Subject:AC3 (DD EX) Dropouts

Posted by: alk3997

Date:11/3/2004 12:30:21 PM

I did a search on "dropouts" and didn't find a reference to this possible issue...

I'm attempting to make a test file, so that I could make sure that my 6.1 channel mixes had the correct levels when encoded through the Vegas AC3 encoder. The 6.1 channel test includes a voice phrase through each channel then all channels, followed by 1kHz tone (80Hz on the LFE) through each channel and then all channels, and then white noise (fc=500Hz) through each channel and then all channels. The channels include rear center tests.

The peak for each of the segments is set at -3dB. I've verified that I'm hitting the -3dB peak on the audio mixer indicators. I've got normalization set to -28 dB. The EX flag is set to "On" for 6.1 channel sound.

Unfortunately, by the time I get to the white noise section, I'm getting constant dropouts of the digital audio to the AVR-5803 receiver. I can see the AC3 (DD) signal indicator LED flashing when the dropouts occur. Playing back the file again results in the dropouts in the same locations. So, I have to believe this is something in the encoding process that I'm doing wrong. When the AC3 light is lit, the display indicates a Dolby Digital EX signal. Obviously when it is not lit, there is no sound and no indication of an AC3 signal.

Any help is gratefully appreciated. I can't think of what setting would be causing this type of problem. One more item - I think I may see a single dropout during the earlier tests but by the white noise test the dropouts occur more than 50% of the time.

Andy

P.S. I accidentally placed this into the DVDA forum - I appologize if this turns out to be a dup.

Subject:RE: AC3 (DD EX) Dropouts

Reply by: BJ_M

Date:11/3/2004 5:27:23 PM

1. try with no compression setting and see if same problem
2. convert the ac3 file back to wav files and bring back into vegas and see if the drop outs are audio related (will see in vegas) or header related (wav will be fine in vegas) .. use besweet to convert ac3 to wav ..

Subject:RE: AC3 (DD EX) Dropouts

Reply by: BJ_M

Date:11/3/2004 5:28:31 PM

also -- if you have a section on a timeline with no audio -- vegas will sometimes choke up on such areas , its best to add something in a track

Subject:RE: AC3 (DD EX) Dropouts

Reply by: alk3997

Date:11/3/2004 6:14:48 PM

I think you may have hit it a possibity. There are areas with no sound for up to 10 seconds. I'll try to eliminate those tonight. Thanks!

Subject:RE: AC3 (DD EX) Dropouts

Reply by: alk3997

Date:11/4/2004 12:24:58 PM

That was the bug. I encoded a simple chord in the front center channel that lasted the whole 5 minutes of the test tone file. Sure enough, the AC3 was encoded properly.

My next problem is that it isn't very useful to have test tones with a chord in the front center the whole time.

I haven't had time to test for the bug conditions yet. Is there any documentation on this bug? How much audio is required for the bug to go away? In other words if I put a 20kHz signal in one channel that is down lower than -85dB, would that prevent this bug?

Thanks again for pointing me in the right direction!

Andy

Subject:RE: AC3 (DD EX) Dropouts

Reply by: ScottW

Date:11/4/2004 2:36:05 PM

You might try just dropping an empty event on the track and then resizing it as needed. Insert/Empty Event.

Subject:RE: AC3 (DD EX) Dropouts

Reply by: alk3997

Date:11/5/2004 8:46:39 AM

ScottW, I tried that last night. Unfortunately, this actually made things worse. The AC3 light on the receiver during the empty track sections flashed in a regular pattern. When the actual audio started, it was broken. I then tried an AC3 track without silence and it played fine.

I'm trying out some other ideas on how to bypass the error.

Andy

Subject:RE: AC3 (DD EX) Dropouts

Reply by: BJ_M

Date:11/5/2004 6:01:08 PM

an empty event doest work -- i use a file i made of white noise at -75dB and just stretch it as needed or duplicate it ..

this bug doesnt ussually alway show up in short ac3 encoding , ussually only longer encodes .. i have not alway been able to duplicate it -

i also see sometimes the encoder get stuck on events with almost square wavs or no noise tracks ... (square wavs are from some encoding i do to ac3 of smpte time code)

Subject:RE: AC3 (DD EX) Dropouts

Reply by: alk3997

Date:11/9/2004 11:08:15 AM

BJ, I did the same thing - a -75dB random noise signal coming out of the center channel and that works perfectly. For the purposes of a -3dB tone, the -75dB noise can't be heard. So, it is a good work around.

I guess the next step is to see what Sony plans to fix this bug...

Andy