| 摘要 |
Method and apparatus for secure transmission of analog signals, particularly facsimile signals. The analog signal is first converted to a frequency-modulated signal, if not already one, and is then converted to a sequence of digital words representing the instantaneous frequency thereof, by sampling at a fixed rate. Next the digital words representing the sample values are written in sequence into a buffer memory, at the constant sampling rate. The digital words are then read out of the memory at a different, pseudo-randomly variable rate and converted back to an analog FM signal for transmission. The transmitted signal is, thus, a time-delay-modulated version of the original FM signal. Optionally, the digital word values may be transformed before being converted back to analog frequency form, so that the modulation content of the transmitted signal is scrambled, as well. Further security may be obtained by reading out from the memory in a pattern which differs from the write-in address pattern. The read-out rate from the memory is varied by changing the clocking rate of the read-out operation at pseudo-random times, to select the read-out rate from a plurality of available rates. The available rates include at least one which is faster that the write-in rate and one which is slower than the write-in rate. To avoid underflow or overflow of data at the memory, detection of an incipient underflow or overflow condition causes the read-out rate to be changed, respectively, from a faster-than-write-in rate to a slower-than-write-in rate, and vice versa; when the fast/slow nature of the rate is changed, the selection of the new rate is made pseudo-randomly from among the available choices. Also, the fast/slow nature of the rate may be reversed pseudo-randomly, either at random times or at preselected points.
|
