Digital signal processors (DSPs) are a foundation element of virtually every type of consumer electronics, especially in digital media and communications. The form that these DSPs take is shifting as the standalone processor gives way to multicore and system-on-a-chip (SoC) designs. Vendors are striving, though, to ensure that consumer device developers have as easy a task as possible when utilizing this once-arcane technology.
Open up any electronics device in the home and chances are good that you will find a DSP inside. You may not recognize it as such, however, because the DSP is so deeply embedded within the application. In a cell phone, the DSP lies at the core of the baseband processor, where it handles tasks such as the audio codec. In a PC, DSL interface, or wireless router, the modem devices hide secret DSPs inside.
Every portable multimedia device uses a DSP to handle image processing and compression. This includes both simple media players and multifunction playback devices such as Apple’s iPod, as well as capture devices like digital cameras and camcorders. Many of the DSPs inside these devices are known by other names, though, such as video processors or image encoders.
The demands on these DSPs are growing. Increasing adoption of high-definition video has multiplied the pixel rate that video processors must handle both at the set-top and increasingly among mobile devices. Similarly, high-definition audio with surround sound has increased the precision and performance demands on audio processing DSPs. Even relatively simple systems such as digital cameras are expanding their needs with the advent of features such as image stabilization and automatic face detection.
Internet convergence is providing an additional opportunity for these DSPs to proliferate. Television set-top boxes, Internet Protocol (IP) telephones, cameras, gaming systems, and media players are blending together in various combinations, interchanging information, and collectively connecting to the Internet. As once standalone consumer devices evolve, their core DSP functions need augmentation to handle wireless communications as well as the new media they must now manage.
STANDALONE DSP EVOLVING
To meet this growing array of needs, the traditional numbercrunching DSP has had to evolve. Yet the standalone, generalpurpose programmable device is still in use. The Analog Devices (ADI) Blackfin DSP family, for instance, is available in broadly targeted versions. Similarly, the Texas Instruments (TI) C64x and C674x families have general-purpose members. Increasingly, however, the DSP functionality a consumer device needs is arriving in another form.
Where the number-crunching requirements are small, the DSP functionality may be as simple as a multiply-accumulate (MAC) block or other DSP-like extensions to an otherwise conventional RISC processor. ARM offers the V5TE DSP extensions for several of its processor families including the ARM9. It also incorporates the DSP-based Neon media processor in the CortexA8 processor family. Similarly, MIPS has DSP application-specific extensions for its MIPS32 and MIPS64 architectures. Tensilica can add DSP functions to its Xtensa LX3 and Xtensa8 customizable cores as well as offer the VectraLX DSP engine.
A common form in applications that have relatively stable, well-bounded requirements for DSP functionality is the accelerator coprocessor. In a DVD player, the microcontroller handling the user interface and system control can have coprocessors for handling demanding functions such as video and audio decoding. These engines are typically DSP-based but target specific application requirements, often by the mix of integrated peripherals they include.
For example, Freescale offers its Symphony DSP family for high-definition and surroundsound audio processing. Such coprocessor engines are optimized to handle just the one task, but retain the flexibility for reprogramming to accommodate requirement changes such as increased resolution or to handle new codecs for improved playback quality.
Often, vendors don’t even identify these targeted devices as DSPs. Instead, these products are known by other names. The DSP Group’s XpandR II family, for instance, targets Digital Enhanced Cordless Telecommunications (DECT) and Wi-Fi wireless telephony. The XpandR II incorporates a CEVA Teak- Lite DSP core, but the company literature simply calls it a wireless chipset processor (Fig. 1).
Where bounded functionality has stabilized to the point of being a commodity requirement, the DSP has become a fixedfunction device that’s scarcely distinguishable from hardwired logic. It is preconfigured for a specific operation, with its programming stored in on-chip ROM.
Simple, low-cost devices such as MP3 players and telephone modems use DSPs this way. As with other targeted devices, vendors do not typically identify them as DSPs. Examples include Zoran’s line of digital camera image processors and Vimicro’s line of IP camera processors.
Vimicro’s latest camera processor, the VC034, is dedicated to handling a digital still image and sending it over a USB link to a host. It includes an integrated interface for direct connection to the image sensor, hardware for creating the JPEG header, and a USB slave interface, all surrounding a ROM-based image signal processor that handles the JPEG compression.
The device’s design could also have implemented JPEG compression in hardware. But since it’s firmware-based, it can offer developers some flexibility for the implementation of additional processing, such as face tracking, as well as the addition of new features. Such flexibility is one of the key reasons that DSPs remain in fixed-function devices rather than migrating all the way to hardwired designs for lowest cost. Consumer device developers need the ability to customize their product’s features to remain competitive.
EMBRACING THE MULTICORE TREND
At the other end of the consumer spectrum, DSP requirements are both unbounded and in continual flux. This is the case with Internet convergence devices, which are seeing a proliferation in the types of media they must support. They’re also seeing many changes in the algorithms for handling the media due to evolving compression techniques. Further, consumers are demanding evergreater image resolution and sound quality in the media playback, increasing the performance demands on the DSP.
The answer that has evolved to address these shifting requirements and growing demands is the multicore embedded DSP. In such devices, one or more programmable DSP cores join a RISC CPU core integrated into a single chip. The TI OMAP L-138 combines an ARM9 CPU with a C674x DSP. The devices in the Zoran Quatro 4500 series, which target printers, have an ARM11 CPU and four DSP cores.