BD Rebuilder strongly insists on encoding 5.1 LPCM audio to AC3

[Sebarino]

To be honest, I'm not sure BD Rebuilder is designed to accept uncompressed 24 bit 48 Khz 5.1 audio. So I'm not exactly complaining, but I would like to know if there's a way to force it to not touch audio tracks like the one I described, kind of like the FORCE_NOENCODE=1 option does for video.

This is a 10 minute test file for a personal project. The plan is to author a 4K Blu-ray, so I'm running different tests to see what works best, but in this case it's an export from DaVinci Resolve Studio to MKV, a 2160p 24 fps 8 bit Rec.709 video at 50 Mbps with LPCM 5.1 24-48 audio. So I made sure the option to keep BD audio intact was enabled, but that kept encoding.

So I looked into the hidden options doc to see if anything else could help and saw the KEEP_HD_LPCM=n option, which I copied and pasted into the ini file and setting it to =1. I thought that would do it for sure, but nope. Still encoding to AC3.

So, is it possible for BDRebuilder to author a Blu-ray or 4K Blu-ray with LPCM audio untouched? If so, what other options I have to set?

[ProWo]

So, is it possible for BDRebuilder to author a Blu-ray or 4K Blu-ray with LPCM audio untouched? If so, what other options I have to set?

QUESTION: The Audio checkboxes under SETUP are a little confusing. Can you explain them?

ANSWER:

The switches have a different impact depending upon your output selection. You also have to remember that because of its huge size, LPCM is treated differently

If rebuilding output greater than BD-9 (e.g. BD-25):

1. Default (all audio options unchecked)
---- All audio is converted to DD AC3. Bitrate is 640Kbs for 5.1 and 224Kbs for 2.0.
---- All 7.1 sources are down-converted to 5.1
1.1 448Kbs selected -- 5.1 = 448Kbs, 2.0 = 224Kbs
1.2 192Kbs selected, all sources down-converted to 2.0 @ 192Kbs

2. "Keep HD" overrides all other audio settings. If selected:
---- DTS HD keeps a bit-for-bit copy of the original audio
---- DD HD keeps a bit-for-bit copy of the original audio
---- LPCM keeps a bit-for-bit copy of the original audio
---- You may get failure due to inadequate target space if much LPCM is used

2.1. If you use "Keep HD" and also add "KEEP_HD_LPCM=0" as a hidden option:
---- LPCM is converted to AC3 at a bitrate determined by other audio selections
---- Other audio tracks are bit-for-bit copies of the original

[Sebarino]

2. "Keep HD" overrides all other audio settings. If selected:
---- DTS HD keeps a bit-for-bit copy of the original audio
---- DD HD keeps a bit-for-bit copy of the original audio
---- LPCM keeps a bit-for-bit copy of the original audio
---- You may get failure due to inadequate target space if much LPCM is used

2.1. If you use "Keep HD" and also add "KEEP_HD_LPCM=0" as a hidden option:
---- LPCM is converted to AC3 at a bitrate determined by other audio selections
---- Other audio tracks are bit-for-bit copies of the original

Well, therein lies the problem. If Keep HD is enabled, and the audio is LPCM, it should do a bit by bit copy of the original audio. But, in the log I can see it as it goes through the tasks, and it says Track 4352 (und): Reencoding audio to AC3.

And while the KEEP_HD_LPCM=0 option may convert LPCM to AC3, it seems obvious that if KEEP_HD_LPCM=1, then it should tell BDRB to not convert at all. But I still see the same thing as above.

Does that make sense?