Putting Portable Storage in Perspective

Tom Coughlin is president of Coughlin Associates, an engineering consulting firm specializing in storage technology. Contact him at www.tomcoughlin.com.

Computer scientists often refer to the characteristics of memory devices as constituting a "storage hierarchy." The concept of a storage hierarchy lets you sort memory products based on important characteristics for the applications for which they are to be used. For example, among the important characteristics for consumer electronics storage devices are:

• Price
• Size
• Power
• Capacity
• Data rate
• Reliability and environment

Currently, the two main approaches to addressing storage in portable consumer devices are hard-disk drives (HDDs), like the 1-inch HDD in the first-generation Apple iPod, and high-capacity flash memory, such as that used in the recent iPod nano. In particular, Apple's move to flash memory raises the questions of whether the days of rotating magnetic memories are numbered, and whether flash technology really can outpace HDDs.

At this point, HDDs still have the edge on flash memory, mainly in terms of cost. However, HDDs also have performance advantages over flash memory, including:

• Suitability for streaming and multitasking, especially if multiple writes are required (such as when recording programs while listening to music).
• Better for applications that demand a large number of overwrites (HDDs have unlimited rewrites, flash has a 10,000 to 1 million write limit).
• Higher write bandwidth for transfer and synchronization of content (faster time to sync with wireless, for instance).

That said, HDDs won't tolerate going through a washing machine and won't operate well if the ambient temperature is very low or very high. Additionally, HDDs are somewhat more sensitive to shock if they're dropped (although rapidly developing zero-g sensors and other technology can pull the head from the disk before shock can occur).

Still, the biggest difference between HDD and flash memory is power usage. Because of the much higher data rate of HDDs, they don't need to be continuously powered up; for instance, in Table 1, which compares important characteristics of a typical 1-inch HDD to a 2-level per cell SD flash (MLC), such as would be used in a music player. Although HDDs use more power when on, in practice they aren't kept on at all times. For MP3 and other highly compressed musical digital content, an HDD may only be on 5 percent of the time. When on, the drive fills a semiconductor buffer with content, then turns off. The content is then streamed from the buffer and the HDD is turned on again only when the buffer needs to be refilled.

All of these factors—as well as the needs of the application—must be taken into account when creating a storage hierarchy for mobile systems. Figure 1, for instance, illustrates a mobile digital storage hierarchy that shows the proper choice of digital storage as a function of storage capacity, price, data rate and environmental performance. I've included possible optical storage products because they could offer very low-cost content distribution. Note that for a particular device, application and price point, one or more storage devices can be used. For instance, a consumer electronics device could have an embedded 1-inch HDD drive for mass storage and a removable flash-memory device for data sharing.

Hybrid Drives

Flash memory can also be incorporated into HDD circuitry, creating hybrid drives such as those announced by Samsung and Hitachi to utilize the hybrid drive support in Microsoft's Windows Vista. Such combinations of storage components in a single application take advantage of the strengths of both types of storage. At last year's Windows Hardware Engineering Conference (WinHec), Microsoft and Samsung demonstrated one such prototype. The HDD architecture incorporated a "OneNAND" device from Samsung that works within the hard disk's architecture. The ultra-high-density benefits of magnetic storage technology are preserved, while the ultra-low-power, ultra-high-reliability and fast read/write access of advanced NAND technology (such as OneNAND) enhanced the overall value of the hybrid drive at little additional cost. The HDD eliminates costly inefficiencies caused by the need for the HDD to continue to spin whenever the computer is on. Additionally, the HDD design also provided significantly faster boot times for computers running Windows Vista.

Microsoft reports that the HDD prototype uses 1GB OneNAND Flash as both the write buffer and boot buffer. In the hybrid write mode, the mechanical drive is spun down most of the time, while data is written to the flash write buffer. When the write buffer is filled, the rotating drive spins and the data from the write buffer is written to the hard drive. The hybrid drive saves power by keeping the spindle motor in idle mode almost all the time, while the operating system writes to the OneNAND write buffer.

While the cost of an HDD increases with the addition of OneNAND, Microsoft believes the increase is mitigated by several factors, including lower maintenance costs, 95 percent power savings when the disk is not spinning, faster boot time and substantially increased reliability.

