GB2173675A - Communication system - Google Patents

Abstract

A communication system for sending video and audio signals to a telephone line 20 supplies a digitized video signal from a camera 7, A-D converter 25, and frame store 26 to a parallel-series converter 29. The byte signals from the converter 29 and sync signals from a timing generator 31 are supplied to a frequency shift keyer 30 which produces an output signal at any one of several distinct frequencies within the audio frequency band of the telephone line. This output signal is mixed with an audio signal from a telephone apparatus 2 and sent to the line 20. The telephone apparatus is connected to a filter 23 for removing the predetermined frequencies from a received signal. A filter 24 passes the discrete frequencies from the received signal to apparatus 32-37 for processing the received video signal to provide a display on a monitor 38. <IMAGE>

Description

SPECIFICATION Communication system The present invention relates to a communication system. For instance, the system may be of the type designed to transmit video signals via a communication path, such as in a telephone network.

One previously known system for sending video signals along a communication path of relatively limited bandwidth, in particular a telephone line, is disclosed in US Patent No 4485400. This known system captures an image in a frame store of digital type and then sends video signals to the line at a rate at which the line can cope. In particular, the frame store is read at relatively low speed and the digital output signals are supplied to a digital-analogue converter. The analog output is used to frequency modulate a carrier so that the maximum deviation is within the available bandwidth. The FM signal is then converted into sound and acoustically coupled to a conventional telephone bandset for transmission. At the receiving end, the signal is reproduced by the handset earphone which is acoustically coupled to a microphone.The signal from the microphone is demodulated, digitised, written into a frame store and then displayed at the normal scanning rate on a raster-scan monitor. However, because the signal is transmitted in analogue form, it can be substantially degraded by the poor performance of many standard telephone lines and systems.

The system disclosed in the above-mentioned US patent is notable in that it permits both audio and video signals to be carried by a single connection by transmitting the video signal in the blank portions of the audio signal corresponding to natural pauses in speech. However, relatively complicated apparatus is necessary in order to multiplex and de-multiplex the audio and video signals, and the rate of transmission of complete video frames varies according to the speech activity of the audio signal.

According to one aspect of the invention, there is provided a communication system for transmitting a video signal along a communication path, comprising means for converting the video signal to a serial signal having at any time anyone of a plurality of predetermined discrete levels, and means for converting the signal at each of the predetermined levels to a signal having a respected discrete frequency.

The video signal may be supplied to an analoguedigital converter whose parallel output is supplied to a parallel-series converter. The video signal is thus converted into a serial signal whose level switches between first and second predetermined levels, corresponding to binary 0 and 1, respectively, and preferably among the first and second predetermined levels and a third predetermined level for synchronization (sync pulses).

According to another aspect of the invention, there is provided a communication system for transmitting first and second signals along a common communication path, comprising means for supplying to the communication path the first signal, whose component frequencies are capable of assuming any value within a range of values within the communication path passband, and the second signal, the or each component frequency of which is capable of assuming any one of a plurality of predetermined discrete frequencies within the communication path passband. Preferably the second signal has at any one time a fixed level at any one of the predetermined frequencies. Such a signal is essentially digital in nature and may be binary.

The predetermined frequencies may fall within the range of values if interference between the first and second signals at the predetermined frequencies is unlikely to cause problems. Otherwise, the predetermined frequencies should be outside the range of values. The range of values may comprise a plurality of non-overlapping subranges separated by respective ones of the predetermined frequencies.

The communication system according to the first aspect of the invention may be the communication system according to the second aspect of the invention, with the video signal being the second signal. The first signal may be an audio signal so as to provide what is known colloquially as a video telephone.

The communication path may be electrical, such as a copper or other conductive wire, electromagnetic, such as radio frequency transmission including signal sideband, infrared, or visible light, optical such as an optical fibre, acoustic such as ultrasonic or any other type.

