NOISE REDUCTION
To reduce noise levels and produce an accurate sound, try to use the full range of -128 to
127 when you represent a waveform. This reduces how much noise (quantization error)
will be added to the signal by using more bits of precision. Quantization noise is caused by
the introduction of round-off error. If you are trying to reproduce a signal, such as a sine
wave, you can represent the amplitude of each sample with only so many digits of
accuracy. The difference between the real number and your approximation is round-off
error, or noise.
By doubling the amplitude, you create half as much noise because the size of the steps of
the wave form stays the same and is therefore a smaller fraction of the amplitude.
In other words, if you try to represent a waveform using, for example, a range of only +3
to -3, the size of the error in the output would be considerably larger than if you use a
range of +127 to -128 to represent the same signal. Proportionally, the digital value used
to represent the waveform amplitude will have a lower error. As you increase the number
of possible sample levels, you decrease the relative size of each step and, therefore,
decrease the size of the error.
To produce quiet sounds, continue to define the waveform using the full range, but adjust
the volume. This maintains the same level of accuracy (signal-to-noise ratio) for quiet
sounds as for loud sounds.
ALIASING DISTORTION
When you use sampling to produce a waveform, a side effect is caused when sampling
rate "beats" or combines with the frequency you wish to produce. This produces two
additional frequencies, one at the sampling rate plus the desired frequency and the other
at the sampling rate minus the desired frequency. This phenomenon is called aliasing
distortion.
Aliasing distortion is eliminated when the sampling rate exceeds the output frequency by
at least 7 KHz. This puts the beat frequency outside the range of the low-pass filter,
cutting off the undesirable frequencies. Figure 5-5 shows a frequency domain plot of the
anti-aliasing low-pass filter used in the system.
- 150 Audio Hardware -
To Next Page
Loading...
