Another aspect of prefiltering deals with repeated fields. When transferring film to video, some fields are repeated. MPEG allows users to just repeat the field at the output, rather than processing it again. The iTVC15, then, identifies repeated fields.
The chip also looks for major scene changes. Typically, MPEG does its work by predicting across frames. Predicting across a scene change is absolutely useless, though. So, the iTVC15 has to recognize a scene change and start over again.
The motion search range is a key improvement as well. MPEG tracks motion from frame to frame. A good encoder will track motion perfectly and only send the decoder a vector and some refinements—a little rotation, a small change in shading, and other updates of this kind.
Basically, the decoder moves the object along the vector to get to the new frame and then refines it using the difference information. If the object moves too far and the encoder isn't able to track that motion, this technique can't be used. The whole object becomes a refinement, which is very inefficient.
The motion search range has been dramatically increased to deal with this problem. Using more queues or buffers between the different stages of the algorithm provides a chance to look further ahead. More information can be obtained, and better decisions can be made. In effect, iCompression has decoupled motion estimation from the encoder.
The iTVC15 also has improved quantization. This is the fundamental compression block that makes the tradeoff between how much information is going to be kept for a particular detail within the image and how much is going to be thrown away. "You have to be very careful to throw away information where the eye is not going to be so sensitive," Farrelle notes.
The company uses an adaptive quantization scheme that analyzes the image and makes the appropriate decisions based on the context. iCompression says it has improved the scheme, compared to its previous generation of products. Also, the company has concentrated on optimizing the iTVC15's silicon to cut the price. This, it believes, is a fundamental difference between this new chip and its competition.
"The way to do this is to trade off hardware versus software," Farrelle says. For example, iCompression's adaptive quantization is based heavily on hardware, but there are just enough hooks for the software to make appropriate changes.
Farrelle notes that wherever the hardware can be left unsupervised, a function will stay in hardware. But the firmware monitors the various hardware blocks many times per picture. It gathers statistics, runs background processes, monitors changes, and controls the hardware as necessary. To facilitate this, the image is partitioned into slices. At this slice level, the firmware becomes involved in the process.
The firmware handles rate control, which is the quantization and bit allocation. "You've got a certain amount of bits, and you have to put them where they belong. That's done dynamically based upon what's in the picture," Farrelle states.
The quantization happens in the encoder block. The preprocessing supplies the information that lets the encoder make the right decisions about the quantization. This controls the quality of the final stage. To perform the preprocessing, the chip employs three proprietary DSPs. Each DSP is assigned to a specific task, such as motion estimation.
The encoder is a full MPEG-2 encoder, meaning audio and video in and MPEG-2 out, which can be stored or transmitted. The decoder is exactly the same. It's not just a video decoder or audio decoder—it's a full system-level decoder. It can be used for DVD and storage or transmission applications.
Although the encoder and decoder sit on the same die, they are totally decoupled. The chip can decode video streams that are, for instance, transmitted by satellite. Video information doesn't have to be encoded by the chip itself. This gives the DVR user the ability to watch one program while recording another, for example. In other words, the iTVC15 can decode one program while encoding another in the background (Fig. 2).
The chip is built on a 0.25-µm process. As a point of comparison, iCompression says that the iTVC15 integrates five chips that were used to build the TiVo system that was on display at the Consumer Electronics Show last January.
Complete software support for the Windows environment is provided, including the latest Direct Show filters and software decoders. To facilitate rapid application software development, the company supplies source code for sample player/recorder applications. For embedded applications, an application programming interface (API) supports a variety of processors and real-time operating systems.
Price & Availability
The iTVC15 comes in a 456-ball, plastic ball-grid-array (BGA) package. Samples are available now, with production scheduled for the third quarter. The price is $29 each in 100,000-unit quantities.
iCompression Inc., 2500 Walsh Ave., Second Floor, Santa Clara, CA 95051; (408) 855-2556; fax (408) 727-5078; info@icompression.com; www.icompression.com.