Preferably the system includes at least one terminal having a frame store arranged to write a video frame from a video camera, video cassette recorder, facsimilie head reader, or the like, at a normal scan speed and to read the frame at a slower speed so as to provide the video signal. preferably the terminal includes a telephone apparatus. The terminal preferably also includes a further frame store for writing a frame at relatively low scan speed from the communication path and for reading the frame to a monitor at the normal scan speed of the monitor.

The connection from the line to the further frame store preferably includes a filter having a multiple bandpass response with each passband being centered on a respective one of the predetermined frequencies. The connection from the line to the telephone may include a filter arranged to pass the range of frequencies or each of the subranges and to reject the predetermined frequencies.

It is thus possible to provide a system which allows a video signal to be transmitted in digital form along a communication path of relatively narrow bandwidth. It is also possible to use such a system to transmit audio signals along the same communication path, effectively by frequency division multiplex, without any interaction between the audio and video signals. The source of video signals may be any raster-scan type apparatus such as a video camera or recorder or a facsimilie reader.

The invention will be further described, by way of example, with reference to the accompanying drawings, in which Figure 1 shows an external view of a communication system terminal constituting a preferred embodiment of the invention; Figure 2 is a block diagram of the terminal of Figure 1; Figure 3 shows four graphs relating to the operation of the terminal of Figure 1; Figure 4 is a graph illustrating a signal produced in the terminal of Figure 1; and Figure 5 is a flow chart illustrating operation of the terminal of Figure 1.

The terminal shown in Figure 1 is capable of transmitting and receiving audio and video signals along a conventional telephone line. The terminal comprises a housing 1 containing a telephone apparatus 2. The telephone apparatus 2 may be of any suitable type for cooperating with a telephone network. The apparatus shown in Figure 1 is of relatively recent type and comprises a hand set 3 containing a microphone and earphone, a key pad 4 for conventional dialing so as to establish a connection, a 16 digit 16 number memory key pad 5 to permit automatic dialiing of regularly used numbers by single keystroke, a loudspeaker 6, and various other key switches for selecting special functions.

The operation of the telephone apparatus 2 is not substantially changed by its presence within the terminal.

The housing also contains a video camera 7 which has a fixed-focused wide angle lense with a depth of focus extending from 0.2 metres to infinity. The camera has a fixed iris and utilises electronic automastic gain control in orderto adapt to the level of illumination of the subject.

The housing further contains a television monitor including a screeen 8 and controls 9 for controlling the brilliance and contrast of the displayed image.

Although the preferred embodiment illustrated includes a monochrome CRTtelevision monitor, any type of raster-scan display unit may be used.

The front of the housing 1 also mounts four controls whose operation will be described in more detail hereinafter. Briefly, a video on/off switch 10 allows the video part of the terminal to be switched on or off, a freeze frame switch 11 allows the picture currently being displayed on the screen 8 to be frozen or held, a scan rate switch 12 allows the scanning rate or time to be selected, and a grab switch 13 allows the output of the video camera 7 to be routed directly to the monitor. The circles below the switches illustrate light emitting diodes used to indicate the state of the functions selected by the switches.

With reference to Figure 2, the terminal is connected to a conventional telephone line 20 by means of a jack or other connector shown at 21. The connector 21 is connected to a separator/combiner 22, which has two inputs and an output. One of the inputs is connected to the telephone apparatus 2 whereas the other input is connected to the video circuitry described hereinafter. The output is connected to a multiple notch filter 23 whose output is connected to the telephone apparatus 2. The output is also connected to the input of a multiple bandpass filter 24, whose output is connected to the video circuitry. The separator/combiner 22 combines the audio and video signals produced by the terminal and sends them to the telephone line 21. The separator/combiner 22 separates the signal received from the line 20 and supplies this to the inputs of the filters 23 and 24.

The video circuitry of the terminal may be notionally divided into transmit and receive sections.

