ProRes bitrate lower than expected on Alexa Mini

Sat Feb 27, 2016 11:56 pm

I'm working with two Alexa Minis right now. We are shooting 2K ProRes 444 XQ with Sandisk 128GB cards, all 23.98fps. Every time I format a new card in camera, it says 31min remaining. But when I fill up a card and download it, there is about 42min of footage. This seems to be a bug in the camera.
It is interesting, so I started doing some research.

I calculated the average bitrate for each card spanning 2TB of footage. The average is 386Mbps, with standard deviation of 14. This footage is all ProRes XQ 2048x1152. Apple's white paper doesn't list that frame size exactly, but for 2048x1080, it says the data rate should be 453Mbps (ProRes XQ no alpha).

Then I looked at some footage from a different job and a different Mini. This is 3840x2160 ProRes 444. The average bitrate is 934Mbps. Apple's white paper says it should be 1061Mbps.

Why is the bitrate so low? I'm afraid it might be somehow losing quality by recording lower than it should. The three cameras I've observed were running firmwares 3.01.01 and 3.00.24.
Elliott Balsley
 
Posts: 22
Joined: Mon Jan 18, 2016 8:39 pm

Re: ProRes bitrate lower than expected on Alexa Mini

Tue Mar 01, 2016 5:47 pm

Hello Llama Films,

Apple's ProRes is a variable bitrate codec therefore you will end up with different data rates "by design". This is also why the recording length varies. As long as you are not experiencing any artifacts or image quality problems, a lower datarate is not an issue.

Kind regards,
Jan
Jan Heugel
Application Engineer
Jan Heugel
 
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Joined: Wed Aug 13, 2014 3:15 pm
Location: Munich, Germany

Re: ProRes bitrate lower than expected on Alexa Mini

Wed Mar 02, 2016 8:54 pm

I know it's a VBR codec, but the remaining time indicator is consistently wrong in my tests, with a variety of indoor and outdoor shooting. It always says 31min for a fresh card, and I always fit about 42min. This is a difference of 35%, and I don't think the bitrate ever varies THAT much.
Elliott Balsley
 
Posts: 22
Joined: Mon Jan 18, 2016 8:39 pm

Re: ProRes bitrate lower than expected on Alexa Mini

Sat Mar 05, 2016 12:15 am

Hi,
the time indicator is designed to be conservative and to show the worst case scenario. For ProRes XQ you have to shoot something with a lot of detail or high entropy to get to the target data rate, e.g. very high contrast leaves.
The compressor will not use the bandwidth until it actually needs it and ProRes XQ is a pretty high data rate already.

-michael
Michael Jonas
 
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Joined: Thu Mar 26, 2015 3:28 pm

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