iām confused by this, and Iām quite curious. Iād like to understand better, if youāll indulge me.
In what situations is Opus the best transcoding target? I agree that itās a great codec. I donāt understand when itās the best transcoding option.
modify rankings based on compatibility concerns
3a. ARC and S/PDIF only support DD 5.1, DTS 5.1, and LPCM 2.0
3b. Donāt send multichannel AAC to some TVs, they eat it.
3c. Device-specific quirks, ad nauseum
identify best common denominator
4a. most channels ā
4b. no further conversions required ā
4c. best conversion method ā
4d. match original codec family ā
4e. best codec quality
profit
I assume itās something like that, but exponentially more complicated.
Is EasyAudioEncoder capable of converting E-AC3 to AC3 without performing a full decode and re-encode? Thatās one of the slickest features of E-AC3. If itās supported by the Plex tools, it could make E-AC3 ā AC3 a preferred path compared to other options.
EAC3 isnāt backwards compatible with AC3 - though an EAC3 decoder can play AC3. For playing back EAC3 it requires a full decode and encode to go from EAC3 to AC3 - the only codecs with āsimpleā backwards compatible is DTS HD formats, they can be stripped back to just DTS Core without needing a full decode and encode run - we can do this on the client on Plex for Android. As far as Iām aware this is because DTS HD formats are extensions to DTS Core, so we can skip the extensions and just pull down the core, EAC3 however has a different way of wrapping the packets so we need to decode in order to be able to rebuild the packets as AC3.
Opus is preferred mostly because of the bitrate performance, simply put it can hold higher qualities with less bitrate, itās also very fast to encode compared to other formats.
Itās pretty much decided how you pointed out, in the case of the issue above I didnāt take into account that some setups (like optical) can still play Opus but the channels would be restricted to stereo when itās passed over PCM (optical supports 2 channels over PCM at the most). So even though Opus would usually be a better target, if your stuck with stereo output itās not the right target when playing surround sound.
Player 28473 is now showing an error when trying to play an EAC3 file, so this issue does not seem to be fixed. Switching to HDMI instead of optical results in 2ch audio.
EAC3 from Blu-ray is a funky hybrid. Itās a 5.1 AC3 legacy stream, plus 4 channels of dependent stream EAC3 giving the rear and side upgrades. The EAC3 can be discarded and the AC3 can be used as-is.
And (Or?) pure EAC3 was also designed to be easily converted to AC3 without a full decode & encode. Iām referring to section 6, here:
Unlike standard transcoding processes for converting between incompatible formats, the E-AC-3 to AC-3 conversion process does not require a full decode to PCM samples followed by a full encode to AC-3. Because E-AC-3 and AC-3 use the same filterbank, bit-allocation process and framing structure, the conversion device needs only to perform a partial decode followed by a partial encode.
That should preserve quite a bit of quality, compared to switching codec families entirely.
But Iām only guessing at which libraries Plex uses for EAC3 conversion - and maybe Iām assuming they can do things that they canāt.
Iām not aware of any open source tools that can do such conversion. Being technically possible doesnāt mean that itās practical with the tools available.
Huh, TIL will have to run this by someone to see why we canāt play EAC3 as AC3 on the client side, it might be due to the patent restrictions (even though AC3 is now patent free). On Plex Media Server (which is licensed), when converting EAC3 to AC3 we should be using the process youāve highlighted via easy audio encoder. This would mean EAC3 to AC3 is lighter on transcoding, though this is quite trivial for audio decoder / encoding in terms of server resources and since Opus provides better bitrate quality selection itās not worth the benefit of EAC3 to AC3.
