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A COMPARISON OF MICROCONTROLLERS AND DSPs

A Case Study
by Jerry Horn

Start ý Fine Tuning ý Turn Up the Volume ý Timing is Everything ý Storage ý Pricing ý Encore ý Sources and PDF

PRICING

Table 3 shows a bill of materials for the three different system configurations. Prices generally consist of a single unit when purchased in lots of 100 (except for the DSP56364, for which I could only find a single unit price). The SRAM is overkill for this application, but all the configurations use it. Also, the DSP56364 works with only a single SRAM device even though it operates on 24-bit data internally. The DSP hardware can be programmed to save data as 8-, 16-, or 24-bit words, storing and retrieving the data in successive addresses automatically (if larger than 8 bits).

Unlike the SX28AC, which can be programmed by the end user, the DSP56364 requires an external EPROM to store its program (in quantities of approximately 10k units or more, the DSP56364 can be manufactured with a specific program in mask ROM, removing the need for the EPROMýI am assuming that is not an option).

Of course, the unit would also require pedals, knobs, sliders, a case, jacks, and so on. However, these should be approximately the same regardless of the design approach to the effects circuitry. One difference would be power consumption. It seems like the DSP would require more power. Although it is possible that the DSP might be able to enter a sleep mode for a significant amount of time, it is unlikely that the SX28AC would be able to sleep.

A Scenix microcontroller, along with an audio ADC and DAC, would provide the lowest cost, but the savings might only be minimal. I do not honestly believe that the Scenix microcontroller can perform the necessary functions for all of the guitar effects unless they are severely limited in functionality. It is possible to get a useable system, but I would be concerned about the amount of work required to put 10 lbs of stuff in a 5-lb bag. You would have to be creative, careful, and meticulous in solving a large number of problems.

It is also my opinion that the DSP will make the whole design simpler and straightforward, leaving considerably more time to play with the actual effects and allowing different types of effects to be implemented, which is the real goal here.

I have used the Motorola DSP56x and DSP563x family of DSPs for almost 10 years. Do not let the number of pins fool you, the circuitry is straightforward, and these devices are more like microcontrollers on steroids than mysterious DSPs. They are not for every application, but where they are needed, they shine.

In addition, I typically program them just like microcontrollers. That is, the code is simple and does not use the signal processing hardware at all. This type of code probably makes up 95% to 99% of the program and is generally the user interface software and initialization code. The actual digital signal processing routines are a little more difficult to code, but those routines are usually small. And, after everything has been set up, you can play with the signal processing code, endlessly trying almost any type of signal processing you can think of.

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