Photography by Bas Bettenhaussen

Yes I do!!!!

The acronym ‘UDMA’ - which stands for Ultra Direct Memory Access - has become the new watchword for high performance in compact flash memory cards. Several newly launched DSLR cameras support the UDMA protocol and UDMA memory cards rated at 266x, 300x and higher are now on sale.

UDMA - what’s it all about?

So just what is this exciting new UDMA technology? Actually, in IT terms, it’s pretty ancient and became prevalent over ten years ago as hard disk drive technology evolved. History is repeating itself as solid state flash memory devices play catch up with electro-mechanical disk storage technology. DMA is a process by which data can be moved from a storage device very efficiently, without labouring the host device’s processor. Ultra DMA is a set of definitions for faster and faster theoretical transfer rates ranging from Mode 0 (16.7 megabytes per second or MB/s) to Mode 5 (100 MB/s). You may have heard of ATA ratings for hard disk drives and these mirror UDMA Mode numbers, so UDMA 3 is the same as ATA 3 or even ATAPI 3.

But enough of the jargon - how fast is a UDMA card? Most card manufacturers, the one exception being SanDisk, rate their cards with ‘x’ numbers, 60x, 80x, 100x, 133x, etc. These numbers represent the theoreticaldata transfer speed performance compared to a standard CD music player, which plays data at a rate of 150 kilobytes (Kb) per second, or 0.15 MB/s. A 100x card is a hundred times faster than a CD music player and so is rated as being able to transfer 15MB/s. The very fastest Compact Flash cards currently available are rated at 300x, or 45MB/s.

High speed reality

So what does all this performance mean to photographers? In theory, if you can copy your photos off a card at 45MB/s, a 1GB card will only take 20-odd seconds to empty. However, typical previous generation 133x high speed cards tend to take about a minute and a half to unload using a USB card reader. That’s around five times slower despite a rating that is only just less than half as fast.

Rated card speed is just one factor that determines actual transfer rates. The speed of the host computer does affect transfer rates, or more notably the kind of system interface that the USB port is connected to internally. USB also erodes raw speed through protocol latency - basically it’s never 100% efficient. In our recent tests using a state of the art PC, we achieved just over 17MB/s with a 133x category card (SanDisk Extreme III), or about 113x.

We managed to achieve a transfer rate of 31.3MB/s with a 300x Lexar Professional UDMA card, or 209x, but only using a Lexar UDMA compatible card reader connected to a FireWire 800 port, itself connected to a high bandwidth PCI Express bus on the PC motherboard. The same card read using a standard USB 2.0 High Speed card reader only managed a 16.9MB/s transfer rate - slightly slower than the Extreme III card on the same reader. But in turn, the Extreme III card was notably slower when read using the UDMA reader compared to a standard USB reader. We also discovered wide variations in the speed that cards could be read via the USB ports of our test cameras.

Write performance

Reading a card is only one side of the coin. Write performance is important when the card is in the camera and being bombarded with shots produced continuously at high speed, as the latest cameras are capable of. In continuous shooting mode, images are first shunted into the camera’s internal memory, or buffer, before being dumped onto the card. The buffer to card interface can be critical to the sustainability of continuous shooting. Both the Sony Alpha A700 and Olympus E-3 we tested are UDMA-compatible, but it was the Canon EOS-40D that impressed the most, despite not being UDMA compatible. Instead, the Canon relies on a more efficient onboard JPEG compression and buffer management system. The 40D does eventually choke during a lengthy continuous JPEG shooting burst, and the shooting rate drops dramatically, but it’s capable of many more more high speed shots before this happens. Only when shooting for long stretches in RAW mode, over 15-16 continuous shots, do the UDMA DSLRs show better performance. (Source: Sandisk.com)