Dolby digital 5.1 and decoding...

[scottylans]

Disclaimer: this may well be a long long post, PLEASE bear with me.

I need to understand Dolby Digital 5.1, and how it works with Decoders / Signals etc.
Before you read too far I'm a GAMER and I will be analysing this from a GAMERS perspective. (this is for a PC audio setup)


I wish to know for this for the primary reason of purchasing the right gear to get the most from something many of you may respect - Doom 3.
(and no doubt the many games which use the engine of it, and the games after that which use true dolby 5.1 after it finally takes off in the PC scene)



Now,....




What my problem is - not with the technlogy of how this works, but more the DOLBY side of things - the specification and what it's going to do to my signal (if anything) on it's way.


Are all Dolby decoders the same? - in saying this I mean will a dolby decoder always perform the same operations as another when it comes to decoding, and if not can they be setup to do this?

I notice that some "proper" receiver / decoders will decode the signal and offer an option of 10 / 15 / 20 ms delays on the surround signal - why not 0ms?

Does my decoder "get smart" on me and try to perform "tricks" on my 6 independant signals?
Where does the crossover / splitting of the low frequency stuff occur? - does my sub channel (coming from the digital source) only contain low frequencies, or does the actual sub channel simply get high frequencies stripped via the decoder?


Could I in theory connect up 6 identical speakers to my 5.1 setup (yes a normal sat to my sub channel)
Then after doing this could I send a DD 5.1 signal to the decoder (assuming I have an nforce which has a dolby ENCODER on the fly) and have 6 levels of identical volume - then hear it roughly identically on each speaker?

OR

Will the dolby signal be "parsed" by the decoder and have all kind of "filters" applied to the audio? (sending audio to the centre channel like pro-logic - or seperating low / high frequencies?)




As far as I'm aware, a 5.1 DD signal coming out of my PC via my SP/Dif connection contains 6 independant channels which will be decoded via a decoder and sent out (analog) to my speakers (5 sats and a sub)


Also, as far as I'm aware a 5.1 "analog" setup will be sending out pre decoded signal to 6 independant analog channels..... (3x headphone jacks on some nforce / 5.1 based boards) - which can then be amplified etc (see Klipsch 5.1 speakers without the DD addon decoder)



What I really want to know is this.


Can the dolby signal send this signal out and have the decoder "obey" the following rules.


centre - volume 0
left - volume 0
right - volume 0
rear left - volume 50
rear right - volume 0
sub - volume (whatever)




OR will my decoder "share" some of the signal with some of the other channels as per part of the Dolby specification - because that's just "how it's meant to be" ??


I'd LOVE (LOVE!!) to think that the standard is purely 100% 6 fully independant channels which can be manipulated in any particular way.

What this means is you could re-create the sound of an object circling you perfectly on a 360 degree field.
See example

Front left speaker - 10% volume of helicopter
Front right speaker - 75% volume of helicopter
Rear right speaker - 10% volume of helicopter
Rear left speaker - 0% volume of helicopter

Now. if we "gradually pan" this signal like this (over a few seconds)

Front right speaker - 10% volume of helicopter
Rear right speaker - 75% volume of helicopter
Rear left speaker - 10% volume of helicopter
Front left speaker - 0% volume of helicopter

You will notice the sound has moved from 01:30 "o-clock" to 04:30 "o-clock" - and has swept past us - if we continue this pattern of adjusting the volume of the helicopter - now moving the focus to the rear left, front left - and finally returning to the front right - we have done a 360 degree sweep of the sound of a helicopter moving around us in a circle.(I'd damn well love it if this is possible)



The thing I've recently discovered (according to a diagram I saw)

http://www.msicomputer.com/Newsrelease/images/dolby.jpg
http://www.programpower.com/Living_room_diagram.gif
http://www.programpower.com/diagram.gif



is that the "official" layout for the 5.1 spec of speakers is to actually NOT have rear left and rear right like most gamers would have, but to actually have speakers ("the sorrounds") which are at 3'oclock and 9'oclock to our sides - not meant to be exactly around us like a cube - this disapoints me, however if the specification complies with what I listed above, we could easily move the speakers into a cube like shape (I'm ignoring the centre here, yes) and technically create a "box" we are sitting inside for perfect 3D (well 360 degree) sound.


I will admit I got a copy of the Doom 3 Alpha, and I do know for a fact that the /sound/ dir contains audio in files named like this.
(this is an example mind you)

id_logo_l.wav
id_logo_r.wav
id_logo_c.wav
id_logo_rs.wav
id_logo_ls.wav
id_logo_lfe.wav



So can anyone answer my questions on Dolby 5.1 for gaming and the abilities of the Dolby specification and decoders?
I'm actually looking at a small home theatre setup for my 5.1 setup at the moment - using a Conia receiver (accepts 5.1 DD in optical and coax form) and 5 kit based speakers (small but nice for the price) as well as a woofer (obviously) I'm _NOT_ looking at a Logitech Z-680 "dedicated PC" 5.1 setup, nor a Creative 5700 / Klipsch 5.1 or any other PC based setup.

Can anyone (perhaps a nvidia DD driver coder or complete audiophile) help me out here with my questions?

thank you very much for your time.
(hopefully there's other Doom / 5.1 / 3d gaming / 3d audio freaks out there who will also get a kick out of any replies to this question)


- AbRASiON:confused;

[Sparhawk]

Wow... that's quite a post. I can't answer all your questions, but I can certainly answer a few.

