Let Wiim or Apple TV downmix 5.1 to stereo PCM?

[Krabin]

Hi! Should I let my Apple TV downmix the 5.1 surround sound to stereo PCM or leave it on auto and let my WIIM Ultra do it? (My LG tv could also do it, but I'm guessing it's probably the least correct option.) I can definitely tell a difference, mostly in the treble. When Apple does it, the highs seem a bit rolled off, smoother, and darker than when I set its output to the default auto and let the WIIM do the downmixing, which is a bit brighter and maybe slightly grainier. I can't decide which sounds better though, and I'd like to go with the one that's technically better.

 

[Wiimer]

Hi! Should I let my Apple TV downmix the 5.1 surround sound to stereo PCM or leave it on auto and let my WIIM Ultra do it? (My LG tv could also do it, but I'm guessing it's probably the least correct option.) I can definitely tell a difference, mostly in the treble. When Apple does it, the highs seem a bit rolled off, smoother, and darker than when I set its output to the default auto and let the WIIM do the downmixing, which is a bit brighter and maybe slightly grainier. I can't decide which sounds better though, and I'd like to go with the one that's technically better.

Has the latest firmware already arrived for your Ultra? If not, the new FW may improve HDMI ARC.

Nevertheless, unless you are using wireless surround on your Ultra, you will have less trouble if you choose PCM on the Apple TV side. This is just my opinion based on my experience..

If you don't like the sound quality of the PCM transmission from Apple TV, please consider setting an EQ for the HDMI input.

Unfortunately, it is not clear to me which is technically superior. (I don't have Apple TV to begin with.)

I hope that a member in the know will respond to this.

 

[90sNintendoKid]

The last time I checked, I'm pretty sure that a single Wiim can't downmix the .1 from Dolby 5.1 and I've verified that using the the Spatial Audio Tool Kit which means might I recommend that you set the Apple TV to downmix.

 

[Mr Ee]

The last time I checked, I'm pretty sure that a single Wiim can't downmix the .1 from Dolby 5.1

No need to check; see the wiim article on this in the 'Wiim Spport' section which explains why, and why it's as expected.

 

[Krabin]

@Wiimer @90sNintendoKid @Mr Ee thanks for the replies! I just read about the LFE being left out, but I also read that it's standard practice for a lot of devices to do so. Do you guys know if the Apple TV leaves it out or mixes it in? I can't find much online about this stuff for some reason. I've read that the Apple TV downmixing is solid, but that's about it.

 

[EddNog]

@Wiimer @90sNintendoKid @Mr Ee thanks for the replies! I just read about the LFE being left out, but I also read that it's standard practice for a lot of devices to do so. Do you guys know if the Apple TV does leaves it out or includes it? I can't find much online about this stuff for some reason. I've read that the Apple TV downmixing is solid, but that's about it.

I switched mine (Apple TV 4K) to forced Stereo this evening to watch the first episode of Star Trek: Strange New Worlds season 3 and I assure you, LFE is included in the downmix.

-Ed

 

[harkpabst]

I just read about the LFE being left out, but I also read that it's standard practice for a lot of devices to do so. Do you guys know if the Apple TV leaves it out or mixes it in? I can't find much online about this stuff for some reason. I've read that the Apple TV downmixing is solid, but that's about it.

This is a difficult topic. According to my own personal research the situation is as follows:

Dolby Labs should know, but unfortunately their specs are ambiguous. Regarding the handling of the LFE it's not entirely clear if the docs just describe the hardware decoders produced by Dolby (which did not include the LFE into a stereo mix) or if doing so was a requirement of the standard.

Fact is, pretty much all AVRs do include the LFE in a stereo downmix. Some other devices do as well. Others take the Dolby documentation literally and leave out the LFE.

Who's right? That's almost impossible to tell. If all studios kept true to the idea of the LFE (only use it for additional effects the sound system (excluding the subwoofer) might not be capable of, but not for essential content) we probably wouldn't even have to ask this question.

 

[marsboer]

I switched mine (Apple TV 4K) to forced Stereo this evening to watch the first episode of Star Trek: Strange New Worlds season 3 and I assure you, LFE is included in the downmix.

Actually it is not. It is easily verifyable by playing e.g. a True HD 7.1 calibration video. You'll get no sound when the LFE channel is playing with output set to stereo. I guess you were lucky that the sound guys put ample bass in the front channels in that show.

Also chatgpt agrees, but that does not mean much

The only "workaround" I know of on the Apple TV 4K is to use Infuse with the include LFE option in downmix enabled. It does not cover streaming services of course, but at least works for your own media files. In my case that is where the highest quality playback are done anyways, but it would have been nice if the Apple TV could do this for all media.

 

[EddNog]

Actually it is not. It is easily verifyable by playing e.g. a True HD 7.1 calibration video. You'll get no sound when the LFE channel is playing with output set to stereo. I guess you were lucky that the sound guys put ample bass in the front channels in that show.

Also chatgpt agrees, but that does not mean much

The only "workaround" I know of on the Apple TV 4K is to use Infuse with the include LFE option in downmix enabled. It does not cover streaming services of course, but at least works for your own media files. In my case that is where the highest quality playback are done anyways, but it would have been nice if the Apple TV could do this for all media.

