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Fitting DSP to application no easy task

Posted: 16 May 2003 ?? ?Print Version ?Bookmark and Share

Keywords:dsp? networked multimedia device? nmd? ethernet? lan?

Selecting a DSP for wired and wireless networked multimedia applications is a complex endeavor. First, a thorough analysis of the processor's core architecture and peripheral set must be prepared in the context of both present and near-term industry interface needs. Second, it is crucial to understand how multimedia data - video, images, audio, and packet data, for example - flows through a DSP-based system to prevent bandwidth bottlenecks.

Among the first measures that system designers should analyze when selecting a DSP are the number of instructions performed each second, the number of operations accomplished in each processor clock cycle and the efficiency of the computation units. The merits of each of these metrics can be determined by running a representative set of benchmarks - for instance, video, and audio compression algorithms - on the DSPs under evaluation.

The results will indicate whether the real-time processing requirements exceed the DSP's capabilities and, equally important, whether there will be sufficient capacity available to handle new or evolving system requirements. Many standard benchmarks assume that the data to be processed already resides within internal memory. This technique allows a more direct comparison among DSPs from different suppliers, as long as the designer reconciles the I/O considerations separately.

The right peripheral mix saves time and money by eliminating external circuitry to support the needed interface. Networked multimedia devices (NMDs) draw from a universe of standard peripherals. Prominent among these is, of course, connectivity to the network interface. In wired applications, Ethernet (IEEE 802.3) is the most popular choice for networking over a LAN, whereas IEEE 802.11a/b is emerging as the prime choice for WLANs. Many Ethernet solutions are available as a direct extension to the DSP. In addition, on DSPs that can support microcontroller functionality equally well, a TCP/IP stack can be managed right on the DSP.

Also necessary for linking the DSP to the multimedia system environment are UART serial ports. In NMD systems, audio codec data often streams over synchronous 8-bit to 32-bit serial ports, whereas audio and video codec control channels are managed via a slower serial interface such as SPI.

DSPs suitable for the NMD market will include an external memory interface that provides asynchronous and SDRAM controllers. The asynchronous memory interface facilitates connection to flash, E2PROM and peripheral bridge chips, whereas SDRAM provides the necessary storage for computationally intensive calculations on large data frames.

A new peripheral that has started to appear on high-performance DSPs is the parallel peripheral interface (PPI). This port can decode ITU-R-656 data as well as act as a general purpose 8-bit to 16-bit I/O port for high-speed ADCs and DACs or ITU-R-601 video streams. It can also support a direct connection to an LCD panel.

Additional features are available that can also reduce system costs and improve data flow within the system. For example, the PPI can connect to a video decoder and automatically ignore everything except active video, effectively reducing an NTSC input video stream rate from 27Mbps to 20Mbps and sharply reducing the amount of off-chip memory needed to handle the video.

- David Katz & Rick Gentile

Senior DSP Applications Engineers

DSP and System Products Group, Analog Devices Inc.

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