Crystal Semiconductor of Austin, Texas, a wholly owned subsidiary of Cirrus Logic Inc of Fremont, California, has introduced the CS4920 Audio Decoder digital-to-analogue converter, a single chip audio decompression system. The CS4920 can produce stereo analogue output, and includes a fully integrated signal processor that supports a wide range of digital audio decompression standards. The CS4920 is claimed to be the first device to incorporate all of the decompression and conversion functions into a single chip. Into its 44-pin package have been integrated a proprietary programmable 24-bit signal processor with 18Kb of program and data SRAM; a 16-bit, CD-quality stereo digital-to-analogue converter; a programmable clock manager; a compressed audio input port; a digital audio output generator and a serial control port. Current audio subsystems with such features typically use a combination of ASICs and discrete components that require 200 or more pins. The CS4920 eliminates these high-pin-count devices, reducing the form factor, power consumption, design complexity and overall cost of implementing an audio subsystem. With the CS4920, compressed digital audio that is broadcast on such networks as cable and satellite television, AM and FM radio, and telephone lines, or that is contained on such media as CD-ROMs, can be cost-effectively decompressed and converted to standard analogue or digital audio signals, Crystal says.
Meeting new standards
The CS4920 supports both industry-standard and proprietary signal processor algorithms, and is available with software that performs industry-standard MPEG layers 1 and 2 and Dolby AC-2 decompression. The market for audio decoder chips is expected to grow from about $23m in 1993 to almost $200m in 1997. Although there is a trend toward digital audio transmission in AM and FM radio, there are presently no clear-cut standards, Crystal says, suggesting that as the CS4920 is fully programmable, it can meet current market needs, and aims to also be adaptable to standards as they emerge. In the personal computer world, the CS4920 is suitable for decompressing the audio data that is widely available on CD-ROMs for multimedia applications. The CS4920 is based on a programmable signal processor core developed by Crystal Semiconductor that operates at up to 16.5 MIPS. This core has a 24-bit fixed-point data path, 4K by 24-bit program RAM, and 2K by 24-bit data RAM. It processes and decompresses audio data according to the algorithm loaded into its program RAM by an off-chip microcontroller. The algorithm can be changed on the fly to accommodate data using different compression formats. The 16-bit digital-to-analogue converter of the CS4920 converts the decompressed digital audio data into a two-channel, CD-quality analogue audio output signal. Using proprietary delta-sigma conversion techniques, it has a signal-to-noise ratio of up to 90dB and a total harmonic distortion specification of less than 0.01%. The serial control port supports communication between the on-chip signal processor and off-chip microprocessor. Such decompression algorithms as MPEG and Dolby AC-2 are stored in the microprocessor’s system memory and downloaded to the signal processor program RAM through the serial control port. When not communicating with the microprocessor, this port is available for such other functions as balance control. The programmable audio input port accepts and double-buffers the compressed audio input data and presents it to the signal processor for processing. The digital audio output generator creates from the decompressed audio a serial digital output that is available for application to an external device, such as a digital compact cassette player, that requires a digital input. The clock manager accepts a synchronous input of any frequency from 7KHz to 40MHz and produces all of the internal clocks required to run the signal processor and audio functions. The CS4920 is packaged in a 44-pin plastic leaded chip carrier. Samples will be available at the end of October, and volume production is
planned for the first quarter of 1994. It will be priced at $38 each in quantities of 1,000 or more.
