AC3 + Best Kbps for music

craftech wrote on 5/24/2006, 8:53 AM
I never actually like AC3 audio compared to the PCM audio they come from when I render DVD theatre productions. They just take up less room. I don't usually crank up the Kbps when I render the DVDA NTSC Stereo audio template.

I was thinking of trying it so I was wondering:

What is the limit in terms of potential compatibility issues? I know DSE was taken back last time I posted the low number of Kbps I generally use for the render. He said I was "throwing away" musical data, but didn't make a recommendation. I don't want to create compatibility issues so what do you all recommend I render the Kbps to?

Thanks,

John

Comments

John_Cline wrote on 5/24/2006, 11:53 AM
Uncompressed 48k stereo audio is 1536 kbits/sec. I normally use 192k for voice and other "non-critical" stuff. I use 256k, and occasionally 384k, for music and audio which has come from "pristine" sources. I have never had any compatibily issues and the higher bitrate sounds great.

Just remember to get into the custom settings of the .AC3 encoder, go to the "Preprocessing" tab and set both of the profiles under "Dynamic Range Compression" to "None."

John
craftech wrote on 5/24/2006, 1:43 PM
Uncompressed 48k stereo audio is 1536 kbits/sec. I normally use 192k for voice and other "non-critical" stuff. I use 256k, and occasionally 384k, for music and audio which has come from "pristine" sources. I have never had any compatibily issues and the higher bitrate sounds great.
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Thanks John,

That's what I usually use (192 and 256). I have also used 384K, but it still doesn't sound as good to me as the PCM audio. I was wondering about higher than that. Have you ever tried it?

John
John_Cline wrote on 5/24/2006, 3:30 PM
Dolby Digital AC3 is a 24bit system and I have encoded a stereo project that was recorded and mixed to 48k 24bit trying both 512k and 640k. If there was any audible difference between the AC3 audio and the 24bit PCM, no one around here, including a few "Golden Ear" types, could reliably discern the difference using either bitrate. The resulting DVD was played on a variety of systems with no problems. Your mileage may vary...

John
riredale wrote on 5/24/2006, 4:27 PM
Why not do a simple test?

A double-blind test would be the most accurate, but something far easier would be to just put the raw wav file on the timeline, and the ac3 version directly underneath. Then invert the phase of one of them. What you hear is the extent of the difference between them.

I suspect that any hash or junk that you hear will be way below the level of either by itself, which to me implies that it will be effectively masked. I would also assume that a higher bitrate for the ac3 will result in an even quieter difference signal.

What I don't know is whether both signals will be exactly 180 degrees out of phase. It could be that the ac3 waveform could be slightly delayed, rendering this test useless unless one goes in and carefully ensures that peaks are exactly out of sync--then there won't be any flanging effect.
craftech wrote on 5/24/2006, 7:44 PM
A double-blind test would be the most accurate
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I can pick out the PCM file pretty easily. We are talking AC3 stereo here, not surround. AC3 works very well for surround and despite endless hours spent trying to make a surround music soundtrack I end up hating the effect and in fact I do not choose 5.1 surround when I play a commercial music DVD. It just sounds too unnatural to my ears.

John
riredale wrote on 5/24/2006, 10:02 PM
I know what you mean. My "specialty" is 4-channel surround, shot real-time with a dual Sony stereo mic setup on top of my camera. The first time I heard it, I didn't necessarily like it, but after hearing it for a while, it now sounds very strange to just hear the front speakers only. Kind of like the effect of going from stereo to mono.

DJPadre wrote on 5/24/2006, 11:02 PM
Dolby Standards are -
384 and 448 for 5.1 (inc EX) mixes
192 for Stereo or Stereo surround
96 for mono (not that DVDA supports this..

Stick to what works and you wont have a problem
johnmeyer wrote on 5/25/2006, 7:57 AM
According to MPEG.ORG, the maximum legal data rate for AC-3 is 448 kbps:

Max Audio AC-3 Bitrate
craftech wrote on 5/25/2006, 7:36 PM
Thanks for the input everyone.

"According to MPEG.ORG, the maximum legal data rate for AC-3 is 448 kbps:"

I noticed that Vegas issues a warning message when you go that high.

John

320 seems to sound better so I'll use that this time around.
johnmeyer wrote on 5/25/2006, 8:55 PM
I noticed that Vegas issues a warning message when you go that high.

Yeah, until I read the MPEG.ORG page, I'd never seen anything higher than 384.