GB2330969A

GB2330969A - Transmission of an additional signal in a television signal

Info

Publication number: GB2330969A
Application number: GB9722983A
Authority: GB
Inventors: Simon William Day
Original assignee: Silverstone Holdings Ltd
Current assignee: Silverstone Holdings Ltd
Priority date: 1997-10-31
Filing date: 1997-10-31
Publication date: 1999-05-05
Also published as: GB9722983D0

Abstract

An additional signal, eg audio or data, is added to the back porch of a conventional video signal in the form of at least one voltage pulse. Preferably there are two pulses, the difference in amplitude between which represents the amplitude of the additional signal. The pulses have a duration of approx 2Ás and a constant average voltage of approx. 0.35V. The pulses are inserted a predetermined time delay, of approx 1Ás after the rear edge of the line sync pulse. An audio signal for addition to the video signal may be sampled at line frequency.

Description

METHOD OF. AND APPARATUS FOR, TRANSNITTTNG A TELEVISION SIGNAL The present invention relates to a method of transmitting a television signal containing a video signal and at least one additional signal, and an apparatus for transmitting such a signal. The additional signal may be an audio signal and/or a data signal. In particular, but not exclusively, the invention relates to a method and an apparatus for transmitting such a television signal within a narrow transmission bandwidth.

The incentive for the present invention came from the announcement by the UK Radio Communication Agency in 1997 that a frequency band having a bandwidth of 10MHz would be made available for closed circuit television signals at a frequency of 1.394GHZ. This bandwidth is sufficient for a black and white video signal but is too narrow for a conventional video plus audio signal.

Closed circuit television systems are often used for applications such as entry cameras, security cameras and baby monitors where it is desirable to be able to transmit an audio or data signal in addition to the video signal.

This is not however possible with existing technology without increasing the bandwidth of the signal. It may in due course be possible to compress conventional television signals using digital techniques but such techniques are available yet and development costs are likely to be quite high.

It is an object of the present invention to provide a method and an apparatus for transmitting a television signal containing a video signal and at least one additional signal, that mitigates at least some of the afore-mentioned disadvantages.

According to the present invention there is provided a method of transmitting a television signal containing a video signal and at least one additional signal, wherein said additional signal is transmitted by inserting at least one voltage pulse into the back porch of the video line signal.

The term "back porch" as used herein is well known in the art and relates to the part of a conventional television signal within each line making up the TV picture that follows the sync pulse and immediately precedes the luminance signal. More specifically, the "back porch" is the region between the synchronising pulse trailing edge and the blanking pulse trailing edge (see Electronics Engineers' Handbook by Donald G. Fink, published by McGraw Hill, section 20-10, the contents of which are incorporated by reference herein).

The present invention makes it possible to transmit an audio signal and/or a data signal within a black and white video signal, without increasing the bandwidth of the signal. It is thus possible to transmit a video signal and an audio/data signal within the 10MHz band made available at 1.394GHz by the UK Radio Communication Agency. The invention is not, however, limited to that particular frequency band and may be applied to television signals transmitted at many other frequencies. The invention is applicable to both the European PAL television system and also the North American NTSC system.

The invention may be implemented using simple analogue electronic circuits. Development and production costs are therefore relatively low.

Advantageously, at least two voltage pulses are inserted into the back porch of the television line signal, the amplitude of the additional signal being represented by the difference in the voltages of those pulses. The use of a differential voltage to represent the additional signal helps to prevent visible interference with the video signal and also prevents base-line wander, since absolute voltage measurements are not needed.

Advantageously, the pulses each have a duration of approximately 2ups.

Advantageously, the average voltage of the pulses inserted into the back porch of each television line signal is constant and is preferably equal to approximately 0.35V.

As the maximum positive voltage of a television signal is normally approximately 0.7V, this makes full use of the available range.

Advantageously, the at least one voltage pulse is inserted into the back porch after a predetermined delay from the rear edge of the line sync pulse. The predetermined delay is preferably approximately 0. 5-lps.

The additional signal may represent an audio signal, which may be obtained by sampling the audio waveform at the line frequency of the television signal, which is approximately 15.6kHz. This gives a maximum audio frequency of about 7kHz, which is satisfactory for most purposes.

The additional signal may alternatively represent a data signal, which may be obtained by sampling the output of a sensor at the line frequency of the television signal. The sensor may for example sense speed, temperature or any other variable quantity, or it may simply be a binary sensor such as a switch.

An audio signal and a data signal may be inserted into alternate lines of the television signal, so allowing both audio and data to be transmitted with the video signal.

Similarly, two or more audio signals and/or two or more data signals may be transmitted.

