Signal splitter

1 SIGNAL SPLITTER

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No. 11/204,608, filed Aug. 16, 2005, now U.S. Pat. No. 7,746,194.

FIELD OF THE INVENTION

This invention relates to a signal splitter for reducing noise ingress and a cable television network incorporating such splitters.

BACKGROUND TO THE INVENTION

Cable television networks are no longer purely distribution networks used for TV and radio distribution, but now also provide access for the customer to the networks. Thus TV and radio signals are distributed from a local centre or optical node by way of a signal splitter with an output connected to each customer. Retum traflic from each customer is retumed through the splitter to the local centre or optical node and thence to the rest of the network. Such retum traflic might include requests for pay-per-view television programmes.

Usually the traflic from the customer to the local centre or optical node is called “return path traflic” or “upstream signals”. The upstream signals are transported using a different frequency range than the distribution signals (usually called “downstream signals”) originating from the network provider. Modern cable TV networks typically use 5 MHz to 65 MHz for upstream signals and 85 MHz to 862 MHz for downstream signals, although other frequency ranges are also used.

All upstream signals, no matter how they originate, are transported to the local centre or optical node. Thus unwanted noise in upstream signals will also be injected into the network. The unwanted signals originate from various sources but a major part is due to radiation of outside transmitters in the used upstream frequency range. The total sum of these unwanted signals is known as “ingress”. The majority of ingress originates from the in-house installation of the customer and is therefore injected into the network at a customer access point. This ingress is a major problem in the network since all these unwanted signals are summed and will limit the signal to noise ratio (and therefore the capacity) of the upstream signals.

It is an aim of the present invention to provide a signal splitter which reduces noise ingress into a cable television distribution network.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided a signal splitter comprising an input and a plurality of outputs, wherein altemate outputs are connected to phase shifting devices. Where such a signal splitter is used in a cable television network, the phase shifting devices ensure that noise ingress in upstream signals, i.e. those originating from the customer, passing into the network is substantially reduced. The up stream signals are made up of signals from a number of different customers, each customer signal including data and noise components. The data components from different customers are unrelated in amplitude, phase, and frequency as they originate from different subscriber equipment. However the noise components in each customer signal are similar to one another because they originate for the most part from the

2

same source, namely radio frequency electromagnetic radiation picked up by the equipment of the subscribers and the cables connecting such equipment to the outputs of the splitter. Introduction of a phase shift into the upstream signal before it reaches an output ensures that the noise components cancel one another when the upstream signals originating from the customers are summed by the splitter.

Preferably the phase shifting devices introduce a phase shift of 180°, so that noise components of altemate outputs are antiphase and cancel one another out almost entirely when the signals are summed.

Each phase shifting device may comprise a phase shifting transfonner.

Where the splitter has an even number N of outputs, N/2 phase shifting devices will be required, N/2 being a whole number. Where the splitter has an odd number X of outputs, then the number of phase shifting devices used will be the nearest whole number above or below X/2.

If required the phase shifting devices may be permanently connected to their respective outputs and secured within a common housing to the outputs, so being built into the splitter. Altematively the phase shifting devices may be separable from their respective outputs.

In accordance with another aspect of the invention, there is provided a cable television network incorporating a plurality of signal splitters comprising an input and a plurality of outputs, wherein altemate outputs are connected to phase shifting devices, the phase shifting devices acting in use to ensure that noise ingress in upstream signals, i.e. those originating from the customer, passing into the network is substantially reduced.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

The invention will now be described by way illustrative example and with reference to the attached drawing figures, in which:

FIGS. 1 and 2 are schematic diagrams of prior art signal splitters;

FIG. 3 is a schematic diagram of a signal splitter in accordance with the invention; and

FIGS. 4 and 5 are spectrum analyser traces showing the powers of the summed signals at the inputs, respectively, of a prior art signal splitter and a signal splitter according to the invention, when used in a cable television network.

DESCRIPTION

The prior art signal splitter 10 of FIGS. 1 and 2 comprises an input 12 and a large number of outputs, of which only a first output 14 and a second output 16 are shown for the pmpose of clarity. In use these passive signal dividers 10 act as an interface between a local centre or node and a number of customers, each customer connected to one output of the splitter 10, with the splitter input 12 connected to the node. Arrow 18 represents transmission of television signals (downstream signals) from the service provider to the input of the splitter where the signal is divided or split for onward transmission to the customer, arrows 18a and 18b representing transmission of split television signals from the first and second outputs 14, 16 of the splitter 10.

Dotted arrows 20a and 20b represent the retum transmission of data signals (upstream signals) from the first and second subscribers to the first and second outputs of the splitter.