GB2279206A - Video-Telephony Network - Google Patents

Abstract

A number of video-telephony stations are connected via analogue links 10 to a common node 12 where the incoming video signals are multiplexed, successive frames are compared, and the resulting difference signals are compressed and digitized for onward transmission along high-grade digital links 14 and 14. The node may include a digital exchange. <IMAGE>

Description

VIDEO-TELEPHONY NETWORKS TECHNICAL FIELD OF THE INVENTION This invention relates to video-telephony communication networks.

BACKGROUND Until now, video-telephony systems have been largely digital and make use of specialised compression techniques to reduce the required bandwidth.

On the one hand, some systems attempt to use existing data telecommunication cabling, but these systems suffer from poor picture quality because current compression techniques only allow a maximum of 20 frames per second using conventional narrow bandwidth 64 kbit or 128 kbit digital telephony links (e.g ISDN 2 cabling). On the other hand, much better picture quality is obtainable when the available bandwidth is increased to around 2 Mbit, but the installation of high quality ISDN 30 cabling, or similar, is prohibitively expensive for small users, as are 2 Mbit video digital encoders/decoders. As a result, anyone contemplating installing such a link has the added disadvantage of a very limited communication network, since the number of subscribers is presently very small.

An aim of the present invention may be viewed as being to provide a system which provides high quality video-telephony communication over considerable distances, and which is not prohibitively expensive.

SUMMARY OF THE INVENTION The present invention proposes a video-telephony network in which a plurality of video-telephony terminals are coupled by respective local analogue cable links to a common local node which converts the incoming analogue signals from the said terminals to a digital form for transmission over longer distances via high grade data links. The local node also receives incoming digital signals from the high grade links and converts them to analogue form for transmission via the short-distance links to said videotelephony terminals.

The high grade data links can be existing ISDN (Inter Services Digital Network) links which typically make use of optical fibre and microwave radio transmissions to achieve high quality communication over long distances.

Because several subscribers can all be connected to the shared node facility, the cost of video telephony to each subscriber can be made very competitive because the cost of the node and ISDN links is shared.

In each node the incoming analogue video-telephony signals are preferably multiplexed before being digitally encoded. Generally, the multiplexed signals will be compared frame-by-frame and the resulting difference signals transmitted digitally. The signals are preferably compressed prior to transmission in accordance with a compression algorithm such as the internationally recognised H261 standard.

BRIEF DESCRIPTION OF THE DRAWINGS The following description and the accompanying drawings referred to therein are included by way of non-limiting example in order to illustrate how the invention may be put into practice. In the drawings: Figure 1 is a general pictorial representation showing two typical node arrangements comprised in a video-telephony network of the invention, Figure 2 is a more detailed block diagram of each node arrangement, and Figure 3 is a schematic block diagram giving further detail of the node arrangement of Fig. 2.

DETAILED DESCRIPTION OF THE DRAWINGS Referring firstly to Fig. 1, a number of buildings B1-3 in an area Al are provided with a system of the invention which permits direct video-telephony communication with each other, and with buildings C1-3 in a remote area A2. The buildings will usually be private dwellings and/or small businesses, although the system could also be made available to larger corporate users.

The distance between the areas Al and A2 will generally be at least several kilometres, and the areas will typically be in two cities or even in different countries.

Each group of buildings in the respective area Al, A2 is connected by low cost data grade cabling 10 such as eight wire twisted pair, typically having a bandwidth of up to 100 MHz, to a shared node 12, which will generally be located within 300 to 400 metres of the buildings. Such cabling can be laid in at relatively low cost along routes used by existing service providers such as electricity or cable television services. Each node is in turn connected directly to a high grade multiplexed digital communication network using ISDN 30 or similar fibre optic links 14, typically having a bandwidth of 2 Mhz. Where two adjacent service areas are relatively close together they may be connected directly via appropriate ISDN links, which generally include strategically placed digital exchanges (not shown).Where the areas are more widely spread the ISDN network may employ microwave radio links between base stations 16, and for inter-continental or international transmission, geostationary satellite relay stations (also not shown) may also be employed.

Referring to Fig. 2, each of the users is provided with a video-phone which will typically comprise a conventional telephone 20, a small video camera 22 and a monitor 24 connected to a separate or integral audio-visual exchange (AVX) 26. A television receiver can be used as the monitor 24, or a personal computer can be provided with a special printed circuit card to permit its use as a monitor, allowing the display of a "window" image in part of the computer screen. Video recorders can be included for transmitting prerecorded tapes.

Each AVX sends and receives conventional wide-band analogue audio-visual information along the local links 10 to the common node 12. Each node 12 includes a multiplex/codec unit 30 which multiplexes the incoming analogue video signals and then compares them frame-by-frame to extract the difference information. The difference signals are digitally encoded and compressed according to a conventional compression algorithm, to be sent by X21 CITT synchronous transmission to a digital ISDN exchange 32. The incoming compressed digital information is also decoded by the codec 30, demultiplexed, and the resulting analogue information is sent down the local links 10 to the appropriate AVX 26. The exchange 32 is conveniently situated at the node 12 and routes the digital signals down a national ISDN link 14A or an international ISDN link 14B, as appropriate.

Further detail of the node is shown in Fig. 3. It will be noted that a synchronising link 40 connects the digital exchange 32, the multiplex/codec unit 30 and each AVX 26. Audio signals which accompany the video signals can be routed in both directions through the codec 30, but where telephony communication is required without video, the audio can be routed along line 50 directly from the AVX to the codec 30 for digital encoding or along line 52 to the digital exchange 32 for direct ISDN transmission.

A further audio route 54 is also provided in the link 10 for connecting a PBX telephone exchange 56 to the ISDN network separately from the AVX.

The digital exchange 32 included at the node 14 also has facilities for connection via a Digital Private Network Signalling System DPNSS, if required.

It will be appreciated that although the above example makes specific reference to video and voice telecommunication, the present system can also be used to send and receive data transmissions, thereby making the advantages of encoded digital communication networks such as ISDN 30 available to large numbers of subscribers at reasonable cost.

Claims (8)

1. A video-telephony network in which a plurality of videotelephony terminals are coupled by respective local analogue cable links to a common local node which converts the incoming analogue video signals from the said terminals to a digital form for transmission over longer distances via a high grade data link.

2. A video-telephony network according to Claim 1, in which the local node also receives incoming digital video signals from the data link and converts them to analogue form for transmission via the short-distance analogue links to said video-telephony terminals.

3. A video-telephony network according to Claim 1 or 2, in which said node includes means for multipiexing the incoming analogue signals before being digitally encoded.

4. A video-telephony network according to any preceding claim, in which said node compares successive frames of each incoming analogue video signal and digitizes the resulting difference signals.

5. A video-telephony network according to any preceding claim, in which said node compresses the signals for digital transmission, in accordance with a compression algorithm.

6. A video-telephony network according to any preceding claim, in which said node includes a digital exchange for routing the digitized videotelephony signals.

7. A video-telephony network according to any preceding claim, in which each analogue cable link is fed by a plurality of video-telephony terminals via an analogue audio-visual exchange.

8. A video-telephony network substantially as described with reference to the drawings.