S2 IS: card speed requirements - revisited

[cvicisso]

I have always been in the camp that says "I'm no expert when it comes to digicams, so I'm going to rely on the advice of people who are." That's why I come to review sites and forums like this (duh). Here's my concern:

Darn near every professional review that I've read on the S2 IS recommends a large, high-speed memory card in order to take full advantage of the incredible video and continuous drive modes.

I got into a somewhat heated discussion with someone who claimed that this was not necessary - that 'normal' (non-high-speed) cards would work just fine. I contended that it really didn't make much difference - just pay the extra few bucks, play it safe and buy the high-speed card. Then you know that you'll never have a problem. Jeesh.

However, now I'm starting to wonder if that guy had a point after all. A few bucks is a few bucks after all, and I like to keep them whenever I can. :D The maximum write speed of the S2 IS during video (640x480 - 30fps) or continuous drive mode (including 'continuous-high') is only about 3MB per second. That's it. It will never write anything to the SD card any faster than that NO MATTER HOW FRIGGIN FAST YOUR SD CARD IS.

A 60X SD card (like the SanDisk Ultra-II) can accept written data at 9MB per second. Now I know that it's good to have a bit of a fudge-factor (get a slightly faster card than the minimum required) in case there's a hiccup or something, but that's THREE TIMES FASTER THAN THE CAMERA CAN EVEN WRITE! :eek: That seems a bit excessive (IMHO).

According to the camera's specs and observed performance (max 3MB per sec), the minimum speed SD card should be 20X (150k X 20 = 2MB per sec). Again - not saying we should go with the bare minimum, but 60X-66X-and-beyond seems a bit crazy to me.

Can someone PLEEEEASE comment on this?? Why are all the experts telling us to get the fastest cards we can??? And please don't tell me that it's for the added speed when transferring the images to our computers - because that's ridiculous.

[dadof4]

The card has to be fast enough for "bursts"...

The write speed is not constant. It can vary depending on the amount of motion and the amount of change from frame to frame. Typically a frame is analysed for content every X number of frames. Only the areas of subsequent frames that change are "written". For example if you are making a video of a wall with no motion the compression techniques are such that a large amount of video requires a small amount of writing.

If it was not for the compression a 640 x 480 / 30 Fps would require...

640*480*30 = ~9MB/S and that is only considering one byte per pixel. It requires more than one byte per pixel for more than 256 colors.

[bubble.tea]

interesting...do you know if the write speed of the S1 is the same?

[bigrig]

I'd suggest since you seem to be very dubious of the claims, get a basic card and try it out.

I just went with a fast card, since they are barely more expensive than the plain janes.

50x 1GB for $56 -
http://www.newegg.com/Product/Product.asp?Item=N82E16820211309

66x 1GB for $63 (what I use) -
http://www.newegg.com/Product/Product.asp?Item=N82E16820183156

Matt

[Balrog]

A couple of points here .. first, the maximum data transfer required for video might only be 3MB per second, but that's not the worst case scenario .. worst case is the continuous high-speed picture taking, which is 2.4 full-res SuperFine jpegs per second, working out closer to 5MBps.
Second, (this isn't really relevant, but) about what dadof4 said regarding data rates being different depending on the amount of motion going on in the video - that isn't actually the case here. Canon records video in a Motion-JPEG codec, which basically compresses each frame separately as a JPEG and strings them all together; there isn't any temporal redundancy compression involved (as in MPEG codecs - this is one of the biggest drawbacks of Canon's video mode..)

[Ferdster]

I wondered about the same thing, wondered if SanDisk's Ultra II SD was as fast as the camera could go, or if it could use the twice as great (20MB/s) speed of the Extreme III SD. I e-mailed SanDisk to ask them that, and after yanking me around with their eBox way of giving out information, they simply said that the Ultra II & Extreme III were compatible. Well, duh?

I then wrote to Canon: "I certainly expect you to know the maximum data writing rates of your cameras, and if you supplied me with this information for the PowerShot S2 IS then I could answer my own questions about performance with different memory cards." Their reply: "The information that you are requesting is not available for the camera."

Maybe I need to hack the CIA or NSA to find this information. In any event, the postings here have been rather helpful, and certainly much more so than Canon or SanDisk.

[Balrog]

Well, for the S2, the Ultra II should be just fine; no need to go overboard with the Extreme III ... I cannot conceive of any possible situation where the camera will use 20MB/s write speed. It *might* just make your transfers from camera to PC faster, but that's hardly worth the added expense, imo.