Editorial: September 12, 1996
The
relentless trend toward more integration and greater functionality per IC has
now reached communication ICs, having conquered video and audio devices. Vendors
of digital protocol/format ICs are adding line drivers/receivers to their
devices--which are inherently analog functions--and vendors of line
drivers/receivers are moving up the capability chain by adding digital
functions.
As a designer, ask yourself if vendors who straddle both worlds provide a design that performs well in each. Although a line driver/receiver seems achievable with the right mixed-signal process and design, the harsh reality is that these circuits face unique problems, such as slewing, noise, transient and overload situations, and similar real-world miseries.
Even if vendors can meet the challenge of providing digital ICs with effective line-interface circuitry, you should consider whether this is the best choice for the mid- and long terms. If you need a different line driver/receiver, such as for fiber rather than copper media, you may be out of luck. Although you may be counting on the vendor's promised next device, most complex mixed-signal designs have subtle performance surprises that are apparent to the design team only after testing the device.
By using separate devices, you can retain flexibility, simplify layout, ease the upgrade path, and achieve better performance, in most cases. Although two packages are usually more costly than one device, the virtues of two ICs often outweigh the apparent benefit of the more integrated solution.
Recognizing this, the latest proposal for PC-based audio, Audio Codec '97 ("PC-audio standard defines, splits analog, digital roles," EDN, July 18, 1996, pg 16), from Intel and leading IC vendors, mandates two ICs, one for signal processing and the other for A/D and D/A analog-intensive functions, with a simple five-wire path between the two.
Regardless of Intel's stated or presumed "real" motives (such as pulling the sound-processing functions into the CPU), designers are often better served by the two IC approach. You can upgrade the signal-processing IC to include 3-D sound and other special effects and still retain your analog channel IC. Conversely, you can upgrade your audio IC to more channels and retain the signal processing IC you've finally debugged and integrated with your software. Best of all, you'll have more sources to choose from because there are more analog- and digital-only sources. And, the vendors can develop new products more quickly and with greater certainty when they concentrate on one side of the mixed-signal world.
Bill Schweber
Technical Editor