[Technology Report]
All-Digital Approach Hikes Audio Quality In Consumer Products
Adding a digital power amplifier to a DSP-based audio processor keeps the signal in digital format from the source to the speakers.
From professional studios to car radios to home entertainment systems, the demand for high-quality audio is on the rise. The increasing performance of low-cost programmable DSPs is helping makers of these systems meet customer demand.
In fact, digital techniques have reinvigorated audio processing, bringing professional-quality surroundsound and 3D effects to consumer products. They're spearheading the convergence of PC and consumer audio products in applications like Internet audio by using MP3-like compression techniques. Moreover, the addition of digital amplifiers creates complete digital solutions, starting from the source and going all the way to the speakers.
To keep the external component count low, developers are bundling optimized peripherals and functions, including interfaces, around high-performance DSP cores. Highly integrated system-level audio processors are the result. Also, advances in process technology permit designers to pack Class D digital amplifiers on the same audio processor chip. Unlike their analog counterparts, digital amplifiers eliminate radiation and dramatically cut power dissipation. Placing them inside speakers creates a new class of audio transducers called digital speakers.
Embarking on that all-digital audio paradigm, Texas Instruments (TI) has bundled a fully digital audio amplifier with its TMS320C54x DSP, and a portfolio of algorithms to offer a complete hardware/software solution that includes digital audio decoding at the speakers (Fig. 1). Some standard formats handled by the DSPs include Dolby Digital, 3D Surround Sound, DTS, MP3, and Advanced Audio Coding (AAC).
There's enough horsepower available from the DSP chip to perform other tasks, such as filtering and high-precision parametric equalization, as well as artifact-free volume, bass, and treble control. So, a host system can send digital data directly to the speakers from a CD, DVD, TV, game console, or the Internet.
"By having a digital connection, designers avoid the power and signal losses associated with transmitting analog signals in audio systems," notes Chris Schairbaum, worldwide marketing manager for TI's Internet audio products. "Also, the system noise and other faults inherent to analog transmission are eliminated to greatly improve sound quality. Plus, integrating codecs with audio processors on the same silicon simplifies design and cuts board space requirements."
Direct-Digital Amplification This all-digital audio chain is making the speaker enclosure an attractive place for amplifying the sound and for achieving a balanced mix of signal-to-noise ratio (SNR), dynamic range, and frequency response at a reasonable cost. According to TI, audio designers can now enclose a complete system inside a speaker with higher than 90% efficiency and 95-dB SNR while eliminating bulky and expensive heatsinks.
With the ability to sample at rates as high as 192 kHz, and a dynamic range of 110 dB, TI's TAS5015 digital audio amplifier can now support sound reproduction that's compatible with DVD-audio standards. Additionally, the amplifier dissipates about one-tenth the power of conventional Class A and Class AB amplifiers.
Combining audio-friendly peripherals around its cores, TI continues to offer power-efficient, application-specific DSPs for a variety of consumer and PC applications. The company is looking into bringing the digital audio-power amplifier on board too.
"End-to-end digital solution" also is the buzz term at Cirrus Logic, a major contender in the audio arena. Em-ploying DSP-controlled digital-switch techniques, the company has developed a fully digital Class D amplifier, dubbed TrueDigital. To offer a platform ap-proach to building audio systems, Cirrus has combined its True-Digital power amplifiers with DSP-based audio processors and created a total entertainment (total-e) solution.
Moreover, the company has integrated this technology with a DSP/RISC core to create a highly integrated Maverick processor, the EP7409. The processor is aimed at Internet audio players and many other portable digital audio applications. Some key functions combined on this processor include a TrueDigital Class D headphone amplifier, the AMR7TDMI CPU, a 24-bit audio DSP, MavericKey security technology, 104 kbytes of SRAM, and 60 kbytes of ROM (Fig. 2).
"This integrated Internet audio processor approach provides cost savings due to a lower bill of materials (BOM), and substantially reduced power consumption," says John Marc Woosley, business development manager for Cirrus. The Maverick processor dramatically reduces the number of external parts required to build a player, thereby cutting the system cost, he asserts. It eliminates the need for external NOR flash to program and store data and requires a low-cost 32-kHz crystal clock. An internal PLL generates all clock frequencies needed by the system. The EP7409 is designed to bring the cost of flash-based Internet players below $100 (retail price).
"Three things contribute to the overall reduction in system power consumption," notes Woosley. These are a 90% efficient on-chip TrueDigital Class D headphone amplifier, low-voltage operation, and fewer clock cycles to perform MP3 tasks. While the employed 0.25-µm CMOS process lowers the supply voltage, the dual-MAC-based 24-bit DSP architecture uses an 18-MHz clock to perform MP3 tasks.
Compared to first-generation products, power requirements were slashed in half to 170 mW in the latest third-generation players, claims Cirrus. That translates to double the run time in portable Internet audio players.
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