We've been regularly shooting 4k 8 bit on the C200. We have exclusively been recording using Sandisk Extreme Pro 128gb 95MB/s SD cards. Since checking the c200 data specs online, I realised we are nowhere near the apparent 150MB/s write speed that the c200 outputs.
But we have never had an issue, not even write speed warnings, or noticeable video compression.
Can anyone shed some light on this? thanks!
It is definitely possible that you won't experience issues with your current card, but it could depend on many factors. The camera has its own buffer built in so it is likely that in the time you have been shooting with it, the card and buffer are able to work together and keep things moving fine for the duration of the clip, for example.
One important thing to keep in mind especially when using a slower writing card is to leave the camera powered on for about 30 seconds to a minute after you finish recording so that the camera can clear the buffer and finish writing and finalizing the data onto your card.
Unfortunately there's a ton of confusion about bit rates, which is understandable, as manufacturers seem to make this as confusing as they possibly can. Sadly I think you've been a victim of this.
First, the C200's bit rate (shooting MP4) is 150 Mb/s -- that's 150 megaBITS per second. Not 150 MB/s -- which is 150 megaBYTES per second, or 1,200 megabits per second. Lower-case 'b' is bits, upper-case 'B' is bytes. A byte is 8 bits.
Second, the speed "rating" advertised on the memory card -- 95 MB/s, which is 95 megaBYTES per second, or 760 megaBITS per second -- is total garbage. This number is made up by marketing people, based on the absolute best-case possible write time for a single block of data. It has no relevance for video. Ignore it or risk frame dropping and/or corrupted video.
Basically this is saying that once in a blue moon, the card may actually be able to take data at that rate. If capturing a video frame correctly once in a blue moon is good enough for you, then I don't know what you're doing. For video, you need the memory card to record EVERY SINGLE FRAME, even in the WORST-case scenario.
The ONLY speed rating which matters is the "v" speed class, which you will find on the front of any good video-rated SD card. This rating is based on the WORST case, not the best case. If your card doesn't have a "v" rating, then you risk losing data, if a frame needs to be written while the card happens to be doing some housekeeping. There is a LOT more going on inside SD cards than most people realise.
The number in the "v" rating is MB/s; i.e. megaBYTES per second. So you need to convert the camera's bit rate to a byte rate -- divide by 8. For the C200, at 150 Mb/s, that's about 20 MB/s, so you need a card rated at v30 or higher. Anything less is a risk. Note that I would always allow some headroom on these speeds, depending on how much I trust the manufacturer, so v30 for 20 MB/s seems OK.
A U3 card may work for you. This is technically the same speed as v30, but the "U" rating criteria are looser, and may not guarantee performance in high-end video recording. But it may be OK.
Of course you have to choose your SD cards based on the modes you're going to be recording. On my C70, in slow&fast mode, it can record up to 545 Mb/s, so a v90 card is required for that mode. I use v60s for normal recording.
Hope this helps.