AC3 Encoder is putting AGC on my sound track

MartinE wrote on 7/29/2015, 2:25 AM
Hi folks. Using VP13 and DVD Architect 6. When rendering out sound track using AC3 using the Pro encoder I've deselected the compression options but notice that some peaks above 0db are causing AGC action with a very long release time. I don't want any AGC at all. Will try getting the peaks down (clapping etc) and see if this works but it looks like there is AGC applied above 0db, is this what you would expect?


Geoff_Wood wrote on 7/29/2015, 6:00 AM
Are you expect output over 0dB ? AGC , or digital clipping - which could be considered an extreme form of AGC ?!!!

rraud wrote on 7/29/2015, 8:24 AM
To begin with, you cannot have audio above 0dBFS.
Beware of the AC3 'factory' presets, the audio level is set for 'theater' PB loudness standards so I recommend disabling the encoder's dynamic range compression for DVDs and such. These settings are in the 'Custom' settings in Dolby Digital AC-3 Pro.
BTW, these settings are not available in the Dolby AC-3 Studio version
ChristoC wrote on 7/29/2015, 3:52 PM

First, I re-iterate Digital Audio 1:01: you cannot go above 0dBfs, ever.

Musicvid posted this helpful little gem a long time ago:

Reply by: musicvid
Date: 9/10/2008 10:45:10 AM

" Lou et al,
I've run some exhaustive tests with calibration test tones and done extensive reading of the Dolby metadata specs, and in reference to your question about the most accurate reproduction with the AC-3 Pro Encoder, I am prepared to make the following recommendations with notes:

If you're doing AC-3 Stereo, you can ignore everything I say about the Phase Shift option, it's greyed out of course.

Dialog Norm = -31 (this sets it at unity gain)
RF and Line Mode DRC = OFF (WYHIWYG)
DC High Pass Filter = ON (this operates at <=3Hz and takes DC bias out of the mix)
Bandwidth High Pass Filter = ON (this takes aliasing noise at >20kHz out of the encode, but it also lowered the final volume by -0.1dB in my tests)
90 Deg Phase Shift = ON (makes it easier for Pro Logic decoding on older equipment, Dolby says it is inaudible in 5.1 except in "very rare" cases)

Everything else can be left just as it is in the 5.1 DVD template; the only difference I can determine compared to the Studio encoder is the 90 Deg Phase Shift, but turning it off replicates the downmix pan bug I reported in the Audio forum. Interesting, huh?"

musicvid10 wrote on 7/30/2015, 12:24 PM
If the peaks are clipped, they are clipped.
There is no AGC in the Dolby encoder, but I bet there was on your camera source.
MartinE wrote on 7/31/2015, 6:57 AM
ChristoC and others, many thanks for the replies. I realise that I worded my post rather badly. What I was hearing was a sudden drop in level on the finished DVD (which slowly recovered) when a transient loud sound (clapping I think it was) in VP went beyond 0db (and therefore would be clipped in the rendered file).

Anyhow I am reworking the track to peak at no greater than -3db and use the settings by musicvid and see what I get.

Again, many thanks for the help.