Change Audio Format to Dolby Digital 5.1 - Output Bitrate?

[Jim Lahey]

Hello. Although my AV system is fully capable of playback of the more modern audio formats - up to but not including Atmos - when using the Apple TV Plex client I have to change audio format to AC3 5.1 or it will only output stereo PCM to my receiver irrespective of the format of the file coming from the Plex Server. That is the thrust of the issue but it's not the purpose of this topic. Since my gear is a very capable if ageing separates system then it always sounds great anyway, even with trusty old vanilla 5.1.

My question to you good people is this: out of curiosity, when the Change Audio Format is switched in settings to Dolby Digital 5.1, what is the output bitrate of the stream? I'm hoping it's at least 384kbps as per the older DVD typical norm? I would like to know the maximum bitrate of this AC3 downmix on ATV because it's relevant when I'm encoding HEVC videos from my purchased Blu-rays...no point ripping passthrough audio tracks with mega bitrates if I cant hear them, as it's just wasted file size. I can always re-rip them at a later time if my audio situation changes.

If anyone can shed some light on the Plex issue then that's a bonus. It is only the Plex app - Dolby Digital Plus etc. plays just fine from iTunes or ATV+ sources without me having to use this workaround.

TLDR; what is the maximum bitrate of Apple TV's Dolby Digital 5.1 mixdown capability?

 

[Wombert]

Two possible reasons why it's passing stereo LPCM only.

First, maybe your AVR does not support multichannel LPCM.

Second, you have the ATV connected to the TV, and the TV talks to the AVR via optical or regular HDMI ARC (not eARC). S/PDIF (and thus ARC) only has bandwidth for (E)AC-3, DTS or stereo LPCM.

In both cases, the Apple TV detects the lack of LPCM and falls back to the next best supported format, which is DD+. Hooking up the ATV directly to the AVR would solve it for the second case; in the first case, you're out of luck.

The Apple TV, if supported by the device chain, always decodes audio and sends it to a TV/AVR as multichannel LPCM, as that's the only way to get sound overlays like remote navigation sounds or Siri, or features like the dynamic range compression ("reduce loud sounds" option). Even Dolby Atmos - E-AC3 is decoded to 5.1/7.1 LPCM, transmitted in format called Dolby MAT, which can incorporate Atmos metadata.

I presume Plex simply always outputs LPCM, without attempting DD/DD+ as a fallback - or because the input format is not DD+, and it can't/won't encode whatever the input format is (DTS, AAC, whatever) to DD+ for the Apple TV to hand down the HDMI chain.

FWIW, the maximum possible bitrate for AC-3 over S/PDIF and thus ARC is 640 kbit/s, but I'd really solve the multichannel LPCM issue instead.

By the way, Infuse Pro can decode even Dolby TrueHD and DTS-HD MA and pass it through the ATV as multichannel LPCM. It can connect to Plex servers, and you no longer need to worry about video transcoding either.

 

[Jim Lahey]

Thank you for your reply. Yes at the moment the audio goes back to the receiver via eARC but I had considered this not to be the problem since it’s only Plex that causes the issue. However, the receiver does support LPCM so who knows. Unfortunately, although it supports 4K/60 pass-through, it’s fairly old and doesn’t do HDCP 2.2 which is why I rewired everything some time ago in order to send video direct to the TV. But as I obviously don’t play UHD Blu-ray discs from ATV then I could in fact pass the video through the receiver for that one piece of equipment.

I’ll give it a trey when I acquire another HDMI cable. Thanks again for your help ?

 

[HDFan]

I'm encoding HEVC videos from my purchased Blu-rays

You aren't just ripping them with MakeMKV? I play my .mkv files via Plex and my receiver. Depending on the client I can get the full DTS-MA audio bitrate, for example on an Nvidia Shield TV or doing a DNLA stream to my Blu-Ray player.

I obviously don’t play UHD Blu-ray discs from ATV

Why not? I play them via Plex all the time.

