Convert 5.1 to 2-stereo, without loosing quality or wasting Mo?

[seewen]

If I convert a 5.1@448 Ac3, for example, into Stereo Ac3.

What is the minimum bitrate I need to "don't loose anything" ?

Is it 448kbps ?
or is it 448/5 = 89.6 -> *2 = 179.2, rounded = 192kbps
or is it something different, beccause there's more than 2 channels informations in it ? ( something like : "(original/2)*2.5", etc.. )


And is there a difference between a MP3-CBR@192 and Ac3@192 ( stereo), or 256-256, etc..

EDIT after reading posts about Ac3-re-encoding, it seems that it's better to get MP3 from Ac3 5.1 than Ac3-Stereo.
But read nothing about the minimum bitrate..

A+

[Sadamitsu]

Encoding to AC3 will always result in a loss of data, regardless of the source, because AC3 is a lossy compression format.

[AsukaFan]

It's really a matter of taste... IMHO, using AC3 2.0 (converted through a quality program, perhaps BeSweet), 192 kbps is more than enough, even for a hardcore audiophile. If you ask me, even 160 kbps would be more than enough, in fact.

(BTW: a 5.1 AC3 file at 448 kbps has -in real life practice- almost the same quality as the original WAV file, so your re-encoding would be almost a first-time encoding; that's why many people uses 384 kbps instead for 5.1 tracks.)

...What's more: 192 kbps is the bitrate used for the cinema Studios in almost every AC3 2.0 audio track embedded into a commercial DVD. & that's also the bitrate recommended at some Q&A.

And AC3 has some advantages over MP2 & MP3: it's more "universal", if you're going to use that audio track for a DVD (for example, some US players can't read MP2 audio, not to say DVD's without any AC3 tracks, just MP2). But, if you're going to use the audio track for a PC, MP3 is far more universal...

[DJ Bobo]

160kbps AC3 will cut frequencies over 16KHz -> Not Hi-Fi
192kbps AC3 provides full frequency range, BUT the sound is not alive enough, if you understand what I mean (a little bit muffled). Starting from 224kbps, it begins to sound similiar to the original source.

For AVI delivery, better use MP3. MP3 has better quality than AC3 by the same bitrate.

[AsukaFan]

OK, DJ Bobo, you ARE a hardcore audiophile, to say the least... though, in theory, you're right about the 160 kbps bitrate: it really isn't the ideal bitrate if the audience are bats, dolphins & dogs.

But if you can distinguish between 192 kbps & 224 kbps without special sound machinery... perhaps you ARE a bat!! Of course, I'm just joking, PLEASE don't get annoyed; but the FACT is that, for most humans, 192 kbps is more than enough for a film's AC3 2.0 sound track; that's the opinion of cinema Studios, and that's also mine (in fact, I'm Tony Motola!! Hail me!!).

Anyway, you're quite right about MP3: it's far better for AVI conversions, specially because it can be used in almost any PC without having to install any new codec.

...BTW, DJ Bobo, greetings to Kauro, Yahiko & Kenshin. And pay the meal at least once!!

[seewen]

Ok, thank you very much for these explainations. As I can see, their's no "magic-conversion-formula" ;).

I don't have 5.1 sound reproducer system :/. So for a simple HIFI, I think that their's no advantage to use Ac3 stereo..
I didn't noticed at once, but Ac3 seems to be always less loud than MP3 ( when both are Converted from 5.1).

Thank again.

[DJ Bobo]

@ AsukaFan
Shimpai suruna! I'll go lend some money again :D
BTW, check the DivX5 forum for some hearing test we talked about (it is hidden under a divx related question, it will be hard to find, well search for... sweep), I was the only one able to hear frequencies above 20KHz, so I guess God blessed me with fantastic ears :D

@ seewen
When you compress to AC3, you can choose the "compression characteristic". Setting it to none will keep the volume as is. But forget about it, just stick to MP3 as long as you're dealing with AVIs and you're not interested in 5.1 sound.

[seewen]

Originally posted by DJ Bobo

@ seewen
When you compress to AC3, you can choose the "compression characteristic". Setting it to none will keep the volume as is. But forget about it, just stick to MP3 as long as you're dealing with AVIs and you're not interested in 5.1 sound.

Yes, when re-encoding Ac3->Ac3, you can only deal with Boost and OTA ( to adjust the sound). But with MP3, you can use the same + Azid. That's why I prefere the result (loudness), maybe. ;)

[DIggedy]

192 kbps is more than enough for a film's AC3 2.0 sound track; that's the opinion of cinema Studios, and that's also mine
192 is not always used by studios... you will quite often find 224kbps 2.0 tracks on commercially produced dvds as well. Also, your playback equipment will play a big part in your perception of the audio as well. Some people can't hear a difference simply because their equipment can't reproduce the subtleties of the audio enough to make a discernable difference between different bitrates. AND also we are humans, we are all different, some peoples senses are more sensitive than others... the amount of times I've heard a strange noise outside and commented to my other half 'whats that noise' and they say 'what noise, I can't hear a thing!'... Just my 2 cents.

[Valex]

@seewen
448/5 = 89.6 -> *2 = 179.2, rounded = 192kbps

It is not right. More correct equation is:
448kbps/6*3 = 224kbps.
The result is the same as DIggedy said.

Why? Because in most cases each channel is first divided into two big frequency bands (roughly equal). First parts are coded independent, and second is mixed together and coded once for all channels (it is like joint-stereo in mp3, but with some differences). So we have 6 really independent 'channels' (LFE does not matter because of low bitstream usage). And in final product we'll have 3 (2 stereo channels and one coupling channel) of this 'channels'. In case if you want to make independent stereo channels you'll need:
448kbps/6*4 = ~300kbps