Why use AC3 2-channel instead of PCM on a DVD?

[alenhard]

Is it just me or does Dolby Digital 2 channel surround leave a lot to be desired? (Especially after being spoiled by 5.1 sound for so long!)

Why in the world would anyone want to encode to AC3 when a regular PCM/wav sounds so much better? For example, 2 channel AC3 doesn't deliver to your subwoofer but PCM does.

So why bother with BeSweet, AC3Encoder, etc?

I just don't see the use...

[adam]

Because AC3 is a heck of a lot smaller. If you have room for PCM than go for it, but I'd much rather use all that space for a higher video bitrate.

[thayne]

I can't hear the differnce. I use AC3 or mpeg-1 layer 2 sound to save disk space for a higher video datarate

[alenhard]

So Thayne, you are implying that 2 channel Dolby gives the same acoustic range (especially lows for bass)?

Not true- it won't! (at least not without being downmixed to PCM anyways). I think there is a huge difference!

[Cornucopia]

alenhard,

It's just you.

The only quality difference between 2.0 AC3 and 5.1 AC3 is the # of channels (assuming ~ same bitrate per channel). Neither I nor my clients have ever had trouble with sound spectrum/quality, given a reasonably high enough bitrate.

Maybe it's the encoder you're using. If it's encoded right, it's possible to have BETTER bass management with AC3 than with PCM.

Scott

[thayne]

That's what I'm saying. I can hear no difference. Maybe my ears aren't as decirning as yours, but I can't tell the difference.

[dcsos]

GET THE ZEPPELIN DVD
its the first commercial one I could really hear the better quality of the PCM

[roach76]

I thought in order of best to worst quality was....

1.PCM Audio - wav
2.DTS
3.Dolby Digital - ac3
4.MPEG1 Layer 2 - mpa

I thought this was the order because, PCM is completely uncompressed, being the original unaltered. Any better bass mangement is just an enhancement that is a created effect. Just like when you have a stereo that enhances stereo, or tries to create the illusion of surround sound if not encoded as 5.1 or Pro-Logic. Never sounds as good as 5.1/6.1..... because the channels are separated, and the Player or encoder doesn't have to try and "fake" it.

[alenhard]

Cornucopia-

Grab a comercially authored DVD with both a 2 channel AC3 soundtrack and a 5.1 soundtrack. Forward to an action scene in 5.1 sound. Lots of bass- explosions, etc. Now switch the audio in the setup menu of your DVD and rewatch the same scene. What happens? The audio cuts out of the rear speakers and subwoofer. You can see on your receiver that the audio input changes from AC3 2.3 to AC3 2.0.

Again, unless you have your receiver set to upmix to 5.1 from 2 channel or use some other simulated surround from the 2 channel, you will not get the same output from your speakers (and I refer again to the lack of subwoofer output because a movie with no bass is the worst thing in the world!)

It has nothing to do with the AC3 encoder I use. 8)

[lordsmurf]

If WAV was best, I think it would be in heavier use. It's just older and uncompressed. Uncompressed doesn't always mean better. It just means it's not compressed. Compression doesn't have to make things bad, especially when talking about things like sound, as the compression mostly removes things that we cannot hear anyway.

Maybe my dog notices a difference, but she hasn't said anything to me about it.

I use AC3 because it's smaller. I see no difference between WAV, MP2, MP3, and AC3, given proper bitrates and comparing the same sound file that comes from the same source.

My hearing isn't perfect, too many concerts and working in loud enviroments, but I'm not deaf or anything. Sounds the same for me. AC3 let's me put more into the video.

As far as more/less bass, well, not all of us can afford surround sound just yet. I do well with a mono tv and a stereo tv. Though my computer has surround sound, and that's where I do all my audio work.

[flaninacupboard]

alenhard,
if a commercial disc has a 2 track DD file, it is usually encoded in Dolby Surround. Played back on a surround amp, the correct mode to use is Dolby Pro Logic (or PL II if you have it)
This will give you left, right, centre and rear sound. PLII recievers also try and do some seperation between left rear and right rear. success varies.

Bass management in this mode is 100% down to your reciever setup. if you setup your reciever wrong, it will sound bad. i suggest you go through it's setup and adjust the sub size and satellite size to the correct values.

If it still doesn't give you any joy then either the 2 channel file is -very- heavily compressed (if it's 192kbps or lower, it won't sound too hot) or the audio is encoded badly.

Listening to my Laserdisc copy of Close encounters of the third kind in ProLogicII mode i can tell you the sub -definitely- works on my system.

[Cornucopia]

alenhard,

I've made some of those "commercial" discs you talk about, so I know what I'm talking about.

flaninacupboard is giving you the facts, correct bass management is mainly determined by your receiver setup. It does also have to do with choices in downmixing and Dolby Surround encoding--whether the LFE are incorporated in the main program (shouldn't usually be), what compression characteristic is used, whether HP and LP filtering is incorporated during preprocessing, whether the DS flag is specifically indicated, etc.
But also you can't compare apples to oranges, bitrates should be comparable. If you've got a 5.1 track at 512kbps and a 2.0 track at 192 kpbs, guess which one will sound better?

