Maintain Dolby in DivX 4 encoded movie?

[Jackie]

Hi,

I use to encode movies using DVD2AVI and Virtual Dub, Audio-Encoding with Lame/LameGUI.

Unfortunately, when encoding MP3, the Dolby-Digital (AC5.1)information is lost, right?

Now, how can I maintain this information? I am aware that I cannot mix the full 5.1 information into 2 stereo channels, but I remember there used to be some "analog" Dolby surround years ago which used only the two stereo channels to encode a surround signal.

First question: are modern Dolby Digital Decoders (I have a creative DTT 3500 on my computer) apable of decoding that "old" format?

Second, and most important: is it possible to downmix AC5.1 to a Dolby Surround format in some way that I can then multiplex into my movie? Maybe even in AC5.1, but analog Dolby Surround would be sufficient, too.

Thank you for your ideas,

Jackie

[Kandor]

Use azid for the dolby sorround part.
and if you have a dtt3500 or what its named you can make a divx with the original dolby digital 5.1 audio so it will have the dvd sound.
if your in too the latter then read the guide on doom9s page before asking more questions

Use BeSweet plus BeSweetGUI, both from Doom9 download section.

BeSweet uses the Azid.dll for decoding the AC3 to Stereo, and by default this will be done by downmixing to the good old Dolby ProLogic ( DPL ) format.

To be able to hear that you will need to feed the analog output of your soundcard into a DPL capable device like a Dolby Surround Receiver or TV. They will detect there is DPL information in the Stereo stream and decode to the individual channels ( L, center, R , Surround ), an indicator light should come up confirming this.

Make sure you use bitrates > 160 kbps for your MP3 and set the 'Stereo' mode in Lame ( thats the MP3 encoder of BeSweet ) to 'Stereo' instead of 'Joint Stereo' to avoid the encoder does 'channel coupling', because this could destroy the DPL matrix and lead to sonic artefacts on center and surround speaker.

A good mode to set Lame is VBR q = 1 .... 3 , i always use 'old routine' , its slow but very good. Dont use the 'R3MIX' preset because this one uses 'Joint Stereo' by default. Its excellent for music or if you wnat a normal Stereo movie track ( no DPL ), but not for surround.

[tangent]

Originally posted by ChristianHJW

Make sure you use bitrates > 160 kbps for your MP3 and set the 'Stereo' mode in Lame ( thats the MP3 encoder of BeSweet ) to 'Stereo' instead of 'Joint Stereo' to avoid the encoder does 'channel coupling', because this could destroy the DPL matrix and lead to sonic artefacts on center and surround speaker.

A good mode to set Lame is VBR q = 1 .... 3 , i always use 'old routine' , its slow but very good. Dont use the 'R3MIX' preset because this one uses 'Joint Stereo' by default. Its excellent for music or if you wnat a normal Stereo movie track ( no DPL ), but not for surround.

I have to disagree here. If you understand how Dolby Surround and Mid-Side stereo coding works, you will realise that Lame's Joint Stereo makes encoding of Dolby Surround encodings much more efficient. How Dolby Surround basically works is that rear channel information is encoded into the left and right channel with a 180 degrees phase shift. The easiest way to describe it is that you place the sample into the left channel, and place the inverted version of the sample into the right channel. During decoding, the difference between the left and right channel (L-R) is taken to obtain the rear channel sample.

There are two types of stereo coding modes used in MP3. They are intensity and Mid-Side stereo. Intensity stereo is meant to be used only for low bitrates because it kills phase information. LAME does not implement Intensity stereo. Mid-Side stereo frames are used by LAME when Joint Stereo is enabled. Mid-Side stereo works by converting the left and right channels into mid and side channels. Mid=(L+R)/2 and Side=(L-R)/2. The idea is that there is usually a lot information which are similar between the left and right channels, therefore a near mono sample window will probably have most of the information in the mid channel and nearly nothing in the side channel. This allows the information in the mid channel to be coded with higher quality because more bits are available to encode them.

Now look what happens with a Dolby Surround sample is being encoded: Redundant data actually appears, but this time not in the Mid channel but in the Side channel! In fact, the Side channel is now a perfect representation of the Dobly Surround rear channel! As you can see, Dolby Surround encodes is likely to have better quality using Joint Stereo than simple stereo. It's almost like MP3 Mid-Side stereo was designed with Dolby Surround in mind

The main worry about badly implemented joint stereo is the loss of stereo seperation and a bad JS/SS threshold, and not really the loss of Dolby Surround information. Joint Stereo does not use 100% MS frames, but a decision is made whether to use a MS or SS frame by looking at the amount of redundant data. If the left and right channel are very dissimilar (occurs more often in music), then it would revert to a SS frame. Normal VBR is known to have some issues with this decision process, but this is fixed by using the --nssafejoint switch together with the --nspsytune switch. With this combination, you will never have to worry about joint stereo problems, and yet benefit from the more efficient coding of joint stereo. --r3mix does not use this --nssafejoint, but it is used in "--alt-preset standard" and "--alt-preset extreme". The ABR --alt-presets are designed to use this only from 160kbps onwards.

Thanks for this brilliant explanation. I stopped using DPL in Lame MP3 tracks with bitrates < 160 kbps in 'joint stereo' because i had artefacts in the center and surround speakers ( on my dad's Sony TV by the way, playing from my laptop ). But i never tested --nssafejoint .

[tangent]

Are you using the --alt-preset <abr bitrate> for those encodes? They should work fine.

[Taric25]

Since ISO MPEG-4 will probibly be able to be supported by DVD players in the future, is there any investment in making Besweet multichannel MP3 compatible? Power DVD 4 is. I just think it would be too cool to have 5.1 MP3s in our DivX files.

EDIT - 6.1 would be even cooler.

Vorbis audio is to replace MP3 for DivX, this is 100% certain. It supports up to 255 channels in one audio stream. 5.1, 6.1 or even 7.1 is no prob at all with Vorbis ....

Originally posted by tangent Are you using the --alt-preset <abr bitrate> for those encodes? They should work fine.

Not yet, but will do. I am a lazy bump and using BeSweet or HeadAC3he for transcoding to lame, and both didnt support the --alt-presets until lately.

Now both do and i will use this on my next encoding i guess. Am i correct to say --alt-preset 128 is based on ABR ? If so, great, because i was using ABR ( old routine ) on most of my encodings lately, by advice from R3MIX forum members some time ago ....

[tangent]

Yup.. "--alt-preset <bitrate>" is ABR
"--alt-preset CBR <bitrate>" gives you CBR
"--alt-preset standard/extreme" are the two VBR presets
"--alt-preset help" gives you all the above information

[grivad]

I'd recommend checking out a tool called HeadAC3he.

Description:

"WAV/MP2/MP3/AC3 multi-functional tool including fast and high quality 2-pass decoding using azid/mpalib engine, DS2 downmix, and straight mp2/mp3/ogg encoding using MP2enc/LAME/Vorbis engine."

It allows you to encode in Dolby ProLogic II -- DPL2 is like DPL (surround information in an 2 channel signal), except it seperates the left and right surround channels instead of having a single rear surround channel like regular DPL. The encoding is backwards compatible with DPL, so if your receiver supports either DPL or DPL2, which it undoubtedly does, you will get surround.

Works pretty damn well in my opinion.. no substitute for 6 discrete channels of sound, but if you want to get it down to 2, I'd say this is your best option.