Does AC3 cause "Audio" Compression?

[Jack Robertson]

Hello,

I know AC3 is a form of compression, but I have just finished working on a project which was edited in DV format and the 2 channels of audio was mixed in 48kHz 16bit. The audio had no compression applied to it in the mastering stage and sounded fine when going to a DVD authoring house.

We had 50 DVDs duplicated and since then I had a listen to the project on DVD and the sound is slightly "pumping" as if audio compression was applied during mixing. At this stage, this confirms on a couple of DVD players and home theatre set-ups, but for some strange reason it is less evident (and even undetectable) on other DVD players/home theatre set-ups... weird but read on!

So my main questions is whether AC3 compression could be the cause of this "compression" problem.

You see the DVD authoring house are saying that they applied no pre-processing to the audio before it was encoded, but there is slight pumping, especially in scenes that have lower audio.

I originally thought it might have been these "night mode" settings on either DVD players or Amplifiers, but I have all of it turned off for full dynamic range, so it doesn't seem to be those.

I know I handed the DVD authoring house a master DV tape that sounded excellent, so it must either be AC3 encoding or the way they digitise (or capture) the project into their PC. I know for a fact that they transferred the MiniDV tape to DigiBeta and then from DigiBeta into their PC. I have suspicions that this "DigiBeta" step could have been where the "audio" compression occurred....

any thoughts???

Regards,
Jack

[Rob Lohman]

I can't comment on the beta step, but yes, a Dolby Digital
encoder *can* introduce such "issues". It depends ofcourse
on the encoder used and the settings. AC3 encoding is an
"art" as well (as is good MPEG2 encoding). Since the house
has done this for you it will be very difficult to lay the finger
on where the problem originated. Perphaps they can work
with you?

Normally a house will give you a trial run first before pressing
the final discs. Then you can "fix" such problems.

Try to get a listen session to the Beta tape. If that hasn't
got the problem then it must be in the capture or AC3 encoding
stage.

[Rob Lohman]

Don't forget that a lot of cheaper audio equipment do things
like auto levelling and other equalizing stuff. Only the higher
end equipment is usually trustable on the quality of your tracks.

[Jake Russell]

Yeah A.Pack(mac) can cause this problem if you don't know what settings to use but that said the issue is usually only heard when playing in Apple DVD Player....

Jake

[Jack Robertson]

First of all, I thank you all for the replies especially you Rob.

I have called the DVD Authoring house and they apparently didn't use the DigiBeta step and so the project was captured straight from the master DV tape into their PC for encoding, and the audio was digitally transferred via AES/EBU.

They say they have done no pre-processing again. They are however saying that it could in fact be the AC3 encoding which might have caused this "audio compression" and obviously they cannot do anything about that as they didn't write the algorithm.

At this stage I agree with them, but I'm still not 100% convinced about it being solely AC3.

I did ask whether we could have had the project done as PCM and he said yes, but the resulting audio would have been up to 10 times larger in comparison to AC3.

So, although this is not totally solved, it is something to think about and as Rob says, get an approval DVD disc before mass duplication, we didn't have that luxury as we had not time :(

I will try and find out from Dolby them selves to get their view about this matter and AC3 encoding.

Cheers for now,
Jack

[Rob Lohman]

If possible try some other AC3 encoders yourself. If you don't
have any send me an e-mail, I can do a testencode with two
packages for you on a piece you have problems with.

[Jack Robertson]

(Sorry Loooong post)

I've been researching this DVD Audio problem and to tell you the truth audio on a DVD is a very technical thing especially if AC3 encoding/decoding is concerned!

I have extracted the AC3 file (digitally) from from the actual project DVD from my DVD-ROM drive in my PC on to my Hard drive (using an app called DVD Decrypter available from: http://www.doom9.org/) and then played the extracted AC3 via Power DVD player (v4) and to my surprise the audio compression was still there...

So now I think to myself that it must be encoded that way, but it still baffles me that I do not hear this compression on certain systems... keep reading, the fun starts...

I called the DVD Authoring house (again) and spoke to the DVD guy and I basically told him that there is nothing we can obviously do now, but perhaps there *is* a problem that may need to be looked at since we've come across it... and he in the end said (and which is true) that AC3 is never going to be *exactly* the same and the anomalies we hear are most likely a result from the AC3 coding, which he also has no control over.

So my search continued for answers on the net...

I wanted to convert the extracted AC3 file back into 2 channel wav to see how it looked and sounded in my original mixing program and I came across a plugin for winamp (called WinampAC3 available from http://sourceforge.net/projects/winampac3/) which seemed interesting. I downloaded it and installed it in my winamp (has to be v2.xx) and played the extracted AC3 file in it and guess what... it still had compression! BUT I opened the AC3 plugin's options window and there was a check box for DRC (Dynamic Range Control) which was checked. So I unchecked it and BINGO the extracted AC3 audio file played back normally as it should so this confirms the fact that the audio was encoded *without* compression.

