U. Glaeser

FIGURE 39.66
pitch
Address Looping
The phase accumulator of the sample rate converter must have some provision for looping back to a lower
address. Clearly, it cannot continue to increment to infinity. The finite number of bits used to represent
the integer part of the phase precludes that possibility. In addition, it is not useful simply to rely on binary
wraparound of the address from its maximum value back to zero. This would imply all channels of sample
rate conversion are reading from the same input waveform. At a minimum, the phase accumulator contains
a loop address and loop size for each channel. When the value of the phase accumulator crosses the loop
address, it loops back by the loop size and continues from the beginning of the loop. This enables both
streaming audio and wavetable synthesis.
Many natural musical instrument sounds can be characterized by an attack phase, sustain phase, and
release phase. The attack phase is often a primary cue to the listener as to the identity of the instrument.
Usually, it consists of a rapidly changing and nonrepeating waveform. Conversely, the sustain phase is
often a steady state that can be easily described by a repeating waveform. This is also true of the release
phase. During sustain and release phases, the phase accumulator can loop to create the repeating waveform,
saving considerable memory. Besides, the length of the sustain phase is usually unknown because
it is controlled by the length of time the musician presses the key.
When streaming audio, the software fills a circular buffer with a continuous waveform to play. The
phase accumulator loops at the boundaries of the circular buffer and plays the stream. The software must
be careful not to overwrite audio that the sample rate converter has not yet played.
Envelopes and Modulation
It is often necessary to control various aspects of a sound, such as pitch, amplitude, and filter cutoff
frequency with time-varying signals called envelopes. The audio system may use these envelopes to simulate
the changes in sound that occur when a 3-D sound source moves, or as an integral part of the music
synthesis process.
A typical music synthesizer envelope generator has four segments designated attack, decay, sustain,
and release (ADSR) as shown in Fig. 39.67. These four segments are a reasonable approximation of the
amplitude envelopes of real musical instruments. The first two segments are attack and decay, and are
usually a fixed duration. During these segments, the sound is changing rapidly, often containing transients
and wideband noise corresponding to the initial strike of a drum, or pluck of a string. The decay segment
leads to the sustain segment, a variable duration, steady state corresponding to the portion of a note that
is held for a length of time. The final segment, release, occurs after the musician releases the note.
© 2002 by CRC Press LLC
phase accumulator
fraction integer
index
counter
Sample
Memory
Coefficient
ROM
Gossett–Smith interpolator block diagram.
n
k n+1
k n
x T-n
Linear
Interpolator
a n
Output
Accumulator
output