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PCM vs AC3 recording for DVD

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Brent Hilgenkamp
PCM vs AC3 recording for DVD
on Oct 11, 2010 at 10:44:39 pm

There have been multiple posts about using AC3 audio recording over the default PCM because of the time required to import using log and transfer. My question is if I record AC3 will there be any discernible difference when my final product is a DVD using AC3 anyway? I shoot long form video and not only does FCP constantly crash when trying to log and transfer the clips but the time increases exponentially when trying to import long clips vs shorter clips.


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john lenihan
Re: PCM vs AC3 recording for DVD
on Oct 11, 2010 at 11:01:47 pm

Brent,

You will have to test that yourself with your own material.

AC3 uses digital signal processing techniques which actually perform calculations in hardware that involve multiplication, and division.

If your signal is always less than zero db ( I think they actually give about 6 db guard band more), then those calculations work fine.

If your signal has a loud spoken word , or music that peaks over that, the calculations have a point where they divide by zero. It causes a loud click in the audio that is VERY obvious.

If you have a fast responding autogain on your camera you will be ok. If you manually adjust the volume during recording to stay below that zero db, you will be ok.

So, It depends on the control of your source, and type of source, and type of recording setting to answer your question.

If you record a sample and cannot hear clicks, then you are safe.

PCM encoding does not have division in its computation, so when you have too high of a signal, you will get distortion, but it will be clipping distortion, and you can often use a filter to compensate for the clipping.

The reason why we can get by with AC3 on the output side, is we as editors can make sure no signals are too high.

I hope this helps

John Lenihan

LeniCam Video Productions
972-378-0446

John Lenihan

LeniCam Video Productions
http://www.lenicam.com


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Brent Hilgenkamp
Re: PCM vs AC3 recording for DVD
on Oct 11, 2010 at 11:10:51 pm

Very informative explanation, thank you for that.

Our source footage is basically live music events - do you think the home viewer would be able to perceive the difference between recording in AC3 over PCM? Unfortunately all of our cameras are out around the country and I have no way to test the difference between the two.


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john lenihan
Re: PCM vs AC3 recording for DVD
on Oct 11, 2010 at 11:30:06 pm

When the AC3 distortion happens, it is not subtle.

Try this experiment with your editor and representative recorded music you already have.

Raise the gain on the editor so your peaks are well past plus 12 db.

Then render the audio first as a wave file.(PCM)
Then render the audio as an ac3 file.

listen to the difference in distortions.

John Lenihan

John Lenihan

LeniCam Video Productions
http://www.lenicam.com


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Brent Hilgenkamp
Re: PCM vs AC3 recording for DVD
on Oct 12, 2010 at 12:02:40 am

If there is no clipping/going over 0dB onsite during the recording would the recorded AC3 audio be nearly identical to what the PCM would have been? I know this is a somewhat subjective matter though so any thoughts would be appreciated. Again since I don't have the cameras in house I'm kind of going on others' thoughts. Thanks


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john lenihan
Re: PCM vs AC3 recording for DVD
on Oct 13, 2010 at 3:29:03 am

That would be correct. If you have someone watch the level meter on the camera while the shooter shoots, and it never gets above 0db on the camera meter, than I would expect it to ultimately end up without the either kind of distortion, pcm clipping or ac3 popping.

John Lenihan

John Lenihan

LeniCam Video Productions
http://www.lenicam.com


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Devin Terpstra
Re: PCM vs AC3 recording for DVD
on Oct 15, 2010 at 12:26:07 am

Sounds like Brent is asking about the quality of the final audio, and doesn't have any issues with clipping or bad audio. I'm curious if any one is unhappy with AC3 audio that has been recorded at proper levels?



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