About a year and a half ago, a new class of product was introduced at the Consumer Electronics Show—the digital video recorder (DVR). Connected to an ordinary TV, this device digitizes the incoming video signal and stores it to a hard disk. It can be used to pause favorite TV shows and create instant replays. Of course, it also can record programs and replay them later.
First to market with these products were Replay TV Inc. (formerly Replay Networks) of Mountain View, Calif., and TiVo Inc. of Sunnyvale, Calif. Their recorders were quite expensive when they debuted, starting at about $700. But prices have dropped and performance has improved, thanks to new, less-expensive ICs and the constantly declining cost of mass storage. As DVR prices tend toward what many people believe is the magic $299 figure, the heavy hitters of the consumer electronics business are being drawn to this arena.
One company hoping to drive down the cost of DVRs while improving quality even further is iCompression. This manufacturer has just announced its iTVC15 MPEG-2 encoder/decoder chip. It's the first device to integrate audio and video encoding and decoding, transport, and a controller for the on-screen display of advanced menus containing broadcast and service information. The iTVC15 can be combined with an inexpensive, high-capacity hard-disk drive and a few support chips to create a very low-cost DVR (Fig. 1).
A key challenge in developing this chip was figuring out how to process the incoming data—the prefiltering. If the IC doesn't preprocess the data well, especially in extended-play modes at low bit rates, terrible results such as blocks and distorted images may follow.
"It's not standard, it's not specified," says Paul Farrelle, vice president of software and algorithm development at iCompression. "That's where we add a little innovation—and then we have to find a way to get that into silicon." Hence, the prefiltering has three different components.
The first is temporal recursive noise reduction—the current picture is weighted against previous pictures. Rather than just encoding the current picture, previous pictures also are taken into account. The current picture is weighted more than the other pictures when the average image is composed. If this isn't correctly accomplished, "tails" are produced. As objects move, a trail is left behind.
This method also removes noise, since it typically isn't in the same place in every picture. Consequently, the IC constantly measures motion and noise level within the picture. "This is all automated and very dynamic. As a result, we can be very aggressive in removing noise without introducing the tails," Farrelle says.
A linear filter is another part of the prefiltering. Generally, the picture starts out as analog. It goes through a video digitizer to produce a frame, which is usually a standard resolution—720 pixels by 400 lines for NTSC (576 lines for PAL). Compressing this at 8 Mbits/s produces excellent pictures after preprocessing. But if the rate is lowered to 1.4 Mbits/s, artifacts of compression such as blocks will appear. In effect, there are too many pixels to process.
To solve this dilemma, the picture's resolution has to be modified before it is compressed at the lower rate. "Softening" the image makes the job easier for the encoder. iCompression has linear filters built into the preprocessing stage to dynamically change or soften the image on the front end. As a result, the end user obtains the best possible picture, even as the data rate is reduced.
A third preprocessing filter—a spatial nonlinear filter—removes noise in the current picture. If there are speckles in the image, for instance, this filter removes them.