Flash memory is available in so many formats that it can be difficult to know what will work with any particular device. Devices such as MP3 players, PDAs, mobile phones, digital cameras, and personal computers can take advantage of flash memory to bolster their storage capacity, but selecting the right format may be easier said than done.

To try to address all of the common formats in one Tech Tip might be quite a read, so we're doing a two part series on Flash Memory. Part I of the Flash memory series will focus on two similar, very popular and generally interchangeable formats: MMC and SD.

The Basics

Before getting into the details, some background on each card may be appropriate. The letters ‘MMC' stand for MultiMedia Card, which is a format that was developed jointly by SanDisk and Siemens in 1997. The letters ‘SD' stand for Secure Digital, and this format is an improvement on the original MMC design, and was developed jointly by SanDisk, Matsushita Electronics (better known as Panasonic) and Toshiba.

Both formats are quite durable and the solid state (no moving parts) components are protected by a rigid plastic shell. The devices are generally unaffected by extreme temperatures, and should withstand a drop of 10 feet without experiencing any damage from shock.

Physical Features

Both MMC and SD flash memory units measure approximatly 24mm x 32mm x 2.1mm, about the size of a typical postage stamp, and weigh a mere 2 grams. This miniature footprint may make them about the easiest way to misplace your data, but also allows the devices that accept them to be smaller. Personal electronics are shrinking as they get more powerful, and the necessary accessories need to keep pace. Although they share the same basic form factor, MMC and SD cards can be distinguished by two physical features, a sliding tab and the number of connections.

When looking at an MMC or SD card so that the label is facing you, and the electrical connections are facing away from you, there will be a notch in the upper right corner of the card. From this point of reference there will be a small sliding tab on the left edge of an SD card, not found on an MMC card. Compare this 512MB MMC card with this 512MB SD card and you can see the difference if you look closely at the enlarged images. This tab slides into two positions, locked and unlocked. It allows the user to manually write protect the data on the card, which means with the tab in the locked position data can be read from the card, but nothing can be written to or erased from the card.

The other physical difference is on the backside of the card. An MMC card features seven electrical connections (small rectangular pads for data transfer and receiving power), whereas an SD card has nine.

* While there may be rare exceptions, for all practical purposes, SD & MMC cards may be used interchangeably on current devices, especially if they indicate "SD/MMC" compatibility.

Transfer Rate

SD and MMC cards are capable of similar data transfer rates, with a slight edge going to the SD cards. SD cards are now available with write speeds rated at 60x (9 MB/s) and read speeds rated at 66x (10 MB/s), while MMC transfer rates seem to peak at 9 MB/s in either direction. Not much of a difference and both are quite fast, but end user results will vary and may not reach these speeds in real world use, regardless of format chosen.

SD and MMC cards should reference a speed as part of the technical specification, and it is an important thing to consider when shopping around. Lower speed cards are still commercially available, and can have an impact on the performance of digital cameras or other devices where speed may be critical. Each ‘x' in the speed rating represents 0.15 MB/s, so if 45x compared to 66x doesn't sound like a big deal to you, maybe putting it in terms of 6.75 MB/s compared to 10 MB/s will. Instead of actual speed ratings, some manufacturers will use words like “High Speed” or “Ultra” when referring to the faster cards.

Note: Check the actual write speed specs of your device before purchasing "Ultra" or "High Speed' chips. You could be putting a Hemi engine in a AMC Gremlin. Don't spend the extra money if the camera does not support it.

Capacity

SD cards are readily available in sizes up to 1 GB, 2 GB models are starting to show up, and the SD Card Association states that models with up to 4 GB and 8 GB of storage capacity are also on the way. In contrast, MMC cards have a maximum capacity of 512MB, making the SD technology much more appealing.

Security

As mentioned in the physical features section above, SD cards offer the benefit of write protection. By ‘locking' the card, a user can be assured that the data is secure until they take the necessary step to un protect it. Fears of accidentally losing or changing data can be eliminated by using an SD card over an MMC card, thus improving the security of the data.

Another feature supported by SD, but not MMC, involves copyright protection. The SanDisk web site refers to this feature as "cryptographic security for protection of copyrighted data", and other locations reference it as DRM, or Digital Rights Management. Basically, licensed content can be written to an SD card and it can not be executed except from that specific card.

Applications

In general, SD and MMC cards are interchangeable and either can be used in a compatible device. An SD card may generally cost more than an MMC card with the same capacity, but as seen in this Tech Tip, it does offer more for the money.

Many card readers are available for personal computers that promote the ability to read and write to a variety of common flash media formats. A 15-in-1 reader/writer, such as this one, can be made quite compact thanks in part to the fact that two of the 15, MMC and SD, can be read from the same slot on the device.

MP3 players generally come with a base amount of memory to store music files, but having an expansion slot allows users to increase the capacity, and play time, by adding flash memory of their choice. The Pogo RipFlash MP3 Player is such a device, providing 256MB onboard as well as an SD/MMC slot for easy expandability.