Now there is still the "why" it plays compressed on some systems and not on others...

I went to www.dolby.com and found out more info about this Dynamic Range Control... here is 3 paragraphs extracted from an information PDF (pages 14-15) on AC3 coding, located at: http://www.dolby.com/tech/ac3-flex.pdf

*********************************
The AC-3 coder contains an integral dynamic range control system. During encoding, or at any point thereafter, dynamic range control words may be placed into the AC-3 bit stream. These control words are used by the decoder to alter the level of the decoded audio on a block basis (every 5.3 msec). The control word may indicate that the decoder gain be raised or lowered. The control words are generated by a level compression algorithm which may be resident in the AC-3 encoder, or in a subsequent
bit-stream processor.

The default for the decoder is to use the control words which will result in the reproduction of the audio programme with a compressed dynamic range. The exact nature of the compression is determined by the algorithm which generates the control words, but in general the compression is such that headroom is reduced (loud sounds are brought down towards dialogue level) and quiet sounds are made more audible (brought up towards dialogue level).

...the listener may instruct the decoder to partially or fully remove the dynamic range compression which has been applied to either the loud (above dialogue level) or soft (below dialogue level) sounds. (Actually the audio is coded without any level alteration, and level alterations occur at the decoder. However, this is irrelevant to the listener, since the default for the listener is to reproduce the programme with the compression characteristic which has been intended by the programme originator. The listener must take some action to remove the compression.)
*********************************

So as you see the actual encoded audio has *no compression* applied to it, but the encoded AC3 stream can have these things called "control words" that tell your decoder of how to listen/compress the audio. And as from the above extracted paragraphs, most decoder's default is to using these "control words", BUT the listener is (or should be able) to alter or disable any DRC (Dynamic Range Compression)!!!

I have spent hours trying to find all this and hopefully you see that the compression in fact is done by the decoder. However for decoders that do not allow you to control the DRC then the default is used, ie if the AC3 signal has been encoded with these "control words", then your decoder will compress the signal... :(

So at this stage I have two things to find out:

1, Whether the DVD Authoring house in fact used these *control words* in the AC3 stream.

2, Whether all decoders have the option of disabling DRC (compression), which could actually be applied inside a DVD player and or in a Amplifier decoder….

Hope you can make sense from all this info, as I'm still trying to digest it myself! :)

Cheers,
Jack

[Peter Moore]

Jack,
I was just about to reply with a lenghty explanation until you found out the exact right answer on your own - it is DRC.

DRC is the devil, because oftentimes you don't know it's being applied! You mix your audio perfectly and then use the default 2.0 or 5.1 encoding settings, and suddenly on some (not all!) DVD players or AC-3 decoders, you get pumping of softs and softening of louds.

It drove me NUTS trying to author my own DVD. I couldn't understand why the mixes were always perfect, until they were encoded (I was using Vegas) to 5.1, and then they were off. Then I'd spend so much time trying to fix them, and then they'd still be off! I WANTED louds and softs at different times.

Now I will give you my opinion on your situation. This was the authoring house's fault for using AC3 encoding with DRC. They should replace your disks for free, IMO. They should have asked you whether you wanted it, and you could have said, "No my mix is good as is." I now know to disable DRC in the AC3 encoding - everything sounds better that way! But it's not just that - DRC is objectively wrong if you've already mixed your sound the way you want it.

Oh and by the way, sometimes it's not something that can totally be disabled once it's done, because certain encoders do it in a way that actually affects the way the audio is encoded. Vegas is one of them. Some players make it much less pronouced though - how, I have no idea.

[Rob Lohman]

Good to know. Kinda was what I expected. So can we turn this
off in Vegas, Peter?

[Peter Moore]

Yes absolutely. When you go to your AC-3 encoding settings, there will be an option for Dynamic Range Control. Some of the options will be "Music", "Film", "Dialogue," etc. Just select "None." I would also suggest everyone make their own templates with it turned off, so you don't ever forget.

You'll also find that your tracks are much louder that way. Ever wonder why some DVDs' audio is so soft when played on a TV? That's why.

Again, this is assuming you're doing a good mix yourself, which will always require some sound compression on the individual tracks. Range compression is absolutely essential in modern recording. Otherwise just casual dialogue will have clipping even when it sounds perfectly normal volume. So you do need it, and you'll find all your sound is much more professional sounding, but you then never want to apply it again to the entire mix with across-the-board settings. I honestly don't know why ANYONE would ever want this, let alone why it's set on by default.