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Thread: PCM vs AC3

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  1. Member Reading Bug's Avatar
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    Is there any true "sacrifice" in creating DVDs with PCM audio as opposed to AC3, in regards to playback and longevity?

    I know AC3 is compressed to just a fraction of what PCM requires, giving the disc more room. But for argument’s sake, if I fit three MPEGs on a single-layer disc using Sony Architect under the following conditions, will using PCM be ok? Even if it pushes my DVD settings to the 9800 kbps max?

    MPEG Video: MPEG-2, 720x480, 29.97, 8500 kbps (VBR)
    MPEG Audio: MPEG Layer 2, 44.1 KHz, 384 kbps

    DVD Video preset: 8264 kbps (VBR)*
    DVD Audio preset: PCM, 1536 kbps

    *I set the video to 8500 kbps to match the MPEG, but Architect reconfigured it to 8264 when I selected PCM. Since the video doesn't re-encode, I know my picture isn't compromised. But can anything negative result from this "downgrade," in regards to playback, quality or longevity?
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  2. Member wulf109's Avatar
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    PCM is uncompressed and takes up more room than AC3 or mpeg audio. It is commonly used on music DVD's to preserve maximum audio quality. Unless your source is PCM there no point in using it in authoring a DVD.
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  3. Member PuzZLeR's Avatar
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    Originally Posted by Reading Bug
    Is there any true "sacrifice" in creating DVDs with PCM audio as opposed to AC3, in regards to playback and longevity?
    PCM is lossless while AC3 isn't. But, assuming the same source, the higher the bitrate for AC3, the closer to the quality of the PCM version. A good compromise for small file size, and near-equivalent quality to PCM, is AC3 at 256kbps IMO.

    Also both, AC3 and PCM, are the two "no-trouble" formats in NTSC-land (which I assume you're from) and require no conditions other than space. The other two, MP2 or DTS, have "conditions". And anything else must be re-encoded to a compliant format to be accepted.

    Here's how I see it IMO:

    1) If your source is AC3 there is no reason to re-encode. Even AC3->PCM offers no benefit, only bloat.

    2) If your source is MP2 (your case), or DTS, don't re-encode, but you must have a secondary stream otherwise most players will give you problems. Use AC3 to satisfy that condition to save space since you probably may only want the MP2, or DTS for playback anyway.

    3) If your source is PCM, then you have two choices:
    -Keep it as PCM if you have the space to keep the full uncompressed quality.
    -Encode to AC3 to save space. The bitrate and compromise on quality loss is your choice.

    4) If your source is something else, then encode to PCM or AC3 with similar thinking to 3).
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  4. Member edDV's Avatar
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    Originally Posted by Reading Bug

    *I set the video to 8500 kbps to match the MPEG, but Architect reconfigured it to 8264 when I selected PCM. Since the video doesn't re-encode, I know my picture isn't compromised. But can anything negative result from this "downgrade," in regards to playback, quality or longevity?
    If you set Architect to other than the MPeg2 video specs you are importing to the project, it will either reject the project or recode. As said above there is no advantage to converting audio to PCM. Either leave the MP2 audio as is or convert to AC3 in Vegas then import that to Architect.

    You don't have bit rate on the DVD-5 for both your 8500Kb/s video and 1536Kb/s audio. You would have to recode the video. There is no good reason to do that.
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  5. Member PuzZLeR's Avatar
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    Originally Posted by edDV
    Either leave the MP2 audio as is or...
    But doesn't that give some NTSC DvD players problems playing it back?
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  6. Member edDV's Avatar
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    Originally Posted by PuzZLeR
    Originally Posted by edDV
    Either leave the MP2 audio as is or...
    But doesn't that give some NTSC DvD players problems playing it back?
    Very few but he could add a second set of AC3 tracks as you suggested. Architect allows multiple audio sets.
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  7. Member Reading Bug's Avatar
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    Thanks all for the suggestions. My original stand-alone audio tracks were PCM, but putting them to new MPEGs required me to compress to MP2. I know I can't convert back to PCM on the DVD (or can I?) but thought perhaps an uncompressed setting would prevent further sound loss.
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  8. Member t0nee1's Avatar
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    IMO, just K.I.I.S, convert PCM--> AC3
    "I typed the word Google into Google. Guess what came up? Everything."

    "What we've got here is failure to communicate"
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  9. Member edDV's Avatar
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    Originally Posted by Reading Bug
    Thanks all for the suggestions. My original stand-alone audio tracks were PCM, but putting them to new MPEGs required me to compress to MP2. I know I can't convert back to PCM on the DVD (or can I?) but thought perhaps an uncompressed setting would prevent further sound loss.
    You can use the MP2 as is. It will be playable on ~98%of DVD players.

    If you want to convert to PCM, you would neeed to recode video to less than 8264Kbps. No reason to do either.
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  10. Member Reading Bug's Avatar
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    Originally Posted by edDV
    If you want to convert to PCM, you would neeed to recode video to less than 8264Kbps. No reason to do either.
    Actually, Architect doesn't force a re-encode when bumping the video down from 8500 to 8264. I guess it's too small a difference.

    That's why I'm wanting to know if I'm "safe" using PCM - I can avoid the compression on the audio and not sacrifice video, right? Safe or no?
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  11. Video Restorer lordsmurf's Avatar
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    PCM takes away space that could be occupied by video, if you're putting more than 90 minutes on a disc.
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    If your audio source is 2 channel audio, then encode it to AC3 192kbps or if you're a little generous, at 224kbps is more than sufficient.
    If you have a surround audio source with more than 2 channel, and it's important for you to maintain that multi channel audio, then AC3 is a no brainer choice, because you cannot author a DVD with multichannel PCM.

    Using a PCM track will also significantlly affect picture quality as well.
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  13. Member PuzZLeR's Avatar
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    [quote="Reading Bug"]
    Originally Posted by edDV
    That's why I'm wanting to know if I'm "safe" using PCM...
    Maybe I'm misunderstanding but if you're worried about DvD compliancy you are perfectly safe with PCM. Space compromise would be your only internal negotiation.

    As for MP2, perhaps I was thinking in the past since it was a bigger issue with the early runs of players. I guess today, with most of the later DvD player models playing several "non-compliant" formats like DivX, Xvid, MP3, WMA, etc, that too may not be an issue any longer.
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  14. Member edDV's Avatar
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    Originally Posted by Reading Bug
    Originally Posted by edDV
    If you want to convert to PCM, you would neeed to recode video to less than 8264Kbps. No reason to do either.
    Actually, Architect doesn't force a re-encode when bumping the video down from 8500 to 8264. I guess it's too small a difference.

    That's why I'm wanting to know if I'm "safe" using PCM - I can avoid the compression on the audio and not sacrifice video, right? Safe or no?
    I don't understand why it doesn't recode the video. I also don't understand why you want to convert mp2 to PCM. You gain nothing. I'd use mp2 or ac3.

    The other option is to start over and use PCM audio from the source and encode the video to 8200Kb/s.
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  15. Originally Posted by Reading Bug
    Even if it pushes my DVD settings to the 9800 kbps max?
    The DVD max allowable bitrate for video, audio, subs, and muxing overhead isn't 9800 kbps, but 10080:

    http://www.videohelp.com/dvd
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