Which SD card should I buy?--Audacity Audio compares.

Some people think that an SD card is an SD card and just give me the cheapest one. My purpose is to help educate you so you buy the one that will work for you. Not only are SD cards very different, but also the hardware you put them in is very different, and the applications you run have very different requirements as well.

I'll start with the applications. I represent Audacity Audio, a developer of PDA-based dictation and voice recorder applications. Audio capture usually creates very large files and needs to be stored in real time. I say usually because different audio formats have different requirements. The typical PCM 8 kHz format takes about 950K to store a one-minute recording. If I record at PCM 16 kHz, a higher quality format, it takes double the storage. I can also record at PCM 22 kHz and use up over 2.6MB per minute. Conversely, I can record in a compressed format and create very small files. We offer a compressed 6 format with a one-minute file using under 50K, a compressed 11 comes in under 90K, and a compressed 24 at about 175K. If I use a compressed format, my card writing requirements are much less stringent than if I am recording in a PCM 22 kHz format.

Generally, audio and video applications are very data intensive and you will want a "fast" SD card. This usually equates to more expensive, although SD cards are pretty inexpensive. Data intensive applications are also the ones that require a lot of memory and you probably want to store them on a card. Writing to the card taxes the card more than reading from the card. For instance, displaying a map page, also very data intensive, is easier than recording an audio or video.

The critical factor is how fast can data be written to and read from the SD card. With some cards this "speed" will vary from one test to the next. Sometimes, how full the card is affects its speed, and the hardware the card is inserted into also affects the performance. Below I chart 4 Palm hardware devices and 4 different cards:

Palm Hdwe SanDisk 64M SanDisk 256M Lexar 32X SanDisk Ultra II

Tungsten/T 99 k 22 k 152 k 140 k Write
364 k 416 k 494 k 430 k Read

Tungsten/T3 121 k 23 k 198 k 165 k Write
936 k 1.14 k 2.62 k 2.62 k Read

Zire 72 114 k 20 k 201 k 168 k Write
794 k 1.09 k 1.75 k 1.75 k Read

Treo 600 59 k 21 k 70 k 194 k Write
312 k 323 k 595 k 639 k Read

As you can see both device hardware, SD card, and the combinations vary significantly. On the Treo 600, the SanDisk Ultra II is clearly the winner, but on the other models the Lexar 32X card (really made by Panasonic) has a slight edge.

I would recommend either a 32X card (designed for faster access) or the SanDisk Ultra II. I personally use the Lexar 32X card in my Treo 600 and I record audio just fine. If the Ultra II had been available at the time I purchased my 32X card, I would have chosen it. The other two cards just don't work for me.

You might be saying, "this is all pretty complicated, how do I know if it works?" Audacity Audio has a free utility that will help assess your card and hardware compatibility. It is called Card Speed and can be downloaded from the Audacity Audio web site, www.AudacityAudio.com. Another way is to test with a "critical ear". If you are recording audio and your card is too slow, you will get "dropouts"--places where the file did not get recorded. The file appears to play back faster than it should. I call it "a chipmunk effect".

A final thought. You probably use your PDA for many things. With some applications any card will work, but for others, a good card is required. I recommend investing in a good card.

Fred Clark, President
Audacity Audio

ps Sorry about the formating of this table. It seems to get all messed up and I can't figure how to fix it.