[MPlayer-dev-eng] pl_surround questions/suggestions

[Stephen Davies]

Hi Felix,

On Wed, 5 Dec 2001, Felix Buenemann wrote:

> I've today tested the pl_surround stuff and it seems to work quite good (but 
> I must say 5.1 version sounds even better :)

Yeah - 5.1 will definitely sound better; the channels are completely
independent.  With "matrixed" surround sound there is lots of cross-talk
between channels, especially with a simple "passive" decoder such as I
built so far.

> I've got a question to the channel mapping you do:
> 
> For 4/6 channel dolby digital my sblive channel mapping is:
> L/R/Ls/Rs/Center/LFE (yeah, I changed my cabling after seeing windooze 
> decoding also had swapped channels)
> Of course for 4 channel only L/R/Ls/Rs are really used and the last two 
> channels are dead.
> 
> Now as Dolby Surround (and Pro Logic) uses a 4 channel technique with this 
> splitting: Left, Right, Rear, Center, I'm curious how you decode this.

As you say, surround encoding encodes L, R, C and S.  My decoder makes 4
outputs, L, R, Ls and Rs.

L and R come from the input signal "as is" - so contain C in equal parts,
together with the S in anti-phase.  This gives a "phantom" centre, and a
diffuse surround in the front speakers.  I derive S from L-R, delay by
15msec and feed to Ls and Rs in anti-phase.

This is the dolby surround decoder as described by Dolby, except for a
missing 7kHz low-pass filter.  The 15msec delay takes advantage of
something apparently called the Haas effect - sounds from the front that
leak to the rear will be heard first in the front, which creates a
perceptual effect that keeps them in the front.

The code could easily also decode a C channel, (L+R)/2, but Dolby
recommend to not do so with basic surround decoding because its only got
3dB separation from the L and R and so just tends to narrow the
soundstage.

Your suggestion to extend to a 4, 5 or even 6 channel mix is a good
one.  A C can be easily made, but needs the Pro-Logic steering to help
the separation.  And LFE with a low pass filter like you say.

First, though, I want to try make the existing decoder more Pro
Logic-like.  This will help increase the channel separation.  I have just
been looking at the Pro Logic and Pro Logic II principles to understand
the detail and I'll have a try at that.  The Dolby site has good
information but doesn't spell all the Pro Logic subtleties out.

> Another nice thing would be configurable delays and amplification of all 
> channels, but that's something for the future (My Dolby Digital/Dolby 
> Surround Pro Logic Amplifier has that, it's used to make sure signals arrive 
> at the listener at the exact time, delays are calculated by distance from the 
> listener).

Yep - Anders already suggested this stuff when we were talking about the
surround decoder.  To my mind perhaps its better done as a separate
plugin, then it can be used whether the 4 or 5 or 6 channels come from ac3
decoding, the surround plugin or whatever.

As it is the surround levels from this decoder are quite different to the
ac3 one: but the pro-logic stuff should help balance things out.

> Btw. I found very usefull page on many video related subjects including dolby 
> techniques at http://www.labdv.com/ (also has nice thx dolby digital and 
> dolby surround (downmixed) samples).

I'll check that out, thanks.

Glad someone else tried it with some success.

Steve