SA9401
Tone Waveform
The frequency content of any desired tone is defined by taking one cycle
o
f the tone
(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
DR-01087
1 / 400Hz = 2.5ms
30ms @ 8k SAMPLES / sec = 240 SAMPLES = CYCLE
FIGURE 4
400Hz Tone modulated at 33.33Hz.
sames
NOTE THAT SIGNAL
AND ENVELOPE
START ON ZERO
CROSSING POINT
11
SAMES [ SAMES ]
