How should 7.1 be downmixed to 5.1

I hooked up an Oppo 83 to a pre-pro via MCH analog. The pre-pro only has 5.1 in and so I set up the Oppo to downmix 7.1 to 5.1. The speaker setup is 7.1 (using rear surrounds) and Dolby PL IIx is applied. I then played a disk that identified the channels. The rear surround channels played through the respective L or R surround channel speaker. The rear surrounds remained silent. It seems to me that the down mix in the Oppo isn't doing a job as well as it can. Instead of merely redirecting the rear surround to its respective surround channel, it should process a rear surround channel and send appropriate signals to both surround channels such that the application of Dolby PL IIx would then route the signal to the correct rear surround speaker.

Shouldn't that be possible? Do any players do that?

Ed

Kal,

You didn't read my post carefully. Of course it can't send a signal directly to the rear surround. But it can process the signal and send appropriate signals to the surrounds such that Dolby PL IIx would direct it to the correct rear surround instead of the surround.

Ed

Yes. It's an Anthem AVM 20 which is one of the few that can do it.

I think I agree. But at a minimum, it could have easily weighted volumes to L & R surrounds so that a 5.1 speaker system would have phantom imaged between the surrounds, and maybe PL IIx in a 7.1 speaker setup might have redirected to a rear speaker.

Yes.

Ed

I don't think so. The player is down-mixing using the most simple algorithm. It just sends L rear-surround to L surround and R rear-surround to R surround. The pre-pro then does only what it can do - reproduce through the L or R surrounds. The pre-pro has no way to know that these signals originated as rear surrounds.

Ed

But as I said above, even if you only had a 5.1 speaker system, you'd probaly want the rear channels to phantom image between the surrounds.

Ed

I didn't mean to imply digital encoding. I meant that in principle the analog signal can be encoded by phase and amplitude distribution such that the DPL IIx can figure out that the signal should come from the rear surround speaker.

This is my point. If the down mix simply redirects the rear signal to the surround, the pre-pro cannot distinguish that signal from one that is meant to be in the surround channel. I'm wonder whether it's too much to expect a down mix that is more elaborate.

No, the Anthem does not do that. But that capability wouldn't really be a solution to the problem here.

Ed

I haven't asked them. I assume that the down mix is something that is "hard wired" in a chip that they purchase and they have no way of changing it.

Ed

It is set to Analog DSP.

Yes - I'm not saying that I can't get sound out of the rears with 5.1 analog in - with Dolby PL IIx I do get sound when the input is a movie or a concert disk. The problem is on a test BD when only one rear surround channel is being identified. In other words, the down mixing is not happening in an optimal way. With "random" multichannel audio, someting will stimulate the DPL IIx to put something in the rears.

OK - I set that aspect up a long time ago and don't quite remember. But those are hokie settings IMO.

Ed

The tone for the rear right comes out the right surround speaker in your example. That's because the Oppo's down mix simply redirects the rear right to the right surround. Therefore the Anthem only knows that it's getting a right surround and plays it there as it should. I think that a proper 7.1 to 5.1 down mix should somehow put signal in both the R and L surround channels in such a way that Dolby PL IIx will (mostly, maybe not perfectly) decode it as a rear right channel.

Ed

When it sees complex signals in both L&R channels then invariably something gets routed to the rears. But the way in which the signals are routed is not what is intended by the original 7.1 mix.

Certainly Dolby PL IIx does the best it can with the signal it gets. But I feel that things have been comprimised. For example, if you had a disk that had a 7.1 mix where a sound is panned across the back from L to R, that pan would not be reproduced faithfully, even though at some point I'm sure sounds would come from the rear surrounds.

Ed

No I don't - but then I haven't played with any BD players other than the Oppo. It just seems odd that when the engineers were deciding how to down mix 7.1 to 5.1 they didn't do anything more than simply copy L to L and R to R.

Regarding encoding analog signals, didn't the old Dolby Pro Logic do that to get a center and a mono surround channel?

Ed

I feel like I'm beating this to death - so I'll stop soon - but let me just repeat something I said above: Even if you have a 5.1 system, you'd want something originating in one of the rear surround channels to be redirected to a weighted volume between the 2 surround channels so that it phantom imaged behind you instead of to the side surround.

Ed

For the case that I'm talking about, I don't see any reason why the proper downmix wouldn't send signals to 2 surround channels. After all, the original mix had a signal to a channel that does not exist in the downsampled space. Clearly, sending the signal completely to one downmixed channel cannot be correct since then it should have been in that channel in the first place.

Anyway, if the downmix is processed in an appropriate way and sent to the 2 surround channels, playback with PL IIx ideally would place it back in the single rear surround channel that it originated in.

Ed

I don't know why this is so hard to understand. I'm trying to explain as clearly as I can. When I say that the "proper downmix could be sent to 2 surrounds, I'm talking about the source material originating in one of the rears. Of course when both L & R rears are downmixed, both surrounds get a signal even with the simple scheme that is implemented.

I've already said this several times. The problem is not in the pre-pro. The player is down mixing in a trivial way - just copying L rear to L surround and R rear to R surround.

I realize that this is not happening. I started this thread because I was surprised to discover that the downmix is not this way but instead the trivial copying - which happens to lead to undesirable effects - like poor panning.

Ed