Requirements and Trends

Over time, consumer storage applications will become richer and consumer expectation will become greater. Furthermore, the growth of new ways to interact with consumer devices, as well as technology improvements in making high-resolution content in mobile and fixed applications available to mobile consumers, will increase resolution demands. This will drive demand for higher capacity storage. The net results are various market levels for economy and premium consumer devices with various storage requirements. Examples of high-resolution devices include personal media players and HDD cell phones. In fact, I suggest that the Apple iPod nano may represent the last major development in what we will call the "low-end media player market." This market is characterized by lossy music formats for mobile applications that economize on scarce and relatively expensive digital storage. This has characterized the history of portable music players until now. Lower capacity flash memory that is cheaper than a comparable HDD dominates this market, and the iPod nano represents what may be the last major innovation for this type of product. That said, lossy compressed format music players will represent the highest volume market for some time. Compressed music formats, such as MP3 and AAC, allow a lot of music to be stored in a smaller storage capacity than is the case for a lossless music format, thus providing lower capacity and lower cost products for less discerning and price-conscious listeners.

Lossy music compression technologies include MP3, AAC (used by the Apple iPod iTunes service) and Ogg Vorbis (an open-source compressed music format). These lossy compression technologies remove some of the music "information" in the compressed music file. Once this music information is removed, it cannot be recovered; hence the term "lossy." Lossy compression takes advantage of the fact that the human auditory system doesn't notice certain types of signal degradation. However, depending upon the compression algorithm used and the compression ratio chosen, lossy compression can introduce artifacts that can be noticed by a keen ear, particularly in a quiet setting. These popular lossy compression technologies can reduce the file size by 80-90 percent.

On the other hand, lossless compression compresses audio without losing the original audio signal's integrity. Thus, an audio track compressed with lossless compression can be decoded to its original uncompressed form without artifacts. Lossless compression technologies reduce file size by about 50 percent.

I believe that with the very large digital storage capacity available with small form factor HDD, a new class of portable media player will develop. This player would provide significantly higher resolution audio and video content on an appropriately sized screen or external viewing device. Just as when greater memory became available on personal computers, leading to new features and higher performance on these products, so too will these new consumer electronic products provide higher resolution and lossless content storage, providing a more refined user experience.

The end result is to provide a portable device offering home-stereo quality music and video experience. Perhaps these lossless portable media players will be introduced by traditional high-end audio providers such as Panasonic, Sony and the like, rather than the traditional mobile consumer companies. These sophisticated media player products would likely provide ready access to photographs, music and video.

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Table 2: Media units vs. storage capacity for various resolution photo, music and video files.

Table 2 presents projections for the number of digital photographs, music files and video files for different resolutions and device capacity. The gray areas for each media format show an estimate of the desired unit content capacity ranges. These are based on an estimate of how much content an individual would like to have available on demand from a local portable device. For instance:

• Up to 20,000 photo images.
• Up to 10,000 songs.
• Up to 100 movies.

Table 2 presents an estimate of the acceptable size of storage for various applications at various resolutions, including:

• A pure 4MP photo viewer with 20,000 maximum images has 20GB.
• A combination camera and photo viewer with 8MP resolution and 20,000 images has 40GB.
• A 10,000-song MP3 player has 40GB.
• A 10,000-song lossless compression player has 140GB.
• A CD-quality 10,000-song player has 280GB.
• A 100-movie player at VGA resolution has 70GB.
• A 100-movie player at DVD resolution has 417GB.
• A combination 20K 4MP photo, 10K MP3 song, 100 VGA movie player has 130GB.
• A combination 20K 8MP photo, 10K lossless compressed song, 100 DVD movie player has 597GB.

Based on Table 2 and Figure 2, the storage device for a 20K 4MP photo, 10K MP3 song, 100 VGA Personal Video Player (PVP) (130GB) using a 1-inch HDD would have a storage cost of less than $55 by 2010, enabling a consumer product with these characteristics selling for well under $200.

Audio and video player technology built into cell phones may also demand significant amounts of digital storage. Nokia, Samsung and others introduced several of these products in 2005.

Looking to the Future

From the analysis I've presented here, you will find that in the future:

• Different storage devices will compete for portable applications depending upon trade-offs of various specifications and characteristics. These specifications and characteristics will be organized as a portable digital storage hierarchy.
• Hard-disk drives will enable richer media applications much more cost effectively than flash memory. Flash memory will dominate for commodity player products, while HDDs will define high-end products.
• Both HDDs and flash memory will increase in capacity in the coming years; HDDs will maintain their advantage in $/GB for some time to come.
• Small form factor HDDs will most likely be embedded mass storage in most mobile applications where they are used with removable storage being either flash memory or even small form factor optical disks.
• Higher-capacity HDDs and suitable human interfaces will allow portable products with very high-resolution audio and video, appealing initially to a high-end audience.