The improved power consumption comes from the integration of a dc-dc converter on the same die. On-chip registers control the converter, ensuring that the Class AB amplifier is on only when necessary. Other on-chip functions include a 56004-compatible DSP core, a USB interface, an LCD interface, a memory interface that supports NAND flash, and a codec (Fig. 4).
"With this compact and inexpensive solution, SigmaTel has the potential to expand beyond MP3 audio players into other systems like cell phones, PDAs, and digital cameras," says Will Strauss, president of Forward Concepts, a market research firm. The flexibility and programmability of an on-board DSP allows the STMP3400 to handle other decoding formats, like Microsoft's Windows Media Access (WMA) and Dolby's AAC.
Meanwhile, Motorola has taken a new direction with its audio solutions. Until now, the supplier had focused on solutions based on its 24-bit DSP architecture, the DSP56300. Combining this powerful DSP core with specialized peripherals, software, and development tools, Motorola offered its Symphony audio processors for a myriad of audio applications. Now, it's complementing this line with a new family based on a 32-bit microprocessor architecture, called ColdFire. By running both types of code efficiently, ColdFire enables the replacement of two separate processors by a single CPU, Motorla contends.
Aiming for higher levels of integration and lower system power, Motorola is readying an MP3 decoder using the ColdFire architecture. Incorporating an enhanced multiply-accumulate (MAC) unit, custom audio peripherals, and system software, this new audio decoder uses just 36 kbytes of memory and a 19-MHz clock. The decoder can run from on-chip memory and requires fewer cycles than typical decoders to provide longer battery life for consumer products, says Flip Lockhoof, marketing manager for Motorola's audio and gaming products.
Because external memory and other interface devices aren't required, the ColdFire-based decoder also cuts system costs. "We could have chosen to implement it on our 56300 Symphony DSP family. But we selected the ColdFire microprocessor as the optimum architecture, due to a large amount of control code that must be dealt with," says Ken Obuszewski, Motorola's audio solutions operation manager.
In short, it's a popular embedded controller that's easier to code for audio algorithms and supports high-level languages like C and C++, Lockhoof says. It's also being targeted for applications that require significant control processing for file management, data buffering, and user interface, along with signal processing, he adds. However, there are a number of applications where DSPs offer a better solution. And, Motorola will continue to serve those systems with 24-bit DSP cores in its arsenal. Even so, integrating a Class D amplifier on an audio processor chip is not yet on the supplier's drawing board.
The first ColdFire audio derivative should be unveiled later this year, with general sampling beginning in the first quarter of 2002. In addition, Motorola is readying an MP3 encoder employing the ColdFire microprocessor.
While most suppliers have moved from 16-bit resolution and a 44.1-kHz sampling frequency to 24 bits and 96 kHz or higher, Analog Devices (ADI) continues to field 32-bit DSP solutions. With the advent of DVD audio, 20- or 24-bit resolution and 96- or 192-kHz sampling rates are becoming the norm. Advances in data converters are helping designers to go for 20- and 24-bit ADCs and DACs.
"Digital processing of these higher-resolution audio signals is requiring the use of 32-bit processing to ensure that quantization noise artifacts won't exceed the 20- or 24-bit input signal," explains Colin Duggan, product manager for ADI's digital audio group. "Plus, the 32-bit DSP provides sufficient headroom to achieve the desired dynamic range, and it doesn't limit the SNR of data converters. In addition, it uses less MIPS to do the decoding job."
Therefore, it's not surprising that ADI is pushing the 32-bit single instruction, multiple-data (SIMD) SHARC DSP into this space. Also, according to ADI, SHARC prices have dropped in the last few years, making this DSP architecture attractive for low-cost consumer applications.
SHARC was first introduced into professional audio equipment and has now gotten into high-end audio due to price cuts. ADI is planning further reductions in cost to make it attractive for price-sensitive consumer applications. The company's goal is to bring the price of 32-bit floating-point SHARC DSPs to under $5.00 within three years.
All of this activity in audio chips comes against a background of optimistic market forecasts by audio chip manufacturers. Despite this year's decline in PC sales and associated audio chips, they see the market for audio chips as "booming." A recent report by Forward Concepts shows that the total market for all varieties of audio chips reached nearly $1.7 billion last year, and it's projected to go over $4.9 billion by 2005. This represents an annual compound growth rate of about 20%.