GB2117210A - Video transmission system - Google Patents

Abstract

A video transmission system, for example a cable television system, comprises a transmitting centre 1 with an internal source or sources of program channels for transmission, a plurality of distributed end user receivers e.g. 16 and a plurality of cables 10 each leading from the centre 1 to one of (or to a group of) the receivers. The centre 1 includes selection equipment 3 responsive to a signal from each end user to select a channel, from the plurality of available video channels, and transmit it to the respective end user over the respective cable (or over a channel, specific to that end user, of the cable associated with the respective group of end users). A reduction in the bandwidth requirements of the cables 10 and transmission equipment is achieved particularly where a large number of channels are to be made available for the users. <IMAGE>

Description

SPECIFICATION Video transmission system This invention relates to a video transmission system and enables reducing the bandwidth requirements of cable-based video systems, such as cable television or similar applications where users are given access to a variety of channels of video bandwidth, although not necessarily transmitted in the format usual for video reproduction.

There are a wide range of television-like applications the popularity of which are currently on the increase partially as a result of technical innovations and cost reductions. Principal application areas are closed circuit television (referred to as CCTV here), cable television (CTV), forms of television similar to cable television (also referred to as CTV here) and other forms of monitoring television, i.e. remote site monitoring.

In the case of any system which is effectively transmitting a number of channels of video (i.e.

television type) information over any appreciable distance a practical limit may be reached due to considerations related to the bandwidth requirements, i.e. the effective number of channels to be transmitted. In the case of a system using any form of electromagnetic radiation (visible infra-red or ultra-violet light, radio waves, electrical signals and so on, either within or without special conductors such as electrical cables, wave-guides and fibre-optic cables) a practical limit may be reached due to bandwidth consideration either limiting the number of possible channels, making the system impossible or making the system expensive due to a need for expensive components. It should be realised that bandwidth limitations can be due as much to political reasons such as transmission licencing and other users as to engineering limitations.The main problem associated with high bandwidth transmissions is bandwidth availability but other associated problems become particularly noticeable with dealing with cable transmissions.

The primary problem is the cost of high capacity cable, other important problems being the bulk of a single high capacity cable or a combination of lower capacity cables, and the weight or mass of the cable. Another problem may be high power comsumption due to the wide bandwidth of the system or large number of channels.

One situation of interest occurs when there are only a few users connected to a particular CTV route or branch, or the similar situation when although there may be many users on a particular branch, they are either watching a smaller number of channels or have switched off their receivers. In either case the number of TV channels supplied exceeds the number of channels required. For instance if a 10 km CTV branch is supplied with 10 TV channels while the viewers are only watching 3 of these channels -- perhaps because there are only 3 users on that branch, or perhaps because the many users watching are only interested in 3 of the 10 channels, then only 30% of the branch capacity is being used while some 70% is being wasted.Potentially this allows the CTV company or operator to use 70% less cable if one cable per channel is used, or to use a cheaper cable with 70% less channel capacity if a single high capacity cable is used. Potentially cheaper transmission equipment and channel selection equipment may be used since only 3 instead of 10 channels need to be handled. This is cheaper simply by virtue of reducing the bandwidth of the signals which need to be transmitted and handled, allowing cheaper cables to be used.

If the channel carrying capacity is to be increased by having a number of separate cables - perhaps one per channel - then a practical limit to the number of channels may be reached due to the bulk or weight of the cables. A bundle of, say, 200 co-axial cables would be virtually impossible to route through an urban centre without causing massive disruption, and probably quite expensive due to labour costs. The weight of such a bundle would almost certainly prevent it being routed along roof tops or being strung from telephone poles, and such a bundle would also be very unsightly.In addition the sheer cost of the cable - or suitable alternatives such as fibre optic cable -- would make a 200 cable or even 200 channel video system out of the question for the majority of possible present applications such as cable television. Although non-cable alternatives such as laser or microwave links over all or part of the route overcome problems due to weight and bulk they do have problems associated with handling a large number of channels and high bandwidths; engineering problems and power consumption being the major problems.

In accordance with this invention, as seen from one aspect, there is provided a video transmission system comprising a centralised transmitter apparatus with an internal source or sources of program channels for transmission, a plurality of distributed end user receiver apparatus, and a plurality of cables each leading from the centralised transmitter apparatus to one of (or to a group of) the end user receiver apparatus, the centralised transmitter apparatus including selecting means responsive to a signal from each end user to select a channel, from a plurality of available video channels, and transmit it to the respective end user receiver apparatus over the respective cable (or over a channel, specific to that end user, of the cable associated with the respective group of end users).

In a video system transmitting a number of selectable channels to a receiver end, transmission limitations are thus overcome by shifting the process of channel selection from the receiver end of a limiting transmission link to a point previous of the limiting transmission link in such a way that the number of channels of video information may be reduced and possibly minimised.

