Understanding HDMI audio and downmixing

3rdDegree

I have a problem understanding how HDMI audio is processed by your average TV. In fact, I'm not even sure how to ask this question so couldn't even Google for an answer (trust me, I have tried!).

Here's the scenario:

I have a Panasonic TX-P46G10L TV which is nearly four years old now. It's a great TV with great picture and I'm very happy with it. But I just can't figure out how it handles HD, Dolby Digital and DTS audio. It seems to me that it can only receive and process stereo signals via HDMI. Unless something is down mixed to stereo from HD, DD or DTS, my TV wont play the audio correctly. Let me give some examples:

The TV is connected to my PS3 and a Dune Media player via HDMI. It is connected via amplifier (eg: PS3 -> Amp -> TV). That shouldn't matter, but it is related.

1. Unless the PS3 is set to send stereo audio only (no DD, no DTS or any HD formats), when I play a DVD or blu-ray , it plays without the centre track. I can hear no spoken dialogue. That makes sense to me. The audio must be down mixed before my TV can reproduce it properly. So, if the PS3 is sending a 5 or 7 channel PCM signal, my TV can't handle it. Must be 2 channel. Fine.

2. If I set the Dune media player to output Bitstream, then I get the same problem as the PS3. No center channel. But if I set the Dune to output PCM, I am still getting 5 or 7 channel HD audio from the Dune and the TV can handle it? It plays perfectly! Why? Is it possible that the Dune is sending more than one audio channel? One stereo and one multi-channel and my TV magically picks the one it wants? Seems a little far fetched to me!

3. To add to the complication, I had a Popcornhour media player and when that was set to output bitstream via the HDMI, the TV played the audio correctly. Very strange. Was that also sending multiple signals and the TV picks the one it could use?

This wouldn't be an issue except that sometimes I wish to listen to audio from the PS3 through the amp and then I want it in full HD. But to achieve this, I need to manually change settings on the PS3 every time I switch from listing to a movie through the TV speakers and when I listen to one through the amp. It's a pain. I don't have to do that for the Dune, so want to understand why I need to do it for the PS3 (and an Xbox One I also tried this with). The reason I am asking is I want to replace the PS3 with a dedicated BD player and I don't want to replace it with something that will also require me to change the settings to manually down mix when switching from amp to TV. I read the manual of the BD player and although it does discuss down mixing, I'm not sure if it will do what I want. So, basically, I would like to know why my TV can play multi-channel audio in scenario 2 above a not scenario 1.

TLTR: Is there such a thing a auto-downmixing when playing DD, DTS or HD audio to a TV. I seem to have the feature on a Dune media player but when using a PS3, I have to manually change settings every time I switch from amp to TV audio.

This has been bugging me for years and now I've finally asked! Would be so greateful if someone could explain this to me!

Many thanks

andy1249

What you are talking about here is essentially the HDMI handshake.

Any two devices connected by HDMI have a handshake , or an exchange of information that is meant to tell each device what the other can do.
For example , when you connect a "source" such as a PS3 to a "Sink" such as a TV , the PS3 asks various questions , such as what resolutions can you handle ? , what audio types can you handle ? and so on.
The Source then typically only sends out what the Sink can handle.

For each source , the handshake happens with the next HDMI device in the chain.

With a Receiver or amp in the chain , the handshake happens between PS3 and amp , not PS3 and TV , and this may the root of your problem.

Most TV's only accept stereo from external sources and most only accept it in PCM format data.

Dolby Digital , DTS and their respective HD flavours are encoded data , and this must be decoded to PCM before being sent to the TV if the TV cannot decode the format , if you bitstream it , this means it is not decoded at source , and the sink device must have a decoder on board or you get nothing.

Amps or receivers can decode anything these days , so it would seem in your case , that the setup for the amp needs adjusting , not the setup of your sources.
Check the setup of your amp , and check it for each HDMI input , as usually it is possible to set each input individually.

The amp does matter , in fact it is critical , because its the amp that is "handshaking" with your TV , not the sources !
It is the amp deciding which data to send to the TV.

3rdDegree

Thank you very much Andy!

3rdDegree

andy1249 wrote: »

What you are talking about here is essentially the HDMI handshake.

Any two devices connected by HDMI have a handshake , or an exchange of information that is meant to tell each device what the other can do.
For example , when you connect a "source" such as a PS3 to a "Sink" such as a TV , the PS3 asks various questions , such as what resolutions can you handle ? , what audio types can you handle ? and so on.
The Source then typically only sends out what the Sink can handle.

For each source , the handshake happens with the next HDMI device in the chain.

With a Receiver or amp in the chain , the handshake happens between PS3 and amp , not PS3 and TV , and this may the root of your problem.

Most TV's only accept stereo from external sources and most only accept it in PCM format data.

Dolby Digital , DTS and their respective HD flavours are encoded data , and this must be decoded to PCM before being sent to the TV if the TV cannot decode the format , if you bitstream it , this means it is not decoded at source , and the sink device must have a decoder on board or you get nothing.

Amps or receivers can decode anything these days , so it would seem in your case , that the setup for the amp needs adjusting , not the setup of your sources.
Check the setup of your amp , and check it for each HDMI input , as usually it is possible to set each input individually.

The amp does matter , in fact it is critical , because its the amp that is "handshaking" with your TV , not the sources !
It is the amp deciding which data to send to the TV.

