Audio DSP Perks Up Sound-Quality Perception

Perfecting the art of audio challenges the best of design engineers. Some help in that process may be in store with the SigmaDSP multichannel 28-bit audio processor. Developed by Analog Devices, the AD1940 chip performs equalization, multiband dynamics processing, delay compensation, speaker compensation, and image enhancement.

By applying these algorithms with 28-bit resolution, the SigmaDSP can compensate for real-world limitations of speakers, amplifiers, and listening environments. Automotive sound systems, digital TVs, home theaters, musical instruments, and many other products can use the chip to dramatically improve perceived audio quality.

The signal-processing engine in the chip is comparable to engines in high-end studio equipment, with most of the processing performed in full 56-bit, double-precision mode. This produces strong low-level signal performance with an absence of limit cycles and idle tones.

The AD1940 features a fully programmable DSP engine optimized for audio processing. It can process up to 16 time-division-multiplexed (TDM) serial digital audio channels, each handling up to 192-kHz sample rates. A 28- by 28-bit multiplier and full 56-bit accumulator offer a wide dynamic range for the audio signal processing. Input and output word lengths are actually 24 bits. The four extra internal bits supply headroom to permit internal gains up to 24 dB without internal clipping.

The fully programmable DSP engine runs a rich array of algorithms from Analog Devices. Such algorithms also can be developed using the company's SigmaStudio graphical design environment for audio engineers. The easy-to-use software lets designers graphically configure a custom signal-processing flow using various blocks, including biquad filters, dynamics processors, and surroundsound processors.

Three main memory blocks surround the DSP core. First, a program RAM (1536 words by 40 bits) is initialized by an internal boot ROM on power-up. Second, a parameter RAM (1024 words by 28 bits) holds various signal-processing parameters. Third, a coefficient ROM (512 words by 28 bits) stores predetermined audio characteristic information to better optimize the audio processing.

Parameters stored in the parameter RAM control individual signal-processing blocks such as IIR equalization, filters, dynamics processors, audio delays, and mixer levels. Words held in a small target/slew RAM (64 words by 28 bits) can be used as channel volume controls or for other parameter updates. A data memory (6 kwords by 28 bits) buffers audio data and can serve as a delay buffer (up to 128 ms at a 48-kHz sampling rate).

To manage all data transfers to the internal memories, a sophisticated control port supports read and write capabilities to all memory locations. Very flexible serial input and output ports provide glueless interfaces to various analog-to-digital and digital-to-analog converters (ADCs and DACs), general-purpose DSPs, S/PDIF receivers, and sample-rate converters.

The SigmaDSP, designed to run from a 2.5-V supply, comes in a 48-lead LQFP. It operates over the automotive temperature range of ­40°C to 105°C. Samples of the AD1940YSTZ with its SPI-compatible control interface, and the AS1941YSTZ with an I²C control interface, are immediately available. In lots of 10,000 units, both chips cost $5.96 each.

Evaluation boards for both devices, which include the SigmaStudio design tool, cost $499. The AD1938 advanced codec supports the SigmaDSP. It packs four ADCs and eight DACs and costs $5.88 in 10,000-unit lots. The AD199x series of Class D amplifier chips, in development now, will pack two channels each with 5, 10, 25, or 40 W per channel.