AC3 and linear PCM, compressed or not?

[pcexpress-guy]

I notice AC3 and linear PCM type audio formats in DVD streams.

Question: is AC3 a compressed audio stream?

is a linear PCM audio stream a compressed audio stream?

are either of these a true loseless compression(i.e. no quality lost), if it is compressed?

Thanks!!

[johns0]

ac3=compressed
pcm=not compressed

[BJ_M]

ac3=compressed
pcm=not compressed

ac3= lossy

lpcm = uncompressed, therefore lossless

[pcexpress-guy]

thanks folks!

IS AC3 the same as MP3 or is AC3 its own compression type?

also would you say that most commerical movies out there are in AC3 format? or l-pcm?

thanks again!

[BJ_M]

ac3 is a compression type developed by dolby labs ...


it is not like mp3


ac3 is the most common type of audio on dvd's

[DRP]

thanks folks!

IS AC3 the same as MP3 or is AC3 its own compression type?

AC3 is the same sort of thing as MP3 yes, but it is not the same. It is similar in that both are lossy methods of audio compression which take advantage of the psychoacoustic limitations of the human ear to permanently remove those portions of the sound which can't be heard over the other sounds playing at the same time, thus reducing significantly the amount of information being stored.

You can think of AC3 as being similar to MP3 if that helps you to comprehend what it is, but the two formats are different and not compatible in any other way. Like ZIP & RAR are similar as well. They both do the same thing in a similar way but they are different.

also would you say that most commerical movies out there are in AC3 format? or l-pcm?

Definitely AC3. The reason being that LPCM can only contain 2 channels (conventional stereo) while still being small enough to store on a DVD in addition to the video information required. AC3 is like an MP3 file that can contain 5.1 surround sound. Most people perceive a lossy surround sound format to sound better than a completely uncompressed format which is only conventional 2 channel stereo. Indeed, most people are not even interested enough to find out what "lossy compression" even means or recognise that AC3 is one of these formats. If you ask most people to tell the difference between a properly encoded MP3 file and the original CD, they won't be able to pick the difference with just their hearing, but they certainly can hear the difference between 5.1 surround sound and normal stereo, so they conclude that the one with 5.1 (AC3) is the superior quality - even though technically it isn't.

[pcexpress-guy]

excellent explanation!

if you have two high quality encoded MP3 and AC3 files side by side? is the sound quality (from a technical standpoint) about the same?

also is DVD-AUDIO disk, ac3 file format? or something different? (or can it vary?)

[Cornucopia]

Side by side is hard to tell, but most would probably say that (given similar bitrate and # of channels) AC3 would be better than MP2 or MP3.

DVD-Audio is a completely different beast. It uses ONLY (uncompressed) LPCM and/or losslessly-compressed MLP (sort of like a zipped LPCM) in the AUDIO_TS folder. For DVD-Video player compatibility purposes, it may also include some AC3 in the VIDEO_TS folder's VOBs.


HTH,
Scott

BTW, LPCM is supposed to be able to allow 5ch DVD-Video audio. It's just not very well supported at all.

[DRP]

excellent explanation!

if you have two high quality encoded MP3 and AC3 files side by side? is the sound quality (from a technical standpoint) about the same?

That I can't answer. I don't know enough about the AC3 format nor have enough experience of using it to comment with any confidence. If I had to make a guess though I would say that the main advantage of AC3 is the capacity to store the audio as 5.1 surround sound. As you are no doubt aware surround sound is the way forward and it's what everyone wants these days with home theatre systems etc. No-one really wants to know what an audiophile thinks of the format with regard it's lossy status. To 99.9% of the consumers out there it is perceived to sound better and that's what matters.

MP3 is only 2 channel conventional stereo but it is open source thanks to the LAME project. That being the case, it is probably much more finely tuned towards complete audio transparency to the original PCM stream than is AC3. AC3 wouldn't need to be so throughly tuned to perfection because it has the ace up it's sleeve of 5.1 support.

It wouldn't really be fair to compare AC3 to MP3 because they're not exactly the same thing by virtue of the 5.1 support included with AC3. If it was me and 2 channel stereo was good enough for what I wanted, then I would use MP3 rather than AC3 for that purpose.

also is DVD-AUDIO disk, ac3 file format? or something different? (or can it vary?)

DVD-A is a relatively new format where the DVD disc is used solely for storing audio only. The storage capacity of a DVD is massive, so when there's no video to include, there is more than enough space to store the audio in a completely uncompressed state (like a conventional CD-DA disc). These type of discs trade on having the ultimate in high quality audio currently available. If they used *any* form of lossy compression then they wouldn't be able to claim this.

Thay contain LPCM streams only, sometimes in much higher resolution than conventional audio CD-DA. Conventional CD-DA is 16bit, 2 channel stereo @ 44.1kHz. DVD-A discs typically use 24bit, 2 channel stereo @ either 48, 96 or 192kHz sampling frequencies.

In truth, conventional CD-DA audio is very, very good. It's true to say that the developers (Sony & Phillips) made a mistake in making it so good when they developed it, because the format is now well over 20 years old and it shows no sign of dying yet. The fact is that while DVD-A is technically speaking much better quality than conventional CD-DA you'd need a very high end stereo system and golden ears to seriously be able to claim that you could hear the difference.

[sacajaweeda]

thanks folks!

IS AC3 the same as MP3 or is AC3 its own compression type?

also would you say that most commerical movies out there are in AC3 format? or l-pcm?

thanks again!

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