99 of 286
85 FreqOut Type Subroutine Invocation FreqOut(pin, freqA, freqB, duration) Parameter Method Type Description pin ByVal Byte The pin on which the signal will be created. freqA ByVal Integer The primary frequency, in Hertz. freqB ByVal Integer The secondary frequency, in Hertz. duration ByVal Single or Integer The duration of the signal, in seconds or units. See the discussion below for more details. Discussion This routine generates a signal on the specified pin that is a digital approximation of two superimposed sine waves having the specified frequencies. The method used to produce the signal is a pseudo-PWM technique similar to that used for DACPin(). The output signal is actually purely digital, consisting of a series of precisely timed pulses that have an average value approximating that of two superimposed sine waves. This signal must be filtered to get an analog approximation. Depending on what you want to do with the signal, it may need to be amplified as well. The duration of the signal may be specified in seconds by providing a Single value. Alternately, the time may be specified in units of approximately 1 millisecond by giving duration as an Integer or UnsignedInteger value. In either case, the valid range is approximately 1ms to 32 seconds. Before beginning the frequency generation, the specified pin will be made an output. When the routine returns, the pin will still be an output. If the pin is invalid, or both frequencies are zero, or the duration is zero, this routine does nothing. The maximum frequency that can be produced is approximately 14.4KHz. Requesting higher frequencies will produce undefined results. Resource Usage This routine uses the I/O Timer and disables interrupts until the signal generation is completed. RTC ticks are accumulated during the process so long signal durations should not cause a loss in RTC accuracy. Example Call FreqOut(pin, 440, 880, 5.0) ' play middle C/high C for 5 seconds Because of the high frequency nature of the pulse train used to synthesize the waveform some filtering is required. The example circuit below may be used to couple the output to a high impedance speaker (> 40O) or an amplifier. Note, however, that the signal is too large to be fed to the microphone input of an amplifier. Instead, the Auxiliary or Line input should be used.