AC-3 pan downmix error - Reproduceable Bug?

musicvid10 wrote on 9/8/2008, 10:52 AM
Can someone please try to confirm this for me:

1) VP8.0b, new project, 5.1 Surround audio.
2) Drag a stereo .wave file onto the timeline, in my case 16/44
3) Confirm No envelopes, no automation, "Show automation controls" off, panned dead center, everything left at default.
4) Render in AC-3 Studio encoder, either Default or DVD 5.1 template, AGC off. Don't close the Vegas project yet.
5) Insert the .ac3 file into a DVDA5 project, note that the stereo downmix appears and plays mostly in the right channel, not balanced.
6) Go back to the Vegas project, move the surround pan all the way Front Dead Center, confirm no L-R pan, no envelopes, no automation, "Show automation controls" off.
7) Render the project again, repeating step 4.
8) Insert the .ac3 file into a DVDA5 project, note that the stereo downmix now appears and plays balanced L-R.
9) Go back to the Vegas project again, only this time move the surround panner all the way to Rear Dead Center, and render again.
10) Insert the .ac3 file into a DVDA5 project, note that the stereo downmix again appears and plays mostly in the right channel, not balanced.

I think this is a problem with the embedded downmix in the 5.1 file when rendered in the AC-3 Studio template, and affects playback in applications that can access it. It does not seem to affect applications and players that create their own stereo downmix directly from the 5.1 surround file, as is often the case. It is worth noting the downmix looks and plays properly (the bug is not reproduceable) when rendered to the AC-3 Pro encoder.

EDIT: See additional observations 2 and 4 posts down.

Note: I have independently confirmed the behavior in my software DVD player. When it is set at "Stereo" the playback is balanced and correct. When it is set at "AC-3 compatible stereo downmix" the playback is unbalanced just like in the DVDA5 timeline.

If this can be reproduced in someone else's setup, I would like to hear from Sony or be directed to other responses regarding this apparent bug. If, however, it's something just on my system, I will welcome the opportunity to dig deeper.


newhope wrote on 9/9/2008, 3:54 PM
Just a quick not to say I haven't forgotten this but I'm in the middle of delivery of a project.
I'll test the problem as soon as I get my edit out to the client.

New Hope Media
musicvid10 wrote on 9/9/2008, 6:15 PM
A bit of additional information -- I've found that I can replicate the bug using the AC-3 Pro 5.1 encoder if I turn the 90 Deg Phase Shift option to "Off."

Edit: BTW, good luck on delivery of your project. You're earning more $ on this than I am!
newhope wrote on 9/10/2008, 6:35 AM
****You're earning more $ on this than I am!****

hehehe...Yeah possibly but I doubt you recently spent a few days down corroded storm water tunnels videoing.

I'm making a corporate for a client that specialises in horizontal boring.

Yep it's all about trying to make an interesting boring video.....

Then I have to package the videos into an interactive application.

Quite an interesting problem you've discovered with the AC3 encoders.... I may have time to check it out tomorrow as I've just uploaded a Flash preview of the videos and am in standby mode waiting on my clients comments.

New Hope Media
musicvid10 wrote on 9/10/2008, 8:44 AM
My guess now is that it is the Lt / Rt downmix that is ruined with the phase shift "off," and the Lo / Ro (ordinary stereo) remains OK. This would make sense considering the purpose of the rear channel phase shift is to facilitate decoding of a Pro Logic signal for compatibility.

If this is correct, it makes a strong case for leaving the rear channel phase shift "On" in the Pro encoder, and for turning it "On" in the Studio encoder in the next update. Appreciate your interest in this, Steve, and it would be great if Peter or someone from Sony would jump in and try to validate my observations.

EDIT: Ouch!! It takes twice as long to render a 2 hr 5.1 project in the Pro encoder. I wonder if there is some "back door" to edit the preprocessing bits in the Studio version?
newhope wrote on 9/10/2008, 9:28 PM
I've now had the time to repeat your tests and have come up with the same results with a few additions.

1) I chose a stereo mix from a recent video with a reasonable amount of mono and stereo sources mixed together.

2) I added this to a new Vegas 8 Pro timeline with no other processing.
3) I left the panning set to the default centre position and set the surround panner to FILM as this is the type I would normally use.

I checked the mix in the other types and Constant Power placed the signal into my Centre and Surround channels but FILM and ADD Channels 0 at Centre played normally through the front, surround and centre channels

4) I then rendered an AC-3 file using the AC-3 Studio 5.1 preset with AGC OFF. The resultant AC-3 file was imported into DVDA5 and was heavily right balanced with alomost no left channel signal.
5) I rendered the same tracks using the AC-3 Pro encoder with DRC set to none and the 90degrees phase shift on. The resultant AC3 when imported in DVDA 5 was normal and correctly balanced.
6) I again rendered using the AC-3 Pro encoder with DRC set to none and 90 degrees phase shift OFF and the resultant AC3 file when imported into DVDA5 showed similar characteristics, heavy right balance, as the AC-3 Studio rendered file.
7) I rendered the track again using the AC-3 Pro encoder with 90 Degrees phase shiift OFF and set the Ext. Bitstream Stereo Mixdown to LoRo (stereo). The AC-3 file was imported into DVDA5 and had normal left right Balance.
8) I rendered the track with AC-3 Pro with 90 phase shift OFF and Ext Bitstream Stereo Downmix set to LtRt (Prologic surround). The resultant AC3 file whenimported into DVDA5 was heavily right balanced as before.
9) Final test was to render with the AC-3 Pro encoder and turn the 90 phase shift back on and set the Ext. Bitstream to LtRt Stereo downmix. The result was a normally balanced AC-3 file when imported into DVDA5.

My interpretation of this is that the AC-3 Studio 5.1 encoder is probably NOT applying the 90 Phase shift but is defaulting the downmix to LtRt. There are of course other combinations which may apply in this encoder that can have the same results but this combination seems the most likely as the encoder is designed to create, initially, a 5.1 Dolby Digital mix and then possibly a LtRt downmix. It's bit hard to tell when you can't look at the internal paramter settings.

Similarly the AC-3 Pro encoder defaults to adding the 90 degress phase shift and automatically sets the downmix bitstream to LtRt correctly unless you choose to change it.

The short answer is, use the AC-3 Pro encoder if you are mixing a combination of stereo and mono sources into a 5.1 mix and make sure you set the Extended Bitstream and Preprocessing paramteters correctly.

If you want to use the AC-3 Studio encoder stick to the stereo preset (AGC OFF being my preference) for stereo.

If you are encoding 5.1 I'd recommend the AC-3 Pro encoder.

You may have success with the AC-3 Studio 5.1 if your 5.1 mix is created from discreet mono sources without any stereo material though I didn't have the source material to test this.

New Hope Media
musicvid10 wrote on 9/10/2008, 9:37 PM
Yep, thanks for the validation of my issue and expansion on some areas I would not have investigated. Submitting a support ticket to Sony now, essentially a link to this thread . . .
musicvid10 wrote on 9/20/2008, 8:18 AM
After a few rounds of communication with Sony to confirm the bug still exists in 8.0c, I finally got this response:

"Thank you for writing back.