The transmit section comprises the video camera 7 whose output is connected to the input of a flash anaiogue-digital converter 25. The output of the converter is connected to the input of a frame store 26 constituted by a semi-conductor random access memory (RAM). Atiming generator 27 controlled by clock pulses OK controls the camera 7, the converter 25, and the frame store 26. The timing generator 27 supplies address signals to an electronic selector switch 28 whose operation will be described hereinafter.

The output of the frame store 26 is connected to a parallel-series converter 29, whose output is supplied to a frequency shift keyer 30. A timing generator 31 which also receives the clock pulses CK controls reading from the memory 26 by supplying address signals to the switch 28, and also controls the converter 29 and the supply of sync signals to the keyer 30. The output of the keyer 30 is supplied to the separator/combiner 22.

The receiver section of the terminal comprises a frequency shift detector 32 which receives signals from the filter 24. The detector 32 has a first output supplying digital signals to a series-parallel converter 33 and a sync output supplying sync signals to a timing generator 34, which receives the clock pulses CK. The output of the converter 33 is connected to a frame store 35, which is shown distinct from the frame store 26 but may in practice merely form a different section of a common frame store for the transmit and receive sections. The timing generator 34 supplies timing signals to the converter 33 and supplies address signals to another electronic switch 36 whose output is connected to the address input of the frame store 35.

The output of the frame store 35 is connected to a digital-analogue converter and sample/hold circuit 37 whose output is connected to the monitor 38. An electronic switch 39 is provided for selectivity connecting the input of the monitor 38 directly to the output of the camera 7. A timing generator 40 receives the clock pulses CK and controls the circuit 37, the monitor 38, and reading of the frame store 35 via the switch 36.

The terminal further comprises a microprocessor controller 41 which controls and oversees operation of the terminal. The microprocessor controller 41 contains a microprocessor, read only memory, random access memory, interface circuitry, and a clock generator, the internal circuitry of the controller is essential conventional and will not be described further. The controller 41 supplies the clock pulses CK and is also connected to the switches 10 to 13.

The controller 41 has various input and output ports Ato G which are connected to the various parts of the terminal as shown in Figure 2, and also receives the read and write addresses supplied to the electronic switch 36 by the timing generators 40 and 34, respectively.

Figure 3(a) of the drawings illustrates the band width of a conventional telephone line such as the line 20 to which the terminal is connected. It is known that a band width extending from approximately 300 Hz to approximately 3 kHz is sufficient for the transmission of intelligible speech, and telephone networks are designed on this basis so as to minimise cost. The output signal of the frequency shift keyer 30 is a signal which may at any time have one of three frequencies, namely 1.2 kHz, 1.5kHz, and 2.3 kHz. The frequency shift keyer receives a serial bit stream from the converter 29 and transmits a signal at 1.5 kHz corresponding to binary 0 and at 2.3 kHz corresponding to binary 1.Synchronization signals are supplied by the timing generator 31 to the keyer 30 and are converted to a signal of frequency 1.2kHz. In order to permit these signals carrying video information to pass along the same communication path as audio signals from the telephone apparatus 2, the signals are mixed and sent to the line 20. In practice, the amount of energy at the above-mentioned three frequencies in the audio signal will be relatively small and will not interfere substantially with the signals from the keyer 30. However, it would be possible to remove these three frequencies from the audio signal supplied to the separator/combiner 22 by the telephone apparatus 2, for instance by inserting a multiple notch filter similar to filter 23 between the separator/ combiner input and the telephone apparatus.

When the combined video and audio signals are received from the line 20, they are supplied to the filters 23 and 24 so as to separate the audio and video signals. The filter 23 has a multiple notch characteristic as illustrated in Figure 3(c), whereas the filter 24 has a multiple bandpass characteristic as illustrated in Figure 3(d). Thus, the filter 23 removes the video signal components, or at least attenuates them so that they do not interfere with the audio signal. The audio signal is similarly attenuated at these three spot frequencies but this does not substantially affect the intelligibility. The filter 24 passes these spot frequencies but rejects, orsuffi- ciently attenuates, the remainder of the passband of the communication channel so that the audio signals do not substantially interfere with the video signals.

