Flash RAM Overview

SDHC flash cards are the most suitable form of flash card memory storage, widely used in digital cameras and other applications. Smaller miniSD and microSD formats are more popular to fit cell phone handsets. These can be read and written using special adaptors that fit into an ordinary SD card slot as found on laptop (or notebook or netbook) computers including the XO and on Cardpod MP3 players and other USB card readers. They are slightly more expensive and more awkward to handle than the larger SD and SDHC cards which are themselves only the size of postage stamp.

Currently (February 2009) the lowest cost per GB for flash RAM is for size 16 GB however much smaller sizes (eg 1 GB) are adequate for use with Cardpods and still widely available as commodity items. About 70 hours of audio can be stored in 1 GB (14 MB per hour). This is more than 4 months worth of 30 minute daily programs. Prices are continuing to decline so procurement decisions will have to balance the lower cost of getting larger capacity immediately against the lower cost of waiting. Most likely the best strategy is to simply buy exactly the capacity required when it is required.

A commodity market, dramexchange.com provides both spot and futures prices for large quantities sold from flash RAM manufacturers to Consumer Electronics manufacturers. For example the average spot price on 2009-02-05 was $2.35 for SD 1 GB.

The cheapest quality (lowest speed) are adequate for use with Cardpods.

A major advantage of using Cardpods with SD cards instead of internal flash with ordinary MP3 players is that a larger capacity flash card can be used later when more space is needed and costs have fallen instead of replacing the whole MP3 player. The increased cost for this benefit is negligible.

A more important advantage for Universal Communications projects is that it is not necessary to take a Cardpod to a computer or other copying device with a USB port in order to update the contents. Simply exchange the flash card for an updated one that has been copied separately. This can be combined with the recording capability on a Cardpodcaster for feedback. Simply use a separate second flash card for recording. The Cardpod can continue to be used for listening to the contents of the main flash card while any recordings made are being passed on via postal and courier networks. Then when the recordings have been copied to send to their destinations and the SD card they were on has been updated to the latest content it can be returned and used as the main flash card for listening. Files on the previous main flash card for listening are simply deleted and that card now becomes the card used for recording.

This approach, including regular feedback through recordings, requires two SD cards per cardpod. A much smaller ratio, close to 1, can be achieved if updates are initiated from upstream instead of by the downstream user sending recordings upstream. For example with one extra card being used to successively update each of 4 teachers at the same school

Careful monitoring of supplies and postal or courier transfers will be necessary as flash cards could easily be stolen for resale by criminal gangs. Copying will also be a bottleneck as it would take something like 20 minutes each to transfer 1 GB at about 1 MB per second through each computer USB port used for copying.

For both reasons it may be desirable to provide DVD writers at schools as the main distribution interchange mechanism between the school and the rest of the system.

Broadcast DVDs for all schools can be mass produced for less than $0.30 cents each in quantities of thousands or tens of thousands both cheaper and quicker than updating similar numbers of flash cards. The DVD player at each school is then used to copy the files to flash RAM on the school XO. This only needs to be done once since the DVD player uses much more power (eg 25 W) than flash cards do. Then the XO can be used to update copies for each teacher, and later for each student, with minimal use of battery power. The mass produced DVDs have no resale value and are therefore safer for postal and courier exchange.

DVD players would suffice for this. But the extra cost of DVD writers is justified to also use them for interchange upstream and for local archives etc. Using rewritable media that can be reused many times this could be cheaper than flash as well as less liable to theft.