Handbook for Sound Engineers

1176 Chapter 31

Further complicating the delay measurements is the
possible requirement of sending information to an
external system. This could be in the form of sending a
bitstream to a remote decoder, receiving a bitstream
from a remote encoder, and also any error detection
and/or correction on a bitstream that may be required.

31.8.4 Choosing a DSP

The choice of which DSP to use for a particular applica-
tion depends on a collection of factors including:

• Cost. DSPs range in price from several dollars to
  hundreds of dollars. Low-cost DSP processors are
  typically 16-bit fixed-point devices with limited
  amounts of internal memory and few peripherals.
  Low-cost DSPs are typically suited for extremely
  high volume applications, where the exact capabili-
  ties required, and no more, are built into the chip.
  High-cost DSPs typically are newer processors
  that have a great deal of internal memory or other
  architectural features including floating-point arith-
  metic and high speed communication ports.


• Computational Power: MHz, MIPs, MFLOPs. Com-
  putational power is measured in several different
  ways including processor speed (MHz), millions of
  instructions per second (MIPS), and millions of
  floating-point operations per second (MFLOPS). The
  computational power of a processor is usually
  directly related to cost. An MIP means that one mil-
  lion instructions can be executed per second. The
  instructions that can be executed could include
  memory loads and stores or perhaps arithmetic opera-
  tions. An MFLOP means one million floating-point
  operations can be executed per second. A
  floating-point operation includes multiplies and/or
  adds. Often the architecture of the DSP allows the
  DSP to execute two (or more) floating-point opera-
  tions per instruction. In this case the MFLOPs would
  be twice (or more) the MIPs rating of the processor.
  Higher-speed processors allow the user to pack
  more features into a DSP product, but with a higher
  cost.

Power Consumption. Depending on the application,
low power may be important for long battery life or
low heat dissipation. DSPs will have a power rating
and, often, a watt/MIP rating to estimate power
consumption.

Architecture. Different manufacturers’ DSPs have

different features and trade-offs. Some processors

may allow extremely high-speed computational rates

but at the expense of being difficult to program.

Some may offer ease of multiprocessing, multiple

arithmetic processors, or other features.

Arithmetic Precision. The use of floating-point arith-

metic simplifies arithmetic operations. Fixed-point

processors often have lower cost but often require

additional instructions to maintain the level of

numerical accuracy that is often required. The final

production volume of the end product often dictates

whether the added development time is worth the

cost savings.

Peripherals. Certain features of processors such as the

ability to share processor resources among linked

processors or access to external memory/devices can

have a significant impact on which processor to use

for a particular application. Integrated timers, serial

ports, and other features can reduce the number of

additional parts required in a design.

Code Development. The amount of code already devel-

oped for a particular processor family may dictate the

choice of processors. Real-time code development

takes significant time and the investment can be

substantial. The ability to reuse existing code is a

significant time saver in getting products to market.

Development Tools. The development tools are critical

to the timely implementation of an algorithm on a

particular processor. If the tools are not available or

are not functional, the development process will most

likely be extended beyond any reasonable time

estimate.

Third Party Support. DSP processor manufacturers

have a network of companies that provide tools, algo-

rithm implementations, and hardware solutions for

particular problems. It is possible that some company

has already implemented, and makes a living out of

implementing, the type of solution that is required for

a given application.

31.9 Programming a DSP

DSPs, like many other processors, are only useful if

they can input and output data. The software system

used to input and output data is called an I/O system. As

shown in Fig. 31-17, a DSP application program typi-