Abstract: An optical transmission system comprises a concatenated chain of amplifiers disposed along a fiber transmission path. The signal to noise ratio is measured at each amplifier by selectively notch filtering a transmitted signal and measuring the power of the filtered and unfiltered signal within the notch bandwidth. A supervisory system controls the gain of each system amplifier whereby to maximize the aggregate signal to noise ratio of the system.

Abstract: An optical transmission system comprises amplified forward and return paths between a local station and a remote station. Each amplifier of the forward path has an attenuated loopback coupling its output to the return path. Frequency modulated supervisory signals generated at the local station are returned via the loopbacks. These signals are analysed e.g. by Fourier analysis to obtain a measure of the performance of each amplifier. The system may also be employed for the location of fiber breaks.

Abstract: An error detecting arrangement for a digital transmission line, wherein signals are transmitted in the form nBIP, and which also provides the possibility of signal validation and a signalling channel. The output of a toggle (4) has a line spectrum at fo/2(n+1) or fo/(n+1), where fo is the line digit frequency, depending on whether the parity is odd mark or even mark. The toggle output is bandpass filtered (at 5) as appropriate and applied to envelope detector (6). Errors can be counted (at 10). The envelope detector output is low pass filtered (at 7), the output of which is an analogue measure of the error rate. The latter is compared (at 9) with a threshold BER and the result can be used for validation or providing a signalling channel (FIG. 2).

Abstract: A method of locating an intermittent electrical fault in an optical undersea transmission system, comprises measuring the time difference between two signals emanating from the fault along two different routes of the system. In one embodiment one of the signals is a voltage transient and the other signal is an optical error signal created in a repeater adjacent the fault as a result of surge protection in the repeater responding to the electrical fault, and in another embodiment both signals are optical signals, once again emanating from adjacent repeaters and caused by the effect of the electrical fault on the adjacent repeaters. It is envisaged that such a transmission system could be permanently provided with apparatus to detect such a fault in which synchronized counters are permanently running and frequently re-set and which stop in response to error signals being received from a fault when it occurs.

Abstract: A supervisory method for a digital optical submerged transmission system in which a multi-bit test signal is detected by a repeater while receiving a loop-back signal for causing loop-back condition, and the mix of the bits of the test signal is changed to change a d.c. level derived in the repeater from the test signal. This is compared with a local reference level and after comparison a sign change is indicated the loop-back condition is either enabled or disabled and that event is detected enabling the required input or output level of the repeater to be checked.

Abstract: A repeater for a submarine cable installation which allows remote control of amplifier gain, equalizer losses and oscillator frequency. A control signal of selected frequency is transmitted to the repeater and converted to a d.c. control signal to adjust the selected component. The signal from the remote control may be in the form of pulses of a desired frequency or signals of longer duration.