What's the difference between AC3 Audio and MPEG2 Audio?

[G13]

What's the difference between AC3 Audio and MPEG2 Audio?

[redwudz]

Have you looked in the 'Glossary'? To the left. <<<<<<<<<<<<<<<

[G13]

No, I didn't look under "Glossary," but I did look under "What is DVD?"

So they are the same thing?

[terryj]

No, one is a proprietary format of Audio for DVD,
the other is referncing the encompassing standard. AC3 audio is a form of Dolby Digital Audio, which can vary in Channel, Bitrate, and it's algorithym [sp, sorry it's early]
makes it where it has to be decoded to be played back,
unlike PCM audio, which is a high bitrate AIFF.

MPEG-2 audio refers to, in general the audio file itself,
whether it is Ac3 or PCM, the Standard if you will.

Knida like saying "this is a Quicktime file." but in
actuality it might be .mov, .avi, .mpg4, or .mpeg-1 or -2.
You can say "this is Mpeg-2 audio.", but in actuality it is
either PCM or Ac3.

[Case]

MPEG-2 audio is not AC3, PCM or any other, it's a format on it's own. It's a fading DVD audio option though, few commercial DVDs use it. MPEG-2 audio can be quite good (high bitrate, 7.1 channels), but it lost on popularity even to optional and proprietary format DTS.

http://www.dvddemystified.com/dvdfaq.html#3.6.2

[Cornucopia]

It's not fading anytime soon...

The (fairly new) HDV spec (HiDef signal on DV tape) is MPEG2 Transport stream with MPEG2 14Profile@HighLevel video and MPEG1 Layer2 audio.

Scott

[frankiec]

What's the difference between AC3 Audio and MPEG2 Audio?

No, I didn't look under "Glossary," but I did look under "What is DVD?"

So they are the same thing?

If they were the same thing then why do they have different names?

(Using a little logic goes a long way in getting to the answer faster.)

[G13]

Thanks for the responses.

I read the DVD FAQ that Case referred to. It was very informative.

I am putting together a DVD compilation of music videos taken from some commercial music DVD's. Most of the original DVD's have the sound in 2 channel AC3. However, when I saved the individual videos to my hard drive I converted the VOB's to files containing muxed MPEG2 video and MPEG2 audio using MPEG Streamclip. The reason I did this was so that the files would play in Quicktime (actually Cellulo) with sound. If I'd left the Audio in the native AC3 format the files wouldn't play with sound in Quicktime.

What I really want to know is how much of a difference will there be on my compilation DVD if I make it out of files containing MPEG2 audio instead of the original 2 channel AC3 audio?

Will it not play in surround sound if I make it out of files containing MPEG2 audio instead of 2 channel AC3?

I am hoping that there is no difference (or very little difference) between the 2 audio formats so that I can get away with not having to re-rip the files onto my hard drive in AC3.

[redwudz]

If you have 2 channel sound (stereo) in AC3 the main difference with it and MPEG1-Layer 2 is the size. AC3 is more compact, allowing more space for video encoding, IE, a higher bitrate or more video runtime.

MPEG1-layer 2 is not in the DVD NTSC specification, though most all NTSC players should work with it. I use it all the time with no problems. For PAL, it is in the spec. If your video file is not big, Layer 2 will be OK.

Neither one will be 'surround sound' if there are only two channels. But your amplifier may make sort of a surround effect in that all speakers may output sound.

In Windows, I use ffmpeggui to convert to AC3, not sure what the Mac equivalent is. But if you have the room, stay with Layer 2.

[Case]

What I really want to know is how much of a difference will there be on my compilation DVD if I make it out of files containing MPEG2 audio instead of the original 2 channel AC3 audio?

Will it not play in surround sound if I make it out of files containing MPEG2 audio instead of 2 channel AC3?

MPEG Streamclip can save audio as MP2 (MPEG layer II). For two channel audio, there is no real difference between MPEG-1 layer II and MPEG-2 layer II: MPEG-2 audio extends on MPEG-1 audio.

According to the MPEG Streamclip Help file, the app takes AC3 2/0 audio (two channels, left and right, thus regular stereo or Pro Logic encoded surround sound) "as is" when encoding to MP2, preserving Pro Logic if present, when selecting one of the first four Audio Modes.

A slight reduction in bitrate (448 (most likely) to 384 (in Prefs)) seems to be the most notable difference.

[Cornucopia]

If you have 2 channel sound (stereo) in AC3 the main difference with it and MPEG1-Layer 2 is the size. AC3 is more compact, allowing more space for video encoding, IE, a higher bitrate or more video runtime.

MPEG1-layer 2 is not in the DVD NTSC specification, though most all NTSC players should work with it. I use it all the time with no problems. For PAL, it is in the spec. If your video file is not big, Layer 2 will be OK.

Neither one will be 'surround sound' if there are only two channels. But your amplifier may make sort of a surround effect in that all speakers may output sound.

In Windows, I use ffmpeggui to convert to AC3, not sure what the Mac equivalent is. But if you have the room, stay with Layer 2.

Not exactly correct:

Both AC3 and MP2 have a range of available bitrates. The unbreakable rule is AT ANY GIVEN BITRATE, THE FILESIZE IS THE SAME, no matter what the format (ignoring minor header/wrapper overhead). What I think redwudz is trying to say is that a AC3 2 channel file at 192kbps bitrate is going to be the Quality Equivalent of an MP2 2channel file at 224 or 256kbps. This reduction in bitrate will give you your filesize savings.

Also,
MP2 is "officialy" not supposed to be a part of the PAL spec either anymore (thanks to heavy Dolby lobbying ), but because of grandfathering, it's still considered part of the spec by most people (incl. player manufacturers).

re Surround:
There's 5.1 surround and there's 2.0 surround (e.g. "ProLogic"). Only DTS, AC3 or MPEG2Layer2--with the v2.5 "7.1 channel" extension--can be multichannel surround. Unless you used Hypercube Transcoder (the only straightforward and reliable method), I'm pretty sure that ANY MP2 file that you've created is NOT a multichannel surround file--even if the source AC3 was.
With 2channel surround matrixing techniques (like Dolby Surround/ProLogic), ANY 2channel audio file could be set up as surround, no matter what the file type (WAV, AIF, DTS, AC3, MP2, MP3, AAC, OGG, etc). However, running a 2channel surround file through an additional generation of psychacoustic processing and compression will likely DESTROY the delicate mix balances that the surround matrixing has set up and need to correctly decode back to it's individual channels. This is compounded if you are lowering bitrate.
Therefore, if you are trying to maintain any surround-ness, almost NEVER recompress to lower bitrate--whether the same or different file format.
Note: it sometimes possible to tell what kind of channel structure by the filesize. Here is a synopsis of common ranges...

• AC3 5.1 = 384, 448, 544 kbps
  AC3 2.0 = 192, 224, 256 kbps
  MP2 2.0 = 192, 224, 256, 384 kbps
  AC3 1.0 = 96, 112, 128 kbps
  MP2 1.0 = 96, 112, 128, 160 kbps

Scott