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Increasing mass storage densities for Digital Still Cameras

There’s no doubt that the digital still camera (DSC) revolutionized the photography industry. The ability to easily review, edit, and share photos has proven to be a significant advantage for consumers. As consumers and professionals became savvier with digital photo editing and publishing, the need for higher resolution pictures also grew. DSCs started to increase the resolution of the pictures they were capable of capturing in order to meet this demand, with resolutions of 10 Megapixels not uncommon today.

For maximum photo quality and photo manipulation flexibility, some DSCs also feature raw uncompressed “digital negative” formats. The result of increasing photo quality and resolution has also led to increasing file sizes, which can reach upwards of 20 MB in size. To accommodate these increasing files sizes, memory storage sizes have also been increasing. With the larger memory capacities, consumers have been able to maintain, or increase the number of photographs they may take for a given storage medium.

While increasing storage capacities have solved the photo storage challenge, new challenges arise. DSCs have not been optimized to handle the processing and transfer of such large amounts of data. The first challenge lies in transferring large amounts of data to a PC for editing, printing, or publishing. While USB has become the de facto standard for connecting a DSC to a PC, the USB performance in DSC’s may not have been optimized for handling large amounts of data. To illustrate, the graph in Figure 1 below shows USB transfer times for a few of the DSCs available on the market today.

Transfer times can take from 2.5 min upwards to 16 min for 950 MB, translating to transfer rates of 6

MB/s to under 1 MB/s. With more users transferring larger amounts of data, consumers will not tolerate longer wait times for the data transfer.

Longer transfer times also have the side effect of increasing the battery drain on the DSC. DSCs today do not charge while connected to a PC, therefore the longer a DSC stays connected to a PC, the more battery drain occurs. For a consumer, one of the most frustrating experiences while using a DSC would be to have the battery drain completely during a file transfer.

With consumer demand for transfer times in seconds, and not minutes, transfer rates of less than 10 MB/s are no longer acceptable in order to achieve acceptable transfer times. To understand the reasons for the current slow transfer rates, it is useful to take a closer look at the data transfer path when copying files from the DSC to the PC.

Most DSCs today feature removable storage in popular formats such as SD or Compact Flash. The removable storage is typically connected directly to the processor via the corresponding storage controller. To copy files from the DSC to a PC, the data is first accessed from the removable storage by the processor. Next, it is buffered into SDRAM, before it is sent to the PC via USB. This data path is not straightforward, and thus impacts the USB performance.

The second challenge that arises with increasing file sizes is in regards to a DSC’s multi-shot mode. Multi-shot mode is a feature enabled on many DSCs which allow a user to take multiple pictures in quick succession. Multi-shot mode is used commonly for action shots, where it’s desirable to capture many pictures in a short time span.

In multi-shot mode, pictures are captured in rapid succession and buffered into SDRAM while images are written to storage. As writing to storage is slower than the rate at which the pictures are being taken, the DSC will stop taking pictures once the SDRAM buffer is full. While this may not have caused a significant degradation in the consumer experience in the past, the large amounts of data transferred with larger file sizes are now causing increased challenges in the number of pictures that can be taken in multi-shot mode. The buffers in the DSCs are filling up at a much faster rate, which in turn limits the number of photos that can be stored.

For example, some high end DSCs can shoot up to 10 pictures/second in multi-shot mode. With raw picture formats up to 20 MB, this would translate to 200 MB/s of data being written to the SDRAM. Given an SDRAM size of 512 MB, the SDRAM buffer would be full after just 2 seconds of shooting pictures. The storage interfaces found on DSCs today provide a typical transfer rate from the buffer to the storage of 5 MB/s, clearly leading to a bottleneck when writing to storage. Once the buffer becomes full, the camera would stop taking pictures until the data has been written to storage and the buffer has been emptied.

To enable more pictures to be taken in multi-shot mode, a designer could either increase the size of the SDRAM buffer, or deploy a method to write to storage faster. Increasing the size of the SDRAM buffer can be a costly solution, thus it is worthwhile to explore solutions to improve the transfer time to storage.

West Bridge Architecture

In 2006, Cypress Semiconductor introduced the West Bridge architecture. Similar to the North Bridge and South Bridge in PC architectures, the West Bridge was introduced to allow embedded processors to evolve independently from quickly changing memory and peripheral interfaces. The West Bridge is designed as a companion chip to an embedded processor to free it from data-intensive operations. Processor intensive functions, such as USB or mass storage control, may be offloaded to a West Bridge for maximum performance. In a DSC, a West Bridge can be used to address the two discussed challenges faced by DSCs today.

A West Bridge consists of three ports: one for connecting to the processor, one for connecting to mass storage, and one for USB for external connections. Since the West Bridge consists of both USB and mass storage control, a direct path from mass storage to USB is provided, thus offloading the main processor, and providing for maximum USB performance. Consumer devices using Cypress’ West Bridge Antioch, have been tested to reach USB transfer speeds up to 18 MB/s. At that transfer rate, 950 MB of pictures would take just 53 seconds, enabling the best consumer experience when transferring photos

The independent three-port nature of the West Bridge also enables high performance transfers from the processor to mass storage. With flexible high speed interfaces and a dedicated mass storage controller on the West Bridge, performance of up to 20 MB/s writing to storage has been achieved. By writing to storage at the maximum rate, the DSC is able to empty its SDRAM buffer at a faster rate. In a DSC’s multi-shot mode, faster writes to storage would reduce the bottleneck which can occur. A user would thus be able to take more photos under multi-shot mode than they could without a West Bridge architecture.

Conclusion

With average DSC processor life cycles spanning years, the challenge of keeping up with the latest consumer demands becomes a significant issue. It is clear that picture quality, resolution, and size will only continue to increase as consumers and professionals alike strive for the best quality photographs possible. With the new challenges brought forth in an ever changing market, a West Bridge architecture can be adopted to best meet consumer requirements.

About the Author

Timothy Kung is a product manager at Cypress Semiconductor Corp.
Visit www.cypress.com for more details on various storage technologies

Pro Comp Techs, READ INSIDE. Expert Opinion please.?

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1)Dell Inspirion E1505 Dual Core or
2) Alienware Area-51 m5550

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