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Originally Posted by Nankai  DO NOT USE ANY CLASS 6 SDHC CARD! Class 6 SDHC card might fail the firmware. We are working on this problem now. Class 4 kingston is recommended.
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I was curious and did a few minutes research about class 4 vs class 6 cards. First thing was to look look up some SDHC card benchmarks online. For example:
Benchmarks SDHC Memory Card Charts at Tom's HardwareKingston 32GB Class 4 SDHC Review - Hardware Bistro
A couple of things stand out. Most benchmarks (I looked at half a dozen others) are based on writing images on a camera. For that you want a fast random access write speed. The SDHC class ratings are based on this kind of use. (
SD Speed Class - SD Card Association). When it comes to reading--which is what we care about for music--the rating seems to be a good guide to random access reads on fragmented cards (where you've been copying and removing songs for a long time) that are formatted with small sector sizes. At larger sector sizes on cards that are less fragmented, the SDHC ratings are pretty useless.
I don't know the exact numbers, but I think a WAV at 96/24 would have a theoretical maximum rate of under 12MB/s, and more realistically under 8MB/s. Tom's Hardware lists an average read speed of 15MB/s with a class 4 Kingston card. The class 6 card only improves this to 19.6MB/S. Both of these have enough speed to read the music, although these ratings assume optimal conditions.
The Hardware Bistro benchmark relates read speed to sector size. The CrystalDiskMark numbers suggest that the ratings only hold up by formatting the card with a large sector. The class 4 Kingston card achieves a read speed of 6.5 MB/s with 512kB sectors, but only 2.5kB/s with 4kB sectors. I think the difference between these is that the 4kB mimics a full and fragmented card. Note how this number is below the SDHC required write speed. That's because the specification allows the device to ask the card for the best place to write, and that assumes there is a best place to write. This also suggests that you want to leave enough free space on the card to move at least one large song around. (Fang, are your engineers listening?)
Even at 2MB/s, the speed issue should never come up on a class 4 card using 44/16 CD quality FLAC. With high resolution music, a class 6 card might buy some more speed, or might not. There's no way to tell from the card's rating. Some of the brands don't improve sequential read speed much at all.
While there's not enough detail in the benchmarks to go further, there are some things we can do. There's no reason to assume we'll get much benefit from using a class 6 card. The average read speed (it's really the optimal sequential speed) of a class 4 card is more than adequate for 96/24 WAV. The SDHC cards we're likely to use will be formatted with an average sector size of probably about 32kB, which means we won't see those low read speeds that show in the fragmented benchmark number. The actual read speed might be anywhere from 2.5 to 15MB/s, and more than likely won't drop much below the rated 4MB/s, given our larger sectors. For fragmented cards, the speed probably has more to do with seek time than anything else.
One thing that's clear is the actual read speed we get with our DAPs depends more on how we load the card than on the card's class, and we can do something about that.
1. If the card's already fragmented from moving a lot of songs around, erase the card and reload the songs.
2. If it still skips, and you're using high resoltuoin formats, reformat the card using a larger sector size, using some formatting tool, if necessary. Then write the songs over again to keep the data together.
3. Use flac instead of wav to cut the file size in half.
4. Don't fill the card, but always leave space for one last song.
5. If you constantly update a few cards, leave a lot more free space. Remove songs before adding new ones. The card will still get fragmented this way, but it will be less fragmented if you do this.
My conclusion for now is that it doesn't make much sense to buy a class 6 card. They're twice the money right now, it's hard to see how the extra speed would be used, and there are workarounds for the worst cases. Being able to use a class 6 card you already have is a different story.
By the way, this might be useful to people outside this thread. I'm not sure how to handle that, or which forum is better to post it. Since this thread inspired the research, I put it here. Any suggestions?
)thanx!
