Downmixing AC3 2.0 to PCM - why?


#1

Hi folks!

First of all (since this is my first post here) a big hello to all of you. I’ve been reading here for quite a long time now and this forum with its very helpfull members is my number one information source, when it comes to the WDTV Live box.

Although, I never had that many problems with my little box, reading the forum was always interesting.

Unfortunately I never found an answer to the following question:

Why is the WDTV Live downmixing an AC3 2.0 audio stream to PCM stereo instead of bitstreaming it right to the digital connected AVR?

My WDTV (running the latest official firmware) is connected via HDMI to my Yamaha RX-V 1800 AVR. The audio output setting is “digital”. I have no problems with AC3 5.1 or DTS. But whenever a video comes with an AC3 2.0 audio track (which is true for many DVD releases of older TV shows like “Buffy - The Vampire Slayer”), the WD box downmixes it to PCM.

My older media player from Fujitsu (AMP 150), also equiped with a SIGMA Chipset, did it right - no downmixing to PCM.

So please, can someone tell me, why the WD box behaves like this?

@WD: Is there a chance to get this fixed?

Thanx in advance!

Cheers,
Zap


#2

Welcome to the forums (even though you’ve been lurking a while :>).

Interesting.  I never even thought to check this – but it does beg the question, why should you care?  Do you really think you could hear the difference?   IOW, if someone did an A/B test on you (you can’t do it yourself) with your AMP150, changing from digital to stereo out (to switch to PCM versus AC3) I suspect you’d never be able to tell one way or the other.

I know you’re asking *why?*, but for me it comes down to results.  I just find it very hard to believe it matters.  (And as Spock said, “A difference that makes no difference IS no difference”).


#3

Ooo.  Yeah, that’s a good question.   But first, I don’t know that I’d call that DOWNMIXING, because it’s still 2-channels in and 2-channels out.

But I’m now curious as to what the REAL difference is, other than name.

AC3 2.0 and PCM 2.0 are obviously both two-channels.   However, PCM supports MUCH higher bitrates than AC3 2.0 does. PCM can support 96kHz sampling while AC3 only does 48kHz sampling… and so on.

Does it does this for ALL AC3 2.0 streams?   In other words, if you’re watching a DVD format with AC3-2.0 tracks, does it downmix those?   Or only MKVs?  


#4

Hi again.

@mike:

It’s an easy answer, why I’m caring about this: I don’t like unnecessary processing (converting, filtering, etc.) of digital streams. :wink:

My AVR plays AC3 2.0 streams perfectly and I think every AVR on the market will do so. So why converting the AC3 to PCM with whatever loss of quality might happen? There is absolut no benefit when doing so. You might be right, that mostly you will not here much differences in quality when comparing the PCM with the original AC3 streams. So my question seems a bit academic.

But there is another problem arising from this. I have some tv recordings which have a AC3 5.1 audio stream. When cutting out commercials (which had AC3 2.0 streams) it happens that, after muxing the edited video, the very first haeder of the AC3 stream was wrongly flaged as AC3 2.0. So, the WD box decided to convert the AC3 stream to PCM 2.0. Unfortunately, after this decission is made, the WD box will never change its mind and will happily convert (and in this case downmix) the AC3 5.1 to PCM 2.0 for the whole movie. And, at least in this case we do have a difference… :wink:

@Tony:

You are right. It’s not a downmixing, when converting AC3 2.0 to PCM 2.0. And yes, PCM supports higher sampling rates, but thats no advantage in this case, because the source does not have those higher sampling rates.

As written above, there is no need converting the original AC3 stream and I dislike those unnecessary processing.

To answer your question: Yes, it is done for all AC3 2.0 streams, regardless which container format. At least I have verified this for mkv, avi and mpg.

Cheers, Zap


#5

For your files that have mixed streams you should know that it’s VERY unstandard – which is to say, MOST devices (and even most software) will not correctly handle them.  You should be editing and/or creating your files in such a way that the streams are the same all the way through.

For example, if you try and use MKVMerge to append to such unlike streams it will balk, rightly complaining.  If it were me I’d just make sure things were properly authored (and there are a wide variety of tools that will help with this).

We can see if WD can shed any light on why it does this, but getting the behavior changed is likely to be a VERY low priority for them (because in the vast majority of cases it won’t make any difference at all.  They have many many bigger issues to take care of and even those are stalled at present).


#6

Mike, you get me a bit wrong or I made myself unclear…

My files do not have mixed streams, it was only the very first audio frame that wasn’t cut right, so the AC3 header was set to 2.0. And I DO know, that this is my fault, not WDs. So, no complaining here… Meanwhile I fixed the file…

I made this example, because if the WD would not decide to downmix the AC3 2.0 to PCM, but would bitstream the AC3 stream instead, my AVR would switch to 5.1. You got me? I never noticed this problem when using my old AMP150, because it never converted AC3 2.0 to PCM…

And yes, I’m aware, that this problem is of low priority… but, maybe, it’s very easy to fix and there is a chance, that it gets done in one of the next FW releases… :wink:

Cheers,

Zap


#7

Okay, but I don’t think you quite get me either.

