When testing SDHC card performance, a touch of class helps

Product Kingston 8 GB SDHC cards
Website www.kingston.com

SECURE DIGITAL cards - now in their SD High Capacity, or bare SDHC iteration - have improved in capacity and speed substantially since the first units appeared at the turn of the last century.

The capacity-addressing problem was solved with the block-addressable SDHC, but the confusion about the speeds remains for many, till this day.

Prior to SDHC, the only speed classification from the card manufacturers was related to the old 1x CD-ROM speed, or 150 KB/s. This was just read speed (not the slower write speed). Till today, you may have SD cards classified as 100x - 15 MB/s reads - or 133x, with 20 MB/s reads.

However, card reading only matters when you're copying your photos into the PC, for instance. Write speed is far more important, as, in the case of cameras, the ability to record smooth HD video or quick sequences of hi-res photos will depend a lot on that measure.

With SDHC, the speed classification was revised to use guaranteed minimum write speed as the base instead - so, a Class 4 card must provide 4MB/s or better write speed, while a Class 6 card has to up that to a minimum 6MB/s.

Now, even within each class, your mileage may vary - the flash die and the controller speed both affect the real performance. Generally, multi-level flash memories with 2 bits or 4 bits per cell would be denser but slower compared to the standard single-bit per cell flash.

We were curious to see what's the real difference here, or, to put it this way, what does that extra "touch of class" bring. Kingston, one of the biggest memory industry names, helped out by providing two otherwise similar 8 GB SDHC cards - one Class 4, the other Class 6.

Two "platforms" were used to test the speeds: one a SDHC card reader embedded in the MSI GX600 notebook, the other a Panasonic Lumix TZ2 photo camera - both connected via USB.

The test files - one was a large 700 MB ISO image of an early Suse Linux 11 alpha version. The other one was a 300+ MB directory composed of over 500 smaller files. In both cases, the cards were empty when doing the stuff.

The Class 4 card, on the notebook, managed to write the 700 MB ISO in exactly 142 seconds, or around 5 MB/s - so, it was exceeding the Class 4 minimum spec, but now anywhere close Class 6. The Class 6 one did the job in just 104 seconds - nearly 6.8 MB/s write speed.

Reading back? That was faster - just 42 seconds on Class 4 and 37 seconds on Class 6 here, or nearly 20 MB/s in the latter case.

The full directory took far longer time to write: 9 minutes on the Class 4 and 7 minutes on the Class 6 drive. We have to attribute this to the crappy FAT file system still being used here. Small files do slow down things.

What about the Panasonic camera? Oh well, it seems its USB interface, despite being advertised as 2.0, still ran at 1.1 speeds, so that test portion was discarded.

In any case, both Kingston cards exceeded the required specs by over 10 per cent on a notebook with the native SDHC reader.

As you can see, there is benefit in going for Class 6 - critical when doing hi-res recording of any kind. For VGA-class video, on the Panasonic camera, we could smoothly record 640x480 30 fps video on both cards, while the generic SD that came with the camera had to go back to 10 fps.

In fact, Class 6 SDHC should be able to do VC-1 or MP4 compressed HD recording, whether 720p or 1080i, with some buffer still left. As these cards proliferate, there will be less need for HDD or tape-based HD videocams too. 8GB ones should be able to give us over an hour of full HD stuff, and 16GB ones are coming too, in Class 6.

And, once the 32GB units become more commonly seen later this year, well, it may make both BluRay and HDDVD recorders kinda obsolete - why carry a big disc when you can stuff in all the stuff on a tip of your finger? µ