Tue Aug 25, 2020 10:13 pm

I think the SUBJECT title say it all...
I would like the choice to save with no audio to keep file size down, as I like at do multiple versions of a track to edit up in a to drop on a video editor timeline to mess with..(audio track is on the time line).

Posts: 9
Joined: Tue Apr 03, 2018 6:15 pm


Wed Aug 26, 2020 9:50 am

The audio contributes a very small percentage of the file size. A quick estimate from the relative bitrates suggests a maximum potential saving of 2% for a very modest video bitrate of 10Mbps and a audio bitrate of 192kbps.

Much more effective at reducing file size would be an option for Variable Bit Rate (VBR) encoding of the video.
Terry Payman
Posts: 598
Joined: Sun Sep 14, 2014 8:15 am
Location: UK


Wed Aug 26, 2020 2:41 pm

In Magic, x264 is VBR. The Quality parameter attempts to minimize the data in each frame while still achieving fidelity above a certain threshold. Thus, each frame can vary widely if it is (for example) a smooth gradient (very little data needed) vs a forest scene (lots of detail).

From https://slhck.info/video/2017/02/24/crf-guide.html:

CRF versus Constant QP

CRF is a “constant quality” encoding mode, as opposed to constant bitrate (CBR). Typically you would achieve constant quality by compressing every frame of the same type the same amount, that is, throwing away the same (relative) amount of information. In tech terminology, you maintain a constant QP (quantization parameter). The quantization parameter defines how much information to discard from a given block of pixels (a Macroblock). This typically leads to a hugely varying bitrate over the entire sequence.

Constant Rate Factor is a little more sophisticated than that. It will compress different frames by different amounts, thus varying the QP as necessary to maintain a certain level of perceived quality. It does this by taking motion into account. A constant QP encode at QP=18 will stay at QP=18 regardless of the frame (there is some small offset for different frame types, but it is negligible here). Constant Rate Factor at CRF=18 will increase the QP to, say, 20, for high motion frames (compressing them more) and lower it down to 16 for low motion parts of the sequence. This will essentially change the bitrate allocation over time.
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