Volume 36, Number 1, January-February, 2002
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Simplify Audio Setups with a SigmaDSP Pre-Programmed Fully Configurable Digital Audio Processor
While audio sources at present largely consist of digital media (CDs, DVDs, Internet), most of the audio processing has stayed in the analog domain. It is high time to migrate the audio processing to digital as well.
In the last two decades, audio technology has experienced many advances. Starting 20 years ago with the introduction of the CD, and progressing to MiniDisk and DAT, today we have a choice of high-resolution audio formats such as DVD-Audio, SACD, and the incredible flexibility and storage density of MP3. All these advances concentrate on storage media for the music. But how does the audio signal get processed once it comes off the storage media? How does it get to the output of a system? Are current "digital" systems truly digital? The vast majority of systems today are not.
In audio/video (A/V)-receivers digital signal processing became popular due to the nature of Dolby Digital decoding, but just about all popular audio systems, such as mini-components, car stereos and PC-add on speakers, are still using analog signal-processing technology.
The reason for this is that earlier digital solutions, based on general-purpose digital signal-processors (DSPs), with separate D/A and A/D conversion ICs, carry significant overhead in hardware and programming. Accordingly, the implementation of digital solutions has been difficult and prohibitively expensive.
Now, at a time when major consumer systems are completing the crossover to all-digital media, Analog Devices introduces the AD1954, the first member of the new, cost-effective SigmaDSP family—the first solution to deal directly with the essential problem of integrating an audio-specific DSP, together with high-performance audio converters, on a single IC.
This family of digital sound processing devices, led by the AD1954, offers:
What’s inside the AD1954?
All parameters are fully configurable by the system designer. This allows quick design cycle times while permitting full flexibility in adjusting the system to the specific requirements of each market and customer.
Figure 1. SigmaDSP architecture.
Which DSP core is used in a SigmaDSP?
All filters are calculated in 48-bit double-precision resolution, utilizing special hardware accelerators. Double precision insures that low frequency IIR filters can be operated without limit-cycle problems, which cause audible artifacts.
The core memory comprises 2.5 KB of program RAM, 2.5 KB of program ROM and 1 KB of parameter RAM. All memory is directly accessible through the SPI interface, which uses a self-addressing 32-bit format (8-bit address, 24-bit data) that allows single-cycle access to any memory location. The internal clock rate of the AD1954 is 25 MHz—equal to about 50 MIPS of a general-purpose DSP due to the hardware acceleration.
Figure 2. SigmaDSP processing core.
SigmaDSP Graphical User Interface (GUI) gives the designer Total Setup Control in real time.
The solution is the AD1954 graphical user interface (GUI)—shown in Figure 3. It graphically represents the signal flow of the AD1954 and thus its use is truly intuitive. Every parameter in the signal chain, including filter coefficients, volume settings and dynamic processing functions, can be directly accessed and altered in real time. The GUI connects through the printer port of the PC to the AD1954 evaluation board. In this way, any parametric changes are sent to the AD1954 in SPI format and become immediately effective (in real time).
While the SigmaDSP GUI is intended as a dedicated tool for the system designer, it could also be made available (in a modified version) to the enthusiastic end user. With this PC interface, a user can have total control via a notebook PC.
Figure 3. SigmaDSP graphical user interface.
Why is professional-quality dynamic processing so essential?
This combination of limited amplifier power, heavy bass equalization, and significant total loudness of the system leads readily to a situation where the amplifiers are saturated and start to introduce heavy distortion—resulting in an unsatisfactory and annoying listening experience. In the past, attempts to solve this problem typically used primitive clipped-signal detectors, which avoided the clipping but led to artifacts that were nearly as bad as the clipping distortion itself. The professional-quality, dual-band dynamic processors of the AD1954 SigmaDSP make it possible to control the system limits without artifacts.
Increase clarity and loudness of your system
This is particularly critical in smaller systems with limited amplifier and speaker power.
With a proper transfer function programmed into the AD1954, signals below a certain threshold can be compressed and the music level can be amplified to maintain a level above the road noise. The ultimate use of this technology can be achieved in OEM car systems, where signals from the speedometer and RPM control signals make it possible for the compensation ratio to be altered depending on the speed (wind noise) and engine RPM (motor noise). Anyone who has experienced a smart road-noise compensation strategy in a car sound system never wants to be without it again (perhaps with the exception of a mid-20th-century Rolls Royce driver.)
A "midnight" mode can handle this annoying problem automatically by reducing the dynamic range of the soundtrack. To implement this feature, a similar transfer function to that used for dynamic clip control is used, but at a much lower threshold. To avoid audible artifacts, a professional-type dual-band dynamic-processor like that implemented in the AD1954 is necessary.
The two pairs of sound tracks (Figure 6) are for voices followed by an action scene (bomb explosion). One can see that, while the dynamic range of the action scene gets reduced when the midnight mode is turned on, the voices stay at the same level.
Figure 6. In midnight mode, high levels are suppressed without affecting normal levels.