Configurable Application-Specific Signal Processors Speed Design and Development
DESIGN AND IMPLEMENTATION ISSUES
The challenges in designing and implementing a configurable ASSP are many. Let's consider a few by walking through the design and implementation of a conferencing chip. We'll assume that we have a software building block that mixes voice signals for the purpose of conferencing. What other features might be useful in a conferencing system?
When you first dial into a conference call, you're asked for a conference number and perhaps a pass code. It would be nice if the conference chip could support voice playback to send out prerecorded messages--like the one that prompts the user to key in the conference ID--to individual channels on demand.
When prompted, you enter your conference number using your telephone's keypad, which uses DTMF signaling to send the information. The conference chip should therefore support DTMF tone detection to identify the conference ID that you keyed in.
Quite often a conference bridge will ask you to state your name so that you can be announced to the conference attendees. Therefore, the next logical feature for the conference chip is voice recording.
Before your name is broadcast to the conference members, an alerting signal is usually sent. In other words, the conference chip should have a tone generator.
Echo cancellation? Perhaps. Noise reduction? Definitely--with people dialing into conferences using speakerphones and cell phones, we need a way to combat the background noise.
Since our configurable ASSP is intended to be used in many different products with different feature sets, interfaces, channel densities, and so on, how do we provide maximum functionality to the high-end application without overburdening the low-end application with unneeded features that waste resources, such as MIPS, memory, and on-chip peripherals, and drive up the cost? The answer is to make the solution available on a number of DSPs spanning the low end to the high end, to make the feature set configurable, and to maintain flexibility in the types of available interfaces.
This leads to a set of key implementation tradeoffs: build-time versus run-time configuration. If a feature is configured at build time, it will reside in the configurable ASSP's software image. Even so, the user may still be given the option to enable or disable the feature at run time--either at reset or later.
Our conferencing chip's software image might include echo cancellation, but it may allow the host to disable the echo canceller on a chip-wide basis at reset time or even on a channel-by-channel basis when a party dials into a conference. If the host knows that a party is dialing in from an analog line, the echo canceller could be enabled, whereas if the party is dialing in from a digital line, the echo canceller could be disabled.
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