I dont think it has anything to do with Card speed. The difference between reading from internal Flash memory and from an SD card is that reading from an SD card will add another layer of jitter... or simply jitter created by the additional current used or surges when reading from the SD card?
Ie; From the design/build blog for the Altman Tera player.
""It turned out that the problem was the flash-card. While reading from the SDHC-card, every time I jumped to the FAT in order to look where the next cluster of data would be, there was a huge current surge from the supply. As the FAT has to be looked up regularly there was a very bad jitter correlation.
I learned that this was not only a problem for the one card I was using, but it is a problem inherent in all SD-cards. Whenever you have to change the reading location (i.e. jump to FAT) there will be a jerk in the supply, causing a significant and badly correlated amount of jitter.
So flash storage, just because there are no moving parts, does not mean that you will not have jitter ;-)
The mess you see below the SD-card socket on the following pic shows my attempt to smooth out the power supply of the flash card, using a simple R-C-R-C-R-C chain.
While the filter chain reduced the jitter, this was not a complete solution. I had to come up with something better ...
Then I had the idea, that if I could reduce the number of FAT reads per time, I would be able to break the jitter correlation.
Up to know I was jumping to the FAT for each and every cluster. That gave me a constant jitter frequency that was very audible also dependent on the sample-rate of the wav file.
The solution was the introduction of a continuity counter. While running through the FAT, I would count the number of clusters that are linearly allocated without fragmentation. This way I only need to jump to the FAT again when the continuous cluster chain is running out.
With the continuity counter the number of power supply jerks that are caused during each FAT read, were reduced from a couple of times per second to about 1x per minute.
This completely solved the SD-card jitter problem."
Edited by Ari33 - 12/8/12 at 8:59am