Abstract: A high frequency communications system comprises eight multiplexed (112) spread spectrum channels with data in each channel transmitted using, for example, 32 semi-orthogonal codes to enable 5 bits of data to be sent for every coded symbol transmitted. The multiplexed data is connected to a phase shift key (PSK) modulator (117) to modulate an hf carrier. At the receiver, inband interference is removed by frequency excision (92, 128) or weighting, making use of the facts that the interference is generally narrow-band and the spectrum of the frequency components of the received signal should be symmetrical about the carrier frequency. The detected signal is Fourier transformed by an FFT processor (91) and then connected to an interference exciser (92, 128) where signal samples symmetrical about the carrier frequency are compared and any sample which exceeds the respective symmetrical sample by more than a predetermined threshold is excised or given a low weighting.

Abstract: An ionospheric sounding system for frequency management of a HF communications system operates between a transmitter and a remote receiver and makes use of code-modulated narrow band sounding pulses transmitted at frequencies throughout the HF band. The code has an impulsive auto-correlation function and the frequency selection is pseudo-random. The code may be a two-part complementary code. Alternatively the code may be a selected one of a family of codes possessing high auto-correlation and low cross-correlation properties, thereby enabling communications management information to be conveyed by the choice of code. Measurements of signal and noise are made at the receiver for each transmitted frequency to assist establishing a HF communications link. Synchronous detection of the received signal is used, employing correlators (103, 104) in phase quadrature.

Abstract: An arrangement to remove drift and wow and flutter in a tape recording comprising recording a control signal tone derived from a reference clock (159) in a dedicated control channel or embedded in a signal channel. The control signal on playback (151) is compared to the reference signal in an up/down counter (172) and the difference is used to correct the speed of the tape drive. The drift corrected control tone signal is connected through a programmable RAM delay (154) and a latch (179) to a D/A converter (156). The delayed analogue control signal and the reference signal are limited then connected to phase comparator counter (158) which is triggered by the leading edge of the reference signal and read by the leading edge of the squared control signal to measure the time delay between them as a number of RAM (154) samples (N) and subsamples (n).

Abstract: A slow speed hf communication modem employs frequency and time diversity to overcome transmission problems due to noise, multipath etc. A digitally encoded signal is transmitted consecutively on five complementary 2-tone channels distributed within the hf band. A unique start-of-message (SOM) signal is generated (9) to precede a message transmission and an end-of-message signal (EOM) follows the message. The SOM signal is detected (18) to produce an accurate timing pulse (20) which initiates a clock in the receiver modem (3) arranged to sample each channel tone at times appropriate to receiving message signals. The noise in each channel tone is also sampled before (pre-data bit) and after (post-data bit) a signal data bit and the measured pre-and post-data bits are combined to assign a weighting factor to the associated message data bits. By providing SOM and EOM signals the modem is able to operate unattended.

Abstract: A time clock modem transmitter comprises a pseudo-random number generator (PNG) which produces a time-spaced sequence of pseudo-random numbers such that each number uniquely represents the time of day. A transmitter PNG(11) capable of producing a polynomial sequence longer than 24 hours is reset every 24 hours by a transmitter reference clock (4). A receiver clock modem includes a receiver PNG(26) having the same polynomial characteristics as the transmitter PNG(11). A received signal number is fed into the receiver PNG and the PNG is then started. The output from the receiver PNG and the received signal are then compared to ensure correct synchronization of the receiver clock modem. A detector is provided which is responsive to the random number representing 2400 hours such that it produces a resetting output pulse at 2400 hours to reset a receiver reference clock.