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Hello everyone, as the title implies, I need help sorting out compression for Dolby AC-3 audio. I've been working on restoring film content and what not from optical media, mainly for a much more extensive project in the future. For the most part my restoration efforts have been relatively smooth (minor format mishaps here and there to deal with, nothing major). One of the main aspects I've been exploring is restoring the contents (mainly films) back to their true 24 FPS form. One of the DVD rips I've been working on obviously uses Dolby AC-3 audio, I did some tempo tweaking to make it sync with 24 FPS (bumped from syncing at 23.976 FPS). I wanted to retain the formatting of the audio for compatibility reasons, and for player / receiver usage. I've basically replicated majority of the configurations for the new AC-3 audio when compared to the original DVD source, except for the compression information. I am having trouble figuring out how to set that information for the encode.
NOTE:
Yes, I am aware that I could just convert everything to PCM and what not, but I don't want to do that. I want to retain the audio format used by the DVD for the new tweaked version of the audio. I'm already doing my best to avoid encoding the audio more than necessary as to prevent further loss (Dolby AC-3 audio is a lossy format after all). I just need help sorting out how to set compression for the encode.
I have metadata to express what I am looking for.
MediaInfo metadata of the original Dolby AC-3 audio: (Normal)
MediaInfo metadata of the original Dolby AC-3 audio: (Debug)Code:Audio Format : AC-3 Format/Info : Audio Coding 3 Commercial name : Dolby Digital Duration : 1 h 9 min Bit rate mode : Constant Bit rate : 448 kb/s Channel(s) : 6 channels Channel layout : L R C LFE Ls Rs Sampling rate : 48.0 kHz Frame rate : 31.250 FPS (1536 SPF) Compression mode : Lossy Stream size : 222 MiB (100%) Service kind : Complete Main Dialog Normalization : -24 dB compr : -0.28 dB Center mix level (cmixlev) : -3.0 dB Surround mix level (surmixlev) : -3 dB dmixmod : Lt/Rt ltrtcmixlev : -3.0 dB ltrtsurmixlev : -3.0 dB lorocmixlev : -3.0 dB lorosurmixlev : -3.0 dB Dialogue normalization, average : -24 dB Dialogue normalization, minimum : -24 dB Dialogue normalization, maximum : -24 dB
Notice the compr and dynrng flags. The dynrng flags are only visible with debug metadata enabled for some odd reason, but the compr flag is visible regardless, and is set to -0.28 dB. I would like to set that compr flag to -0.28 dB with my new Dolby AC-3 audio.Code:Audio Count : 326 Count of stream of this kind : 1 Kind of stream : Audio Kind of stream : Audio Stream identifier : 0 Inform : 448 kb/s, 48.0 kHz, 6 channels, AC-3 (Dolby Digital) Format : AC-3 Format : AC-3 Format/Info : Audio Coding 3 Format/Url : https://en.wikipedia.org/wiki/AC3 Commercial name : Dolby Digital Commercial name : Dolby Digital Format settings, Endianness : Big Duration : 4153600 Duration : 1 h 9 min Duration : 1 h 9 min 13 s 600 ms Duration : 1 h 9 min Duration : 01:09:13.600 Duration : 01:09:13.600 Bit rate mode : CBR Bit rate mode : Constant Bit rate : 448000 Bit rate : 448 kb/s Channel(s) : 6 Channel(s) : 6 channels Channel positions : Front: L C R, Side: L R, LFE Channel positions : 3/2/0.1 Channel layout : L R C LFE Ls Rs Samples per frame : 1536 Sampling rate : 48000 Sampling rate : 48.0 kHz Samples count : 199372800 Frame rate : 31.250 Frame rate : 31.250 FPS (1536 SPF) Frame count : 129800 Compression mode : Lossy Compression mode : Lossy Stream size : 232601600 Stream size : 222 MiB (100%) Stream size : 222 MiB Stream size : 222 MiB Stream size : 222 MiB Stream size : 221.8 MiB Stream size : 222 MiB (100%) Proportion of this stream : 1.00000 Service kind : CM Service kind : Complete Main Bitstream ID (bsid) : 6 Dialog Normalization : -24 Dialog Normalization : -24 dB compr : -0.28 compr : -0.28 dB AC-3 mode (acmod) : 7 LFE on : 1 Center mix level (cmixlev) : -3.0 Center mix level (cmixlev) : -3.0 dB Surround mix level (surmixlev) : -3 dB Surround mix level (surmixlev) : -3 dB dmixmod : Lt/Rt ltrtcmixlev : -3.0 