Can I use AirPlay to my Apple TV 4K with HouseCurve app on my iPhone and run a sweep as a test? It feeds through ARC to the WiiM and then out to my speakers. Would that be a good test? Or does that not count because it’s not streaming video from like YouTube or Netflix?

-Ed

 

[marsboer]

You can't just run a sweep. You need to ensure the audio is actually playing back the LFE channel (and ideally only the LFE channel as it is a simple silent/not silent test) in the source. This has nothing to do about if you have bass or not in your front speakers.

I just downloaded a Dolby test video and played it back using Infuse. I don't know if a bullet proof calibration test exits in Netflix or other services, but that would also be interesting to test of course. I guess it's theoretically possible that a streaming provider could implement it's own dowmix algorithm, but from experience it seems like the norm in the Apple ecosystem is to let the device take care of such things, judged by the complete lack of settings in all of the streaming apps when it comes to Video/Audio quality parameters.

 

[EddNog]

You can't just run a sweep. You need to ensure the audio is actually playing back the LFE channel (and ideally only the LFE channel as it is a simple silent/not silent test) in the source. This has nothing to do about if you have bass or not in your front speakers.

I just downloaded a Dolby test video and played it back using Infuse. I don't know if a bullet proof calibration test exits in Netflix or other services, but that would also be interesting to test of course. I guess it's theoretically possible that a streaming provider could implement it's own dowmix algorithm, but from experience it seems like the norm in the Apple ecosystem is to let the device take care of such things, judged by the complete lack of settings in all of the streaming apps when it comes to Video/Audio quality parameters.

Okay, because my previous testing involved playing bass/surround test videos on YouTube and I was getting quite high volume quite deep bass out of the test videos, with my Apple TV 4K set to forced Stereo output.

-Ed

 

[mgr]

I was trying to decide if I want my TV (LG OLED C4) to do surround downmixing or let the Wiim Amp Pro do it when i discovered something.

I use the "AAC 5.1" test file from here: https://drive.google.com/drive/folders/1JxmeedtAtgmoafXv9rroiDOS2vEX7N4b

When I passthrough audio to the Amp Pro (it says "Dolby" in the app), when the rear right channel is player you can hear the sound in both speakers. Same with the rear left channel.

That seems incorrect according to Dolby standards.

When I make the LG decode the signal and send PCM stereo to the Amp Pro the sound is correctly mixed.

I wonder if this is a bug or there is maybe another standard for downmixing surround that is used here?

 

[Wiimer]

I was trying to decide if I want my TV (LG OLED C4) to do surround downmixing or let the Wiim Amp Pro do it when i discovered something.

I use the "AAC 5.1" test file from here: https://drive.google.com/drive/folders/1JxmeedtAtgmoafXv9rroiDOS2vEX7N4b

When I passthrough audio to the Amp Pro (it says "Dolby" in the app), when the rear right channel is player you can hear the sound in both speakers. Same with the rear left channel.

That seems incorrect according to Dolby standards.

When I make the LG decode the signal and send PCM stereo to the Amp Pro the sound is correctly mixed.

I wonder if this is a bug or there is maybe another standard for downmixing surround that is used here?

This video confirms that the left and right channels of the surround speakers are outputting independent sound and appear to be OK.

Are you linking two WiiM devices for the surround speakers?

 

[mgr]

Are you linking two WiiM devices for the surround speakers?

No I have a simple 2.0 setup. I have changed all my sound sources to transmit stereo but I wanted to make sure I will have a proper mixdown in case of a source the only has a surround stream.

This video confirms that the left and right channels of the surround speakers are outputting independent sound and appear to be OK.

When you say "appear OK", do you mean it works correctly in your setup? I need to check this specific video (I wonder if the LG Youtube app has multichannel audio?) but I would assume it will also do the same (the sound being heard in both speakers when the "ball" in the video is either rear left or rear right in the attached video).

 

[Wiimer]

No I have a simple 2.0 setup. I have changed all my sound sources to transmit stereo but I wanted to make sure I will have a proper mixdown in case of a source the only has a surround stream.



When you say "appear OK", do you mean it works correctly in your setup? I need to check this specific video (I wonder if the LG Youtube app has multichannel audio?) but I would assume it will also do the same (the sound being heard in both speakers when the "ball" in the video is either rear left or rear right in the attached video).

Sorry, I was mistaken. I was talking about wireless Dolby 5.1 channel playback on the WiiM.

 

[Djinn206]

I had my Apple TV 4K set to forced Stereo output for a very long time. On the weekend I tried setting it back to Automatic and in my case audio when watching a Netflix movie (outputting Dolby Digital to my Amp Ultra) sounded a lot better than forced to Stereo output. The dynamics are a lot better and it's much more a cinema feeling this way. Even the dialogues are more understandable than before. With forced Stereo I had to switch on "enhance Dialogues" for most of the time on my Apple TV which now can stay off all the time.

So WiiM seems to have a really good job done downmixing Dolby Digital to 2.1.