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Downsample multi-channel wav

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Terry Barnum
Downsample multi-channel wav
on Mar 24, 2020 at 12:48:19 am

Like others here I'm trying to come up with better remote / proxy workflows. The task for today is to get a Library to an editor via file transfer. I have the video reduced from 60GB to 8GB using Steve @ Ripple's HEVC trick and FCPX is happily relinking, but I'm having trouble reducing several days of multi-channel wav files with ffmpeg that total 10GB.

The ffprobe output shows some differences in Metadata but the reported duration and channels are the same but at about 1/4 the size.

Anybody successfully reducing multichannel wav files that FCPX will relink?

ffmpeg:
$ ffmpeg -guess_layout_max 0 -i file.wav -ar 22050 ../newfolder/file.wav

ffprobe output:
Original:
Input #0, wav, from '101T01.WAV':
Metadata:
comment : sSPEED=023.976-ND
: sTAKE=01
: sUBITS=$00000000
: sSWVER=4.51.01
: sSCENE=101
: sFILENAME=101T01.WAV
: sTAPE=DAY1
: sCIRCLED=FALSE
: sTRK1=MixL
: sTRK2=MixR
: sTRK3=LAV1
: sTRK4=LAV2
: sNOTE=
:
encoded_by : Sound Dev: Mix664 S#KA0713225002
originator_reference: USSDVKA07132250022002198QB1XU 11
date : 2020-02-19
creation_time : 08:26:22
time_reference : 1459265809
coding_history : A=PCM,F=48000,W=24,M=multi,R=48000,T=4 Ch
:
Duration: 00:00:24.02, bitrate: 4610 kb/s
Stream #0:0: Audio: pcm_s24le ([1][0][0][0] / 0x0001), 48000 Hz, 4 channels, s32 (24 bit), 4608 kb/s

Reduced:
Input #0, wav, from '101T01.WAV':
Metadata:
comment : sSPEED=023.976-ND
: sTAKE=01
: sUBITS=$00000000
: sSWVER=4.51.01
: sSCENE=101
: sFILENAME=101T01.WAV
: sTAPE=DAY1
: sCIRCLED=FALSE
: sTRK1=MixL
: sTRK2=MixR
: sTRK3=LAV1
: sTRK4=LAV2
: sNOTE=
:
date : 2020-02-19
encoder : Lavf58.29.100
encoded_by : Sound Dev: Mix664 S#KA0713225002
Duration: 00:00:24.02, bitrate: 1411 kb/s
Stream #0:0: Audio: pcm_s16le ([1][0][0][0] / 0x0001), 22050 Hz, 4 channels, s16, 1411 kb/s

Thanks,

-Terry


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Brad Hurley
Re: Downsample multi-channel wav
on Mar 24, 2020 at 12:38:20 pm

It looks like you changed both the bit depth and the sample rate. It would make sense to reduce the bit depth from 24 to 16 to save space, but if you changed the sample rate isn't that going to affect playback speed? What happens if you only reduce the bit depth (from 24 to 16) but don't change the sample rate? You should still get smaller files and FCPX should be able to relink them.

Another option, but more tricky and tedious, would be to remove the stereo mix from those polywav files if you're only using the ISOs. You can do that with Sound Devices' free Wave Agent app, but I think you'd have to split the polywavs and reassemble them and I'm not sure if all the metadata would be preserved in that process. Note that you can edit metadata in Wave Agent, but this approach is likely too much effort for what it's worth if you have a lot of files.

On some of the Sound Devices recorders you can set them up to record only the ISOs and not the stereo mix, which will reduce the size of your polywav files...that could be an option for future projects.


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Terry Barnum
Re: Downsample multi-channel wav
on Mar 24, 2020 at 5:20:37 pm

Thanks Brad. The sample rate change doesn't affect playback speed. ffmpeg did the bit depth conversion from 24 to 16 on its own--maybe a default behavior when going to wav. I'll give it a shot to just do the bit depth and report back but was hoping for more file size reduction with a sample rate change.

-Terry


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Brad Hurley
Re: Downsample multi-channel wav
on Mar 24, 2020 at 5:53:55 pm

That's odd about the sample rate change not affecting playback. Normally, changing the sampling rate of a file recorded at, say, 96 kHz to 48 kHz would make the file play back at half its originally recorded speed and pitch. I'm assuming Final Cut expects to receive files recorded at 48 kHz, which is the norm for video, but maybe it automatically adjusts the project sample rate based on the sample rate it detects in the file?

Removing the stereo mix (channels 1 and 2 in the example file you converted) should cut the size of your polywav files in half; most people wouldn't use the L-R mix and would use the individual ISOs (channels 3 and 4 in your example, for the two lav mics) instead so they can remix their individual levels to taste.


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Terry Barnum
Re: Downsample multi-channel wav
on Mar 24, 2020 at 6:11:30 pm

Unfortunately, no go. Tried using both sox and ffmpeg. Changing either the sample rate or the bit depth causes FCPX to refuse to relink. I'll submit a feature request.

-Terry


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