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  1. I sent a VHS tape in for professional capture and DVD conversion. I am now in the process of ripping and editing the footage. I noticed that if I demux the audio and video and then open the resulting .AC3 file in Audacity, there are apparently 6 channels/tracks of audio present. Given that the source material was a VHS home movie made in 2001, I know that there weren't 6 channels originally recorded. Weren't most camcorders of that era mono? Audacity reports each of the individual tracks to be: Mono, 48000 Hz, 16-bit PCM.

    Looking at the different tracks in Audacity, it appears that 1 is the "main" track, and the others appear to be copies of the original attenuated to varying degrees. Was this done by the DVD creation program just to fit some standard template (I'm guessing 5.1 channel)?

    Did the DVD capture/creation program literally just copy and paste attenuated versions of the single original audio track to create the others?

    Also if I make any edits to this file in Audacity and export it back to a .AC3 file, I get this warning:

    "Your tracks will be mixed down to a single mono channel in the exported file."

    Is that going to cause any problems if I want to remux it with the original video?
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  2. Member DB83's Avatar
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    The dvd-authoring most likely created a quasi-6ch audio.

    Are you considering editing the audio independently of the video ?

    Yes = Loss of audio/video sync.

    Don't use Audacity but can not that save as 6ch AC3 ?
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    Very interesting. I didn't think a DVD conversion shop would even bother trying to do that. I know that Hollywood sometimes fakes mono into AC3 5.1 because consumers are dumb enough to think it's "better" than the original track, but I'm surprised that they'd even try to do that in a conversion. I suppose there is some possibility that camcorders of that era could try to do stereo, but I'm just guessing. I wouldn't expect much separation even if they did it.

    I think Audacity is basically telling you that it's not fooled by this quasi-6 channel sound nor does it play that game, so it's going to save everything as a mono file and avoid that foolishness. Will it cause any problems? Not at all. But do note that some home theatre setups may only play such a file out of the front center channel. I'm no Audacity expert, but if you can get it to save the file as 2 channel "stereo", even if it's just mono duplicated in both channels, that should play out of the front left and front right speakers in a home theater setup and you might find that more pleasing. It'll still be mono but it might sound more natural than just coming out of the front center speaker. Criterion does this with their DVDs and BluRays.
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  4. You can get dual mono out of Audacity. Me, I'd make it dual mono before sending it to Audacity by decompressing to stereo WAV using BeSweet or Head3che. Then you can reencode back to AC3 using whatever you usually use. And that's only if you need a mono or stereo track to save space or something. As DB83 mentions, editing the audio and video separately can be a tricky proposition
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  5. Originally Posted by DB83 View Post
    The dvd-authoring most likely created a quasi-6ch audio.

    Are you considering editing the audio independently of the video ?

    Yes = Loss of audio/video sync.

    Don't use Audacity but can not that save as 6ch AC3 ?
    Originally Posted by manono
    As DB83 mentions, editing the audio and video separately can be a tricky proposition
    Why is editing the audio separate from the video a bad idea? I thought that was the whole point of demuxing the video from the audio. The only audio edits I want to do are a noise/hiss filter and an amplification of the overall volume. Would performing either or both of those modifications on the audio track and then remuxing the audio into the video cause a problem?
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  6. Originally Posted by Special K View Post
    The only audio edits I want to do are a noise/hiss filter and an amplification of the overall volume. Would performing either or both of those modifications on the audio track and then remuxing the audio into the video cause a problem?
    No, of course not. Earlier you said, "I am now in the process of ripping and editing the footage" which might imply cutting out extraneous parts and the like. That's where doing that to the audio separately from the video might be difficult. And if it's only the audio on which you'll be working, I'd convert it to a stereo WAV file first and then import it into Audacity.
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  7. Member DB83's Avatar
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    Could you post a mediainfo report of that demuxed audio and a mediainfo report of the muxed vob. Something seems odd where you state you have an AC3 file but is also PCM. Maybe 2 audio tracks ?

    And, yes, I fell for your bad choice of words 'editing the footage'
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  8. I'm a Super Moderator johns0's Avatar
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    You have to go into audacity edit/preferences/import/export use custom mix checked or it will give you the mono message with any 5.1 ac3 real or pseudo.
    I think,therefore i am a hamster.
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  9. Originally Posted by DB83 View Post
    Could you post a mediainfo report of that demuxed audio and a mediainfo report of the muxed vob. Something seems odd where you state you have an AC3 file but is also PCM. Maybe 2 audio tracks ?

