Television – Special applications – Manufacturing
Patent
1992-10-13
1995-05-02
Groody, James J.
Television
Special applications
Manufacturing
348724, 370 50, 370 70, 455 61, H04N 716
Patent
active
054124158
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the distribution of signals on a network and in particular to the distribution of AM (amplitude modulated) television signals.
2. Related Art
Conventionally, television has been broadcast as AM signals with different channels modulated onto sub-carriers at different frequencies. Existing cable television services using copper coaxial cable have adopted corresponding analogue AM techniques to provide channels in a region of the same UHF spectrum used for broadcast television.
It is now proposed to use cable networks such as fibre optic networks for the distribution of television signals. The wide bandwidth offered by such networks offers the possibility of upgrading to carry future wideband services such as HDTV and the same network may also be used for services other than television, such as telephony. However whilst the use of AM techniques is necessary if the television signals are to be received by conventionally equipped television sets there are a number of problems associated with the use economically of AM techniques on optical networks. In particular the carrying of AM optical signals requires less splitting in order to allow a high power budget, and the use of highly specified linear opto-electronic devices.
BRIEF SUMMARY OF THE INVENTION
According to the present invention, a method of distributing signals from a head-end station via a network comprises combining a plurality of AM channels modulated on sub-carriers at different frequencies to form a composite analogue signal, and characterised by treating the composite analogue signal to reduce the peak-to-mean ratio, digitizing the composite analogue signal, transmitting resulting digital data onto the network, receiving the digital data at a receiver and reconstituting the composite analogue signal for reception at a termination.
Preferably the network is an optical network and the receiver is an optical receiver. Preferably the composite analogue signal is treated by clipping the signal.
A preferred aspect of the present invention provides a method of distributing, e.g. , television signals which is particularly well-adapted to use with optical networks whilst at the same time maintaining compatibility with conventional analogue television sets. As noted above, television signals used for terrestrial broadcasting are modulated onto sub-carriers at different frequencies. When a composite signal is formed by adding together the different channels the resulting waveform has a much higher peak-to-mean ratio than the individual carriers. It is therefore possible to clip or compress the maxima and minima before quantizing without significantly degrading the performance. The A/D converter used for digitizing the signal can then operate over a more restricted input amplitude range and so requires fewer quantization levels to achieve an adequate output video signal-to-noise ratio.
Preferably the method includes receiving the digitized signal at a node remote from the termination and transmitting the reconstituted signal onwards for reception at the termination.
The detection of the digital data is advantageously carried out at a node such as a distribution box or pedestal near the customer's premises. The television signals may then be delivered over conventional coaxial cable in a standard format, avoiding the need for a special adapter or other equipment at the customer-end.
Preferably the composite analogue signal has a bandwidth of one octave or less and is digitized by sampling at a sampling rate below the Nyquist rate.
Preferably, in addition to or as an alternative to clipping the composite analogue signal the method further comprises compressing the composite analogue signal at the head-end station and applying complementary expansion at the receiver.
Companding may be carried out either in the analogue or in the digital domain and preferably the compression function is the cumulative distribution function of the Gaussian probability distribution function.
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Cook Andrew R. J.
Faulkner David W.
British Telecommunications public limited company
Grant Chris
Groody James J.
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