[MPlayer-dev-eng] pl_surround questions/suggestions

[Felix Buenemann]

Hi Steve,

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 :)

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.

Do you decode as (pcm channels):
1. Left
2. Right
3. Rear
4. Center

(or 3. and 4. swapped)

Because in this case it would need a different channel mapping for the 
sblive. sth. like:
1. (L) -> Front L
2. (R) -> Front R
3. (Rear) -> Rear
4. (Center) -> Front (or Analog Center w/SB Live! 5.1)

Or do you only decode rear channels and send it to both 3rd and 4th pcm 
channel?

How about having this:
4 channel decoding as above, so it works with 4 channel pcm (like -aop 
list=surround -channels 4 or -aop list=surround:channels=4) and also a 5 (6) 
channel decode:
1. (L) -> Front L
2. (R) -> Front R
3. (Rear) -> Rear L
4. (Rear) -> Rear R
5. (Center) -> Front (or Analog Center w/SB Live! 5.1)
6. (unused) ... or setup channels to 5 via ioctl.

Now we could maybe even do a 6 channel pcm by doing some low frequency 
filtering/splitting (I think signals below 70Hz) and send the signal to the 
LFE channel (of course only makes sense with 5.1 cards).

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).


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).
-- 
Best Regards,
	Atmos
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