The spectrum modification technique is a digital procedure for placing the spectrum of any band-limited signal-analog or digital within an arbitrarily chosen set of (nonoverlapping) frequency intervals, the sum of whose lengths is not less than the original bandwidth of the signal. Because it can serve the function of several digital techniques, such as mixing, interpolating, and decimating, and can he realized in a finite impulse response (FIR) filter form, it may be suitably implemented by a programmable FIR filter having a flexible timing scheme. The technique is more computationally efficient for many of its applications than the conventional algorithms it replaces. In the application for which it was developed, it replaced a mixer and the concomitant filtering required to shift a signal spectrum a fraction of its bandwidth and cut the computational rate an order of magnitude.

Introduced here is a new technique for encrypting bandpass signals. The technique linearly filters a sample-masked signal, modifying the encryption component of the cryptogram to place its spectrum within the frequency band of the signal. Sample masking is a recognized means of encrypting baseband signals while containing the spectrum of the cryptogram within the signal bandwidth. A drawback occurs for bandpass signals, owing to the processing required in the mixing procedures used ordinarily to convert the signal from bandpass to baseband and back. In contrast, the linear filter modifies the encryption component of the cryptogram without processing the signal component, reducing the computational burden of encrypting and decrypting by an order of magnitude. When the maximum frequency is an integer multiple of the minimum frequency, the filter can output the same information symbols that appear at the input and replace the redundant symbols with symbols that control the passband of the output signal. For voice bandwidths of 300-3000 Hz sampled at 6000 Hz, redundancy symbols (one in every ten or 600 samples/s) are removed, the information symbols (nine in ten or 5400 samples/s) are encrypted, and the redundancy symbols are replaced by bandwidth-controlling symbols. Experimental results of clear voice transmitred over simulated telephone lines validate the technique.