AC-3, how useful really?

[Hulk]

I've been doing some reading on AC-3 audio.

It seems that this is a better version of the MPEG layer II (better known as mp3) codec that also uses perceptual encoding to decrease bandwidth on PCM files.

Also, this format has the abilty to encode 5 streams and a subwoofer channel (5.1) with very good quality and relatively small storage requirements.

Now, for my questions.

1. I'm currently encoding my stereo DVD's using MPEG layer II at a bit rate of 224kpbs, or 192kpbs if I'm strapped for disc space. What is the bit rate for AC-3 stereo encoding? It doesn't seem like it's possible to really gain that much. I can imagine ANYTHING sounding that great at a bit rate of less than 128kbps. Even the new Windows media 9 codec starts to get a little grainy, swooshy cymbals, at bitrates below and around 128kbps. Deceasing the bandwith 64kpbs would "only" save about 60MB over a two hour period. Really not that significant when you're talking about 4500MB total on a disc.

2. The big thing using this format would be to be able to create 5.1 mixes. A very difficult thing to do, even if there was inexpensive software to support it since you would have to be mixing down all tracks to 5.1


Okay, guys, straighten me out, what is so great for US regarding AC-3?

Since MS Pro 7 does AC-3 I guess you could frameserve the video to TMPGEnc and then add the AC-3 track created in MS Pro later using the multiplex in TMPGENc.

How does one create a 5.1 mix using MS Pro 7?

I'm just trying to figure out how important this new tool will be.

-Mark

[Chief_Inspector]

Howdy,

1 - In the late 90's, dolby digital was limited to 448kbps. I don't know if this has changed.

2 - If anyone thinks that a piece of software can do what a 40k piece of hardware full of dsp devices does is fooling themselves. I haven't tried out the ac-3 thing in MSPro 7 but I doubt it is anywhere near the high end studio quality dsp based machines are.

If anything, it may sound like a cheap crossover separating different levels of frequencies into 5 different speakers.

If you are comparing an ac-3 track to a stereo mp3 track on stereo systems, you won't hear much of a difference due to the down mixing being done to the ac-3 track. If you do hear a drastic difference, then there's a problem with the downmixer itself.

What's so great about 5:1? If it's just a plain Jane music sound track, there's nothing great about it. If you have a sound track with choppers, sirens, sound effects, etc, then it's great provided your software has all the panning features as well.

CI

[Hulk]

Thanks for the comments.

I doubt an AC-3 stereo stream is 448kbps, that is a greater bandwidth requirement than 224kpbs MPEG layer II audio that most of us are using now for DVD's.

I'm pretty sure AC-3 is a method of encoding 5 discrete tracks AND a subwoofer channel in a low bandwidth stream that still sounds very good.

The problem is the mixing. You would need a mixer, digital or analog, that support 6 busses in order to create an effective mixdown. To be honest, I don't think this would be very useful for most consumers and even semi-pro's, it would assume you are scoring your own videos and have A LOT of technical mixing sound chops.

This would mean that you would be mixing down individual sound tracks for video's. Every voice-over, sound effects, each track of the music composition, etc... A BIG endeavor.

Also, dsp has come a looong way since the days of horrible computer eq, reverb, etc... There are lots of reverb plug-ins out there that rival a Lexicon 480 or PCM 42 delay (actually someone makes one licensed by Lexicon). Sure, I'd take a Pultic eq or Fairchild compressor over the digital counterpart any day, but this software has gone from the realm of toys to serious stuff rather quickly.

I owned a studio for a number of years and firmly believe that good production comes from, 1. good songs, 2. good arrangements 3. good musicians and instruments 4. good equipment (mics, mic pre's, processing, etc...)

Here's a good example, take a great recording and copy it to a cheesy cassette tape recorder. Play it back, sure it will be noisy, have limited dynamic range and high frequency content, but it WILL still have all of the "goodness" of the original recording. The technical part of music production is not about the equipment. But I diverge...

Perhaps 3 channel would be good, a dialog center and music in the left and right, but more than that would be quite a challenge.

That being said, I don't see what MS Pro 7 offering full AC-3 5.1 support is doing for us. How can MS Pro mixdown to 5.1, I don't understand this.

BTW, I agree with you on multitrack music. I heard some great stereo mixes ruined by 5.1 remixes. I guess I'm kind of old fashioned, but I like the sound of really good stereo mixes. Actually, there are a lot of really great old mono mixes, the Beatles come to mind.

[Chief_Inspector]

Eeek, I get the shivers whenever I hear someone say software plugins come close to the real thing

I still have a studio (mainly for music) and have access to every single high end vst plugins that are out there. There is no chance I would ever use any of those over my Lexicon. That's just me of course.

You are correct. Unless you have the proper equipment and knowledge to do an ac-3 track properly, don't do it.

"I doubt an AC-3 stereo stream is 448kbps, that is a greater bandwidth requirement than 224kpbs MPEG layer II audio that most of us are using now for DVD's."

True. but I said that 448 was the limit. As to what rate they really use to encode, I don't know.

"I'm pretty sure AC-3 is a method of encoding 5 discrete tracks AND a subwoofer channel in a low bandwidth stream that still sounds very good."

Yes it is, but I personally don't think it's worth it unless your tracks have alot of FX and you have the $$ for the equipment. As you said, a really good stereo mix down onto good quality stereo speakers with a sub is just as good.

"Perhaps 3 channel would be good, a dialog center and music in the left and right, but more than that would be quite a challenge."

I don't know if there are any consoles that will allow you to do this. Closest one I worked with back in the eighties was a Harrison console where it was equipped with Quad sound/panning. I guess the old fasion way of doing a good stereo mix and having your crossover in your sub properly set would do the trick.

I'm going to play around with this new feature and compare it to a stereo mix and see what the big deal is. I'll come back here once I'm done, hopefully by Saturday.

CI

[Flying dutchman]

A good overview of valid DVD audio stream formats can be found
here:

http://www.mpeg.org/MPEG/DVD/Book_B/Audio.html

Note that LPCM may be encoded with 48 or 96 KHz and up to 24 bits! I wonder if a DVD player beats a CD player at those specs...



The "greatness" of AC-3 is, for simple home users such as me, that it's simply the standard audio format for NTSC and PAL DVD's besides uncompressed LPCM. Uncompressed LPCM eats up approximately 1/3 of your DVD bandwidth so AC3 gives you more playing time for the buck. Which is not unimportant given the limited capacity of a DVD+R/-R. That's why an authoring software without a basic AC-3 stereo encoder is second-choice.

PAL DVD players also support mpeg-1 layer 2 (a.k.a. MP2) audio and mpeg-2 multichannel so people in PAL countries don't really need AC-3 for encoding simple stereo. Just tMpeg and a basic authoring software and you're ready to go.

Since the majority of DVD players (also the NTSC ones) support VCD and/or SVCD, which also uses MP2 audio, it's incomprehensible to me why some DVD players don't support MP2 audio in DVD tracks - after all they do have the decoder on-board.

[landrover]

I know why some software FX can rival with their expensive hardware counterparts: they can analyze the audio track and adapt their operation to that analysis. Where a hardware device always has to react to what it encounters. This is why software compressor limiters can meet very high standards at a fraction of the cost.

[wkulecz]

I've not yet run into an NTSC player that couldn't playback layer2 audio.

Some are inconvienent requing an audio setup menu option to enable analog outputs which may then turn off the digital "bitstream" output to your surround sound system.

--wally.

[Dr Mordrid]

The APEX's play layer2 audio just fine. So does my sons Philips and his PS2.

Dr. Mordrid

[Hulk]

So, the benefits for us using AC-3 if we are only going to be using stereo streams is increased compatibility with NTSC set top players (maybe) and the possibility of less bandwidth.

Is there an option in MS Pro 7 to select the bitrate of a stereo AC-3 stream? Or it is set to some standard value? Perhaps someone could encode a minute of audio using AC-3 stereo and tell us the size of the file.


Chief,

I know what you're talking about with the plug-in's. Believe me I used to own a Drawmer 1960 compressor, I loved it on vocals, but l have to say that using that compressor all the time "taught" me what a great compressor sounds like and helps me to get the digital verions closer, if you know what I mean.

I also had a Urei 1176LN that I used on bass and Kick drum all the time, again, it added something that I can't get digitally. I'll be the first to admit that.

I used to record vocals using a Hardy M1 mic pre, Drawmer 1960 compressor, and Neumann TLM 193 mic or 414. I'm sorry I had to sell that stuff.

Still, I'm amazed at how far digital recording has come over the last 10 years.

-Mark

[Topha]

Dont know if it helps, but the ac3 tracks from a DVD movie had 448kbps for the 5.1 track and 95kbps for a mono track. could it be that they use a lower bitrate for the rear channels?

