CF vs. SD speed - why such a diff?

This has bugged me for a long time.
All my flash cards are standard speed. I can r/w 300Kb/s with my CF cards. I can go 250Kb/s with my SM card. The best I can r/w with my 512mb SD card is 95-100Kb/s, either with a reader on my PC or my PPC. I had the same results with a 128mb SD card I used to own.

It's impossible to stream video from the card and even music is choppy. Is the SD technology that much slower than comparable CF? I hesitate to buy a 1-2gb SD card if they are all such slugs.

aren't your SD cards kingston branded? they're said to be very slow.. most of the other SD's are ok (500 Kb/s or even more), according to user reports. so, no, it isn't normal that your cards are that slow.

All my cards are Sandisk, not by plan, it just hapened that way. I'm gonna buy a 1gb SD from a local retailer (different brand) and see how it runs.

I going ahead with my expansion chassis and figure (hope) I can mount multiple cards in the USB multireader for cumulative storage.

From what I know, the normal Sandisk cards are slow, CF, SD or otherwise. Their Ultra or Ultra II series are speced at 50X or something, while some of the generic ones are speced at 40x, 50x or 60x. Newer cards are even speced at 80x.

But note that the fine prints for some of these speeds are for burst and not sustained speed. Also, most are for read speed, write speed are usually lower, sometimes 3/4 of that or half ...

I had a 256MB SD card and it was somewhat slow. A KingMax 60x card proved to be much much faster.

Also, depending on the app, performance may vary, but should be relatively constant between cards.

Thanks Snappy. I'll make sure I get a 40x or better card to try. The current card I have is intolerable.

Well, I don't know this for sure, but based on the number of electrical contacts you can see on the card (50 pins for CF versus about 10 for SD), my guess would be SD is a serial I/O device. CF is definately parallel (16-bit data, right??). Now granted, speed isn't determined just by the number of data lines; you have to consider the bitrate per data line too. But I suppose that could have something to do with SD not performing as well as CF, particularly under sub-optimal conditions.

Then again, I could be way off here...

You are absolutely right, SD cards are based on Serial IO. CF cards support ATA interfaces, which can be pretty speedy. Howevery, serial io interfaces like USB (yes, USB is based on serial IO, think Universal Serial Bus , can reach up to 12Mbps for USB 1.1 and 48Mbps for 2.0.

Granted, these are theoretical speed and mostly can be achieved only in burst mode, still should be fairly fast.

In that sense, any SD can outperform a slower speced CF card, it all depends on the speed rating of the card.