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AC3 always transcodes to AAC

adamska46adamska46 Members, Plex Pass Posts: 61 Plex Pass

Hi,

As iOS devices natively support AC3/EAC3 decoding since iOS 9.3, I wonder why Plex always transcodes AC3 5.1 audio to AAC. The video direct streams, but not the audio.

Was it not direct playing/direct streaming a few builds back ? I remember it doing so.

I am using an iPhone 6S on iOS 11.2.6, latest PMS 1.12.0.4829.

Thanks ! :)

Synology DS218+ (Intel Celeron J3355 Apollo Lake iGPU, 10 GB DDR3 RAM)

Answers

  • dragonmeldragonmel Members, Plex Pass Posts: 692 Plex Pass

    how do you plan on listening to 6 channel audio on a device with 2 speakers?

  • adamska46adamska46 Members, Plex Pass Posts: 61 Plex Pass
    edited March 12

    By a process called downmixing. The idea is to let iOS do this downmixing job, instead of PMS. This way, playback is way quicker to start when no transcoding takes place.

    Synology DS218+ (Intel Celeron J3355 Apollo Lake iGPU, 10 GB DDR3 RAM)

  • dragonmeldragonmel Members, Plex Pass Posts: 692 Plex Pass

    yeah.. but why would I want to pay mobile data charges for content that I am going to throw away..

    audio transdcode on the server is damn near free and I see no delay.

    additionally, I am not positive, but I dont believe ios will even handle most multichannel codecs natively.. i.e. it wont play it and it wont transcode it/downmix it

    additionally.. my server is plugged in and has unlimited electricity.. transocding and muxing/downmixing takes battery power..

    I could go on buy I think you get the point

  • kclimiekclimie Members, Plex Pass Posts: 79 Plex Pass

    iOS handle AC3 and E-AC3 just fine. See https://developer.apple.com/library/content/documentation/MusicAudio/Conceptual/CoreAudioOverview/CoreAudioEssentials/CoreAudioEssentials.html#//apple_ref/doc/uid/TP40003577-CH10-SW57

    @dragonmel I'm going to disagree with you on the mobile bandwidth argument since the Plex iOS client is capped at 2Mbps for cellular data anyway, it will probably force the server to transcode the file regardless. Even if it didn't do that, If you're streaming a 1080p or 4K file to a mobile device, the total size difference between a file with a server-transocoded AAC stream versus the same file with the original AC3 stream is rather miniscule.

    I personally agree with @adamska46 - I'd like to have PMS just stream the file to the device as is. I'd rather just have one audio stream in the file, rather than having the make sure it has AAC and AC3. I personally make sure that my media is in an Apple-ecosystem format (MP4, AAC/AC3, text subs, etc). I'd really rather just have one audio track in the file and let the client decode it, which the Apple player can do (take a video with an AC3 track and sync it to the device via iTunes and the Apple TV app can play it just fine. Add that same file to Plex, and you won't get any audio).

    The best option, imo, is for Plex to add more knobs and buttons for how the transcoding engine will work, so that you can have the server transcode the audio to AAC, and I can have it stream the original AC3, but they seem to want to keep it as simple as possible for the lowest common denominator. Considering their (non-existent) support model, I can understand.

  • dragonmeldragonmel Members, Plex Pass Posts: 692 Plex Pass

    @kclimie

    cant agree with you.. in addition to the above reasons..

    most of us have more than one device, TV, etc. there is no 'one codec' that can be assured to play nativly across all clients.

    let the server do its job.. provide the file's video and audio codec that the client can handle at the highest quality setting commesurate with the hardware.. i.e... only sending stereo to a device with 2 speakers.

    use vlc on your ios device and DLNA server if you want to do something else

  • kclimiekclimie Members, Plex Pass Posts: 79 Plex Pass

    I apologize, I didn't realize the discussion was about cross-platform compatibility, I thought it was that the Plex iOS client won't play AC3 files. My point is that iOS is fully capable of playing the format, but the Plex client won't play it.

    Another argument in favor of including the original audio is that if I stream synced content from my iOS device to, for example, a friends Apple TV, I would lose the multi-channel audio because the server has stripped it out of the media.

  • adamska46adamska46 Members, Plex Pass Posts: 61 Plex Pass
    edited March 12

    I believe the proper way Plex is meant to work is to activate transcoding ONLY when the client device does not support your media codecs.

    It is with that in mind that I decided to ask the question why AC3 transcodes to AAC on iPhone and iPad.

    When I play my media on my LAN, I personally choose to play it in original quality, so I expect my server to leave video and audio untouched whenever it is possible. I do not discuss the operation when I am away from home and play my media, in this case I agree with you and would like my server to do some transcoding to save bandwidth. On my LAN, bandwidth is not an issue and the ability to play my media in the best possible quality should not be questioned.
    Don’t you ever want to play your media in original quality when this option is available ? I believe our request does make sense.

    Synology DS218+ (Intel Celeron J3355 Apollo Lake iGPU, 10 GB DDR3 RAM)

  • dragonmeldragonmel Members, Plex Pass Posts: 692 Plex Pass

    no.. I don't want 5 channel audio being pushed to a device with 2 speakers.. I am an adult.. and don't watch high quality content on my iPhone.. I tend to.. you know.. come out of the basement and turn on a television..

    when plex transcodes.. its not transcoding the video... only the audio if you have your settings right.. and personally the server does a better job of down mixing the 5.1 into 2 channel stereo and again... does not wear the phone battery down..

    enjoy banging your head into the concrete wall here.. this will go nowhere with the plex devs either .. its working they way they intend

  • adamska46adamska46 Members, Plex Pass Posts: 61 Plex Pass
    edited March 14

    @dragonmel said:
    enjoy banging your head into the concrete wall here.. this will go nowhere with the plex devs either .. its working they way they intend

    How do you know that? Are you from the dev team?

    Synology DS218+ (Intel Celeron J3355 Apollo Lake iGPU, 10 GB DDR3 RAM)

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