It doesn’t matter if it’s so easy to fix you or I could do it – trust me, there are things that are five minute fixes we’ve been asking WD for many months now, and they don’t get done.  They prioritize their list based on the problems they cause folks, and what ain’t at the top just won’t get done.  This just isn’t something that one out of a thousand would notice or even care about.

I don’t think it hurts to ask for it (in the Ideas section, tabbed above) but I also think there is about as much chance of it happening as Lebron referring to himself in the first person (namely: none).  I could be wrong, but I’m not.  (But do go ahead and put the idea in the idea section and let’s see).


#8

Good point Mike, I should place it in the Ideas section and just see what will hapen… sooner or later… :wink:

Anyway, thanx for your time Mike.

Cheers,

Zap


#9

I think I’ve figured this out.    I smacked my forehead when I realized what we were talking about as if it were some huge mystery.  

Why is the WDTV Live outputting AC3 2.0 tracks as PCM?   Because it is SUPPOSED to according to the specs and the Dolby Labs License. 

AC3 2.0 is COMPRESSED; PCM is NOT compressed. 

So really, the WDTV is “Decoding” the AC3 2.0 stream and outputting the uncompressed result.

It CAN’T do that with the AC3 5.1 because it’s apparently not licensed for it (nor does it have the hardware capability to OUTPUT 5.1 sound; it’d need discrete channel output to do that) , but it CAN pull the 2.0 CORE out of a 5.1 stream and output THAT, which it will do as, you guessed it: analog audio.)     It CAN however, PASS THROUGH AC3 5.1.

As to why it doesn’t just PASS-THROUGH 2.0 streams when you’re setting to DIGITAL MODE?  It’s probably Po-TAY-to Po-TAH-to;   Your receiver will do it if your WDTV doesn’t, but either way, you’re getting the exact same AUDIO output at the speakers.


#10

zaphod42 wrote:

 

It’s an easy answer, why I’m caring about this: I don’t like unnecessary processing (converting, filtering, etc.) of digital streams. :wink:

 

My AVR plays AC3 2.0 streams perfectly and I think every AVR on the market will do so. So why converting the AC3 to PCM with whatever loss of quality might happen? There is absolut no benefit when doing so. You might be right, that mostly you will not here much differences in quality when comparing the PCM with the original AC3 streams. So my question seems a bit academic.

 

As written above, there is no need converting the original AC3 stream and I dislike those unnecessary processing.

 

Cheers, Zap

As Tony has pointed out something has to do some processing as you cannot hear AC3 naturally. If its not the WDTV Live it will be your AVR. To listen to the sound it must be somehow converted to analogue audio in order for your speakers to work.


#11

Tony, now that throws some new light on it. Reading your explanation, I slowly get the feeling that you are absolut right here… hmmm should sleep over it…

Anyway, it should pass through 2.0 as well… :wink:

Thanx for your investigation.

Cheers,

Zap


#12

I believe that we overlapped in posting so you may not have seen my reply.

http://community.wdc.com/t5/General-Discussions/Downmixing-AC3-2-0-to-PCM-why/m-p/49724/highlight/true#M12294


#13

richUK wrote:


As Tony has pointed out something has to do some processing as you cannot hear AC3 naturally. If its not the WDTV Live it will be your AVR. To listen to the sound it must be somehow converted to analogue audio in order for your speakers to work.

 

Rich, you are right as well.

However, I would like my AVR doing all conversion from the original digital source up to the analog speakers. I think, this should be made configureable…

My DVD players for example will not convert any AC3 2.0 to PCM. They all stream the original AC3 track to my AVR unless I configure them to do PCM output. Exactly that I was going to expect the WDTV Live should do.

Cheers,

Zap


#14

Interestingly, DVD playback (ISO, VOB) will output as AC-3 (over optical at least).


#15

PixelPower wrote:

Interestingly, DVD playback (ISO, VOB) will output as AC-3 (over optical at least).

Is that AC3 2.0


#16

richUK wrote:

 


PixelPower wrote:

Interestingly, DVD playback (ISO, VOB) will output as AC-3 (over optical at least).


Is that AC3 2.0

 

Yep, my Pioneer Receiver says ‘AC-3’ and two of the little speaker icon light up.  

If it were AC-3 5.1, there would be 5 icons lit (plus a dot for LFE).  :)

Most DVD menus are AC-3 2.0, so it’s a quick one to check out.


#17

Hi!> * * *
PixelPower wrote:

Interestingly, DVD playback (ISO, VOB) will output as AC-3 (over optical at least).

Ok, but there are at least two differences between our setups: 1. According to your signature, you use the newer beta FW and 2. you connect your WD via optical to your AVR.

But thanx for the hint, I will check it this evening (CET :wink:) with a ripped tv show dvd, where the episodes are all AC3 2.0. I can’t check this with DVD menus, since my official FW does not support DVD navigation.

I will also check the optical way, since my son’s WD TV Live is also connected via optical TOS Link to his AVR.

Cheers,

Zap


#18

So, just a quick response:

I checked against DVD playback (playing the VIDEO_TS folder), but it’s all the same. I still got PCM instead of an AC3 stream. I also checked with a optical connection to the AVR. Again, no success.

Maybe, this is already  fixed in the current beta…

Cheers,

Zap