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Lossy or Lossless? Please use this forum to post spectral and frequency analysis posts about shows you have your doubts about.

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  #1  
Old 2007-08-10, 11:10 AM
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Is decompressed AC3 lossy?

If the audio of a DVD has been encoded to AC3 and is then decompressed to LPCM, is the audio lossy or lossless? I would think it would no more lossy than the AC3 it was encoded to. Am I wrong?

Thanks.
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  #2  
Old 2007-08-10, 11:24 AM
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Re: Is decompressed AC3 lossy?

It would be lossy. That's like taking an mp3 and decompressing it to wav - you just can't get back the stuff that was removed when it was compressed to mp3 (or ac3).
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  #3  
Old 2007-08-10, 12:01 PM
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Re: Is decompressed AC3 lossy?

Quote:
Originally Posted by U2Lynne
It would be lossy. That's like taking an mp3 and decompressing it to wav - you just can't get back the stuff that was removed when it was compressed to mp3 (or ac3).
Thanks...another question:

What if the LCPM is lossless and stereo, and reduced to mono.

Is that lossy?

Thanks again.
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Old 2007-08-10, 12:07 PM
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Re: Is decompressed AC3 lossy?

Well, it isn't lossy in the sense that if you look at an FA or SA of it, it will show frequency response in the range you want, however, you will never be able to go back to stereo from there. At least that is my understanding - you take the two channels and put them all into one.
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Old 2007-08-10, 12:22 PM
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Re: Is decompressed AC3 lossy?

Quote:
Originally Posted by U2Lynne
Well, it isn't lossy in the sense that if you look at an FA or SA of it, it will show frequency response in the range you want, however, you will never be able to go back to stereo from there. At least that is my understanding - you take the two channels and put them all into one.
What if the original source had mono audio that was encoded to stereo was it was made into a DVD?

Is there a way to tell if the original audio was mono or stereo?
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Old 2007-08-10, 12:34 PM
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Re: Is decompressed AC3 lossy?

Quote:
Originally Posted by tgunn2760
What if the original source had mono audio that was encoded to stereo was it was made into a DVD?

Is there a way to tell if the original audio was mono or stereo?
This I do not know. Someone more knowledgeable about this will have to answer.
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  #7  
Old 2007-08-10, 12:36 PM
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Re: Is decompressed AC3 lossy?

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Originally Posted by U2Lynne
This I do not know. Someone more knowledgeable about this will have to answer.

Thanks for your help.
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Old 2007-08-10, 01:06 PM
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Re: Is decompressed AC3 lossy?

Quote:
Originally Posted by tgunn2760
What if the original source had mono audio that was encoded to stereo was it was made into a DVD?

Is there a way to tell if the original audio was mono or stereo?


do you mean it was 'teased' into fake 'stereo' like TISDU or so you mean they simply doubled the mono track to 2 channels to make the DVD compliant?

If the latter - don't 'fold down', as that doubles the amplitude values - just extract the left channel.

The thing is, if you want it to go to CD, red book says it must have two channels - mono doesn't fly. So, you'd have to copy left to right again (which is what they did in the first place, so maybe leave it).
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Old 2007-08-10, 02:12 PM
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Re: Is decompressed AC3 lossy?

Quote:
Originally Posted by jameskg
do you mean it was 'teased' into fake 'stereo' like TISDU or so you mean they simply doubled the mono track to 2 channels to make the DVD compliant?

If the latter - don't 'fold down', as that doubles the amplitude values - just extract the left channel.

The thing is, if you want it to go to CD, red book says it must have two channels - mono doesn't fly. So, you'd have to copy left to right again (which is what they did in the first place, so maybe leave it).
I don't know if the original recording was mono or stereo.

I have a DVD which has AC3 stereo audio, but the volume is very low.

I extracted the audio in wav and deleted a channel as someone suggested, ran it through Goldwave and then doubled the volume. It sounds great, but it's lossy.

