Defining Data Transfer Read and Write Speeds for Photography and Motion Capture
Wes Brewer, CEO ProGrade Digital
The Value of Data Transfer Speed in Today’s Photo and Cinema/Video Applications
From the earliest days of marketing memory cards for use in digital cameras, manufacturers have used performance (data transfer speed) as a measure of product “value positioning.” One underlying reason for this is that some applications can truly benefit from a faster transfer speed, and so the measure of performance has had real value to certain buyers.
A second reason is that the underlying cost of producing a faster card is generally higher, and therefore creates a need to charge more.
Lastly, there is simply a need for product differentiation by the card manufacturer–faster performance must be better. The consumer will therefore assume greater value in a higher speed and be willing to pay more for it– whether he needs it or not.
The question of whether transfer speed matters or not is best determined by looking at how a memory card is being used. And to truly understand the value of speed, it must be first noted that the industry measures data transfer performance in two ways: “to the card” (writing data), as well as “from the card” (reading data):
In a typical digital camera, a high-speed buffer is used to ensure that, for most normal capture, the speed of data being written to the card is never an issue for rapidly saving a nominal quantity of photos. As photos or video are captured, these data files are immediately stored (temporarily) in a buffer–typically built with DRAM (Dynamic Random-Access Memory). DRAM is considered “volatile memory,” meaning that if power is lost, the data (pictures) will also be lost; it is, therefore, important to empty the camera buffer as quickly as possible by writing buffer data to a non-volatile memory card. Where write speed of the memory card becomes important is when a camera’s buffer becomes full and the user wants to continue capturing photos or video without delay. As the buffer fills, it must begin to transfer (write) data out to the memory card and therefore, if a slow memory card is used, it will noticeably slow the ability to continue capturing beyond the buffer size. A fast memory card can typically allow the user to never notice a slowing of capture rate since its speed is much more closely matched to the speed of the camera’s buffer memory. Examples of cameras currently on the market that require a fast “speed-rated” card for High Definition video or burst-mode capture include the Sony A7R family (SD-UHS U3 speed class or higher required), the Canon EOS C200 (VPG 130 or higher performance guarantee required), plus ARRI and Black Magic.
After photos or video files have been written to a memory card, a user will typically want to move these to a computer for editing, archiving or uploading to the cloud for sharing; this is where read speed becomes important. In fact, when designating card’s transfer performance, many times the read speed will be the most prominent measure of performance stated on the packaging and the card itself. This is because, in most cases, the camera buffer as described above has minimized the need for buying the absolute highest write performance card. More so, the read transfer performance has become the most important speed measure to most users since the real-world value of moving data (photos and video) for editing, archiving and sharing is the most important aspect of a post-capture workflow. To ensure that the quickest download of photos and video to a computer is achieved, a fast memory card reader paired with a fast memory card is recommended.
Until recently, most memory card readers offered a connection to a computer via a USB 3.0 (SuperSpeed) port, which has a theoretical maximum transfer rate of 5Mbits/second (625Mbytes/second). While USB 3.0 provides an adequate level of performance for most consumer applications, some computers, like the Apple MacBook Pro, have recently started to ship with ThunderBolt 3 ports which boast a theoretical maximum 40Mbits/second (5GBytes/second) transfer speed capability. The beauty of Thunderbolt 3 is that it utilizes a Type C USB connector, which is compatible with the newest version of USB (SuperSpeed+); the SuperSpeed+, aka USB 3.1 Gen 2, the port provides 10Mbits/second (1.25Mbytes/second) transfer capability.
Ensuring performance improvements from pairing a SuperSpeed+ reader and capable computer are two-fold: a) cards that can transfer data in excess of 550Mbytes/second will not be limited by the USB bus speed, and b) data from multiple cards may be transferred simultaneously per bus speed limits. For example, a dual-slot reader such as the soon-to-be-released ProGrade PG-02 reader can concurrently transfer data from both a CFast 2.0 card and an SD UHS-II card at speeds of up to ~675Mbytes/second (or should we list’ at speeds of up to’) on each card. The benefit equates to an approximate 35% faster transfer rate than previously possible. In the future, as faster memory cards are released, the benefit of a USB SuperSpeed+ reader will become even greater.
In summary, transfer speed does matter. To professional users, both write and read speed matter. To consumers, read speed is generally the most important measure of performance. For all users, a fast memory card reader is essential to ensure that the least amount of time is required during the post-capture workflow. Newer computer interface technologies such as USB SuperSpeed+ and Thunderbolt 3 provide greatly improved performance for reading data transfers from a card to a computer. As memory card performance continues to improve, the importance of fast memory card readers will continue to grow. At ProGrade Digital, we are committed to optimizing card transfer performance for the broadest spectrum of users, while providing advanced memory card readers that minimize data transfer time in the post-capture workflow.