Quality Loss from AC3 to LayerII

[Offroad]

Had a Vob file that was out of sync and used Womble to correct it during multiplexing. The file had AC3 audio and although Womble would accept it I couldn't adjust the sync between Video and Audio with the audio being AC3.

I converted the AC3 audio to MPII during demuxing then was able to use Womble to adjust the sync during multiplexing.

Just curious how much quality is lost when this is done.
The video was 720x480 VBR and was not re-encoded, the audio was originally AC3 at 256kps.

Also is quality lost when AC3 or MPII audio is converted to LPCM?
It would seem you are just uncompressing the audio so this would not lose any quality.

Any info appreciated.

[daphy]

converting from a lossy to another lossy format always mean losing quality.
PCM isn´t a lossy format so it should be better than converting to MP2.
Take a look a the DVD-Video specifications in PAL-land PCM/MP2 is allowed - don´t know about the US standart
There are also methodes to sync AC3 (without reencoding it), so take a look at the download section of this board or @doom9.org

bye Daphy

[Cobra]

There will be no quality loss when converting to LPCM. It stands for Linear Pulse Code Modulation, and is a non-lossy format. However, you shouldn't really use it for DVD since it eats up ~1400kbps as compared to 256kbps MP2, which is also high quality. Using MP2 will allow you more space on the DVD which you can then use to give the video more bitrate - something that will be more advantageous than slightly better audio.

[offline]

Bitrate is the key here, or rather, bit rate per channel.
AC3 stereo and Mp2 stereo produce similar quality
sound at around 256kps. Converting between the two
at this bit rate should not produce noticeable quality loss.
(at 48Hz)

With converting to LCPM, although you are just un compressing the sound,
there are quality losses that creep in during the conversion. I doubt
that they would be audible.

The problem lies with the fact that "quality" is such a subjective
term. Best always to test for yourself on a good stereo system.

[daphy]

Question:
Is it (meanwhile) possible to go from AC3 to MP2 without first decoding the AC3 to PCM?
It could be possible with besweet but I ain´t sure.
With other words:
decoding the AC3 to WAV and synchronizing it with the Video stream must be the better deal than again going into a lossy format -> because this transcoding leads to two lossy conversions -> PCM only to one
Please correct me if I´m wrong

bye Daphy

[mats.hogberg]

Question:
Is it (meanwhile) possible to go from AC3 to MP2 without first decoding the AC3 to PCM?

No.
And I don't fully agree with offline - take a mp2 or mp3 or AC3 or whatever lossy format. Play it. Decode it to wav. Play it. It will sound (and in fact be) exactly the same. The quality loss uccurs when going from a lossless format to a lossy, not the other way around.

/Mats

[offline]

Umm.. I might need to be corrected but I'm
fairly sure that any conversion process from
a compressed format requires decompression
to raw data (a PCM file without a header) which
is a kind of WAV file anyway. i.e. the answer is No

[offline]

@Mats - you may have misunderstood me.

My 1st point was - take a PCM, convert to ac3, convert to PCM and compare with
the original PCM. You can see differences
but I doubt that they would be audible.
I could not hear them.

My second point was-
In addition, take an 256kbs stereo Ac3 and
convert to mp2 at 256kbs. That is one lossy
format to another lossy format. There should
be a noticeable reduction in quality - in my
tests there was none. Change the bit rate, however,
and there are obvious differences on various
scales between the two formats. There is no
linear scale in quality with psychoacoustic algorithms.

[mats.hogberg]

OK, I see what you mean. I guess you have to run the wave forms thru an oscilloscope (and I don't doubt an audiophile will do / have already done that) to spot the differences...

/Mats