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Conexant tips MPEG coder for STBs

Posted: 16 Feb 2005 ?? ?Print Version ?Bookmark and Share

Keywords:stb? mpeg-2? mpeg? data converter? media gateways?

A year ago, STB architectures were so simple. For a given network, audio and video (A/V) signals came out of a baseband chip in a fixed format, usually some dialect of MPEG-2. The designs implemented a simple MPEG decoderno need for high fidelity or advanced error-recovery featuresand passed the digital A/V on to the data converters. End of task.

But in the race to turn commodity STBs and PCs into Media Gateways and digital entertainment centers (capital letters denoting additional margins for the OEM), things have become messy. One of the biggest problems is local storage. Viewers, so the theory goes, want to raise their arms up from the couch long enough to pause, review or even apply special effects to broadcast signals. They may even want to time-shift content altogether.

That means local storage of the incoming signalsand in many cases, it means random-access storage, since tape won't provide many of the more advanced features without huge buffers. But given the enormous size of disk capacity available on current commodity disk drives, local random-access storage in turn means compression. Unfortunately for those who enjoy elegance of architecture, that means taking the compressed media stream from the baseband chip, decoding it for viewing and then simultaneously re-encoding it to send it off to the disk drive in a sufficiently compact format.

That means the STB, media center or controlling system must now have MPEG-2 encoding hardware. And it has to be pretty good hardware, too. Compression ratio is critical, as it reflects directly on the trade-off between storage capacity (visible to a user if the ratio is low) and image quality (visible to a user all the time). OEMs won't get far selling an appliance if it can't record an entire weekend's feature movies, or if after the second one the films begin to look as if they had been pirated with a VCS camera from a second-run theater screen.

Recognizing this need, Conexant Systems Inc. has launched an MPEG-2 encoder chip designed specifically for these kinds of applications. The CX23418, in true SoC fashion, includes a variety of signal-processing blocks, a CPU to control them and rich choices of I/O formats.

Cleanup time

Specifically, the chip accepts input of NTSC, PAL or Secam and their respective stereo audio broadcast formats. Input may either be analog video, analog audio or IF audio from tuners, or digital A/V. Composite, S-Video and component video formats are supported. The chip uses 10bit video A/Ds internally for the conversion, if necessary.

Once the A/V signals have been captured, the CX23418 offers a variety of options for cleaning up the signal before encodingimportant to both resulting quality and disk space. The chip has 2D and 3D adaptive comb filters for doing general noise-cleanup work. Of particular importance when the signal source is a VCR, the chip also includes a time-base correction block to eliminate the synchronization drift associated with mechanical tape transports.

On the audio side, 6bit resolution is maintained throughout the processing.

So much for cleaning up the signals. Now the problem is compressing them. This is the stage that typically separates the broadcasters from the amateurs. Basically, the more time spent working on a signal stream, the better the job of trading quality for compression ratio. On-the-fly compression, minimal hardware resources and low power all conspire against the task the chip faces.

It responds with hardware. The MPEG blocks offer an extended motion-search range, increasing the probability that a particular cell can be encoded with just a displacement vector instead of a whole new set of discrete cosine transform (DCT) coefficients. Motion estimation is decoupled from the DCT encoding, further improving the compression ratio, according to Conexant.

And the hardware uses an adaptive quantization engine, eking a bit more compression ratio out of the best choice of coding for the coefficients.

Fear of storage

Observing that media center designs which need video compression are often adding digital audio broadcast reception capabilities at the same time, Conexant has thoughtfully included a flexible audio broadcast decoder engine in the chip. The block automatically detects any of a variety of broadcast standards, a useful feature in locations where different audio broadcast channels may use different encoding schemes.

Finally, there is the issue of the terror that content providers feel for the whole idea of digital storage. To prevent the wholesale theft of content and the collapse of the entire entertainment industry, prevailing opinion holds, any content that is stored locally must be encrypted so that only an authorized customer can use it, and only in authorized ways.

To this end, Conexant has provided an encryption block in the chip as well. This hardware handles AES, triple-DES and CPRM encryption so that entertainment industry executives can sleep well.

Taken together, the CX23418 can be thought of as a pretty complete toolkit for upgrading the STB of a year ago into the media gateway motherboard of 2005. Most of the pieces are there, on a single slice of silicon.

The chip is sampling now, with production ramp planned this quarter. The chip is packaged in a 23mm, 388-pin BGA, and is priced at $20 each in lots of 10,000.

- Ron Wilson

EE Times

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