According to a further aspect of the invention there is provided a device for transmitting a television signal containing a video signal and at least one additional signal, wherein said device includes a video input port for said video signal, an additional input port for said additional signal, inserter means for inserting at least one voltage pulse representing an instantaneous value of said additional signal into the back porch of the video line signal, and an output port for the combined television signal.

Advantageously, said inserter means is arranged to insert at least two voltage pulses into the back porch of the television line signal, the amplitude of the additional signal being represented by the difference in the voltages of those pulses.

Advantageously, the additional signal represents an audio signal and the device includes audio sampling means for sampling an audio waveform at the line frequency of the television signal.

Alternatively, the additional signal represents a data signal and the device includes data sampling means for sampling the output of a sensor at the line frequency of the television signal.

Alternatively, the device may include means for inserting said audio signal and said data signal into alternate lines of the television signal.

According to a further aspect of the invention there is provided a device for decoding a television signal containing a video signal and at least one additional signal, wherein said device includes an input port for the combined television signal, extractor means for extracting from the back porch of the television line signal at least one voltage pulse representing an instantaneous value of said additional signal, an output port for the video signal and an additional output port for said additional signal.

An embodiment of the present invention will now be described by way of example with reference to the accompanying drawings, of which: Figs. la and lb are timing diagrams for a single line of a television signal; Fig. 2 is circuit diagram of an inserter circuit, and Fig. 3 is circuit diagram of an extractor circuit.

A television picture according to the European PAL standard is made up of 625 lines and is refreshed at a rate of 25 frames per second. Video information is therefore transmitted at a rate of 15625 lines per second, each line having a duration of 64ups. The US NTSC standard has 525 lines refreshed at 30 frames per second, giving a line duration of 63. 5Us. The following description is based on the European PAL system but applies mutatis mutandis to the US NTSC standard, except where otherwise indicated.

A timing diagram for a single line of a television signal is shown in Fig. la and comprises a -0.3V synchronising pulse 2, which is followed by a 5s slot 4 known as the "back porch", after which comes the active line section 6, which contains the luminance information. There is then another brief slot called the "front porch" 8, which is followed by the next synchronising pulse 2'. In a colour television signal, the back porch 4 contains an eight-cycle burst of colour subcarrier known as the "colour burst". In a black and white television signal, the back porch 4 is empty.

In the present invention, the back porch 4 is used to carry an additional signal containing audio or data information.

This is achieved by inserting a delay pulse 10 and two key pulses 12 14, each of duration 2 s, into the back porch 4.

In a quiescent state as shown in Fig. la (i.e. when no additional information is being transmitted), the two key pulses 12 14 both have the same value, which is +0.35V. A small drop in voltage 16 may be provided at the junction of the two pulses for identification purposes.

When audio or data information is being transmitted, the key pulses 12 14 have different values V2 V2 as shown in Fig. lb.

The value VA of the additional signal is equal to the voltage difference between the two key pulses 12 14: i.e.

V@ = V1-V2.

In order to ensure that audio or data signal does not interfere with the video signal, the average value of the key pulses 12 14 is always the same and equal to +0.35V: i.e.

V1+V2 =0.35V 2 In order to transmit audio information, the audio waveform is sampled at the line frequency of the television signal, i.e. 15.625kHz, and the sampled values are inserted into the television signal, one per line, by adjusting the heights Vl V2 of the key pulses 12 14 such that: V1 = 0.35V + VA/2 and V2 = 0.35V - V@/2 where VA is the audio voltage VA, which is proportional to the sampled value of the audio signal.

The audio voltage VA is thus given by: VA = V1 V2 A similar process is used to transmit data, the voltage VA then being proportional to the sampled value of the data signal.

One slight complication is that the line signals making up a television picture are not all of equal length, as some include an equalisation pulse which results in the sync pulse being longer than normal. This difficulty is overcome by timing the audio information to appear at a preset interval after the rear edge of the sync pulse, so that it always appears in the same place in the television signal. As transmission of the audio information is delayed slightly, the audio waveform is sampled slightly later than normal to prevent distortion.

The sampled audio signal is inserted into the television line signal by means of the inserter circuit shown in Fig.

2. The circuit includes a video input port 20, an audio input port 22 and a video output port 24. The video input port 20 is connected to the video output port 24 via transistor 26 and operational amplifier 28. The audio input port 22 is connected to the video output port 24 via operational amplifiers 30 32, FET switches 34 36 and operational amplifiers 38 28. An offset voltage is provided by variable resistor 40, which is connected to the video output port 24 via operational amplifier 42, FET switch 44 and operational amplifiers 38 28. FET switches 34 36 44 are controlled by integrated circuits 46 48 50.