 

[B/D]

@Jim Lahey Have you test those same files with Infuse to see if they work correctly audio wise?. You shouldn't have to lower the apple tv audio quality settings, and make it reencode the audio unnecessarily.

 

[Jim Lahey]

@HDFan I use a mix of MakeMKV and Handbrake but it's largely irrelevant in terms of the output. Just realised I probably didn't make it clear that even vanilla AC35.1 audio won't play in 5.1 through the ATV Plex client unless I first switch audio format in the device settings as previously described. Otherwise, Plex through ATV simply will not play anything other than LPCM2.0. Unfortunately I have also realised today that my receiver is old enough to only support HDMI 1.4a, so 2160p HDR won't be passed through to the display if I rewire the ATV. Obviously not an option.

@B/D no, I have yet to look into that but probably will at some stage later this week. Honestly though it's probably time to just upgrade the receiver. I had been waiting for more HDMI 2.1 options but maybe now is the time to do some shopping around.

Thanks again y'all. Appreciated ?

 

[B/D]

@Jim Lahey

I told you try the Infuse app on the Apple TV because I had the stereo issue with many multichannel audio tracks on Plex. Plus Infuse does not need to transcode anything ever video wise, play plex libraries, support more audio and video formats natively and so forth.


To me , is worth every penny.

 

[Jim Lahey]

@Jim Lahey

I told you try the Infuse app on the Apple TV because I had the stereo issue with many multichannel audio tracks on Plex. Plus Infuse does not need to transcode anything ever video wise, play plex libraries, support more audio and video formats natively and so forth.


To me , is worth every penny.

Thanks. I will definitely look into it. Sounds great ?

 

[Jim Lahey]

So the plot thickens! I tried using Infuse. It's really pretty neat. Except my audio situation is identical. It all leads back to the problem being with my eARC configuration going through my TV, but then why does all iTunes and ATV+ content play as expected without having to fudge the audio format in device settings. I'm normally fairly good with this kind of thing but I'm fresh out of ideas.

I suppose it's important to remember that I'm not overly bothered since vanilla 5.1 is hardly bad and my equipment is otherwise very good despite being a little vintage by the standards of some. I don't see much hope of ever resolving this 2.0 foible until I buy another receiver, but honestly my life priorities draw attention to other things right now.

Maybe if my existing receiver ever blows up then my hand will be forced, but even if I bought a new receiver it may transpire that the Apple TV just sucks a bit at this sort of thing. I'd then be into trying an Nvidia Shield or some such. For now I think I'll just let sleeping dogs lie, but of course I'm open to further suggestions.

Thanks again all for your help and interest ?

 

[B/D]

@Jim Lahey, very strange indeed.

One last shot: 15.5.1 for the Apple TV just got released. Update it and try again.

Have you check by any chance if a firmware update is available for your receiver and/or the TV?

 

[Jim Lahey]

@Jim Lahey, very strange indeed.

One last shot: 15.5.1 for the Apple TV just got released. Update it and try again.

Have you check by any chance if a firmware update is available for your receiver and/or the TV?

Thanks, I will download 15.5.1 tomorrow but I'm not holding out much hope of it making any difference. Yeah my receiver and TV are both up to date. In any case this topic has been very productive and all your help is appreciated because at least now I'm more informed and have ruled out a lot of possibilities. I'm starting to think the Apple TV just isn't very good.

 

[priitv8]

@HDFan I use a mix of MakeMKV and Handbrake but it's largely irrelevant in terms of the output. Just realised I probably didn't make it clear that even vanilla AC35.1 audio won't play in 5.1 through the ATV Plex client unless I first switch audio format in the device settings as previously described. Otherwise, Plex through ATV simply will not play anything other than LPCM2.0.

With what settings do you encode in Handbrake?
What audio tracks are created in your files?
Standard tvOS encode must include both AAC stereo track and optionally may include 5.1/7.1 Dolby-encoded surround track. Also, since couple of last tvOS versions, the surround track needs to have the AAC track linked as its fallback track.
If you switch manually from AAC to Surround track in the Plex/Infuse client, will that change anything?

 

[Jim Lahey]

With what settings do you encode in Handbrake?
What audio tracks are created in your files?
Standard tvOS encode must include both AAC stereo track and optionally may include 5.1/7.1 Dolby-encoded surround track. Also, since couple of last tvOS versions, the surround track needs to have the AAC track linked as its fallback track.
If you switch manually from AAC to Surround track in the Plex/Infuse client, will that change anything?

Oh. Thanks for the info. For the sake of file sizes I only ever encode the one 5.1 digital surround track. Usually AC3 or EAC3. But what I don’t understand here is why would the ATV want to use a fallback 2.0 track to replace the perfectly fine 5.1?

So if I’m not mistaken, the issue lies with the ATV and would likely be fixed if I used, say, Nvidia Shield? I'm most certainly not going to re-encode hundreds of movies just to add a stereo audio track

 

[priitv8]

Oh. Thanks for the info. For the sake of file sizes I only ever encode the one 5.1 digital surround track. Usually AC3 or EAC3. But what I don’t understand here is why would the ATV want to use a fallback 2.0 track to replace the perfectly fine 5.1?

I do not know the exact reasons, but it may have been since tvOS 14 that the fallback track was made mandatory. It probably is there for compatibility reasons (iPads and iPhones prior to Spatial Audio era).
I used to encode without AAC as well and they became problematic, so I added the AAC stereo track.
There is extensive discussion here (disregard the BS about home sharing not supporting surround audio):

Apple TV 4 with tvOS 14.3 no longer able … - Apple Community

discussions.apple.com

So if I’m not mistaken, the issue lies with the ATV and would likely be fixed if I used, say, Nvidia Shield? I'm most certainly not going to re-encode hundreds of movies just to add a stereo audio track

Don't know about others, I never used them. aTV is special in how it handles audio in that it internally decodes it. Most others just bitstream to HDMI out.
You do not need to re-encode anything. Subler will remux your file and you can let it encode and add the stereo track. It will also set the fallback automatically for you.
For audio it will go very fast.

 

[Jim Lahey]

I do not know the exact reasons, but it may have been since tvOS 14 that the fallback track was made mandatory. It probably is there for compatibility reasons (iPads and iPhones prior to Spatial Audio era).
I used to encode without AAC as well and they became problematic, so I added the AAC stereo track.
There is extensive discussion here (disregard the BS about home sharing not supporting surround audio):

Apple TV 4 with tvOS 14.3 no longer able … - Apple Community

discussions.apple.com

Don't know about others, I never used them. aTV is special in how it handles audio in that it internally decodes it. Most others just bitstream to HDMI out.
You do not need to re-encode anything. Subler will remux your file and you can let it encode and add the stereo track. It will also set the fallback automatically for you.
For audio it will go very fast.

So I tried a file that does also have an AAC 2.0 audio track and the problem remains identical. Not sure about the linking thing you mentioned though. I will look into that and also check out your link later.

Thanks ?

 

[priitv8]

So I tried a file that does also have an AAC 2.0 audio track and the problem remains identical. Not sure about the linking thing you mentioned though. I will look into that and also check out your link later.

So I reckon, changing the audio track to 5.1 one did not change anything?
PS Have you tried the Computers app - the native Home Sharing solution?
Does this reproduce surround sound?
I am using it all the time, all my files are in MP4 and apple-friendly format and I have no problems with surround sound. Agreed, I also have aTV directly connected to the receiver and I do not have eARC on my TV.
I will test later with a 5.1 track from Plex server and see if it plays back correctly in Plex and Infuse.

 

[Jim Lahey]

So I reckon, changing the audio track to 5.1 one did not change anything?
PS Have you tried the Computers app - the native Home Sharing solution?
Does this reproduce surround sound?
I am using it all the time, all my files are in MP4 and apple-friendly format and I have no problems with surround sound. Agreed, I also have aTV directly connected to the receiver and I do not have eARC on my TV.
I will test later with a 5.1 track from Plex server and see if it plays back correctly in Plex and Infuse.