Also in accordance with this invention, as seen from a second aspect, there is provided a video transmission system comprising a transmitter apparatus receiving input channels from a plurality of remote cameras, a plurality of end user receiver apparatus, and a plurality of cables each leading from the centralised transmitter apparatus to one of (or to a group of) the end user receiver apparatus, the transmitter apparatus including selecting means responsive to a signal from each end user to select a channel, from the plurality of available video channels, and transmit it to the respective end user apparatus over the respective cable (or over a channel, specific to that end user, of the cable associated with the respective group of end users).

An embodiment of this invention will now be described, by way of example only with reference to the accompanying drawing, the single figure of which is a schematic diagram of a video transmission system in accordance with this invention.

A video transmission system, in the form of a cable television system, comprises a transmission centre 1 serving a plurality of distributed end user receiver apparatus, one of which is shown at 1 6 as a conventional television receiver typically situated in a user's home. A plurality of low bandwidth cables 10 (e.g. coaxial or optical fibre) lead from the centre 1, each such cable leading to a group of end user receiver apparatus and the centre 1 includes selecting means 3 responsive to a signal from each end user to select a channel, from a plurality of available video channels, and transmit it to the respective end user receiver apparatus over a channel (specific to that end user) of the relevant cable.Alternatively each or any of the cables 10 may lead to a single end user receiver apparatus, The transmission centre 1 includes an internal source or sources of program channels for the transmission, for example video cameras or playback machines for video recordings. It may additionally include means for receiving channels transmitted to it, but in general requires active manual control and operation and is not passive or automatic as would be the case of a routing substation.An unspecified number of source channels V1 of video type material, such as unmodulated video or high frequency digital data, are applied to selection equipment 3, such as a video matrix controlled by data 4 from a computer 5 responding to signals 6 externally or internally sourced and serving to select which of the available channels is to be transmitted to each end user.

Individual selected channels (e.g. V2) pass as indicated at 7 to transmission equipment 8 (e.g. a modulator) and thence to a transmission channel 9 of the cable 10, which channel is specific to the respective end user. The transmitted program channel V3 is converted by converter 11 to a signal V4 for short distance transmission and may require reconversion to a standard format V5 or to be combined with other signals 1 5 by final equipment 13 to be applied by a line 14 to the television receiver 1 6.

The selection signal may be generated by a selector forming part of or associated with the receiver 16, and may be communicated to the centre 1 over the same channel of the same cable 1 0. Moreover, the receiver may be arranged to transmit, over the same cable, audio or low level data signals for other purposes, even whilst receiving a video transmission.

The channels available for transmission will typically include the usual television broadcast channels, but in addition will include pre-recorded television material played back or sourced within the centre 1. Instead of the usual television material, the channel may comprise data signals for display on the television receiver, or alternatively for use by a teletext adaptor or a computer peripheral in place of the television 'receiver. The centre 1 may include a video library the contents of which are selectable by the end users. The centre 1 may further include means for monitoring the channels actually being transmitted to the different end users, to avoid the need for program surveys.

The main benefit of the system described is the reduction in bandwidth requirement of the cables and associated transmission equipment, in particular where a large number of channels are to be made available for the users. Thus, each cable need only have a channel-carrying capacity equal to the number of end users it is to serve.

In a closed circuit or monitoring form of video system, a plurality of video cameras may feed their video signal outputs over respective cables to a transmitter apparatus including channel selection equipment. A plurality of cables lead from this transmitter apparatus to respective end user receivers (or groups of receivers) with facility for communicating to the selection equipment which of the available video signals is to be transmitted for display to that user.

Claims (6)

1. A video transmission system comprising a centralised transmitter apparatus with an internal source or sources of program channels for transmission, a plurality of distributed end user receiver apparatus, and a plurality of cables each leading from the centralised transmitter apparatus to one of (or to a group of) the end user receiver apparatus, the centralised transmitter apparatus including selecting means responsive to a signal from each end user to select a channel, from a plurality of available video channels, and transmit it to the respective group of end users).

2. A video transmission system as claimed in claim 1, in which each said end user receiver apparatus includes means for transmitting, along its respective cable, audio or low level data signals even whilst receiving a video transmission.

3. A video transmission system as claimed in claim 1 or 2, in which said centralised transmitter apparatus includes means for monitoring the channels actually being transmitted to the different end users.

4. A video transmission system as claimed in any preceding claim, in which said centralised transmitter apparatus includes a video library the contents of which are selectable by the end users.

5. A video transmission system comprising a transmitter apparatus receiving input channels from a plurality of remote cameras, a plurality of end user receiver apparatus, and a plurality of cables each leading from the centralised transmitter apparatus to one of (or to a group of) the end user receiver apparatus, the transmitter apparatus including selecting means responsive to a signal from each end user to select a channel, from a plurality of available video channels, and transmit it to the respective end user receiver apparatus over the respective cable (or over a channel, specific to that end user, of the cable associated with the respective group of end users).

6. A video transmission system substantially as herein described with reference to the accompanying drawing. accompanying drawing.