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Audio is slightly longer than video on export?

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John KnowlesAudio is slightly longer than video on export?
by on Jul 24, 2013 at 9:03:19 pm

Anyone ever run into this?

We just did a :30 commercial spot and delivered it to several outlets for regional broadcast. Most of them accepted our compressed output just fine (an H.264 QT, 1080 @ 29.97, PCM audio) but one outlet kept rejecting the file for being TOO LONG. We measured the file in several programs on the Mac (AE, FCP, QT 7, QT X) and it always came up as exactly 30 seconds every time. Even AE pegged it at 899 frames, so we definitely weren't over.

Today we finally got someone on the phone and he said it was the AUDIO that was running long, not the video. He sent us a screenshot from Vegas Pro (I guess their system is all PC-based) and it showed the video stream at :30.00 and the audio stream at 00:30:29 -- nearly a full second longer than the video! How could this happen? I always thought the length of each stream was identical, so why woudl the audio come out slightly longer?

One clue: the actual AIFF file used in the output comp is itself 00:31 long. I shortened it in the composition to fit within the time, and every setting for 'Duration' that I know of was set for 00:30, but it's like it just ignored the duration on output of the audio and exported the entire length of the track into the audio stream.

Here's my workflow -- I did use other programs to make the file so maybe something else is the culprit:
Finish in AECS6, render out to ProRes 4444 Master @ 23.98; re-import the file and drop into 29.97 comp to correct the frame rate for broadcast, render out to another 4444 Master file @ 29.97; dropped that into Apple Compressor to make H.264 file with PCM audio.

TIA, JVK




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John KnowlesRe: Audio is slightly longer than video on export?
by on Jul 24, 2013 at 10:19:35 pm

UPDATE:

The outlet just told me that the audio bit depth (24-bit) may have contributed to the problem. Would the bit depth actually make the audio run a second longer?

JVK



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Walter SoykaRe: Audio is slightly longer than video on export?
by on Jul 25, 2013 at 1:50:21 am

How curious.

I'd suggest downloading MediaInspector for Mac or MediaInfo for Windows -- either should report the length of each stream. Check each of your intermediate files and see where the problem originates.


[John Knowles] "Here's my workflow -- I did use other programs to make the file so maybe something else is the culprit: Finish in AECS6, render out to ProRes 4444 Master @ 23.98; re-import the file and drop into 29.97 comp to correct the frame rate for broadcast, render out to another 4444 Master file @ 29.97; dropped that into Apple Compressor to make H.264 file with PCM audio."

Sidebar #1: you can render the 29.97i with pull-down directly without rendering the intermediate first. Add your 23.976 comp to the render queue, then click into the Render Settings. Change the field order to upper field first, then select the cadence of your choice in 3:2 pull-down. Ae will take care of the 29.97i bit for you.

Sidebar #2: Compressor has about the worst H.264 compressor on the market. Adobe Media Encoder is both faster and generally produces better-looking or more size-efficient output. (Also, I think but am not 100% sure that AME could also add pulldown correctly for you from your 23.976 intermediate.) If you want to stick with an Apple workflow for compression, the x264 Quicktime component might be worth a look.

Walter Soyka
Principal & Designer at Keen Live
Motion Graphics, Widescreen Events, Presentation Design, and Consulting
RenderBreak Blog - What I'm thinking when my workstation's thinking
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