Thanks Andy

That sort of makes sense. There is no option to downmix HDMI audio output from my amp to the TV. That isn't an option. But I guess for now, I'm not too pushed about that. I'm afraid I still don't really understand what is going on.

Let's say I removed the amp from the equation and fed each component directly to the TV. I still see that when the Dune sends a PCM signal to the TV, I have full audio. When the PS3 sends a PCM signal, there is no central channel. When the PCH sends bitstream, I full audio. When the Xbox One ssends bitstream, I have no central channel.

I guess I'm not trying to figure out how to "fix" this (I don't think it can be with my setup). I guess I just want to understand why it works from one source and not another. What is the difference between what the different sources are sending. Does that make sense?

Thanks

Idleater

3rdDegree wrote: »

I still see that when the Dune sends a PCM signal to the TV, I have full audio. When the PS3 sends a PCM signal, there is no central channel. When the PCH sends bitstream, I full audio. When the Xbox One ssends bitstream, I have no central channel. [/URL quote]

The difference between pcm audio and bitstream is like a text document and a zipped / compressed version of the same document.

You can send me either file to print. For the plain text one, you send it, I read it. It takes a bit of bandwidth but I get it.
For the compressed one, we save bandwidth, but you have to tell me what compression method you use so I can uncompress it correctly at my end.

andy1249

Let's say I removed the amp from the equation and fed each component directly to the TV. I still see that when the Dune sends a PCM signal to the TV, I have full audio. When the PS3 sends a PCM signal, there is no central channel. When the PCH sends bitstream, I full audio. When the Xbox One ssends bitstream, I have no central channel.

Have you done this and are those the results ?

What model is your amp ?

Are all devices connected through the amp , and when not using the amp are you using any kind of passthrough mode ? (e.g. amp in standby but everything still connected via it to the TV )

3rdDegree

Idleater wrote: »

The difference between pcm audio and bitstream is like a text document and a zipped / compressed version of the same document.

You can send me either file to print. For the plain text one, you send it, I read it. It takes a bit of bandwidth but I get it.
For the compressed one, we save bandwidth, but you have to tell me what compression method you use so I can uncompress it correctly at my end.

Thanks. That does make sense and I did sort of figure that was what was happening. But it doesn't answer the main question:

When is that when the Dune sends a PCM signal, the TV can handle it, but when the PS3 does, it can not?

Why is it that when the Popcornhour sends a bitstream, the TV can understand that, but when the Dune or the Xbox sends a bitstream, it cannot.

Thanks

3rdDegree

andy1249 wrote: »

Have you done this and are those the results ?

Whether the amp is in the mix or not, the results are the same. With the TV connected directed to the PS3, the audio is not played correctly. With TV connected to the Dune, audio is played correctly. Both these devices send a multi-channel PCM signal

What model is your amp ?

Yamaha RX-V667

Are all devices connected through the amp , and when not using the amp are you using any kind of passthrough mode ? (e.g. amp in standby but everything still connected via it to the TV )

Yes, with the exception of the above test, normally all devices are connected via the amp. The amp supports passthrough. That is: While switched off, it will pass the signal though it from the source to the TV. I have done extensive searching and also checked the options and manual and the amp does not support downmixing of the audio in HDMI out.

Thank you very much.

andy1249

Thanks for clearing that up .... I know the Yamaha's well , there is no downmixing on passthrough !

So , back to the sources .....

For the Dune .... Note that it has automatic downmix on detection of a TV during the handshake.

Setup / Audio

This section of the setup menu allows to set the connection type of

relevant audio equipment.

Digital audio output mode: you may switch the mode of digital optical

output:

•PCM (Pulse Code Modulation, forced decoding): a mode in which the

original sound track, i.e. Dolby Digital or DTS, is converted (decoded)

to the Pulse Code form. It is supported by decoders of virtually any

up-to-date audio equipment. But this mode is only recommended in case

your AV receiver does not support full decoding of multichannel sound

of above formats, or the audio subsystem of a TV set connected via HDMI

cable is used instead of a home theater system.

•Original (Bitstream): a mode in which the original sound track, i.e.

Dolby Digital or DTS, is transmitted to an external decoder in its

original, encoded, form. Refer to the "Transmission of Digital Audio to

AV Receiver" section. HDMI audio: turn on and off transmition of audio

signal via HDMI.

For the PS3 , note that all audio settings must be manually set for anything other than the default option ....the default option is the one which is the detected at first turn on , or by holding the power button down for 10 seconds and accepting the default options displayed.
( Default is listed as Linear PCM 2 Ch. It does not mention downmixing )

http://manuals.playstation.net/document/en/ps3/current/settings/audiooutput.html

Different devices have different capabilities , all of which are supposed to be detected on the handshake.
Some devices dont support some features , like automatic downmixing of multichannel soundtracks to stereo!
This seems to be the case with the PS3 using HDMI audio , you need to choose a stereo soundtrack if the material has one , if not , you use the amp or miss out on the centre channel !!

So summing up , the Dune auto downmixes PCM multichannel to PCM stereo on detection of a TV , the PS3 does not.

3rdDegree

Prefect explanation Andy! That covers all my questions. It was the automatic downmixing during the handshake that I was missing. Now I understand. Thank you very much sir!