Little question about AC-3 Dynamic Range Compression in Sonic Foundry

[FlimsyFeet]

Firstly thanks to SomeJoe for the excellent guide at the top of this forum.

I have one little question: If I sample a Dolby Pro-logic mix off a VHS tape or laserdisc, will that already have been dynamic range compressed? If so, should I therefore select "none" for the DRC option if encoding to AC-3 with Sonic Foundry?

[SomeJoe]

It's possible it may have already been dynamic range compressed, but there's no standard for Pro Logic Encoding that I know of that would dictate whether it was or not.

If you suspect that the material coming off the VHS/LaserDisc is already DRC, then you probably want to set DRC to "None" in your AC3 encoder. Make sure your DialNorm is set properly, of course.

You may have to try it both ways. After AC3 encoding with DRC set to "None", listen to it on your DVD player. If you have trouble hearing soft sounds or the loud sounds are blowing you out, reencode with DRC set to "Film Light" or "Film Std".

Another way to do it would be to look at the captured waveform in Sound Forge/Audition/Cool Edit/etc, and measure the RMS level of typical dialog or other soft sounds vs. the RMS level of explosions/loud music/etc. If the difference exceeds 35-40 dB, you may want to add DRC during AC3 encoding.

A third possibility is that the source might be noisy. For instance, linear audio tracks from VHS tape have a lot of tape hiss, similar to cassette. The audio may be already DRC to compensate for this. After you capture it, you might have only 20dB of difference in soft vs. loud sounds, so if you encoded that to AC3 directly, DRC would not be appropriate. However, if you first run some type of noise reduction on the audio and manage to increase the dynamic range to 35-40 dB or so, now you might want to use DRC. Completely depends on how much audio processing you want to do.

[FlimsyFeet]

[For the pedantic, when I said "Dolby Pro-logic" the correct term I should have used was "Dolby Surround". Pro-logic refers to hardware decoders not the actual soundtrack.]

Thanks for the detailed reply SomeJoe. I think I'll do it with no DRC to begin with, because logically that's how it would sound if I connected by VCR directly to my 5.1 system set to Dolby Pro-logic mode, and you would think that a retail VHS would be mastered for home listening.

I've got an option on both the DVD player and the surround decoder to apply DRC on-the-fly, if need be.

BTW modern VCRs will only play the linear track on a VHS tape as mono only, Dolby Surround being 2-channel must alway be stored as a hi-fi track which is much better quality.