Digital Audio Forges Ahead

Analog audio electronics are approaching "maturity," a state eventually achieved by most technologies, in which almost all the great discoveries have been made and progress becomes a process of increasingly arcane refinements. Digital audio is in no such danger, as evidenced by three new product announcements made the first week of April.

Because of their efficiency, digital amplifiers have long been favored by engineers for portable applications. They haven't made big inroads in the audiophile market yet, but that situation may be changing. Santa Clara, CA–based Tripath Technology Inc. has announced that its "Class-T" advanced 1-bit digital audio amplifiers have been chosen by Aiwa for its new stereo micro component system. Sony and Yamaha also use the devices in their micro components, as does Blaupunkt in some of its mobile systems.

Tripath's 1-bit TA1101B enables Aiwa's XR-V10MD micro component system to put out 10Wpc into a 6-ohm load in a very compact package. "This recent adoption by Aiwa exemplifies the broad acceptance of Tripath's 1-bit digital audio amplifier device. Tripath's device is used widely, from personal computers such as Apple's iMac to audio products like Denon and Sony's home-theater systems,'' said Tripath chief Dr. Adya Tripathi. The TA1101B offers "space-saving technology, low heat dissipation, and high energy efficiency," the announcement stated.

Tripath is the originator of an amplification technique called "Digital Power Processing" (DPP®). The company's Class-T 1-bit audio amplifiers employ a new approach to amplifier design, with multiple signal processing techniques, including adaptive/predicative processing, noise shaping, and pre-distortion and sigma delta modulation, instead of traditional pulse-width modulation techniques.

Geneva, Switzerland–based STMicroelectronics has also announced new audio power amplifier chips that promise further reductions in the cost and size of home theater and home audio systems. The company's new Direct Digital Amplification (DDX) chips can "improve sound quality while reducing power consumption, size, and cost," an April 3 announcement stated. The compact flat plastic devices, capable of generating 100W at very low distortion, were developed by Apogee Technology Inc. and licensed exclusively to STMicroelectronics.

DDX designs allow bipolar-CMOS processing to be combined with DMOS power technology on the same chip, at an affordable cost, STMicroelectronics claims. The technique "improves audio performance by eliminating the need for digital-to-analog converters . . . by replacing conventional class-AB audio amplifiers with more efficient switching amplifiers, the power wasted as heat in the amplifier is greatly reduced, which cuts both power consumption and the need for expensive heatsinks to cool the chips." Going beyond class-D switching power amplifiers, DDX uses a patented "damped ternary pulse-width modulation scheme that further improves efficiency." A typical DDX amplifier will be 20% more efficient than a comparable class-D amplifier and 300% more efficient than a class-AB design, with proportionate savings in size and weight for heatsinks.

Large-scale integration is the key to DDX technology, which divides its functions between control processor and power amplifiers. The control processor performs basic DDX modulation functions plus other DSP-based signal functions for surround sound features, tone and volume controls.

STMicroelectronics has three DDX components available for OEMs: the STA304A controller; and two power amplifier types, the STA500 and STA505. The STA304A converts two serial digital inputs in I2S or S/PDIF format into five channels of "digital drive" for DDX power amplifiers, and performs surround sound processing plus volume, tone, and balance controls. Other functions, such as equalization, can be added by embedding additional software, the manufacturer explains.

The STA500 output device delivers 2x30W or 60W in parallel mode; the STA505 delivers 2x50W or 100W in parallel mode. Both chips offer protection against low or high supply voltages and provide a thermal warning output used by the STA304A to avoid a shutdown by reducing power to a safe level. The STA500 is assembled in a "PowerSO36" package, a standard small-outline plastic package with internal copper leadframe to enhance its thermal performance. The STA505 is in a similar package with a die mounting flag on the upper side for attaching an external heatsink.

San Jose, CA–based Asahi Kasei Microsystems Semiconductor, Inc. (AKM), has introduced a high-resolution digital signal processor (DSP) with integrated analog and digital audio functions. The AK7720A is a single-chip audio DSP with a built-in two-channel ADC and six-channel DAC for use in car audio, digital TV, and home audio products. The diminutive AK7720A supports high-performance 24-bit/192kHz digital-to-analog conversion for six channels, for "compatibility with existing and emerging MP3, CD and DVD audio standards."

The AK7720A utilizes an internal PLL circuit that delivers 50% more processing performance for a given clock speed than competing designs, its maker claims. The lower clock speed is said to reduce electromagnetic interference (EMI) while enabling the AK7720A to deliver "superlative audio performance." The AK7720A boasts dynamic ranges of 98dB in the ADC and 108dB in the DAC, "making it ideal for home theater systems." The chip has a five-channel input and 72kbit delay memory. It is RAM-based, allowing OEMs to implement their own algorithms for surround-speaker surround decoding and bass management. It also offers any combination of digital or analog input/output capability. The AK7720A is housed in a small 100-pin flat package; it is priced at $20.65 each in quantities of 1000. Evaluation boards and samples for both products are available now from AKM.