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Part 2 shows how to choose the right hardware and software architectures, and how to choose an RTOS.
This two-part series examines the principles that guide successful DSP system design. As we will see, these principles are largely driven by sales volume. As shown in Figure 1, DSP systems are broken down into three rough categories by volume. The volume determines the cost margin the system can support. Low-volume designs generally support high margins, while high-volume designs generally require low margins.
Figure 1. DSP applications are segmented by the number of units shipped per design.
It is this cost view of the system that leads us to The 10 Immutable Laws of DSP:
- Volume drives down margin and cost based on both economies of scale and experience effect.
- Custom silicon delivers the best cost savings but little flexibility; it is used for all very high volume applications.
- FPGA implementations provide a lower-risk alternative to delivering custom solutions without the very high cost of custom silicon.
- DSP and digital signal controller (DSC) components offer great performance for DSP applications where the delivered functionality is not clearly defined and changes must be made to adapt products over time.
- Compared to DSPs, FPGAs and other hardware implementation can offer 10X or greater performance improvements and significant cost reductions for highly parallel algorithms.
- Almost all hardware uses fixed-point to reduce costs for volume applications and floating-point for lower-volume quick-to-market applications.
- DSP implementations no longer offer 10X performance over general-purpose processors.
- Algorithm libraries must be hand optimized to offer top performance.
- Algorithm libraries require support libraries to build a real system and get top performance.
- A DSP RTOS can save designers time and money.
Law 1: Economies of scale, experience effect, and decreasing margin
Economies of scale state that if we make a few of something, the unit cost will be high. If we make a lot, the unit cost will be much lower. For high-volume products, key customers will purchase in bulk. Distributors will also take many units for bulk shipping, Specialized equipment can be made to speed production. And so on.
The experience effect comes from making many units over time. The more units you make, the more efficient you become at making them, and the lower your costs become. In fact, there is a mathematical relationship between volume and cost: The log of cumulative volume is proportional to log of cost per unit. This allows us to predict the future price and cost reductions for a product using a simple linear relationship, as shown in Figure 2.
Figure 2. Experience curves relate cumulative volume to production cost. (Source: Wikipedia.)
Experience curves require some attention to detail. A common mistake is to plot cost against time, not against cumulative volume. Another common mistake is to overlook inflation. (It is worth noting that economies of scale and the experience effect apply to many industries, not just to DSP.)
It is also important to note that a product's price and its production cost have different curves. Early in its life cycle, a new product can command high margins from early adopters. As the product begins reaching mainstream markets, the margin will fall. In the long run, price will track cost with some fixed percentage margin. (For details on this phenomenon, see Strategic Market Planning, Abell and Hammond, Prentice Hall ISBN 0-13-851089-X)
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