[FFmpeg-user] No option to specify dynamic range compression profile in AC3 encoding

Townson, Andrew E Andrew.Townson at charter.com
Fri Feb 16 17:03:01 EET 2018

ffmpeg Team,

According to the ffmpeg documentation the AC-3 encoder has options to access all Dolby metadata per the Dolby Metadata Guide link available on ffmpeg.org. This is true for all Dolby metadata except the Dynamic Range Compression (DRC) profile selection. We are experiencing volume discrepancies from set top boxes while playing content encoded from ffmpeg. We also use Minnetonka's Surcode for Dolby Digital 5.1 AC-3 encoding software. It does allow the selection of a DRC profile and there are no audio discrepancies as seen with ffmpeg. Is this an option that can be added or can someone offer a solution to this issue?

Command lines used:

First pass - ouput is parsed as $loudnorm variable with properties:

ffmpeg.exe -i input.wav -vn -af "loudnorm=print_format=summary" -f null -

Second pass:

ffmpeg.exe -i input.wav -vn -c:a ac3 -ac 2 -b:a 192k -ar 48000 -af "loudnorm=I=-24:offset=$($loudnorm.Target_Offset_LU):linear=true:measured_I=$($loudnorm.Input_Integrated_LUFS):measured_TP=$($loudnorm.Input_True_Peak_dbTP):measured_LRA=$($loudnorm.Input_LRA_LU):measured_thresh=$($loudnorm.Input_Threshold_LUFS)"  -dsur_mode 0 -dialnorm -24 -y output.ac3

At a later step this AC-3 is multiplexed into a MPEG2TS stream.

The output file created has a dialnorm stamp of -24 and is read at -24 LUFS with the Dolby Digital Meter utility and ffmpeg's loudnorm audio filter. So all appears acceptable, but the AC-3 consistently plays back at a lower volume level from the STB.

Any assistance would be appreciated. Thank you.
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