Dolby Surround Questions

[RScullion]

Hi,

I've got a few questions about Dolby surround I haven't found the answers to.

1) Is there a directshow filter to decode a dolby surround track (say that's been stored as an MP3) and produce a four speaker output for the Live/Audigy/etc? If not, is there any other way to do it short of buying/building a box.

2) Is the dolby surround track on the DVD already stored as a dolby surround track, or is it three different tracks? (I ask this because using the SPDIF option on the Audigy actually decodes a surround track and sends the result to the correct speakers (as with a 5.1 AC3 track).

3) Is there any way of producing a 4 channel OGG. OggMachine will produce a 6 channel one but it seems a waste of bandwidth. Also, is there any way to set the compression on each channel separately (i.e. compress the rear/LFE channels more than the front)?

I appreciate any help you can give me in answering any of the above questions.

Thanks,

Richard

[MaTTeR]

Hi Richard,

Is there a directshow filter to decode a dolby surround track (say that's been stored as an MP3) The Cyberlink PowerDVD filters have the capabilty to downmix an DS track to stereo and possibly mono if memory serves me right. In the days before Vorbis entered the scene, I use to transcode the DS track from the DVD to MP3. The resulting MP3 is played back through my external amp with full Dolby Pro Logic goodness:) If you have an external amp capable of Dolby Pro Logic then there is no need to build anything as you suggested.

Is the dolby surround track on the DVD already stored as a dolby surround track Most DVD's have both a Dolby Digital and DPL (DS) track on them. Why do you have your Audigy decoding the Dolby Digital track over SPDIF? You don't have an external amp I presume?

OggMachine will produce a 6 channel one but it seems a waste of bandwidth. Your correct of course. Currently multi-channel coupling isn't supported in Vorbis, therefore all 6 channels get full bandwith. 5.1 channel coupling will be implemented once 1.0 has been released. Both HeadAC3he and BeSweet can encode to 5.1 or 2 channel but not 4 channels. Tobias is working on a DSF which might allow 5.1 to be downmixed to 4 or even 2 channel mode.

Maybe you could let us know what your really after hear before you go trying to build some sort of box:D Hope this helped a little bit.

[RScullion]

Thanks for the info. If a send a Dolby Surround track to the Audigy by enabling SPDIF out it creates 3 different channels (two for the front and one for the rear.) Hence I was wondering if WinDVD was sending it those three channels (direct from the DVD) or whether the track on the DVD was already encoded as a two channel Dolby surround track, and the Audigy was decoding that (since it is capable of decoding an AC3 encoded track).

The other question about the directShow filter - I was wondering if it was possible to take a two channel encoded Dolby surround track and pass it through a directshow filter to decode it to a four speaker track, (decoding rather than downmixing). I haven't managed to get the cyberlink filters to do that.

Thanks.

[ChAoS Overlord]

I have a question too, somebody told me that besweet automatically downmixes the 5.1 tracks to surround, but why is there a switch for surround output mode (under stereo) in the azid 1 tab then? Do I have to turn this on? What are the optimal settings, for encoding a dolby surround track for my divx using besweet.

Something different, my setup is a plain old sblive 1024, with four analog speakers connected (4point surround) to playback with this speaker setting. What setup/decoder/filter/... should I use for dolby surround playback (using a two channel mp3-encode) with such a setup? AC3 works using the intervideo filter, but what about this dolby surround then? Wouldn't my player recognize it as a standard 2channel mp3? :confused: I'm confused...

EDIT: also should I use the "center downmix level" since I have no center speaker?

[DarkAvenger]

It should be no problem to encode to 4ch using HeadAC3he:

(based on 2ch config) Just tick rearleft and rearright in output channels, select 0db gain in input ch for both. But you probably have to experiment with the DRC setting. I guess you should choose the same for all output channels, maybe light.

The only problem is to play this, Tobias' DSF doesn't properly support this 4ch Vorbis, yet.

[ChAoS Overlord]

Can anybody share his insights on behalf of my questions?

[MaTTeR]

@ChAoS Overlord

A two channel MP3 with DPL effects will playback in 2ch mode. The decoding of Dolby Surround would then take place at your external amp, sending pseudo surround effects to the rear speakers. If you have no external amp to do the decoding, then just set the InterVideo filter to Dolby Surround instead of 2ch, 4ch, etc.

BeSweet automatically downmixes 5.1 to Dolby Surround? Hmm...I suppose that could be the default setting but it's easy to change to regular stereo downmix as well. I ask a question awhile back about optimal settings while transcoding to MP3, I recall ChristianHJW and I think DSPguru responding with some good insight. Do a search in the audio forums here for DPL and see if you can find it. Are you using MP3 still or Vorbis?

[ChAoS Overlord]

As for now I'm still using mp3. Is vorbis a superior format when it comes to divx or multichannel audio, because as far as pure audio (cd-rips) is concerned I still adhere to vbr mp3's. I will look up info on DPL via the search option thx.

EDIT: I have done a search on "dpl" in this forum, but the answer didn't come up in the 12 found posts :(

please help me out (dspguru?) so that I know
- which options to check for a perfect surround downmix for a 4 speaker setup (actually 5 speaker (2+sub) + 2 connecte to my two analog outputs
- which "center downmix, surround downmix, lfe to l-r, ........ I should use
- which playback filter to use for true dolby surround playback (because dolby surround is under the "two channel" option on the intervideo decoder)
- Is vorbis a superior format for divx? If so in what domains?

[MaTTeR]

Here's two threads regarding DPL-

http://forum.doom9.org/showthread.php?s=&threadid=15328&highlight=DPL

http://forum.doom9.org/showthread.php?s=&threadid=15351&highlight=DPL

[RScullion]

Thanks for everyone who tried to help me. I've found the answers to some of my questions and thought I should post them to complete the thread:

Dolby surround tracks are actually Dolby Pro-Logic sound tracks. DVD's containing these tracks don't have three discrete channels, but two channels with the Dolby Pro-Logic encoding already performed. I've only found three ways to decode these tracks (without a Pro-Logic Decoder):

The first, and only one I've actually tried is to enable the SPDIF output on the DVD player, at which point my Audigy soundcard automatically detects the Pro-Logic encoding and sends the correct tracks to the four speaker outputs. Unfortunately I haven't found any way to send MP3 Dolby Pro-Logic tracks to the Audigy for processing. Possibly it would require on the fly conversion to AC3, and then decoding? (Also, the Running Man DVD claims to have a surround track but it doesn't).

The other two ways involve PowerDVD VR (which claims to be able to decode Pro Logic - but I don't have a copy - and a dolby plug in from SoundScape - which again I wasn't able to test.