Q: AppleTV 4 surround sound issue

So what is the disadvantage, if any, of having the AVR show PCM instead of Dolby Digital? Any explanation would be appreciated in simple terms.

How much of a purist are you?

The fact that the TV4 outputs multichannel audio as PCM or in a "pseudo" Dolby bitstream is not the main issue here. Most of us are more concerned by the fact that the TV4 is decoding the Dolby input bitstreams in the first place rather than passing the audio to downstream devices for decoding and post processing. The quality of the individual audio channels is probably not degraded per se, however, the Dolby bitstreams carry more than just audio information. For instance, dialog normalization, dynamic range control, and active ProLogic II ancillary parameters tell the decoding device how to post-process audio content after decoding. By prematurely decoding these streams, the TV4 is, in effect, stripping the ancillary parameters from the signal stream and preventing the Dolby audio from being played as originally intended.

As you may or may not be aware, a number of users have complained vehemently regarding the TV4's processing workflow and have gone so far as to return their units and revert to the use of TV2/TV3 devices. I myself have kept one unit connected in the hope that Apple will correct matters by adding a dedicated "Dolby Passthrough" option to the current list of "Auto", "Dolby Surround", and "Stereo" Surround Sound options. Luckily, the DialNorm and DRC issues do not seem to affect my library to any major degree since I encoded the content myself. However, I do have a lot of older AC-3 1.0 (center channel) and AC-3 2.0 active matrix ProLogic II files encoded for 3-channel front (left, center, and right channel) and quad (3-channel front + surround rear) playback. Prior to the release of tvOS v9.1, the AC-3 audio bitstream was passed directly to my AVR for decoding and played as originally encoded. Since the release of tvOS v9.1, such audio content is now decoded by the TV4 and passed to my AVR as 2.0 PCM audio which means my center channel audio now plays as mono/mono (Left/Right) audio and my ProLogic II files play as stereo (Left/Right) audio. Sure, I can apply passive PL II enhancement manually on my receiver, but then I am forced to choose from a list of a half dozen or so effects settings. Further, if I try to play a 2.1 Dolby LogicPro II file, then it is passed to my AVR in a 5.1 Dolby bitstream with unused channels empty. And, since the receiver sees this audio as 5.1 multichannel, it will not post process and play matrix encoded center or rear surround channels. In short, the TV4's decoding workflow simply does not "fully" support Dolby content as it should and as my receiver already does in the correct manner. Unfortunately, since the TV4 functions as it is currently programmed to work, the Apple Developer Team does not seem to consider these failings or inconsistencies to be issues that need to be corrected.

Apologies, not a direct reply to your post, just another observation:  B speakers are also broken.

For decades selecting A + B speakers meant "output stereo" to remote B speakers.  Now, sometime I get stereo, but most often just some surround channels.  Makes for a FU'd experience.  Apple you have have broken too much stuff, please think this through thoroughly.

Thanks,

Steve

i agree with you !

i use ATV3 hdmi to sony kdl55 from there go optical out to a sonos system 5.1 > surround works fine

with ATV4 > no Surround only Center left and right

thats very bad :-/

Perfect post from Jon Walker, my ProLogic II files play as stereo (Left/Right) audio, so having to source different ones so that i can encode them as passthrough just to get minimum 5.1. Laborious process caused by limited and/or changed processing by the ATV

i hope apple will fix this soon !

I came across this thread while researching the problem most others here had or are having. My back speakers were pretty much just there. I got a little sound out of them but not much. It wasn't a true surround sound. I tried increasing the rear speaker volume but that didn't work. I then started to think my receiver had issues (10 year old Yamaha) so I was getting ready to buy a new one. I read that advice on page 1 and 2 about changing the ATV4 setting from Auto to Dolby Digital. I cranked up The Good Dinosaur after my kids and I finished watching it. It was like watching it in a theater. Whole difference experience!

It looks like some of you have got a lot deeper into this since the first few pages so I'm going to do some more reading. But for now I'm happy to have my system working like it should without spending any money!

With that set, you are still missing the full dynamic range of DD signal. I have dual subs and explosions or low deep rumble are missing dynamic punch using the ATV DD setting. ATV4 modifies and resends the signal to your AV Receiver lacking full dynamic range. Why Apple decide ATV4 should handle DD signal is beyond any reasoning. Unless they think everyone uses their TV or SoundBar for HT sound. Using ATV4 on a mid-high AV receiver with full 7.1 capabilities is a total Fail on Apple.

