Take a look at the specifications for the Canon R5 C mirrorless cinema camera – it says “DCI 4K (4096 x 2160) at 119.88 fps [810 Mb/s]”. You may already know that “DCI 4K” refers to frame resolution and “119.88 fps” refers to frame rate, but what about “810 Mb/s”? Well, that is the bitrate – a specification that is commonly overlooked and important when it comes to filmmaking. In this article, we’ll breakdown what bitrate is, converting it to Megabytes per second, and how it affects one’s memory card choice.
WHAT IS BITRATE?
Bitrate refers to the amount of data that is created for each unit of playback time (usually in seconds) when recording video files. This number is dependent upon many factors including file type / compression technique, resolution, and frame rate. Generally speaking, the higher the recording bitrate, the higher the quality and file size will be. Most video formats record footage at bitrates measured in megabits per second (Mbps) [1,000 kilobits = 1 megabit], although the introduction of RAW video has increased bitrate usage to gigabits per second (Gbps) [1,000 megabits = 1 gigabit]. It’s important to note though that bits per second (bps) is not the same as bytes per second (Bps).
BITS VS BYTES:
For every 1 byte there are 8 bits, so for example, it would be incorrect to say that the Canon R5 C can record DCI 4K 120 fps video at a bit rate of 810 megabytes
per second. But rest assured, there is a quick and simple formula for converting bits to bytes.
[Bit Rate] / 8 = [Byte Rate]
[810 Mbps] / 8 = [101.25 MB/s]
Now you may be asking, “Why do I need to convert bits to bytes?” Memory card read & write speeds are measured and reported as megaBYTES per second, so it’s necessary to know how many megabytes of data your camera is generating each second (under the various settings) to ensure that the memory card you’re using can keep up.
CHOOSING THE RIGHT MEMORY STORAGE:
After converting from bits to bytes, make sure the memory card(s) you’re using offers write speeds fast enough for the recording mode you wish to use. Every memory card has a maximum potential read and write speed, but what’s even more valuable is knowing the minimum sustained write speed. Minimum sustained write speed is defined as the speed at which data can be sequentially written to a memory device over an extended period of time without ever falling below that performance metric.
Most of today’s memory cards provide a minimum sustained write speed in order to ensure that data is written swiftly and safely to the card, preventing recordings from randomly stopping or frames from being dropped. This can be identified as either Video Speed Class (SD & microSD) or Video Performance Guarantee (CompactFlash, CFast 2.0 and CFexpress Type A & B). Please refer to the table below for today’s commonly used speed ratings:
||MINIMUM SUSTAINED WRITE
|Video Speed Class 10
|Video Speed Class 30
|Video Speed Class 60
|Video Speed Class 90
|Video Performance Guarantee 65
|Video Performance Guarantee 130
|Video Performance Guarantee 400
By knowing the bitrates associated with settings you have chosen for your specific camera and shooting needs, you are able to make wise purchasing choices regarding recording media ensuring it has the bandwidth necessary to accommodate your recording needs. (Note: Card choice may vary from camera to camera and will be dependent on a combination of frame resolution, frame rate, bit rate, and overall capacity requirements)