Mobile phones and PDAs can also take advantage of increased storage space thanks to flash memory slots. The Handspring Treo 600 is a combination phone/PDA that offers an SD/MMC slot for such convenience.

And of course, digital cameras use flash memory as their ‘film', where larger and faster cards are always a welcome upgrade. odak DX7630 6.1MP 3x Optical Zoom 4x Digital Zoom Camera The 6.1 MegaPixel Kodak DX7630 could fill up the same SD/MMC card much faster than the 3.2 MegaPixel Umax AstraPix 640, but one of the great things about these cards is that the user can choose the size, as well as the quantity to have on hand, in order to suit their particular needs and budget.

One word of warning – be sure to check your device for the capacity of the card that it can handle. If your camera can handle only up to a 512 MB card, then using a 1 GB card in the camera will be pointless (depending on the device, some will not even be able to read the card, whereas others will only use up to the capacity that it is rated for. Either way, you want to make sure that you match the card properly to the device). So as always, check your product's manual to be sure that you buy memory that it can support.

Final Words

MMC and SD are two of the more commonly used formats of flash memory, but as mentioned, there are several others. Keep an eye out for next week's Tech Tip: Part II of the Flash Memory Series will address Compact Flash, Smart Media, Memory Stick, and xD formats.

Computer Geeks tech tips and computer advice

The meaning of Bridge Media is that the SD cards can offer a way to connect many sorts of appliances trough one small memory card, without the need to use any interpretation.

An example for sd card use:

You shoot a video clip with your digital camera. You will store the video clip on your SD card which is supported by the digital camera. After that, you can take the memory card out and plug it into your PDA. The video clip is immediately shown on your PDA, without the use of any other device. You didn't need your PC's intervention in doing this - both your digital camera and your PDA know how to read and write data to and from an SD card.

The nest thing you can do is, say, change the video clip is some way sing your PDA (like adding titles). You can than save the new video clip to the memory card and take the SD card to a video camera. The video camera can now read the new video clip and display it on it's screen or on your television.

Here's another example:

You have an important phone call on your cell phone. Your cell phone supports SD memory cards so it's able to record the call. You can later save the phone call audio file on to your PDA memory for future use.

As you can see the possibilities are endless, and memory is something you can easily move from one appliance to the other thanks to the flexibility of PDA Memory Cards

Another thing to remember is that the DS memory cards have very fast read/write speed of X133. This is possible due to the fact that no physical component are active in the read or write process, and the memory is completely electronic.

Ron Heller is the owner and operator of the http://www.pdaonlinecenter.com A website
devoted to PDA, PDA Memory Cards, PDA accessories, PDA computers and more.

1. The SD memory cards are what's called a "bridge media" item. They enable you to pass data between different devices, without the use of a PC. For example, if you want to pass a 2 minute video you shot with your new digital camera, on to your iPAQ, you can do that in a few seconds by taking out the SD memory card from your camera and popping it into your iPAQ. Both the devices support the SD format, and the data is therefore transferred easily. It's an sure way to pass data between different platforms because it's a standard memory format the is supported by so many products and appliances.

2. The size of the SD memory cards is a mere 24mm by 32mm by 2.1mm. It's slim size makes it an even easier "bridge media" because it's so easy to carry around, and it fits into even the smallest appliances. There are also smaller standard of SD cards, such as the miniSD 20mm by 21.5mm by 1.4mm, or the microSD 15mm by 11mm by 1mm, but these also have smaller capacities.

3. It has the ability to play music, video and more without a drive mechanism like those in CD and DVD players. This allows for even more creative freedom for product designers.

4. The capacity of those small SD memory cards is growing, as cards with larger capacity hit the market all the time. For example, the 512 MB and the 1Gb cards are already old news, as the newer 2,4, and even 8 GB capacity cards are now beginning to be seen on the market. This allows for longer and higher quality video clips, for example. It's important to add that many appliances don't support these cards yet, and we should expect this to get better in the future.

5. Fast data transfer rate - these SD memory cards are a lot faster in terms of transferring data than that old CD. This is very reasonable, as there are no mechanical part s involved in the process of reading and writing data. This fact makes the SD memory cards a much faster memory device that the old ones we know. Most SD memory cards have the ability to transfer data at a rate that is 15 times faster than a standard CD-Rom.

There is no doubt that the SD memory cards are an important bridge in the world of digital appliances. They make it possible to pass huge amounts of data easily between different appliances without the need of and medium such as a PC. It's a lot easier to see the pictures your friend took on your PDA without going through a PC, especially if you don't have one around. These advantages make the SD memory cards an essential part of the digital lifestyle.

Ron Heller is the owner and operator of a website
devoted to PDA, palm pilot accessories, PDA accessories, PDA computers and more.