What if we have ATV4 HDMI out go straight into TV HDMI IN and audio set to Auto. Then have Optical audio out from the TV to Optical IN to AV Receiver? Will this then allow the AV receiver to process the audio to whatever we wish it to do?

What if we have ATV4 HDMI out go straight into TV HDMI IN and audio set to Auto. Then have Optical audio out from the TV to Optical IN to AV Receiver? Will this then allow the AV receiver to process the audio to whatever we wish it to do?

No—it's already too late! The TV4 output is already modified and/or degraded. The TV would receive the same corrupted audio that is currently (since tvOS v9.1) being received by the AVR and about which we are complaining here. (I.e., how is a TV supposed to restore missing DialNorm or DRC values or turn stereo LPCM audio back into AC3 1.0 or "active" ProLogic II (3.0 or 4.0) audio bitstreams?) No, the only way to allow the downstream device to process the audio properly is to send it the original bitstream data just as older TV devices did and as the TV4 could before Apple's tvOS v9.1 update. Since (based on Developer bug report responses I've received) it appears Apple is not anxious to restore the "Passthrough" functionality as part of the "Dolby Surround" logic, my recurring suggestion has been to add a fourth option to the "Surround Sound" menu—i.e., a "Dolby Passthrough" option that allows the TV4 user to pass Dolby bitstreams to any downstream device for decoding just as the Infuse media player does when programmed to do so by the TV4 user.

Yeah I don't know. I just had to ask grasping at straws here to make it work. I have two rear speakers dead no sound because of my 7.2 (two subs) setup, when using Auto on ATV4. I guess there is nothing we can really do about it at this point.

Yeah I don't know. I just had to ask grasping at straws here to make it work. I have two rear speakers dead no sound because of my 7.2 (two subs) setup, when using Auto on ATV4. I guess there is nothing we can really do about it at this point.

If your system is set up correctly and other sources (like BD or possibly speaker test apps) provide correct rear speaker output, then the problem would be most likely be the content you are playing. Few commercial files available on iTunes have 8-channel audio to begin with and I don't know of any consumer software apps capable of creating 8-channel TV compatible content. Further, if the TV4 is forced to output more channels of audio than are supplied by the source content, it automatically outputs "missing" input channels as "empty" output channels. Based on what other users have posted, it sounds as if your system is probably working correctly unless or until you can confirm the actual number of active channels in your source content.

Most sources are 5.1, but when fed from ATV3, the AV receiver can then apply PLXII, EX or DTS 7.1 sound. So all speakers are active.

With ATV4 Auto, the receiver cannot process the sound to output PLX II, EX or DTS 7.1. It's stuck on 5.1 only, thus leaving the other two rear speakers silent. The self powered subs are fine since they are split into two from the sub out of the receiver.

Most sources are 5.1, but when fed from ATV3, the AV receiver can then apply PLXII, EX or DTS 7.1 sound. So all speakers are active... With ATV4 Auto, the receiver cannot process the sound to output PLX II, EX or DTS 7.1. It's stuck on 5.1 only, thus leaving the other two rear speakers silent. The self powered subs are fine since they are split into two from the sub out of the receiver.

Mmmm.... Sounds like this is related to or an extension of the 2.1 ("active" matrix 3- and 4-channel audio) problems I've been complaining about. Since the TV4 outputs these older audio formats in what I call a "pseudo" multichannel bitstream, it automatically disables any automatic Dolby "active" matrix post-processing of audio for any formats so encoded. While I can manually apply passive post-processing enhancements, they aren't the same and force me to "color" the audio with, what is to me, an undesired audio "effect" that must be selected by the user. Like your TV3, the TV4 never had this problem prior to the tvOS v9.1 release that eliminated AC3 bitstream passthrough in the "Dolby Surround" mode. Basically, at this point I have to choose between the better video processing and apps of the TV4 and the better audio processing of the TV3—a bitter choice that Apple should not be forcing users to make in view of the fact that there is so little 7.1 TV4 compatible content available. Of course, this is only my personal opinion and I suspect the vast majority of users out there are like my wife who plays little, if any, attention to whether the audio is mono, multichannel, or anything in between.

Sounds like my Mrs, but it's important not just for audiophiles but for those who want to enjoy the blu ray experience with the convenience of streaming from your iTunes library, which is let's face it, the whole point of the device

Unfortunately  lot of my files were, by my oversight, encoded to Dolby Pro Logic II, which one the face of it isn't recognized in auto mode. Maybe I'm wrong as it appears that stereo only is received.

in any case, I'm in the process of obtaining new files and will be encoding them with discrete channels as these appear to come through to my AVR as discrete PCM 5.1

just wish it worked as the ATV3 worked.