5.1AC3 odd downmixing on stereo setup

[pelmen]

I havent been able to find a post that covers this yet so I'm not sure where the problem lies or how best to search for an answer.

Basically I've got DVDs I've made from VHS sources and I've used various methods to upmix the stereo source to 5.1AC3 (mostly bidule and Upmix V0.7x methods).

In 5.1 everything plays fine. All 6 channels are exactly where they should be in the 5.1 file (my first step when using any new upmix method is to use a channel identifier source and check each mono wav and final 5.1 carefully so i know all channels are ending up where they should be.

HOWEVER, I have recently played one of my DVDs on a stereo setup, where the stereo outputs from the DVD player where the only audio. I noticed a sound balance problem though, the right channel was very obviously louder than the left. I retested some of my other DVDs and they too exhibited the same problem but to varying degrees.

Essentially what I've worked out is happening is that the 5.1AC3 is being downmixed but it is using the front right and rear left to form the stereo output. What I'd like to know is why this is happening in the first place? Is it a problem with BeSweet? With ac3enc.dll?


All the upmixing methods I've used for my DVDs have used BeSweet in them (and therefore ac3enc.dll). All the 5.1 files are perfect on four different machines/setups I've tried them on, but when a setup is switched to 2 channel (stereo) then the problem manifests. This problem does not occur on DVDs I've bought.

On my laptop with PowerDVD I switch the 2 channel audio between "stereo", "Dolby Surround Downmix", "Dolby Virtual Speaker", TrueSurround XT" and "Cyberlink Virtual Speaker" and the sound balance is fine in all of these EXCEPT the "Dolby Surround Downmix" then the balance is out.

Each DVD is out of balance by an amount consistant with the level differences between front/rear channels (eg UpMix makes very quiet "ambient" rear channels and the DVDs using this sound like all the audio is being played from the right channel when played in stereo, other bidules though produce rear channels which are closer to the front channels for loudness and so the sound balance is not as bad).
I also use DVD Architect to master my DVDs and when you drop in audio to it is shows a stereo waveform and it is clearly showing me most of the sound is in the right channel.


So it seems there is an issue which the AC3 format file and it is not produced in a way that the Dolby Surround downmix is being done correctly.
So is this a known issue?
Is there a way of fixing it (eg a ac3 file processor that can reset a downmix channel flag or something)?
Is it a limitation of ac3enc.dll?


Anyone else come across this or am I just going mad?

Thanks for hanging on to the end of this post :)

pelmen

[ursamtl]

No need to apologize for the long post. By my sometimes longwinded standards, it was miniscule! ;)

Right off the top of my head, I can't say exactly what your problem is, but if you check the Sticky thread on AC3, GUIDE: How To Properly Encode Dolby Digital Audio (AC3) (http://forum.doom9.org/showthread.php?s=&threadid=56020), you may find some info on downmixing settings. You can also check out the technical papers at www.dolby.com for more info.

[KpeX]

I'd try reencoding a DVD-source AC3 with ac3enc.dll and see if the problem occurs to try to narrow down the issue. If inbalanced downmixing occurs, the problem is with ac3enc. If not, the problem is with the upmixing.

[pelmen]

there didnt seem to be anything related to "downmix"ing in the dolby guide or the threads that could shed light. i vaguely recall though that ac3enc.dll isn't a full blown encoder so the problem seemed likely to lie with that.

as for trying to upmix with a 5.1ac3 source it isn't really possible with the upmixing methods i've used...they all rely on a two channel source. however i do have the first season of the simpsons box set which i re-encoded each episode so it all fit on a single dvdr. for that i re-encoded the video stream seperately and the audio using BeSweet GUI (AC3 to AC3 conversion to lower the bitrate). when i listen to that its fine as well.

this would imply its the umixing methods except that doesnt fit either as i still have my "source" AC3 reference files I created using the various upmixing methods and the channels are perfectly balanced (FL & FR are same level and SL & SR are same level) but the downmixed playback on a stereo setup is still unbalanced by an amount you would expect if one front channel and one rear channel were used to make the two front channels of the stereo.


while browsing the dolby encoding guide though there was a lot of info about dynamic compression. since my AC3 to AC3 source was a "real" AC3 track would its dynamic compression amounts on each channel be retained and being used in the eventual downmixing to stereo i'm trying? so that even if the surrounds sound quieter if a surround was being used as the front left in the downmix would its dynamic compression re-expand it back to a normal level so i wouldnt hear an imbalance in volume between the two speakers?

(all downmixing i refer to is of course only occuring in the dvd players we have here, its not software/computer downmixing in an conversion process).

its still got me mystified. the only common element between the upmixing methods i've used has been besweet (and therefore ac3enc.dll?) to make the final ac3 file. most of what i've made has been using the "5.1_SAD5.1inBidule_by_kempfand_040502" bidule method and UpMix V0.71 (I currently prefer UpMix, I prefer the sound I get from that).

maybe its all related to the dynamic compression settings? but it wouldn't explain why the volume is different when played as stereo by an amount equal to the volume difference between front/rear. that would imply its using incorrect channels to downmix...but if the channels were incorrectly ordered in 5.1 then it would be noticable (and they definitely aren't incorrect, i use a channel identifier test sample on each upmixing method i to just to make sure all channels are playing fine).

it only occurs on "dolby surround downmix" setting in PowerDVD, the other downmix settings it comes out properly balanced and as for the three dvd players i've tried its only occuring on one of them (that player has no problems with any dvds that i haven't remade the ac3 files on) and in its menu it has a dolby downmix feature active for the stereo output. so it would seem that there is something not being set correctly (either a channel flag or dynamic compression or something else) in the AC3 file being made that results in the dolby decoder to not downmix properly. its really weird. not really much of an issue but i'm sure the same is occuring on other peoples discs too (if you have PowerDVD put it into 2 channel output, put on headphones and cycle through the downmix options (it will switch the downmixing as you select each one so you can hear the difference immediately) and the dolby surround downmix is the one where imbalancing occurs).


i'm still learning my way with ac3 and dont have any "pro" packages for making the ac3 files so its hard to know if its the ac3 encoder causing the problem or something else. thanks.