    And, yes, I fell for your bad choice of words 'editing the footage'
    Here is the output from the .MPG file, which should be identical to the .VOB:

    Code:
    Audio
    ID                                       : 189 (0xBD)-128 (0x80)
    Format                                   : AC-3
    Format/Info                              : Audio Coding 3
    Mode extension                           : CM (complete main)
    Format settings, Endianness              : Big
    Muxing mode                              : DVD-Video
    Duration                                 : 1h 16mn
    Bit rate mode                            : Constant
    Bit rate                                 : 448 Kbps
    Channel(s)                               : 6 channels
    Channel positions                        : Front: L C R, Side: L R, LFE
    Sampling rate                            : 48.0 KHz
    Bit depth                                : 16 bits
    Compression mode                         : Lossy
    Delay relative to video                  : -161ms
    Stream size                              : 245 MiB (9%)
    Here is the output from the demuxed AC3:

    Code:
    Audio
    Format                                   : AC-3
    Format/Info                              : Audio Coding 3
    Mode extension                           : CM (complete main)
    Format settings, Endianness              : Big
    Duration                                 : 1h 16mn
    Bit rate mode                            : Constant
    Bit rate                                 : 448 Kbps
    Channel(s)                               : 6 channels
    Channel positions                        : Front: L C R, Side: L R, LFE
    Sampling rate                            : 48.0 KHz
    Bit depth                                : 16 bits
    Compression mode                         : Lossy
    Stream size                              : 245 MiB (100%)
    Does this give you any additional information?

    Also I apologize for the vague "editing the footage" statement when all I really meant was filtering the audio and/or boosting the volume.
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  10. Originally Posted by manono View Post
    Originally Posted by Special K View Post
    The only audio edits I want to do are a noise/hiss filter and an amplification of the overall volume. Would performing either or both of those modifications on the audio track and then remuxing the audio into the video cause a problem?
    No, of course not. Earlier you said, "I am now in the process of ripping and editing the footage" which might imply cutting out extraneous parts and the like. That's where doing that to the audio separately from the video might be difficult. And if it's only the audio on which you'll be working, I'd convert it to a stereo WAV file first and then import it into Audacity.
    is AC3 to stereo WAV lossless?
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  11. Originally Posted by Special K View Post
    is AC3 to stereo WAV lossless?
    Yes, WAV audio is lossless. Later on you'll be reencoding back to AC3 and that's lossy. But with the changes you'll be making (volume and noise removal) the result should be an improvement.
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  12. Originally Posted by manono View Post
    Originally Posted by Special K View Post
    is AC3 to stereo WAV lossless?
    Yes, WAV audio is lossless. Later on you'll be reencoding back to AC3 and that's lossy. But with the changes you'll be making (volume and noise removal) the result should be an improvement.
    extension wav doesn't mean that audio is lossless - WAV is a container - usually wav is PCM and then is lossless but also wav can be compressed audio and as such lossy (depend from codec).
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  13. If you have the ffmpeg plugin for Audacity you can import your audio directly from an AV file, and export directly as AC3.
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  14. Originally Posted by pandy View Post
    extension wav doesn't mean that audio is lossless - WAV is a container
    Okay, point taken. But doing it as I described earlier - decompressing to WAV audio using BeSweet or HeadAC3he - is a lossless procedure. Well, except for the conversion from six identical channels down to two it is.
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    Originally Posted by pandy View Post
    extension wav doesn't mean that audio is lossless - WAV is a container - usually wav is PCM and then is lossless but also wav can be compressed audio and as such lossy (depend from codec).
    I've been guilty of saying that kind of stuff in the past too, but now I have changed my thinking on this kind of response. Is it REALLY helpful to inexperienced members to point out this kind of "one in a million" possibility without any explanation that while this is POSSIBLE, it is NOT LIKELY to be the case at all? For all practical purposes, if someone is dealing with WAV, the odds are huge that it's PCM.
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  16. Member Cornucopia's Avatar
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    I find suspect any transfer service that automatically does 2ch stereo vhs to 5.1ch ac3. It should have been 2ch or 1ch, depending on the true layout of original tracks. That would also enabled better quality with given bitrate.

    Be careful with the combination of those 2? Channels, as there could be some minute but important timing differences between them which could screw up the sound royally.

    Scott

    P.s. some vhs cams were hifi = stereo, some linear = mono. Depending on cam. What kind was yours?

    If mono, use only one of those tracks and then dupe it to 2nd track for 2ch mono..

    Curious, why did you go DVD to edit?
    Last edited by Cornucopia; 5th Mar 2014 at 18:16.
    "When will the rhetorical questions end?!" - George Carlin
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  17. Originally Posted by johns0 View Post
    You have to go into audacity edit/preferences/import/export use custom mix checked or it will give you the mono message with any 5.1 ac3 real or pseudo.
    OK, this makes sense. After applying an amplification mod to the fake 5.1 audio in Audacity and saving it as a new file, the new file is a mono track when I open it in Audacity.

    How does Audacity generate a mono track from a fake 5.1 track? Does it just take the loudest of the 6 available tracks, or does it average them all, or what?
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