The question of data rates arises when it comes to making a decision on the choice of recording, storage media and also for editing. All cameras offer a wide range of recording formats, all of which with their own bit rates.
The data rate is the amount of data written, and is generally expressed in Mb/s or Mbps (megabits per second). And there it is, the confusion – particularly in French!
A bit is a 0 or a 1 and is therefore in binary data – however, do not confuse this with a Byte, which in English represents ‘Bytes’ but in French means a string of 8 bits! The issue is made even more complex by the fact that the cameras indicate the necessary bit rate in Mbps (therefore in bits), whereas storage media provides it in MB/S (and therefore in MegaBytes or Mega-Octets).
In other words, a card given at 95 MB/s (or 95 MB/s) in writing speed for example is theoretically capable of supporting 95 x 8 = 760 Mbps…
Conversely, a camera that requires 150 Mbps of data-rate will need 150 divided by 8 = 18.75 MB/s. To make matters more complicated, not all manufacturers of media specify whether the given data rate is to read or the data or write the data. We suggest it to be necessary to take a margin of maneuver for a minimum of 50%. For example, if we needed to shoot in a format at 150 Mbps then we would choose a card that writes at 18.75 MB/s + a 50% margin of 9.375 MB/s, therefore we would go with media that offers speeds of 30 MB/s.
Vxx to simplify it all?
In an attempt to simplify standards and other formats (UHS, SDXC etc.) and to avoid all of the problems noted above, media manufacturers have decided to simplify everything with the ‘V’ concept. In other words, a V30 card can both read and write at a guaranteed level of 30 MB/s, and the concept is the same for V60 (60 MB/s) and V90 (90 MB/s). This does not necessarily free you to calculate the conversion between the codec rate used by the camera and the selected card, but Panasonic do clearly indicate which ‘V’ card is needed to use its cameras in which mode.
To help you find your way through this, here is a table that shows the different speed classes quite well:
What about Ultra High Speed (UHS)?
The UHS is classified in I, II or II and is the maximum theoretical speed of the SD card bus (in other words, the interface between the memory card and the media reader).
The maximum speeds for the UHS classes vary, from 12.5 to 104 MB/s for UHS I and up to 624 MB/s for UHS III. So do we need to use UHS III cards at all costs as suggested in many forums? Not at all. Be wary that the vast majority of cameras have memory card slots at UHS II, which whilst for the moment are more enough, the maximum speed of UHS II is 312 MB/s giving us 312 x 8 = 2,496 Mb/s.
Here is the recent announcement of the new SD Express Cards. We’ll discuss in detail later: