A Multimedia Chip With Gusto
In the consumer market, target applications can vary over a wide range. These applications can range from low-cost video telephones to high-end electronic products with built-in video instructions.
Unlike other compression ICs that consume silicon area with direct and inverse cosine-transform circuits for intraframe or interframe processing and motion prediction, the 7710 performs frame-based compression. Each line within a frame is stored and compressed on a line-by-line basis. Moreover, the chip architecture uses minimal pipelining.
As a result, the processor requires just 128 kbytes or more of video RAM (VRAM). Its I/O latency time in less than 100 [micro-s]. A patented on-chip compression algorithm, contained in compression-code tables, is simpler than the proposed Joint Photography Experts Group (JPEG) standard. With fewer calculations required, the video processing rate is 12.5 million pixels/s for real-time full-motion video. Although the maximum frame rate is arbitrarily set at 30 frames/s by system software, the company says it can be higher’
Silicon area, which would otherwise be used for complex intraframe or interframe processing, has been devoted to features that reduce system chip count. For example, the processor has a half-duplex 24-bit bidirectional video I/O bus to transfer digital video data between the chip and external video devices. These devices are usually standard digital-toanalog and analog-to-digital converters that supply analog video signals between the 7710 and external devices. The video source can be a scanner, a digital camera, a VCR, a laser disk, or a graphic-image file. The bus is composed of 8 bits of red, 8 bits of green, and 8 bits of blue data. Dataflow direction is controlled by the 7710.
The video interface also produces the timing necessary to generate standard video streams, including vertical sync, composite sync, and blanking functions. This circuitry also controls external video output logic to generate NTSC, PAL, and/ or RGB video formats for a display monitor. With a video input, the processor is typically used in the “genlock” mode, which synchronizes the chip to the incoming video stream from one or more sources.
The genlock mode is useful for combining two or more video images on one screen. In this case, the processor is programmed to act as a video-timing slave. The vertical sync and horizontal syne lines become inputs to the chip, providing timing signals extracted from the incoming video signal by the external video logic.
An external video quantizer controlled by a sample clock from the chip samples the analog video input and transfers the sample to the 7710. At the video output, a pixel clock strobes the compressed output data into the external video interface for conversion back to an analog signal.
Supporting 64-kword-by-4-bit and 256-kword-by-4-bit VRAMS, the memory interface is also designed for system simplicity. A multiplexed address bus connects directly to the VRAM, and compressed video is sent to memory through serial read and write channels. The processor relieves host overhead by performing the refresh function. It also arbitrates host access to its VRAM parallel channel.
In the input mode, the 7710’s audio-processing logic accepts digitized serial audio signals and performs data compression for storage. In the output mode, the processing logic accepts stored and compressed audio data and delivers a reconstructed serial digital data stream for external conversion into analog audio. The serial interface to the external analog circuitry supports the Texas Instruments serial transfer protocol. Although the 7710 is designed to link with the TLC32045 (or equivalent) voice-band analog interface chip from TI, connection to other serial protocols and support chips is possible by using simple external logic, the processor’s compounding technique, or its bypass mode.
To synchronize stored audio and video data, the audio interface can generate and remove audio timemarkers, which are adjustable in 16.6- and 33.3-ms increments. The chip inserts a unique marker code into the compressed audio and video data so that the separately stored audio and video data can be timetracked. The marker is removed by the chip automatically when audio or video is uncompressed. Buffering between the audio-processing logic and host interface includes DMA capability for data transfer between the processor and host CPU.
The host interface is CPU-independent, operating equally well with 8-, 16-, or 32-bit microprocessors. The interface consists of a 16-bit bidirectional data bus, a 4-bit address bus, five control lines, and one line for an external clock.
Working in concert with UVC’s software package, the 7710 has several programmable features that further contribute to implementing a multimedia system with a minimum number of chips Fig. 2). These include programmable display-window size and offset, variable audio-sampling rate, and selectable compression ratio to optimize image quality for any desired communication bandwidth. Other features include chroma-keying and selectable zoom factors of X1, X1.5, and X2. Chroma-keying treats parts of an image as being transparent, based on the image’s color values. As used in the 7710, the technique makes it possible for users to specify background color. The technique also contributes to frame-processing speed by eliminating the need to compress the same solid background in a series of frames.
Another useful feature is a software-controlled, single-step mode to code and decode non-standard video data formats, such as the pixel video format employed in IBM’s video-graphics-array (VGA) display systems. Other modes provide chip and system testability.
The 7710 can be used for data compression or expansion at both ends of a communications link. Alternatively, data expansion at a receive-only terminal, such as a video monitor, can be performed by UVC’s software. Aside from supporting image handling operations, a principal role of the system software is file management.
The software package has a standardized file format to ensure that compatibility exists among applications, and to allow for conversion between standard video file formats and UVC’s format. For example, the software can convert tagged image file format (TIFF) or JPEG files to the UVC format for file transfer. TIFF is a standard file format that stores graphic images in a bit-mapped (raster-graphics) format. A communications spooler manages file transfer between local and remote stations.
Other software tools include high-level function libraries and sample source code to minimize application-development efforts, and low-level function libraries for complex application development. The menu-driven software also has a disk driver that supports data capture and playback using magnetic media. Suites of multimedia applications software provide video and audio capture, editing, video fax, and real-time video conferencing using standard telephone lines. Also available is a video-mail system that manages video and audio messages exchanged between workstations, and optional file-password security and descriptors, which are words that identify files in an indexed storage system for fast data retrieval.