Although some audio signal components may be present within the passbands of the filter 24, the level of these components in general will be sufficiently low so as not to cause interference.

In an alternative embodiment, the filter 24 and the detector 32 may be replaced by three bandpass filters, each tuned to a respective one of the spot frequencies shown in Figure 3(b) and each followed by a respective level detector, with the detector outputs logically combined so as to reconstitute the serial bit stream and the sync signals.

Operation of the microprocessor controller 41 is controlled by a stored program and is illustrated in the flow diagram of Figure 5. When the terminal is switched on, the system is first initialized so as to be ready for use. In particular, the timing generators 27, 31, 34, and 40 are reset. The video on/off switch 10 is then checked to see whether the video circuitry of the terminal is to be operated. Before transmitting and receiving can begin, a telephone connection with a remote subscriber having corresponding apparatus must be established and this is done in the conventional way by means of the telephone apparatus 2. Once a connection has been established and the two subscribers have agreed to use the video terminal, the video on/off switch is turned on at each terminal.The microprocessor controller 41 then supplies a signal at port A which actuates the converter 25, switches the store 26 to the write mode, and switches the electronic switch 28 to supply address signals from the timing generator 27 to the address input of the store 26. The timing generator 27 supplies synchronizing signals to the video camera 7 and to the converter 25 and causes a complete frame to be digitally converted and stored in the store 26. As part of an initial checking procedure, the controller 41 then causes this frame to be displayed on the monitor via the circuit 37.In particular, the frame stores 26 and 35 will normally be part of a common random access memory and the addresses supplied by the timing generator 40 are modified under control of the microprocessor controller so as to read the portion of the memory which received the frame until a frame has been received from the remote terminal.

The controller then checks the scan rate switch 12 to ascertain whether a high or low scan rate (short or long period, respectively) has been selected and supplies a signal at the output port B for adjusting the clock signals provided by the timing generators 31 and 34. The low scan rate is such that a complete video frame is sent or received in 18 seconds and comprises 256 lines of data, each line comprising 256 picture elements (pixels). The high scan rate permits a video frame to be transmitted or received in six seconds, the frame comprising 128 lines each comprising 128 pixels. The scan rate may be set according to the prevailing characteristics of the telephone connection and is verbally agreed by the two subscribers via their telepphones 2.In a possible alternative embodiment, means may be provided for automatically setting the scan rate in accordance with the quality of the telephone connection. When the lower resolution is selected, the monitor 38 effectively blanks alternate lines and alternate pixels.

Alternatively, each line may be repeated once and/or each pixel may be repeated once so as to provide a brighter picture of the lower resolution.

As soon as the frame store 26 has captured the frame of video information, the controller 41 sends a signal to the port A which causes the frame store to read out information under control of the timing generator 31 via the switch 28. The bytes, for instance comprising six bits, are read out in parallel from the store 26 and converted to a serial bit stream by the converter 29 under control of the timing generator 31. The bit stream is supplied to the frequency shift keyer 30 together with a line synchronizing signal provided bythetiming generator 31.

The modulated signal is supplied from the keyer 30 via the separator/combiner 22 to the line 20. As soon as the complete frame has been trannsmitted, a prompt signal comprising a binary code is supplied by the microprocessor to the keyer 30 via port C.

Figure 4 illustrates the digital nature of the last complete line of a frame from the frame store together with a synchronizing pulse and the prompt signal which is illustrated as a binary code 101010101. The controller then times a period of five seconds and, if no signal is received from the remote terminal via the line 20 during that period and supplied to the port D from the detector 32, a further prompt signal is transmitted and a new five second period timed. This is repeated until a signal is received or the terminal is turned off.

