SD cards and speeds

SD card Video Speed Classes are a specific classification system designed to indicate the sustained minimum write speeds of SD cards for video recording. These classes are essential for ensuring that your SD card can handle the data rate necessary for smooth video recording without dropouts or interruptions. This classification is simply indicated by "V" for "Video speed class" followed by the minimum sustained speed measures in MB, or megabytes. For example:

• V30 (30MB/s): V30 cards guarantee a minimum sustained write speed of 30 megabytes per second.

• V60 (60MB/s): V60 represents a minimum sustained write speed of 60 megabytes per second.

• V90 (90MB/s): V90 cards have a minimum sustained write speed of 90 megabytes per second.

As you can see, the classification simply is "V" for "Video speed class" followed by the minimum sustained speed measures in MB, or megabytes.

Now, let's clarify the difference between megabytes (MB) and megabits (Mb):

• Megabyte (MB): A megabyte is a unit of data storage and transfer, commonly used to measure file sizes and data capacity.

• Megabit (Mb or mb): A megabit, on the other hand, is a unit of data transfer rate or bandwidth and is often used to measure data transmission speeds, such as internet connection speeds or video bitrates. 1 megabit is equal to 1/8th of a megabyte (MB), therefore 1MB=8Mb. Mb and mb are often used interchangeably, and both refer to megabits.

All of the bitrates shown in the Sony A7 series of cameras are expressed in megabits. For example "XAVC S 4k 60p 150M 4:2:0: 8bit" means the SD card will be written to at a maximum of 150mbps, or 150 megabits per second, which is only 150mbps÷8=18.75MBps, or 18.75 megabytes per second, which is way below the required 30MBps rating of v30 SD cards by definition.

In summary, understanding SD card Video Speed Classes helps you choose the right card for your video recording needs, while recognizing the difference between megabytes and megabits is crucial for assessing both storage capacity and data transfer rates in various applications, including video production.

References:

1. Speed Class Standards for Video Recording by The SD Association, accessed on September 12, 2023.

2. Quick Guide To Memory Cards For Video With The Sony Alpha 7S III by Sony Alpha universe, accessed on September 12, 2023.

3. "The BEST SD Card for VIDEO & What Do The Numbers Mean?" By Gerald Undone on Youtube.