Do me a favor and try this out...
play a 5.1 track through your system, but physically disconnect the Surrounds and Subwoofer (and maybe the Center as well), use a sound level meter to see what the output is
compare that with the 2.0 track using the same remaining speakers, match the level to the reading on the sound level meter.
My guess is you'll find the tracks almost identical.
The difference is in the "percieved" improvement of sound when played at louder levels (i.e. more speakers) and in the choices made for bass management.
Read up on some of the suggested setups for bass management on the Dolby website for some good tips.

BTW, I was never saying that 2.0 AC3 (compressed sound) sounded better than PCM (uncompressed sound), but then I don't think that's what you were really referring to.

Scott

[digitalmaster]

While I was reading this something came to mind. Does somebody have a link that explains the difference between DTS and AC3? I will do a search in the mean time.

[roach76]

I heard the main difference between DTS and AC3, and I'm sure there's more, is DTS has a HUGE bitrate, something up around 1500. Anyone else? This is interesting.

lordsmurf said

Maybe my dog notices a difference, but she hasn't said anything to me about it.

I think your dog is just an ignorant bitch!

[thayne]

That's not funny...

[CHAOS]

Okay
So the deal is, if i wanted to convert a pal dvd to ntsc and wanted the best bitrate for the video you should convert the audio to ac3 to make more room for the video ?

[flaninacupboard]

roach,
The bitrate used for DTS on CD's and for movies on laserdisc was 1.44mbps, the same as uncompressed 16 bit stereo audio, as found on a CD. Bitrates on DVD are rarely this high, you'll usually find a 768kbps DTS soundtrack, or sometimes even lower.

The main difference between the two compression schemes is the acoustic model they use, AC3 discards a lot more "inaudible" data than DTS will. so if you had a DTS and an AC3 at say 256kbps for 5 channels, the DTS would sound awful, and the AC3 would sound "ok". if you had a DTS file at 1.44mbps and an AC3 file at 1.44 mbps, the DTS file would sound great, the AC3 would sound good, but not noticeably better than a 768kbps AC3.

Music DVD's are a good way to check the difference between the two. The super Furry animals "rings around the world" or nine inch nails "and all that could have been" are good examples.

[EddyH]

either those concerts damaged you more than you think, or you have a very good encoder, if you cant tell between WAV and MP2..

[Gregg]

Hi,

Just a note:
If you compare AC3 5.1 and AC3 2.0 on a home theater setup /5+1/of course AC3 2.0 will sound worse. But try to compare both using a regular /HiFi/ stereo amplifier. AC3 2.0 sounds much better than on the 5.1 setup. Try to play a CD on your DVD player /5.1 setup/ - it sounds like crap.
For the sake of experiment I put 3 audio tracks on my last DivX2DVD project and here's the result:
MPEG2 layer 1 - worst
AC3 2.0 better but low volume /used Besweet to convert/
LPCM best

Enjoy!

[roach76]

I agree with Cornucopia, AC3 2.0 won't sound worse than AC3 5.1 unless the bitrate is lower per channel. If each channel has the same bitrate, the only difference then will be less speakers on AC3 2.0, the opinion of "BETTER" quality comes from the 3D surround sound aspect. In all cases:

1. If bitrate per channel is equal. Then the quality is equal.
2. If 1 track has more channels. Then perception of 3D increases, not quality.

Gregg, when playing AC3 2.0 and AC3 5.1 on a "STEREO" HiFi system. The quality "MIGHT" be worse if the HiFi system doesn't do proper downmixing. But I see your point, AC3 2.0 doesn't need downmixing and will sound as good if not slightly better than AC3 5.1 on ALL stereo HiFi systems.

[flaninacupboard]

Well bass really suffers when downmixed from 5.1
If there is (for instance) a rumbly train coming from the left, honking it's horn, you will hear the horn on the eft (as it should be) but the rumbly bass from both sides because the sub channel is just split between left and right. you also get very quiet sound with a 5.1 downmix, because each channel must be lower to avoid distortion when all channels are active.

I don;t often use the surround system, i much prefer good old stereo

[Cornucopia]

flaninacupboard,
you know why that doesn't sound the same don't you?--because according to Dolby, the "proper" way to downmix is to throw out the .1 channel!

Quoted from Dolby's 5.1 mixing pdf--

Since the LFE signal is discarded in the Dolby Digital downmix process, these bass signals will not present any difficulty.

Yet, it still matters how you set your bass management (both encoding to 5.1, downmixing to 2.0, and decoding). According to Dolby, the reason behind the .1 is to add sub-bass in sufficient quantities, without taxing the main system, even though the main system provides for complete full-range signals to all 5 channels. And they're pretty definitive about using .1 specifically for LFE--Low Frequency FX.

I think most of this thread came about because of a misconception of what the LFE channel is supposed to do, and a confusion with the LFE's .1 signal with the final bass-managed/redirected subwoofer signal.

Scott