Blackmagic Forum

Hi,
I made datarate measurements at three compression rates.
Compression 8 and 5 are fine, but not compression 3:

RAW UHD 3840 x 2160, 50 fps

Compression 8
Datarate calculated: 78 MByte/s
Datarate measured: 79 MByte/s, OK

Compression 5
Datarate calculated: 124 MByte/s
Datarate measured: 123 MByte/s, OK

Compression 3
Datarate calculated: 207 MByte/s
Datarate measured: 120 MByte/s, NOT OK

Same results with SD Card Sony Tough SF-M256T and CFast Card Transcend CFX650 128GB

Measurement done with a 10 sec file, Filesize (GB) devided by 10 sec.

The files look fine, no loss of frames

Any idea, what could be wrong?
Thanks

It depends on the content you've shot. Even when set to 3:1 it does not make sense to artificially inflate the data rate if unnecessary. If you shot something with a lot of bokeh then there is not enough texture and it will compress better than 3:1.

Do your test on a scenery with lots of detailed textures like trees, gras, a landscape.

That is interesting as I thought the fixed ratios had a constant bitrate. So 3:1 is about a third of what uncompressed video would be, 5:1 about a fifth, etc.

The Q0-Q5 rates are variable bitrate within a clip.

That’s not saying Robert is incorrect as it may be 3:1 is a constant bit rate for the clip, just not as many bits as we expect.

Mark Foster wrote:https://www.blackmagicdesign.com/support/faq/59025

see above: CFast Card Transcend CFX650 128GB

Robert Niessner wrote:It depends on the content you've shot. Even when set to 3:1 it does not make sense to artificially inflate the data rate if unnecessary. If you shot something with a lot of bokeh then there is not enough texture and it will compress better than 3:1.

Do your test on a scenery with lots of detailed textures like trees, gras, a landscape.

Thanks, recording a complex test chart at compression 3 indeed leads to 207 MByte/s as calculated.
BMD should not name this mode "constant bitrate", this is misleading.

Frank2021 wrote:

Robert Niessner wrote:It depends on the content you've shot. Even when set to 3:1 it does not make sense to artificially inflate the data rate if unnecessary. If you shot something with a lot of bokeh then there is not enough texture and it will compress better than 3:1.

Do your test on a scenery with lots of detailed textures like trees, gras, a landscape.

Thanks, recording a complex test chart at compression 3 indeed leads to 207 MByte/s as calculated.
BMD should not name this mode "constant bitrate", this is misleading.

Well, it is "constant" where necessary but does not waste bitrate when unnecessary. It is more like a constant maximum bitrate.

Live and learn! Of course I nearly always used to shoot with the Q0 setting on fairly complex scenes so perhaps that’s different given the huge variable bitrate range within a given quantization. I expect when I start using Q1, I’ll save space but I think I should test that to see.

Frank2021 wrote:

Mark Foster wrote:https://www.blackmagicdesign.com/support/faq/59025

see above: CFast Card Transcend CFX650 128GB

yes, this card is NOT in the recommended list

Mark Foster wrote:

Frank2021 wrote:

Mark Foster wrote:https://www.blackmagicdesign.com/support/faq/59025

see above: CFast Card Transcend CFX650 128GB

yes, this card is NOT in the recommended list

(I) In the camera manual from Jan. 2021 this card is recommended 10 times
(II) we found out, that it is not a card issue.

you read an other manual?



and yes, it is a problem when BMD not support the card

the touch is also too slow




and i had never any problems in my 4K and 6K pro with my angelbird CFast cards
in Q0 or 3:1 with 4K DCI or 6K



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Mark Foster wrote:you read an other manual?

Screenshot 2021-06-11 at 21.41.07.jpg

and yes, it is a problem when BMD not support the card

the touch is also too slow

Sony Tough SF-M256T.png

and i had never any problems in my 4K and 6K pro with my angelbird CFast cards
in Q0 or 3:1 with 4K DCI or 6K
.

Yes, Mr. Foster, my eyes are open and I do read another manual: Jan. 2021.
You can easily read this in my message above.
And I know, that the Tough is to slow. But the CFast Card Transcend CFX650 is fast enough!

PROBLEM SOLVED!

ok, i found the CFX650 card in the january manual.
but it seems that it is no longer supported since firmware 7.3.x

THIS WAS NOT MY PROBLEM ; -)


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