.AC3 Quality vs .MP2, and Why Does .MP2 Not Always Play?

[Equus]

Hi again,

A couple questions about audio. Someone told me that .AC3 audio is actually not as good as .MP2 (PCM) audio because it's so compressed but .AC3 is the only one of the two that will do surround sound. Any comments on that?

Also, I'm having trouble getting the audio from DVD's I author with Sizzle or FFmpegX to play audio when I've encoded in .MP2. This is because I have the audio out on my set-top Toshiba to "bitstream" to go out through the optical out. If I change it to "PCM" out, then I can hear the audio, but I don't want to have to do that every time, and I've noticed that when I burn a DVD in iDVD, the audio plays just fine, even though iDVD encodes using PCM Audio (.MP2 I presume) not Dolby Digital (.AC3).

Any ideas about this? I'm tired of having to re-encode the audio from the original .aiff file into .AC3 using DVDSP's A.Pack, cuz it takes as long as the real-time of the file.

Thanks,

Eq

[ravedogX]

Someone told me that .AC3 audio is actually not as good as .MP2 (PCM) audio because it's so compressed but .AC3 is the only one of the two that will do surround sound.

for the most part that is true. some numbers (please not this is just to show an example):
lets take a 90 minute program that you want on DVD, that has 2 channels @ 48Khz/16 bit (DVD spec):

• PCM (AIFF or WAV) Audio: 90 (min) x 11Mb (file size per minute for 2 channels) = 990MB . That will leave you you with 3310MB for your video track.
• AC3: the same audio encoded @ a 256 bitrate in a.Pack will give u a file of 168MB. That will leave you with approx 4100MB for your video... which you can use to create a better bitrate for compressing the video.

Your friend is right about the quality but if we were discussing a CD or DVD-A then you want as much fidelity as you can muster for the final. However, film soundtracks are a much different beast. They contain dialog, music, and effects, use the broad spectrum of the freq range, and play through a variety of channels (remember about 90% of all audio comes from your center channel and its dialog). Not saying that quality has to be less, but in the end, it isn't perceived. Compressing audio with AC3 offers the ability to use less room on the DVD than a standard PCM file leaving more room for the picture. The only other multi-channel solution for DVD is DTS (which works on the same principal as DD, but uses a bit better compression).

Not to mention, there is no file specification for more than 2 channels with PCM (even audio editor and mixing apps that are used to produce surround are going to output the final mix to separate files for each channel, which THEN have to be combined in a format that DVD Players can understand (DD or DTS).

There are many arguments about the various compression settings used when encoding AC3 (voodoo)... regardless, if your source is 2 (Dolby Pro Logic) or more channels - you must use AC3 or DTS (I keep mentioning DTS but i might as well pretend it doesn't exist.... none of the Mac based DVD Authoring apps handle DTS and only a few SUPER HIGH END audio apps with wxpensive plug-ins hadle DTS)

PCM, i am sure, was made for low-end applications (like industrial or instructional dvd's, porn, etc.) and since most DVD players play Audio CD's (uncompressed audio), the capability is there.

That being said, I still encode with AC3 - even on a lot of my mono sources (old shows from 50's, 60's and 70's) - it allows me to specifify a 1.0 spec that will force the audio thru the center speaker (unless the user doesn't have one).

Also, I'm having trouble getting the audio from DVD's I author with Sizzle or FFmpegX to play audio when I've encoded in .MP2. This is because I have the audio out on my set-top Toshiba to "bitstream" to go out through the optical out. If I change it to "PCM" out, then I can hear the audio, but I don't want to have to do that every time, and I've noticed that when I burn a DVD in iDVD, the audio plays just fine, even though iDVD encodes using PCM Audio (.MP2 I presume) not Dolby Digital (.AC3).

Well, the settings on your player are asking you how to route the various types of audio that are on DVD's. You can tell the player "any PCM audio: run thru the analog out" and "any DD run thru optical" or to be honest, any way you like. It's not a one or the other situation. If you have a DVD that is encoded PCM only, then the player will play it back based on your settings... if you have it encoded with AC3 - then it routes THAT accordingly. Each setting should be taken on its own, since you are only going to be able to access one audio stream on the disc at a time anyway (even if u have multiple streams on the disc with different encoding methods - you will see a lot of studio DVD's that encode Menu audio (if they use it) with PCM and the film in DD, etc... although I'm not sure the logic.)/

You have to separate out the 2 diff processes here: The first process is taking your raw souce materials and encoding them into formats that DVD Players understand (MPEG-2, PCM, AC3, etc). Once the files are ready, THEN the DVD "builder" or editor is exactly that: it takes the prepared elements and "builds" them (muxes them together) and usually burns them as well (kind of like building a web page - most editors do just that they edit all the elements that make up the web page (text, images, movies, flash, etc., that are actually created and prepared elsewhere)

iDVD and Sizzle (and even the new DVDSP 2.0) are examples of "builders" that also handle the encoding process as well... and they work with PCM audio only. and Im fairly certain (i know for a fact on iDVD) that its set up only for PCM Audio (except DVDSP)

Even if you manage to take the raw audio and encode it with AC3 you cannot bring it back into iDVD to burn. It's not set up for that (free app)

If you have the ability to use a.Pack (you can only get this with DVDSP), then use DVDSP. Don't bother with iDVD. (If you don't won DVDSP but are using a.pack anyway, save your energy cuz you wont be able to use AC3 in those other apps. You still will have to extract the audio from your souce files (if they are like DV files that have both audio and video in them) because audio and video DVD-ready files must be separate for the muxing process to work.

