sames

SA9401

11

Tone Waveform

(which may be a simple or complex waveform) and calculating the series of PCM sam-

ples which would result if that tone were passed through a CODEC. The definition of a

waveform is perhaps best illustrated by way of an example.

Consider the case of a PTT dial tone which is a signal of 400Hz modulated at 33.33Hz

as illustrated in fig. 4 below. The first step is to identify the lowest frequency in the

composite signal, in this case 33.33Hz. Next the designer should confirm that one

cycle of the lower frequency component of the signal will contain an integer number of

cycles of the higher frequency component. In the example there are exactly 12 cycles

of the 400Hz signal contained within the one cycle of the modulating signal. If this is

not the case then the tone sample should be designed to contain two or more cycles of

the lower frequency tone, subject to the maximum of 512 samples. It is also typical to

take a block of the signal which starts and ends at two similar zero crossing points.

Although any arbitrary point in the waveform may be used a careless choice may lead

to the introduction of undesirable harmonics.

Having chosen a suitable tone sample it is possible to determine the value for CYCLE

as the size of the tone block is simply the number of samples (at 8k samples per

second) which fall into the waveform chosen. In the example CYCLE = 240 samples =

F0H.

1 / 33.3Hz = 30ms

30ms @ 8k SAMPLES / sec = 240 SAMPLES = CYCLE

1 / 400Hz = 2.5ms

FIGURE 4

400Hz Tone modulated at 33.33Hz.