Issues with 5.1 Audio

Stuart Robinson wrote on 10/5/2008, 5:54 PM

I'm working with some AVC files (profile main@L4.0) that have 5.1 Dolby Digital audio, but am having problems getting Vegas to treat the audio correctly.

Dropped onto a 5.1 timeline in 8.0c, Vegas gives me four audio tracks, front L/R, centre, rear L/R and LFE. All the panning schemes are correctly set and the audio emanates from the correct channel... except... the LFE is just a flat line. The track is there but there's nothing on the waveform and in isolation it's just silent.

The source is fine and if I extract the tracks to .WAV in another application, the LFE does have content.

Vegas 8.1 behaves slightly differently. Same project settings, same source file - but when dragged onto the timeline it creates six individual audio tracks, none of which are automatically assigned and they're panned so that the same content appears equally in all channels (I have to go through each and manually pan it). Here again, the LFE is present but nothing but a flat line.

I'm wondering if some weird setting is preventing Vegas correctly reading the LFE or whether this is a bug, given that 8.1 behaves quite differently?

Comments

blink3times wrote on 10/5/2008, 7:00 PM

What is the alternate program you are using for testing? Are you sure it is actually seeing a LFE track or is it simply being extrapolated from the center track? (The center track and the LFE track CAN be looked at as the same track with some programs)

There are also some cams that just plain don't produce a LFE track.

Is it possible for you to post a sample someplace for download? Or.... if you use TSmuxer (free download) you can demux the AC3 from the video of a small clip and email it to me. I can run it through Adobe Audition and see what it shows. ( you have to keep it below 9Mb.... cheap mail server!) ganthoni@mts.net

riredale wrote on 10/6/2008, 9:09 AM

I am mystified by the use of the LFE track at all, by anyone.

Originally it was meant as a mechanism for theaters to inject additional over-the-top sound effects, such as the foot-stomping of the Trex in Jurassic Park. Remember that the other channels are meant to be full-range. LFE is not a woofer channel. It's called a ".1" channel for a reason.

If the above statement is true (and I believe it is), then why does anyone use it at all?

blink3times wrote on 10/6/2008, 9:35 AM

Your statement IS true. Additionally (and believe it or not) the LFE channel is not even actually part of the official Dolby spec.

It's not REALLY needed any way since the lows from all channels in your home 5.1 receiver are sent to the sub woofer quite naturally to start with. It is nice to have however if you want to emphasize an explosion (for example) and make your living room vibrate a lot..... but then how often do you run into this at our level of production?!

None the less.... you can't help but feel a little ripped off if the LFE track isn't there..... if for no other reason than to look impressive on the time line :)

Stuart Robinson wrote on 10/6/2008, 12:39 PM

Blink - thank you for the reply.

>What is the alternate program you are using for testing? Are you sure it is actually seeing a LFE track or is it simply being extrapolated from the center track?<

TMPGenc XPress 4 allows me to extract (or convert to WAV) any of the embedded channels, either to individual files or two-channel combinations. There's definitely a separate LFE. I've been testing with film material, so it's easy to tell the centre - with hard dialogue - and the LFE apart.

>There are also some cams that just plain don't produce a LFE track.<

These particular files are from a Hauppauge. I work with a lot of TV people and this is one of my capture devices. Essentially it can capture an incoming AC-3 stream and just embed that into the TS. The reason I have tested with film sources is because it's a good way of knowing that the source isn't problematic and does have six active channels.

I've uploaded a 30(ish) second clip to:

http://rapidshare.com/files/151510256/Sunshine-Opening.TS.html

...and the demuxed AC-3 audio is at:

http://rapidshare.com/files/151527616/Sunshine-Opening.ac3.html

That should have some LFE content.

Incidentally, there are some legitimate uses for the LFE, added headroom, time alignment etc., it just gets abused and is thought of as the "bass" channel (which it isn't) because that's where playback filtered bass usually ends up. Producers are just as guilty of these misunderstandings as consumers.

blink3times wrote on 10/6/2008, 5:43 PM

Yup... this is clearly a bug.

When I open the audio in Adobe Audition the LFE track is there, has frequency and good amplitude.

But when I open the TS in BOTH 8c and 8.1 the LFE is flat lined. In addition as you mentioned, the panning is not correct when opening in 8.1. All the pan knobs are at center position.

You should file a bug report. Point them to this thread and tell them it has been reproduced.

Stuart Robinson wrote on 10/6/2008, 6:42 PM

Thanks for taking the time to test and reproduce this, I'll file a bug report as you suggest.