ltrtcmixlev : -3.0 dB ltrtsurmixlev : -3.0 ltrtsurmixlev : -3.0 dB lorocmixlev : -3.0 lorocmixlev : -3.0 dB lorosurmixlev : -3.0 lorosurmixlev : -3.0 dB Dialogue normalization, average : -24 Dialogue normalization, average : -24 dB Dialogue normalization, minimum : -24 Dialogue normalization, minimum : -24 dB Dialogue normalization, maximum : -24 Dialogue normalization, maximum : -24 dB Dialogue normalization count : 32 compr_Average : 1.03 compr_Average : 1.03 dB compr_Minimum : 0.53 compr_Minimum : 0.53 dB compr_Maximum : 1.49 compr_Maximum : 1.49 dB compr_Count : 28 dynrng_Average : 0.59 dynrng_Average : 0.59 dB dynrng_Minimum : 0.00 dynrng_Minimum : 0.00 dB dynrng_Maximum : 1.02 dynrng_Maximum : 1.02 dB dynrng_Count : 32
MediaInfo metadata of my new, work in progress, Dolby AC-3 audio: (Normal)
MediaInfo metadata of my new, work in progress, Dolby AC-3 audio: (Debug)Code:Audio Format : AC-3 Format/Info : Audio Coding 3 Commercial name : Dolby Digital Duration : 1 h 9 min Bit rate mode : Constant Bit rate : 448 kb/s Channel(s) : 6 channels Channel layout : L R C LFE Ls Rs Sampling rate : 48.0 kHz Frame rate : 31.250 FPS (1536 SPF) Compression mode : Lossy Stream size : 222 MiB (100%) Service kind : Complete Main Dialog Normalization : -24 dB Center mix level (cmixlev) : -3.0 dB Surround mix level (surmixlev) : -3 dB dmixmod : Lt/Rt ltrtcmixlev : -3.0 dB ltrtsurmixlev : -3.0 dB lorocmixlev : -3.0 dB lorosurmixlev : -3.0 dB Dialogue normalization, average : -24 dB Dialogue normalization, minimum : -24 dB Dialogue normalization, maximum : -24 dB
As you can see with my audio so far, I'm missing all of the compr and dynrng flags. I haven't found a solution yet that lets me apply these flags. I use FFmpeg for the most part with majority of encodes, and the documentation so far hasn't proved anything fruitful for this particular metadata (other than the drc_scale option, which didn't yield what I wanted). I did attempt to research online about these flags, but majority of posts merely talk about removing this information, rather than adding it. What would I need to do to append compr and dynrng flags like the original audio?Code:Audio Count : 310 Count of stream of this kind : 1 Kind of stream : Audio Kind of stream : Audio Stream identifier : 0 Inform : 448 kb/s, 48.0 kHz, 6 channels, AC-3 (Dolby Digital) Format : AC-3 Format : AC-3 Format/Info : Audio Coding 3 Format/Url : https://en.wikipedia.org/wiki/AC3 Commercial name : Dolby Digital Commercial name : Dolby Digital Format settings, Endianness : Big Duration : 4149472 Duration : 1 h 9 min Duration : 1 h 9 min 9 s 472 ms Duration : 1 h 9 min Duration : 01:09:09.472 Duration : 01:09:09.472 Bit rate mode : CBR Bit rate mode : Constant Bit rate : 448000 Bit rate : 448 kb/s Channel(s) : 6 Channel(s) : 6 channels Channel positions : Front: L C R, Side: L R, LFE Channel positions : 3/2/0.1 Channel layout : L R C LFE Ls Rs Samples per frame : 1536 Sampling rate : 48000 Sampling rate : 48.0 kHz Samples count : 199174656 Frame rate : 31.250 Frame rate : 31.250 FPS (1536 SPF) Frame count : 129671 Compression mode : Lossy Compression mode : Lossy Stream size : 232370432 Stream size : 222 MiB (100%) Stream size : 222 MiB Stream size : 222 MiB Stream size : 222 MiB Stream size : 221.6 MiB Stream size : 222 MiB (100%) Proportion of this stream : 1.00000 Service kind : CM Service kind : Complete Main Bitstream ID (bsid) : 6 Dialog Normalization : -24 Dialog Normalization : -24 dB AC-3 mode (acmod) : 7 LFE on : 1 Center mix level (cmixlev) : -3.0 Center mix level (cmixlev) : -3.0 dB Surround mix level (surmixlev) : -3 dB Surround mix level (surmixlev) : -3 dB dmixmod : Lt/Rt ltrtcmixlev : -3.0 ltrtcmixlev : -3.0 dB ltrtsurmixlev : -3.0 ltrtsurmixlev : -3.0 dB lorocmixlev : -3.0 lorocmixlev : -3.0 dB lorosurmixlev : -3.0 lorosurmixlev : -3.0 dB Dialogue normalization, average : -24 Dialogue normalization, average : -24 dB Dialogue normalization, minimum : -24 Dialogue normalization, minimum : -24 dB Dialogue normalization, maximum : -24 Dialogue normalization, maximum : -24 dB Dialogue normalization count : 32
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