Nope. Exactly the same. Not tried the Computers app but honestly I’m losing interest in the whole thing now. Had almost decided to try an Nvidia Shield but it seems virtually impossible to effectively use one without a Google account, which ain’t happening. It can be done but not without jumping through a load of hoops and then ending up with a permanent ‘login to Google’ prompt onscreen.

Apple do my head in sometimes. So antI-consumer it’s unreal. Wouldn’t put it past them to have deliberately done this in tvOS 14 and later in order to nudge everyone into only buying Apple content. Getting 5.1 digital audio from a media box in 2022 should not require weeks of experimenting and trawling the web for workarounds or switching settings every time you want to watch something not sold by Apple.

I’m clearly not alone in experiencing these issues so I’m reasonably confident it’s not my equipment. I will doubtless investigate further at a later time but for now it’s not worth the energy. Thanks again everyone for all your help ?

 

[priitv8]

Weird. I have had virtually zero problems with feeding my content to it using stock AirPlay/Home Sharing solutions.
There are just 2 things it won’t do principally:
*Play UHD bluray rips in Dolby Vision and Atmos
*Play sat-TV recordings with their subtitles

All the rest has become much easier since almost all content is now in H.264 and H.265 so no time consuming video reencode is needed.

 

[Wombert]

As I already explained... the connection from the TV to the AVR cannot carry multichannel LCPM; regular ARC does not have the bandwidth to do that (your AVR does not support eARC).

Only when using fairly recent TVs and recent AVRs together, both with eARC support, will this be possible. Even then, many TVs will still not support passing through multichannel LPCM; same issue with DTS bitstreams, which in order to save licensing costs, most TVs no longer support (Sony being the notable exception).

Mind you that eARC is an HDMI 2.1 feature that many AVR vendors backported to their HDMI 2.0 compatible AVRs, so you could pick up e.g. a used Denon X3400, or newer models (X*500, X*600), which all do support eARC in case your TV does that.

But then, you'd already have an HDCP 2.2 compliant AVR, so you could then just hook up your Apple TV directly to the AVR again, which also lets you adjust other sound options specifically for that source etc.

Anyway, why not always leave the output setting in the Apple TV set to Dolby 5.1 in your case?

 

[Jim Lahey]

As I already explained... the connection from the TV to the AVR cannot carry multichannel LCPM; regular ARC does not have the bandwidth to do that (your AVR does not support eARC).

Only when using fairly recent TVs and recent AVRs together, both with eARC support, will this be possible. Even then, many TVs will still not support passing through multichannel LPCM; same issue with DTS bitstreams, which in order to save licensing costs, most TVs no longer support (Sony being the notable exception).

Mind you that eARC is an HDMI 2.1 feature that many AVR vendors backported to their HDMI 2.0 compatible AVRs, so you could pick up e.g. a used Denon X3400, or newer models (X*500, X*600), which all do support eARC in case your TV does that.

But then, you'd already have an HDCP 2.2 compliant AVR, so you could then just hook up your Apple TV directly to the AVR again, which also lets you adjust other sound options specifically for that source etc.

Anyway, why not always leave the output setting in the Apple TV set to Dolby 5.1 in your case?

Thanks again for your reply. You're right about eARC. That is the setting enabled in my TV but of course the receiver only accepts ARC. Either TV setting 'works' but of course neither solves the issue at hand. But what I don't really get, is that DD+ does come through to the receiver when playing Apple content. But I guess not via LPCM if I understand you correctly? The whole thing is a bit of a mess really and it seems to be a limitation of the ATV. An annoying one. I'm almost certain that I never used to have this issue prior to tvOS 14 using the same equipment.

To your question, I do leave the ATV set to DD 5.1 but it's a little annoying because I then have to remember to change it to 'off' when playing Apple content so that I benefit from DD+

ETA: I feel it's important to roll back a little here and remind myself that the 5.1 foible wasn't really my original concern or the reason for the thread. I was mainly wanting to know the bitrate that ATV uses when it is converting everything to DD 5.1. Especially since it also seemingly has to 'covert' existing 5.1 to 5.1. With that in mind I was wondering if it was cutting the bitrate of 640kbps AC35.1 tracks down to 384kbps or lower.

