Emerging DSP bring DVD-quality to PDAs

As we enter the realm of mobile devices that carry wireless streaming video, it is useful to consider the types of DSP architectures that will be available for such products. We can glean tomorrow's architectures from the solutions that are being developed and extrapolate from there.

Mobile devices are characterized by low-power, low-cost and small-form-factor requirements. To meet these objectives, today's products - from digital cameras to MP3 players to PDAs - typically operate on complex, highly integrated SoC devices that include both the processing blocks, such as DSPs and the I/O blocks specific to the application.

Solutions using a general-purpose processor provide very flexible platforms but are challenged when facing the demands of high-performance applications. Hardwired, fixed-function solutions support high-performance and low-cost objectives but at the expense of flexibility. They also raise development cost and time-to-market risk, among other things, trade-offs that are fairly well understood by system designers.

Attractive solution

The most compelling applications for future mobile devices will be imaging based, whether capturing video for e-mail or videophones or receiving an A/V stream from the Internet or another caller. The advances in networking and peer-to-peer communications brought about by 802.11, Bluetooth, and cellular radio will enable visual communications as simply as we make phone calls.

Video algorithms must process sequences of images that are rapidly generated by an image sensor. Image processing, like A/V processing, must be done in real-time since even digital still cameras offer MPEG-4 video-capture modes and burst-shooting capability. These image-processing algorithms must be executed on all pixels and then compressed according to the appropriate compression algorithm.

Digital image processing is computationally intensive because the data volumes are very large and the operations involve a lot of manipulation on non-32-bit boundaries. General-purpose DSPs and RISC engines are not well-equipped to handle this efficiently. To boost performance, special instructions can be implemented.

Processing requirements

Image-processing algorithms often require only 8bit and 16bit formats, not the 32bit and higher formats upon which modern processors and compilers have focused. Encoding a bit stream during the image-compression process often requires packing bits together. This is typically accomplished with several mask and shift instructions.

Operation on byte and word boundaries is often addressed with single-instruction, multiple-data (SIMD) structures that are supported on top of the general-purpose architecture. Most SIMD implementations take this form but this approach often requires a lot of data reorganization and precision compromises due to the underlying architecture.

A compute engine optimized to support these types of operations well would significantly accelerate image-processing algorithms. Ultimately, processing solutions tailored to the specific computational needs of image processing will bring DVD-like quality to mobile devices.

Consider, for example, that while video processing has always been one of the most demanding applications, it is now commonplace for high-end processors to run MPEG compression algorithms. In the early 1990s, it took multiple ASICs to build an MPEG-2 encoder. Today, high-end processors can run MPEG-2 encode in real-time and many of them can decode multiple video streams in parallel. While today, even MPEG-2 decode is difficult for a programmable mobile device, it is certainly not unreasonable to expect such capabilities to be available in the near future.

And for fully programmable mobile solutions, a keen focus on low power and low cost is given top priority. This means the whole system-level solution needs to be taken into consideration.

For imaging operations, a data memory implemented in SRAM can be more efficient than a data cache since imaging data is not random. The modern very long instruction word (VLIW) architectures store and route wide instruction words and require many instruction decoders but imaging applications benefit much more from data-level parallelism than instruction-level parallelism. Architectures that focus on this aspect of acceleration can be far more efficient since they require fewer transistors to implement.

'Viewing' ahead

Future mobile devices will become very video-centric and with the multitude of compression technologies currently deployed - and the premium value of higher-quality, low-bit-rate coding - we can expect they will require flexible processing solutions. The most successful mobile devices will support high-quality streaming video, both capturing and viewing, since that is what consumers have come to expect.

Designers will be able to select from various approaches to meet their computing needs - from RISC processors and RISC plus DSP, to processor plus fixed-function logic and reconfigurable and application-specific processors.

In five years, the performance curve for mobile applications will rise, largely driven by image- and video-processing requirements. Successful design for this performance level will require a new approach to the problem and the development of a new signal-processing architecture. For example, ChipWrights has recently announced an application-specific processor architecture that strikes a new balance between general-purpose compute power and video-processing compute power.

The CWvx combines a general-purpose RISC processor with an advanced vector array of DSP execution units that are specifically designed to accelerate image processing. The machine operates in SIMD fashion and can be scaled to support between two and 16 DSP execution units. Each 32bit, fixed-point execution unit can support six operations per cycle and four, 8bit MACs per cycle with full precision.

This type of architecture provides a highly efficient compute engine for mobile video encoding and decoding. By leveraging the characteristics of image processing and incorporating a general-purpose processor, DVD-quality video is attainable in a mobile device such as a PDA.

- Steve Wilson

Marketing VP

ChipWrights