View Full Version : AC3 Audio Question

02-22-2009, 09:46 PM
Hey y'all. A quick context to my question: I'm shooting a feature doc with nearly 200 hours of footage, using FCP to transcode to ProRes and then Compressor to convert to another Quicktime-based format (still experimenting... any recommendations?) with a severely restricted bitrate so as to offline the HMC150 footage for storage concerns AND that the current editing machine is a PowerPC G5 (renting a computer to do the initial batch double transcoding). We've shot everything in PH 720p30.

That said, the audio is not something we can or want to "online" in the end. My understanding is that it's typical to offline with low-res video but full quality audio so that the audio track can be sent straight to be edited and mixed, which makes a ton of sense to me. So, my question is, does anyone here know how to use Compressor (or any other Mac-based utility) to convert into a restricted-bitrate video format that uses the original 384kbps AC3 (Dolby Digital) audio? Ideally the master clips would be self-contained and not have split video and audio.

Up till now, I've been converting to 1280x720 MPEG-4 video with a 500kbps bitrate for video and Linear PCM "Best render setting" audio.

Any ideas? Thanks everyone!

P.S. If anyone has a suggestion for a video format that I could convert to instead of MPEG-4 that would run smoothly on a PPC G5 that I can restrict the video bitrate on, I'd appreciate some advice on that too, as MPEG-4 isn't totally smooth on the G5!

And also, I know ProRes is "visually lossless", but I've not heard anything about the audio component. Does anyone know, when using FCP 6.0.5 to ingest my footage, what format and kbps the audio is?

02-23-2009, 02:21 AM
It's just a guess, but I would expect FCP transcoding to an uncompressed audio format. Most probably 48000 kHz LPCM.

02-23-2009, 02:32 AM
Ah I didn't realize that LPCM is uncompressed... so THAT's why I can't set a bitrate, eh? That makes so much sense! Guess I wasn't thinking clearly. In that case, any loss in audio quality would be negligible as far as taking it all the way to the mixing stage and mastering as LPCM. Agreed? Thanks for your reply (and thanks also for your script to combine MTS files - I don't need it but I recognize how valuable it may be to me in the future and how much so it is to everyone else here)!

02-23-2009, 05:12 AM
My guess could also be verified in a very simple manner. Try to locate the transcoded clips with the finder and load them into Quicktime. Quicktime will tell you exactly what codecs have been used for video and audio.

But yes, LPCM is uncompressed, there is still a data rate, but this can easily be computed from the resolution and sample rate.

02-23-2009, 01:26 PM
Thanks again for your responses and help, pailes. In fact, the FCP-ingested resultant ProRes files have an "audio format" (in Quicktime, under Movie Properties and under Inspector) of "Integer" - 16-bit Integer (Little Endian), Stereo L R, 48kHz.

I've never heard of "Integer" as an audio format. It may be 16-bit integer signed LPCM, but it's not specified by Quicktime or anywhere else I can find any properties.

But at any rate, it's no matter I suppose since LPCM is uncompressed and therefore will suffice for my needs of being the final audio format to take into mixing. (Although I still don't know what ProRes' audio format is and whether it's compressed at all. However, I imagine it's far more than sufficient since it's supposed to be an intermediate format.)

By the way, on the other minor question I brought up, do you have a suggestion of a different Quicktime-based (or otherwise FCP-native & fluidly editable) video format other than MPEG-4 that might work better on a PPC G5 for which I can also restrict the bitrate?