Author Topic: Card write speed drops when using crop 50p/60p mode  (Read 287 times)

has_abandonment_issu

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Card write speed drops when using crop 50p/60p mode
« on: April 05, 2019, 12:43:43 AM »
Using a Canon 5dmk3

Experimental 4k raw recording build with lossless compression

firmware 3.123

PixelFlash 1107x 64gb Supersport card

Loading modules crop_rec, mlv_lite, mlv_play, mlv_sound.

My card typical records between 90-100mb/s, but when I switch to the 50p/60p crop recording mode, the write speed drops to 63mb/s ish and stays there. If I try to record larger video it predictably stops recording within a few frames.

Is this a known issue? Am I doing something wrong?


Levas

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Re: Card write speed drops when using crop 50p/60p mode
« Reply #1 on: April 05, 2019, 01:02:25 PM »
I noticed some same behaviour on the 6d, I get lower then expected recording times with High FPS and very high resolution settings in crop_mode.

I'm not totally sure why, but it has to do with lossless recording and how it is implemented in MLV_Lite module in Magic Lantern.
Try using the same settings with raw 10 bit setting (10 bit NON lossless), you probably get longer recording times then with 14 bit lossless.

With lossless recording, the frames must be compressed in camera and after that written to the memory card.
I don't understand this all through, but it sort of works like this:
The writing to the memory card is not done frame by frame with lossless option in MLV_Lite, but in larger chunks. Larger chunks writes faster then smaller chunks.
But while the camera is waiting for a larger chunk, the memory buffer in cam fills up.
Now with high fps, or high resolution, this memory fill up happens really fast.
So now while your camera waits a little time before writing to the card, your memory is filled up. So you end up with an early recording stop because memory buffer is full.

So in this case, try 10 bit non lossless and see if you get more predictable and longer recording times.
 
 

has_abandonment_issu

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Re: Card write speed drops when using crop 50p/60p mode
« Reply #2 on: April 05, 2019, 05:59:00 PM »
Thank you for your response.

 I had considered that the compression may be eating up CPU resources which is slowing down the card. I just tested it at 10bit and unfortunately, the card is still slowed down for some reason, this time to around 73, which is an improvement, but still significantly slowed.

It seems like it's related to setting the camera to 60fps from the Canon menu. Once I turn that on, my card speed drops during recording. Turn it off and the card speed jumps back up to 95ish.

I'm not sure what's going on but that's where I am so far. Because the issue seems to happen when changing settings from the Canon menu, and not from magic lantern's menu, I am going to attempt to downgrade my firmware, and see if that changes anything. I will update if it does.

It's possible that changing the frame rate is also using CPU resources or something like that, but I'm going to try anyway.

has_abandonment_issu

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Re: Card write speed drops when using crop 50p/60p mode
« Reply #3 on: April 08, 2019, 06:33:11 PM »
Okay I downgraded my firmware to 1.1.3 and tried it again.

I am still getting slower rates. However, they are slightly faster. With global draw on I'm getting about 68mb/s, off is 73mb/s. 48p mode jumps to 76mb/s with global draw off.

Still slower than I wanted but a bit more usable. Unfortunately I think there is something inherent to running the camera at a higher frame rate that lowers write speeds. I don't know if this is a bug or a hardware bottleneck, but I am surprised I haven't seen anyone mention it before?