You shouldn't be having this "switching" problem on your DVD - if you set the Audio formats with the Outputs you want, then the player will do the rest based on what kind of disc you feed it.

I'm tired of having to re-encode the audio from the original .aiff file into .AC3 using DVDSP's A.Pack, cuz it takes as long as the real-time of the file.

Why would you be re-encoding anything at all? You either use raw audio (PCM) or AC3 encoded from PCM - where is this RE-encoding taking place? (RE-Anything in the world of DVD's is a BAD things - since AC3 and MPEG-2 use heavy compression it should be the last and final stage of the process (taking a source DV file that is 18GB and making it fit on DVD). As far as the time issue - my Mac encodes 2 channels @ 256 bit at 15:1 (90 min track takes 4 mins) - even if you have a slow mac - its much better than real time (if you really want it to fly - reboot into OS 9 - i get 3X more speed if i do). Unelss you are using true multi-channel audio - you shouldn't exceed about 224 or 256K data rate for your audio encoding (this is something that is hotly debated, but the concept remains).

CLOSING NOTE: I'm not sure what you are doing for your production process but it should go something like this (HUGE ROOM LEFT FOR DEBATE ON THE DETAILS, but the flow is the same):

1. Obtain and edit source material (that can mean from DV Camera, DVD, QT or whatever video source you are using.
2. Convert source material to formats for DVD:[list:2d85a50892]
3. NTSC video: 29.97fps @ 720x480 (PAL has its settings)
4. Audio: either PCM Audio, 1 or 2 channel, 48Khz, 16 bit (RAW) or Multi-channel encoded AC3 or DTS (from the PCM Audio file specs above)

[*]Take converted video and audio elements (and whatever else you may need to build DVD menus etc, if you go that route) and load them into your DVD "builder" program (iDVD, DVDSP, Sizzle etc) and "layout" how the DVD will work.[*]"build" the disc (known as muxing) and then burn the data.[/list:2d85a50892]

Hope this helps and makes sense since i should have been in bed hours ago. Just try to follow the overall concepts of what i suggested and go from there - as I'm sure there will be people who will want to offer corrections or more detailed info (I'm sure its needed) Good luck with it!

[Equus]

Hi,

Thanks for your response!

iDVD and Sizzle (and even the new DVDSP 2.0) are examples of "builders" that also handle the encoding process as well... and they work with PCM audio only. and Im fairly certain (i know for a fact on iDVD) that its set up only for PCM Audio (except DVDSP)

Actually Sizzle ONLY works with AC3 and MP2 (and some other encoding that starts with an L). It doesn't accept any raw audio at all. So what I've been doing with a file that's too big for iDVD's 90 minute limit, is

1) Capture in iMovie and edit
2) With QT Pro MPEG-2 encode the .mov that iMovie created, preserving AIFF audio.
3) Encode the AIFF audio file into .ac3 with A.Pack
4) Mux, author menus and chapters and build the image in Sizzle
5) Burn the image to DVD with DiskCopy.

This produces a good quality DVD with digital audio.

When I have a short enough movie, I do it all with iDVD. iDVD only uses PCM as you said, and this plays just fine on me DVD player as well, with no change in settings on the set-top player's audio.

You shouldn't be having this "switching" problem on your DVD - if you set the Audio formats with the Outputs you want, then the player will do the rest based on what kind of disc you feed it.

Where I'm having trouble is if I encode audio into .mp2 using FFmpegX - the audio won't play on the set-top player unless I change the player's audio output setting from "bitstream" to "PCM" which I don't understand since everyone's been telling me that PCM is RAW AUDIO and .mp2 is another form of compression that is NOT PCM. Bleah! It's very confusing.

Why would you be re-encoding anything at all? You either use raw audio (PCM) or AC3 encoded from PCM - where is this RE-encoding taking place? (RE-Anything in the world of DVD's is a BAD things - since AC3 and MPEG-2 use heavy compression it should be the last and final stage of the process (taking a source DV file that is 18GB and making it fit on DVD). As far as the time issue - my Mac encodes 2 channels @ 256 bit at 15:1 (90 min track takes 4 mins) - even if you have a slow mac - its much better than real time (if you really want it to fly - reboot into OS 9 - i get 3X more speed if i do). Unelss you are using true multi-channel audio - you shouldn't exceed about 224 or 256K data rate for your audio encoding (this is something that is hotly debated, but the concept remains).

Yeah that was my misuse of terms. What I meant was I am tired of having to encode raw audio into .ac3 when it should just play normally on the set-top player if I can preserve it in the authoring program. Is there a way to keep PCM audio preserved in FFmpegX? There is no way to do it in Sizzle that I can see, it will not accept an AIFF file. DVDSP will, but that's a more drawn out process.

Thanks,

Eq