I notice that some "proper" receiver / decoders will decode the signal and offer an option of 10 / 15 / 20 ms delays on the surround signal - why not 0ms?

The original purpose of the surround channel, back in the days of Dolby Pro Logic, was not precise localization. It was to provide an "ambience", rather than a precise sound source. That is the reason for the delay. When the brain hears two sounds with a very slight delay between them, it will assume that the sound originated from the earlier source. This helps to make the surround speakers "disappear", leaving only a general ambience. "Surround sound" was originally intended to provide a sense of ambience, rather than a full 360 degree sound field. Many surround speakers are bipolar designs, which are intended to bounce the sound off the rear walls, further reducing its directionality.

However, with Dolby Digital having discrete right/left surround channels, there are more possibilities, especially for gaming. A surround setup for gaming could be much different than a surround setup for video. I would think that, in a gaming setup, there would ideally be no delay on the surround channels, and, as you mention, they would be set up more as a "box" rather than to the sides as they often are with video.

I think the paradigm is shifting in video, as more and more channels are added (ie. EX, and DTS 6.1). I think things are moving more towards a precise 360 degree sound field. Maybe bipolar surrounds will be obsolete soon?

Does my decoder "get smart" on me and try to perform "tricks" on my 6 independant signals?
Where does the crossover / splitting of the low frequency stuff occur? - does my sub channel (coming from the digital source) only contain low frequencies, or does the actual sub channel simply get high frequencies stripped via the decoder?

Unless there is some additional signal processing (ie, DSP "room ambience" or something), the 6 Dolby Digital channels are completely independent.

As for the sub signal, it is not a full bandwidth channel. Only frequencies below approx. 100Hz are encoded on this channel. The LFE (Low Freq. Effects) channel contains no information over 100Hz, so no filtering is necessary in the decoder, as I understand it.

Could I in theory connect up 6 identical speakers to my 5.1 setup (yes a normal sat to my sub channel)
Then after doing this could I send a DD 5.1 signal to the decoder (assuming I have an nforce which has a dolby ENCODER on the fly) and have 6 levels of identical volume - then hear it roughly identically on each speaker?

No. The LFE channel is always bandwidth limited, and can't be used for full-range material. The encoder will limit the bandwidth.

Will the dolby signal be "parsed" by the decoder and have all kind of "filters" applied to the audio? (sending audio to the centre channel like pro-logic - or seperating low / high frequencies?)

There are various modes that DD can operate in. The normal mode is 6 independent channels, with the LFE channel bandwidth-limited. Most decoders offer other modes to suit specific setups. For example:

If you don't have a centre channel, you can operate in "phantom centre" mode, where all information contained in the centre channel is sent at equal volume to the left and right channels.

If you don't have a sub, you can have the decoder send all information from the LFE channel to the left and right channels.

If you are using small "satellite" speakers with a sub, you can have the decoder filter low frequencies out of the left and right channels and feed them to the LFE channel.

All these modes are optional, depending on the decoder's capabilities.

As far as I'm aware, a 5.1 DD signal coming out of my PC via my SP/Dif connection contains 6 independant channels which will be decoded via a decoder and sent out (analog) to my speakers (5 sats and a sub)

Also, as far as I'm aware a 5.1 "analog" setup will be sending out pre decoded signal to 6 independant analog channels..... (3x headphone jacks on some nforce / 5.1 based boards) - which can then be amplified etc (see Klipsch 5.1 speakers without the DD addon decoder)

This sounds right.

I'd LOVE (LOVE!!) to think that the standard is purely 100% 6 fully independant channels which can be manipulated in any particular way.

It is, with the caveat being that the LFE channel is inherently bandwidth-limited.

What this means is you could re-create the sound of an object circling you perfectly on a 360 degree field.

Yes, DD is capable of this, though as I mentioned above, it traditionally hasn't been used this way. But there is no reason it could not be used this way for gaming. In this case, as you mentioned, you would want the speakers set up in a "box" around you rather than to the left and right as the "official" setup dictates.

Hope this helps.

[tiroth]

The DD spec supports sending each discrete channel to the appropriate speaker. There are a number of details which are implementation dependent, and this will most directly impact things like bass routing and delay lines for the surround channels.

In general if you pay attention only to the bass management and delay all other factors in implementation should be the same. If you need more specific information you will probably find it only in the DD specifications whitepaper; I don't think you'll find much in the way of implementation-specific notes from the manufacturers.

[MWP]

I assume your talking about using a NVidia motherboard to do the dolby digital encoding from game sound. If not, well im confused, cos its the only soundcard or motherboard that will.

Having the PC encode to DD, and then using a reciever to decode will work well, but it wont give 100% accurate sounds that the game intended. This is mainly because of the DSP work done in the DD decoder. Itll be pretty close though.

Pitty no gaming sound cards i know of will put L&R front out on one SPDIF line, and L&R rear out on a second SPDIF line.
Although, im thinking about trying to hack my TB SantaCruz to get SPDIF out from the onboard I2S signals.

[Axeman]

Just a quick note: I believe the SBlive 5.1 card will output front and back spdif signals. The digital comes out of a stereo 1/8" jack. One side is the + for the front speakers, and the other is for the rears.

[Rarkov]

Hi,
From a programmer's point of view, all the channels must be independant. DirectSound or whatever they call it now (changes every release!) has complex alogrithms to enable surround sound systems to pin point where the source is, compared to your position / orientation / height etc etc.

If you are into that side of things, the OpenGL Game Programming book by Primatech is excellent.

Gaz