PALO ALTO, Calif. Dotcast Inc., the company helping Disney deliver video on demand, has opened the hood on a chip that uses existing analog TV signals to ferry digital data to the home.
Dotcast (Kent, Wash.) said it has designed a digital signal processor that harnesses NTSC signals to deliver digital content a rate of one to three Mbits/sec per second. Eventually the company expects to push that rate up to 4.5-Mbits/sec.
The ReX chip, described here at the Hot Chips conference, has been approved by the Federal Communications Commission and has been built into set-top boxes. The boxes should be deployed later this year as part of Disney's roll out of its MovieBeam service, said Slobodan Simovich, Dotcast's vice president of ASIC engineering.
The ReX chip takes a different approach fromother "datacasting" techniques such as the use of DTV signals or unused portions analog TV spectrum. Instead, the ReX chip places data on top of existing video and audio signals.
Specifically, the chip modulates visual data in quadrature with the NTSC visual carrier and is inserted so that it is coherent with NTSC framing. The technology calls for audio data to be modulated on the aural carrier, though only the visual carrier will be used for the first version, Simovich said.
For this to work, the data spectrum has to be prefiltered and the subcarrier carefully spaced to avoid impairing picture quality. Signals can be received by households with a TV station's so-called contour A sphere with a bit-error rate of 10-8. There are 1.67 million contour A households in Los Angeles, for example, according to Simovich.
The ReX chip's "family jewels" reside in its DSP technology, which mixes dedicated logic and programmable elements. At the front end, the DSP handles decoding algorithms, forward error correction and general signal processing functions.
The crux of the signal processing chain is an element called a "configurable stream processor" that blends a unique instruction set architecture, programmable hardware buffers, I/O and buffer synchronization mechanisms The pipeline is designed to handle 52 instructions-including scalar and vector operandsand has set aside memory segments for code, buffers and data. As many as 16 buffers can be configured, and their size may vary. Moreover, the stream processors can be strung together as a multiprocessor subsystem, Simovich said.
In addition, the chip includes a CPU subsystem for system I/O, interrupts and certain DSP applications. For this task the company chose the V850 processor from NEC, which is also Dotcast's foundry partner.
Just as it is trying to breathe new life into old analog TV signals, Dotcast is trying to make the most out of nearly obsolete chip processing technology. The ReX is being manufactured on a five-metal layer, 0.25-micron process from NEC, some three generations behind what some consider to be the leading edge of chip manufacturing. The DSP alone takes up one-third the 12.5-mm x 12.4-mm die area.
Simovich said having access to an advanced process technology was never a priority. Rather, the goal was to make the chip programmable, simple to design and cheap to manufacture. "What we didn't want to do is go for a cutting edge process, which at this point is unstable," he said.
The chip taped out last December and have been in the field since July. In his presentation, Simovich showed the board of a wireless set-top box containing the ReX chip and a separate PowerPC processor from IBM.
The MovieBeam service, announced earlier this year by Disney Chairman Michael Eisner at a National Association of Broadcasters conference, will allow subscribers to download movies to a set-top box over existing airwaves. The datacasting service is being touted as a less expensive alternative to wireline Internet connections.