[Job]

Hot a load of hullaballone!!.....

well - that's my first reaction - then I have to adopt some social skill and recall that I encoded 5.1 using Sonci's Sound Forge - encoded some home movies and burnt my own miniDVD onto CD 2 years ago!!!.......

All special effects laden movies - U571 and Pearl Harbour will use the full 448kbps for the 5 channels.....That's only an average of 90kbps each - but the front center, sub and the rears use much less than the front left and right...

To think a 224kbps MPEG1 Layer 2 audio track is as good as you need is fine...but its a user experience - if you have a $10,000 thearter setup and were a bit of an audiofile into stage imaging etc...(a throw back from master recordings and vinyl) then you would appreciate the difference between MPEG Layer2 and raw 44kHz CD audio tracks.
but for the general user - it seems properly encoded DAC 128kbps mp3's is probably more than enough. 64kbps mp3pro is probably the same....

There been a meteoric rise in the number of 192 and 320kbps mp3's available on servers lately as people think they can tell the difference - I put it down to poorley encoded files are invariably 128 since the people generating them are realatively inexperienced - so 192 is safer and oftne generated by the more experienced user.

I ripped a heap of sound effects from moives years ago in AC3 - you cant beat it. you inherit the full surround sound experience and if you can slip them into your home movie files - all the better.

Unfortunately the whole process is time consuming and there isnt any software that makes it sampler yet. Probably with the advent of 5.1 (or more up to date 6.1) microphones which are omni directional then the car you are recording going past will actually travel from rear to right front to left and back out the other rear will be more of an experience - but thatis probably 5 years away or more and then the software manufacturers will jump on the band wagon....

my 2c

[Hulk]

Not so fast!

"well - that's my first reaction - then I have to adopt some social skill and recall that I encoded 5.1 using Sonci's Sound Forge - encoded some home movies and burnt my own miniDVD onto CD 2 years ago!!!......."

Nothing to say here but you da man!


"All special effects laden movies - U571 and Pearl Harbour will use the full 448kbps for the 5 channels.....That's only an average of 90kbps each - but the front center, sub and the rears use much less than the front left and right..."

Obviously, at the very least, a sub channel would require much less bandwidth, as per Nyquest's theorem.


"To think a 224kbps MPEG1 Layer 2 audio track is as good as you need is fine...but its a user experience - if you have a $10,000 thearter setup and were a bit of an audiofile into stage imaging etc...(a throw back from master recordings and vinyl) then you would appreciate the difference between MPEG Layer2 and raw 44kHz CD audio tracks.
but for the general user - it seems properly encoded DAC 128kbps mp3's is probably more than enough. 64kbps mp3pro is probably the same...."

That's not what I said. I said that I would like to hear the DIFFERENCE between 224kpbs mp3 (on Lame encoder) and AC-3 at the same or lower bitrate.

My speakers are Mirage M5si, preamp is Adcom 565, Hafler 9505 amp, not the best stuff in the world, but I do consider myself an audiophile.

There is a world of difference between ANY high compression encoding scheme and PCM. I didn't make that comparision, I was comparing two different compression schemes.



"There been a meteoric rise in the number of 192 and 320kbps mp3's available on servers lately as people think they can tell the difference - I put it down to poorley encoded files are invariably 128 since the people generating them are realatively inexperienced - so 192 is safer and oftne generated by the more experienced user."

I use the Lame encoder at 192VBR with highest quality settings. Any less and I hear problems, even on my Midiland 4060 computer sound system.


-Mark

[SpiralDragon]

5.1 microphones????? am i missing something here?

[Hulk]

Huh? Not sure what you're asking.

I was talking about the fact that you need a 6 channel master out to really mix down to 5.1. Has nothing to do with microphones. I could record 90 tracks with one mic if I did it all with overdubs.

[Dr Mordrid]

Standard MSPro 7 AC3 encoding methods;

1/0 (dual mono)
2/0 (stereo)

Bitrates: several ranging from 32 to 640 kbps, so there is plenty of bandwidth available.

AC3 audio is the original audio encoding method for NTSC DVD decks, which means that a higher degree of compatability is available when using AC3 with older ones. MPEG audio was added to NTSC DVD decks later on. This means that some decks, mainly older ones, play MPEG audio better than others.

In PAL the reverse is the case. MPEG audio was the first supported standard in PAL-LAND with AC3 audio being supported at a later date.

Today virtually all commercial disks encode audio using AC3 of one profile or another.

Dr. Mordrid