The Signal Processing section, as previously mentioned, serves to transform the received analogue Morse code signal into pulse and space time . duration information for use in the Code Translation section. If the received signal were completely clean and interference free, this transformation would be relatively easy* However, the transformation becomes rather difficult when the effects of noise and signal fading are considered. The main purpose, then, of the Signal Processing section is to recognize the Morse code transmission under realistic conditions of signal fading and distinguish it from pulse interference and “white” noise. The methods used to achieve these tasks will now be discussed.
The incoming pulse-modulated signal is first converted into dc pulses, corresponding to the Morse code being received, by a full-wave bridge rectifier and RC filter. The input signal voltage level is sampled periodically (approximately once every 200 microseconds) and compared to a threshold voltage level. If the input signal is less than the threshold level, a -1 is stored; if the input signal is greater than the threshold level, a +1 is stored. The effects of signal fading may be reduced by proper settings of the input signal voltage and threshold level.
A low pass digital filter technique is employed to limit the effects of noise on the input signal. Rather than base the pulse-space decision on whether the input signal level is above or below the threshold level for any one particular sample, the average of several of the samples must be above or below the threshold before a pulse-to-space or space-to-pulse change is detected.
When an input signal level change is detected, it is checked again to determine if the change is permanent. The checking process is similar to the change detection process, except that twice as many samples are averaged. If the checking process result confirms the input signal change, then the signal is considered to have changed permanently. The time duration of the just-ended pulse or space is then obtained from the realtime clock counter and the clock is reset to begin timing the next pulse or space. If the result of the checking process conflicts with the change detection process, then the input signal change was due to interference rather than a valid Morse code input. In this case, the change detection process is repeated, again looking for an input signal level change.
When a valid input signal change is detected, the pulse or space time duration is stored in a 200g-word memory buffer. Previously stored time durations are retrieved from the buffer as needed by the Code Translation section for processing. The 200g-woru buffer permits the Code Translation section to temporarily lag behind the Signal Processing section without loss of Morse code signal information.
This method of storage and retrieval of Morse code time durations presents an operational limit to the overall recognition program in that it is possible to over-write previously stored time-durations before the Code Translation section processes them. If the interrupts occur too frequently, not enough time will be available for the Code Translation section to keep up with the storage of new time durations, a function of the Morse code transmission speed. Thus, the interrupt frequency upper limit is determined by the execution time of the Cod« Translation section. Fortunately, this presents no real problem to the overall recognition program, since the upper limit is well above the minimum input signal sample frequency needed to ensure proper detection of Morse code signal changes.
The Signal Processing section is divided into two separate routines: 1. the Signal Sampling routine (Fig. 4-4) 2. the Change Detection routine (Fig. 4-5). The particular function of these routines is discussed in the following paragraphs.
The Signal Sampling routine performs the actual sampling of the Morse code input signal. If the sampled input voltage is greater than the threshold level, a +1 is stored. If the sampled input voltage is less than the threshold level, a -1 is stored. If N’ samples have not been taken yet, program operation is returned to the Code Translation section through the Service program. When the N’th sample is taken, program operation continues in the Change Detection routine.