12-12-2017 12:48 PM
"the maximum transfer rate hero6 have to push to the SD card, is around 80-82Mbps when 4K50fps PT on - here we are dangerously close to the max of 90MB/s ...."
Do not confuse bits and Bytes. 1 Byte = 8 bit
80Mbps = 10MBps
12-13-2017 08:36 AM
no, I'm not confusing those.... ....
below is a response I wrote some time ago on this forum.....it is about burst of info, in short time, written by camera on SD card, from multiple source (video, audio, GPS, etc.) in a 1/fps time frame....
this is why, when 4K started to be widely implemented, the old 90MBps is not a good measure for sustaining 4K recording.
Vxx standard cope with, the tests are different, etc.....
a long time ago I started to wonder about this apparent discrepancy.
what is next is not rocket science, is just a way I tried myself to explain this.
I would not point only to GoPro....e.g. see also the Panasonic GH5 specs and requirements for SD cards.
why for an 78Mbps (4k50fps PT on - see below) you need a V30 card, meaning 30MBs = 240Mbps ???
but here the math does not help us....just because the simple math does not explain the complex process of writing a video file.
It is not as simple as transferring, copying a normal file on the SD card. This will be done at those max speeds card is tested
let's take a simple photo, after all the frames are still pictures taken every 1/50sec if we shoot at 50fps...
a 4MB picture.
if I have to write 4MB every 1/50sec I will have a transfer rate needed of 200MBps.
in 1/50sec =20ms the camera will have to read the sensor, apply compression, write on the SD card not only the video, but audio, gps, frame informations, timestanps, etc...multi-source access in the same file.
let's suppose the codecs are much aggressive that the jpeg compression and the video info is much less.
let's say 2MB for each frame we stiil have to have 100MBps transfer rate.
and so on....
and, my feeling is, the maximum or sustainable speed is just a measure of what really counts: the response time.
the process is to write a chunk of info, not continuously, but in bursts on SD card...and SD card have to be responsive enough.
and rated speed is a measure of the responsiveness of the SD card to accepts bursts.
this is why some experienced frames lost... because SD card is not in line with this process....and that frame is dropped, the SD card was not capable to "accept" it in that 1/fps sec
when 4K came, SD card standard was changed to those V...V30, V60 (30MBps, 60MBps, etc...)...for a very good reason.
U3 card is also 30MBps, same as V30
maybe this response time for bursts writing is the answer...just maybe....
so, I made that equivalence 80kbps result in final video file have to be sustained by an 80MBps card (I'm referring at the max read/write speed no at the V)
ofcourse design matters.
buffers both on card and on GoPro, data bus speed, etc.
put here the compatibility and you will find a great mix (mess)....
12-13-2017 09:56 AM
You are right, in a way... But often, the bottleneck is the cameras ability to actially write to the card. It may have all the capabilities in the world inside, to process the raw data from the sensor. But the manufactorer decides how fast they "need" the writing capabilities to be.
I'm not aware of the actual speed of the write interface on the GoPro's, but here's a chart from Magic Lantern, concerning Canons camera's.
Here you can see that every Canon DSLR with SD cards, have a limitation of 40MB/s.
So I guess GoPro has a limitation as well. And therefore must compress 4k video to fit that limitation. And no matter what speed your card is, it cannot write faster than the interface on the GoPro...
Would be nice to know what the max write speed actually is. :)
12-13-2017 10:17 AM
hey @megastorm28161 !
good valuable info! thank you!
and those are SD and CF cards!!!!!!!!
I remember back in my Nikon DSLR period (that ended when I realized that carrying 10kg of optics in city brakes on airplanes, hand luggage, and by no means on my MTB bicycle, is a solution...) , I've tested many SD cards...D750 had a very small buffer, so when continuous shooting, after 7-15 pictures, the DSLR started to hickup...therefore I needed the fastest SD card for RAW (in one slot) and one of the fastest in the other slot (for jpeg).
oh boy, what differences at the same advertised specs...I made tests counting the pictures saved (10 per sec. for example) till started to slow down the pace...
so yes...it is complicated...
12-13-2017 10:37 AM
The first burst is possible because of the cameras internal memory buffer. When it fills up, that's when you can start to "measure" the transfer speed. Limited either by the SD card or the write interface of the camera.
But this is of course a sidetrack of this discussion. :)
Back on track! If GoPro wanted, they could make as fast interface as possible, and enable the raw data from the sensor to be written directly to the card. Wouldn't that be great!!!??? :D
But I guess it will take some years... Considering Canon 5DMk3 needs just a bit (no pun intended) over 100MB/s to shoot 1080P/30 in RAW. Imagine 4K/60...
But still. Give me 1080/30 in RAW, and I'd gladely buy it. It is a nightmare to get acurate colors under water, when the camera have already mistreated and abused the stream from the sensor. I'd ditch 4K Protune over 1080 RAW, any day of the year. One for over water and one for under. :)