of Microprocessors

DIGITAL TONE GENERATION TECHNIQUES 429
time, and return the corresponding ideal tone sample by storing it in S. T
will always be between 0 and 1 but will never equal 1. The range of Ss
returned is not important but should be reasonable. The effect of a table
lookup tone-generation routine is simulated by quantizing T according to
the specified number of table entries and calling the ideal tone-generator
subroutine.
The program executes in two parts. The first part runs 1,000 samples
through and accumulates the mean of the ideal samples and the mean of the
difference. The second part runs another 1,000 samples to compute the rms
difference as described earlier. Phase shift due to truncation in the table
lookup is also corrected. When complete (the program may run for several
minutes on many systems) it prints a single number, which is the SiN ratio
in decibels. Note that this is an absolute worst case, since all noise frequencies
including those that would be stopped by the DAC's filter are included.
A figure ;n better agreement with actual audible noise would be about 5 dB
better.
Figure 13-0 gives some results from running the program. Table sizes
of 256, 512, and 1,024 were tried with no interpolation and with linear
interpolation. Two waveforms were also tried, one being a simple sine wave
and the other being a fairly rich waveform having an equal mix offundamental,
2,3, 5,8, 11, 14, and 17th harmonics.
A. NO INTERPOLATION
1. Sine waveform
a. 256 points 42.99 dB
b. 512 points 49.03 dB
c. 1024 points 55.05 dB
2. Complex waveform
a. 256 points 23.56 dB
b. 512 points 29.55 dB
c. 1024 points 35.41 dB
B. LINEAR INTERPOLATION
1. Sine waveform
a. 256 points 85.19 dB
b. 512 points 97.23 dB
c. 1024 points 109.28 dB
2. Complex waveform
a. 256 points 42.75 dB
b. 512 points 54.76 dB
C. 1024 points 66.82 dB
Fig. 13-6. Worst case table noise for various combinations of table length,
stored waveform, and interpolation. (A) No interpolation. (B) Linear
interpolation.