I know there is a lossless LPCM audio file out there, but the size would make it too big for a DVD5, so I was wondering if after deleting one channel of the lossless stereo LPCM the audio would still be lossless, especially IF the original audio was in mono-and is there a way to tell if it was mono?
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Old 2007-08-10, 06:38 PM
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Re: Is decompressed AC3 lossy?

yes. Take the LPCM audio file you suspect is dual-mono (exact same left and right channels) and open in Cool Edit Pro 2.0 (or Adobe Audition - whichever you have).

Highlight the whole thing and open the "channel mixer". Invert ONE of the channels and play. If the audio is exactly the same left-right, it will be completly silent since you are subtracting the inverse from one side to the other. Get audio, and you do not have a true dual-mono source.


Now, if that is the case and you have isloated a mono source, the only remaining question is whether you can include an LPCM mono track on a DVD and still be compliant with the standard. I know with a CD, that would NOT be compliant - you have to double to 2 channels - back where you started.

Maybe a video mod can chime in about whether a DVD can contain a 1 channel mono LPCM stream - or you can visit www.videohelp.com and look up the DVD-Video standards. I'll try to have a look later if I have a chance.
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  #11  
Old 2007-08-10, 07:26 PM
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Re: Is decompressed AC3 lossy?

Quote:
Originally Posted by jameskg
yes. Take the LPCM audio file you suspect is dual-mono (exact same left and right channels) and open in Cool Edit Pro 2.0 (or Adobe Audition - whichever you have).

Highlight the whole thing and open the "channel mixer". Invert ONE of the channels and play. If the audio is exactly the same left-right, it will be completly silent since you are subtracting the inverse from one side to the other. Get audio, and you do not have a true dual-mono source.


Now, if that is the case and you have isloated a mono source, the only remaining question is whether you can include an LPCM mono track on a DVD and still be compliant with the standard. I know with a CD, that would NOT be compliant - you have to double to 2 channels - back where you started.

Maybe a video mod can chime in about whether a DVD can contain a 1 channel mono LPCM stream - or you can visit www.videohelp.com and look up the DVD-Video standards. I'll try to have a look later if I have a chance.
I don't have either of the programs you suggested, and I don't have the lossless LPCM now so I will worry about it when I get it..IF, I get it?

As for the mono audio being compliant on a DVD, it is. I made one. I overwrote the AC3 audio in Tsunami DVD author Pro with the decompressed AC3. In fact, when I run the DVD files through GSpot it doesn't recognize the LPCM audio, it reads it as AC3.
And I know it is NOT AC3, I burned a copy of the original DVD with the AC3 audio, and played them both, the difference is impossible to miss.

Thanks again for your help.
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Old 2007-08-10, 07:49 PM
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Re: Is decompressed AC3 lossy?

I'm confused.. if you decompress AC3 (I assume you mean to 48kHz WAV), you don't get back the audio data that was discarded when it was compressed to AC3.

It should sound the same. Unless you then RE-encoded to AC3 again.. THAT would sound terrible.
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  #13  
Old 2007-08-10, 08:11 PM
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Re: Is decompressed AC3 lossy?

AC3>LPCM = Not lossy,just a bloated AC3
Some VHS's used to say Dolby on Linear tracks

But tgunn,between you & me(hopefully),i wouldn't do what i 100% know you are thinking.The definitive is being worked on......Shhhhhh

K_K
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Old 2007-08-10, 08:16 PM
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Re: Is decompressed AC3 lossy?

Quote:
Originally Posted by KoolKat
AC3>LPCM = Not lossy,just a bloated AC3
It doesn't make it more lossy, but it doesn't exactly put the audio back either. Unless we are talking about lossless AC3, then? I assumed he was talking about the standard lossy AC3 you find on nearly all DVDs with AC3 sound?
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  #15  
Old 2007-08-10, 08:26 PM
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Re: Is decompressed AC3 lossy?

Quote:
Originally Posted by jameskg
It doesn't make it more lossy, but it doesn't exactly put the audio back either. Unless we are talking about lossless AC3, then? I assumed he was talking about the standard lossy AC3 you find on nearly all DVDs with AC3 sound?
Agreed J,i see what your saying chap

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