1080p > 720p without losing audio sync

[pstnn]

Hi,

I'm currently in the process of converting some m2ts files from blurays to 720p with handbrake. I'd like to know if it is safe to use constant (not variable) "same as source" frame rate when converting? or is there a risk that the audio will unsync?

thank you!

[hello_hello]

Handbrake is very good at maintaining audio sync regardless of the output you use, but be careful with the frame rate you choose. If it doesn't match the actual video frame rate Handbrake will repeat or drop frames as required to output the requested frame rate.

For NTSC hybrid DVDs (combinations of "23.976 fps film" and "29.970fps video") choosing a constant frame rate "same as source" will possibly repeat frames during the film sections for a constant 29.970fps, which isn't ideal. That's the whole point of using a variable frame rate if need be.

For constant frame rate sources, which Bluray sources often are, you should get a constant frame rate even if you choose variable for the output.
If in doubt, check the log file after encoding. I don't use Handbrake much myself but the older version I have installed includes this sort of thing in the log file:

[17:26:11] + Framerate Shaper (1:27000000:1126125)
[17:26:11] + frame rate: 23.976 fps -> constant 23.976 fps

17:26:17] sync: got 1199 frames, 1200 expected
[17:26:17] render: 1199 frames output, 0 dropped and 0 duped for CFR/PFR

The last line isn't included unless you specify a constant or peak frame rate. If there's no frames being dropped or duplicated, ideally that'd be what you want.

[pstnn]

Handbrake is very good at maintaining audio sync regardless of the output you use, but be careful with the frame rate you choose. If it doesn't match the actual video frame rate Handbrake will repeat or drop frames as required to output the requested frame rate.

For NTSC hybrid DVDs (combinations of "23.976 fps film" and "29.970fps video") choosing a constant frame rate "same as source" will possibly repeat frames during the film sections for a constant 29.970fps, which isn't ideal. That's the whole point of using a variable frame rate if need be.

For constant frame rate sources, which Bluray sources often are, you should get a constant frame rate even if you choose variable for the output.
If in doubt, check the log file after encoding. I don't use Handbrake much myself but the older version I have installed includes this sort of thing in the log file:

[17:26:11] + Framerate Shaper (1:27000000:1126125)
[17:26:11] + frame rate: 23.976 fps -> constant 23.976 fps

17:26:17] sync: got 1199 frames, 1200 expected
[17:26:17] render: 1199 frames output, 0 dropped and 0 duped for CFR/PFR

The last line isn't included unless you specify a constant or peak frame rate. If there's no frames being dropped or duplicated, ideally that'd be what you want.

thank you,

so if I understand this correctly, for blurays, same as source + variable should be good as long as there are no dropped frames in the encode logs ?

[hello_hello]

so if I understand this correctly, for blurays, same as source + variable should be good as long as there are no dropped frames in the encode logs ?

No it's the other way around. A VFR output will never cause Handbrake to duplicate/drop frames. If the source is CFR the output should be CFR even if you select VFR. The last line in the log file I quoted tells you what Handbrake needed to do to achieve a CFR (or peak frame rate) output if you selected CFR (or peak frame rate) for the output.

I think the choice of VFR or CFR really only matters if you're using the decomb filter as it's inherently VFR. For hybrid NTSC sources (combinations of film and video) it outputs a combination of 23.976fps and 29.970fps, so when you select CFR as the output, Handbrake has to add/drop frames as required be to give you the desired constant frame rate. If you're not using the decomb filter (and for totally progressive, CFR film on Bluray you wouldn't need to) then VFR/CFR "same as source" should give you the same result. Hopefully that makes sense. If in doubt though, check the log file if you select a CFR output.