Blackmagic Forum

Hi, using the Blackmagic Cam app on my iphone 12 to shoot lectures.
I'm having audio drift problems, and would like to make sure at least my video footage is CBR.
Now when choosing AVC codec at a 30FS in the app settings, it turns out (dixit MediaInfo) that the Blackmagic cam is actually shooting VBR and not CBR :

Frame rate mode : Variable
Frame rate : 30.000 FPS
Minimum frame rate : 28.571 FPS
Maximum frame rate : 31.579 FPS

So is it even possible to shoot video at CBR in the app ?

This is not a BM Cam app issue. The iPhone 12 does not support constant bit rate (CBR) video recording. Instead, it uses variable bit rate (VBR) encoding, which dynamically adjusts the bit rate based on the complexity of the scene. This helps optimize storage by reducing the bit rate for less complex scenes while maintaining high quality for detailed or fast-moving scenes.

The iPhone 12 records video in HEVC (H.265) or H.264 formats, both of which use variable bit rate (VBR) encoding to balance quality and file size. The BM cam just uses this instead . If you need a constant bit rate (CBR) today, you would need to transcode the footage using a third-party application after recording. Unfortunately an in camera app like BM Cam will not likely add a transcoding feature for CBR to their app, at least not to support an older iPhone. I believe CBR is available on the iPhone 15 or newer, likely with intermediate codec format.

Although the OP wrote "variable frame rate" in the post and in the title, the metadata from MediaInfo quoted is regarding the frame rate being variable, not the bit rate.

Variable bit rate encoding does not cause audio drift. It is that variable frame rate, as shown in MediaInfo, that leads to audio sync issues.

And currently, every iPhone, even the latest most expensive 16 Pro Max, records in a variable frame rate.

And Androids too.
Smartphones are not a good solution for the task at hand.

thx for the feedback.
In that case, which budget friendly hardware update (i.e. camera shooting CBR) would you suggest ?

Anything that’s a dedicated camera, rather than a phone that can record video, will work fine.

The BMD 4K Pocket cameras are great, and if you pick up a used one it's also cheaper than an iPhone Pro. The m43 mount allows the possibility to easily adapt pretty much any old/used stills lenses you have access to.