When a signal is received from the line, the controller 41 checks whether the freeze switch 11 has been turned on. If so, the terminal ignores the incoming signal and continues to display the frame stored in the store 35 on the monitor 38. If the freeze switch has not been turned on, the microprocessor controls writing of the received video signal into the frame store 35 via the detector 32 and the converter 33 under control of the timing generator 34. The monitor 38 continues to display the frame currently held in the store 35. The addresses provided by the timing generators 34 and 40 are continuously compared by the controller. Also, data supplied by the detector 32 to the port B and timing pulses supplied to the port F by the timing generator 34 are received by the controller.The timing pulses supplied to the port F indicate when a new data byte must be written in the frame store 35 and the microprocessor then gives priority to the timing generator 34 so as to permit a writing operation.

During this time, the monitor 38 is blanked.

However, at all other times, the timing generator 40 is given precedence by the controller for reading out data to be displayed by means of a signal on the port E which sets the frame store to read mode and switches the switch 36 to the timing generator 40.

While a new line is being written into the store 35 the corresponding line of the display is blanked, so that the display provided by the monitor 38 is effectively updated line by line.

As soon as the prompt signal from the remote terminal has been received and decoded, the microprocessor then causes the frame store 26 to store a fresh frame of video data from the camera 7, and operation is repeated as illustrated in the flow diagram of Figure 5.

When the grab switch 13 is operated, the microprocessor controller enters an interrupt routine and supplies a signal to the port G which either switches on or off the electronic switch 39 depending on whether the switch was previously off or on, respectively. Thus, the output of the camera 7 is supplied directly to the monitor 38, the monitor being tempor arily disconnected from the output of the circuit 37.

Claims (16)

1. A communication system for transmitting a video signal along a communication path, comprising means for converting the video signal to a serial signal having at any time any one of a plurality of predetermined discrete levels, and means for converting the signal at each of the predetermined levels to a signal having a respective discrete frequency.

2. A system as claimed in claim 1, in which the converting means comprises an analogue-digital converter whose output is connected to a parallelseries converter.

3. A communication system for transmitting first and second signals along a common communication path, comprising means for supplying to the communication path the first signal, whose component frequencies are capable of assuming any value within a range of values within the passband of the communication path, and the second signal, the or each component frequency of which is capable of assuming any one of a plurality of predetermined discrete frequencies within the passband of the communication path.

4. A system as claimed in claim 3, in which the range of values is substantially equal to the passband of the communication path.

5. A system as claimed in claim 3 or 4, in which the predetermined frequencies are within the range of values.

6. A system as claimed in claim 3 or 4, in which the predetermined frequencies are between the upper most and lower most values of the range of values.

7. A system as claimed in claim 6, in which the range of values excludes at least one subrange of values including the predetermined frequencies.

8. A system as claimed in any one of claims 3 to 7 in which the second signal has at any one time a fixed level at any one of the predetermined frequencies.

9. A communication system as claimed in any one of claims 3 to 8, comprising a communication system as claimed in claim 1 or 2, in which the second signal is the video signal.

10. A system as claimed in claim 9, in which the first signal is an audio signal.

11. A system as claimed in claim 9 or 10, including a frame store arranged to write a video signal frame received from a local video source at a first speed and to read the stored frame at a slower speed for transmission to the communication path.

12. A system as claimed in any one of claims 9 to 11, including a further frame store arranged to write a video signal frame received from the communication path at a second speed and to read the stored frame to a monitor at a third speed.

13. A system as claimed in claim 12 when dependent on claim 11, in which the second speed is equal to the first speed.

14. A system as claimed in claim 12 or 13, in which a filter is connected between the frame store and the communication path and has a multiple passband response with each passband including a respective one of the predetermined frequencies.

15. Asystem as claimed in claim 14, including a telephone apparatus connected to the communication path by a filter arranged to reject the predetermined frequencies.

16. A communication system substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.