 

[priitv8]

But what I don't really get, is that DD+ does come through to the receiver when playing Apple content. But I guess not via LPCM if I understand you correctly? The whole thing is a bit of a mess really and it seems to be a limitation of the ATV.

You need to know just 1 thing - AppleTV never bitstreams audio out to HDMI.
It always decodes it internally. In case of DD5.1 or DD+7.1 it outputs LPCM multichannel. In case of DD+ Atmos, it outputs Dolby MAT 2.0 (LPCM multichannel with Atmos metadata).
If you set DD5.1 output in tvOS settings, then aTV will re-encode everything back into DD5.1 and send it to HDMI.
According to Apple - this is necessary for e.g. Siri to add the voice overlays to the stream.
As to what bitrate will it come up with - I do not know. And I have no tools available to measure it.

 

[Jim Lahey]

You need to know just 1 thing - AppleTV never bitstreams audio out to HDMI.
It always decodes it internally. In case of DD5.1 or DD+7.1 it outputs LPCM multichannel. In case of DD+ Atmos, it outputs Dolby MAT 2.0 (LPCM multichannel with Atmos metadata).
If you set DD5.1 output in tvOS settings, then aTV will re-encode everything back into DD5.1 and send it to HDMI.
According to Apple - this is necessary for e.g. Siri to add the voice overlays to the stream.
As to what bitrate will it come up with - I do not know. And I have no tools available to measure it.

But then why does DD+ work just fine over ARC when playing Apple content? When I play Apple movies using the same equipment my receiver clearly displays Dolby Digital + (with change audio format set to off).

 

[priitv8]

But then why does DD+ work just fine over ARC when playing Apple content? When I play Apple movies using the same equipment my receiver clearly displays Dolby Digital + (with change audio format set to off).

Hard to tell. Maybe your receiver plays tricks on you?
I only ever see LPCM 5.1, LPCM 7.1 or DOLBY ATMOS - LPCM on mine.
Of course, when you are using ARC, then your TV is also in the signal path and does its processing there.

 

[Jim Lahey]

Hard to tell. Maybe your receiver plays tricks on you?
I only ever see LPCM 5.1, LPCM 7.1 or DOLBY ATMOS - LPCM on mine.
Of course, when you are using ARC, then your TV is also in the signal path and does its processing there.

TV ARC is set to pass-through but it’s the same results when set to PCM or Auto. I mostly understand your points, it’s just that each time I rethink the problem there’s a caveat that calls into question a previous conclusion. It’s a head scratcher that will likely never be solved without first buying a new receiver or a new media player. Until then I’ll just keep switching off DD5.1 in the ATV device settings when playing iTunes or ATV+ media. Thanks once again for everyone’s continued help and interest. I’m gonna draw a line under this now but will remember to update the topic if I ever find a solution ?

 

[HDFan]

Had almost decided to try an Nvidia Shield but it seems virtually impossible to effectively use one without a Google account, which ain’t happening. It can be done but not without jumping through a load of hoops and then ending up with a permanent ‘login to Google’ prompt onscreen.

I can see your resistance to adding a google account, but if you create a nonsensical name one and use it on your Shield what are you risking? Don't understand your "login to Google" prompt - I never see it, but I do login if it is necessary. The Shield TV does have the added capability of playing DTS-MA and other audio tracks which the Apple TV transcodes. Interface isn't too bad. But there are annoying features like no logout from google. Only thing you can do is delete the account. Loses audio so I have to reboot a lot, but other than that plays many of the apps that I use with higher quality audio (Plex, Tidal, etc.).

Not sure why you are going through the pain of creating lossy encodings. You might regret it in the future when you have upgraded you system and the lossy encodings start to be obvious. Disks are cheap. You can get a 16 TB now for a bit over $300. Media libraries go through transitions. DVD>Blu-Ray>UHD. Once you make the transition to the next generation the flaws in the previous generation become apparent. Maybe you don't care.