May
2006, Issue 190
ARM-Based
Modern Answering Machine
Philips
ARM Design Contest 2005 First Prize
GEAR
SHIFT
All
of the TAM-TAM’s DSP functions are performed at an 8,000-Hz
sampling rate except for the FSK demodulator. If the
same sampling rate had been used for the FSK demodulation,
I would have had to sample the bits every 6.66 samples
(8,000/1,200). This would have meant doing it every
seven samples most of the time and every six samples
some of the time. That’s feasible, but the process is
easier if the sampling rate is an integer multiple of
the data rate. For that reason, the caller ID demodulation
runs at a 9,600-Hz sampling rate, which is exactly eight
times 1,200 Hz. (Note that 7,200 Hz would work as well.)
To
avoid potential problems, the sampling rate is changed
the first time an interrupt occurs after the DSP handler
is configured for FSK demodulation. This change of gear
doesn’t interfere with other operations. There is no
requirement for other DSP functions while demodulating
FSK.