The video input signal is buffered by transistor 26 and summed with a composite pulse containing the audio information that is produced by operational amplifier 38.

The combined signal is then amplified back to the standard output level by operational amplifier 28.

Integrated circuit 46 is a synchronising pulse separator that produces the short delay pulse 10 that occurs after the end of the line sync pulse 2. Integrated circuits 48 and 50 are triggered by the delay pulse 10 to generate the two key pulses 12 14.

The composite key pulse comprises the delay pulse 10 and the two key pulses 12 14 and is generated by diodes 52 54.

FET switches 34 36 44 are closed in sequence, controlled by the key pulses 12 14 and the composite pulse to select either audio (positive), audio (negative) and offset level, which is set with variable resistor 40. These switches 34 36 44 feed buffer amplifier 38 to produce the composite pulse, which is combined with the original video signal at the input of operational amplifier 28.

The audio signal is extracted from the television line signal by means of the extractor circuit shown in Fig. 3.

The circuit includes a video input port 60, a video output port 62 that is connected directly to the input port 60 and an audio output port 64. The video input port 60 is connected to the audio output port 64 via transistor 66, FET switches 68 70 and operational amplifiers 72 74 76.

The switches 68 70 are controlled by integrated circuits 78 80 82.

The transistor 66 provides a high impedance tap of the through video signal. Integrated circuit 78 is a synchronising pulse separator, which produces a short delay pulse after the end of the line sync pulse. Integrated circuits 80 82 are triggered by the delay pulse to generate dual key pulses, which are applied to switches 68 70 to select the audio voltages. These voltages are held on capacitors 84 86. The operational amplifiers 72 74 buffer the voltages held on capacitors 84 86 and operational amplifier 76 outputs the differential voltage at the audio output 64. A data signal may be extracted by the same circuit in a similar manner.

Various modifications of the present invention are possible. For example, instead of either audio or data being transmitted with the video signal, both audio and data may be transmitted by inserting the signals into alternate lines of the television signal. Although it is preferred that the additional signal is inserted as a differential signal as described above, since this helps to prevent interference with the video signal, it may alternatively be inserted as an absolute signal (being represented, for example, be a single pulse whose height represents the instantaneous amplitude of the additional signal). The additional signal may also be inserted into the back porch as a digital code.

If the video signal is derived from a colour television camera, a colour burst signal will normally be present in the back porch. This signal will normally be removed (resulting in a black and white picture) before the additional signal is inserted. Alternatively, the additional signal may be added to the colour burst signal and the two separated out later by filtering.

Claims

CLAIMS 1. A method of transmitting a television signal containing a video signal and at least one additional signal, wherein said additional signal is transmitted by inserting at least one voltage pulse into the back porch of the video line signal.
2. A method according to claim 1, wherein at least two voltage pulses are inserted into the back porch of the television line signal, the amplitude of the additional signal being represented by the difference in the voltages of those pulses.
3. A method according to claim 2, wherein the pulses each have a duration of approximately 2ups.
4. A method according to claim 2 or claim 3, wherein the average voltage of the pulses inserted into the back porch of each television line signal is constant.
5. A method according to claim 4, wherein the average voltage of the pulses inserted into the back porch of each television line signal is equal to approximately 0.35V.
6. A method according to any one of the preceding claims, wherein the at least one voltage pulse is inserted into the back porch after a predetermined delay from the rear edge of the line sync pulse.
7. A method according to claim 6, wherein the predetermined delay is approximately lps
8. A method according to any one of the preceding claims, wherein the additional signal represents an audio signal.
9. A method according to claim 8, wherein the audio signal is obtained by sampling the audio waveform at the line frequency of the television signal.
10. A method according to any one of the preceding claims, wherein the additional signal represents a data signal.
11. A method according to claim 10, wherein the data signal is obtained by sampling the output of a sensor at the line frequency of the television signal.
12. A method according to claim 10 or claim 11 when dependent on claim 8 or claim 9, wherein the audio signal and the data signal are inserted into alternate lines of the television signal.
13. A device for transmitting a television signal containing a video signal and at least one additional signal, wherein said device includes a video input port for said video signal, an additional input port for said additional signal, inserter means for inserting at least one voltage pulse representing an instantaneous value of said additional signal into the back porch of the video line signal, and an output port for the combined television signal.
14. A device according to claim 13, wherein said inserter means is arranged to insert at least two voltage pulses into the back porch of the television line signal, the amplitude of the additional signal being represented by the difference in the voltages of those pulses.
15. A device according to claim 13 or claim 14, wherein the additional signal represents an audio signal and the device includes audio sampling means for sampling an audio waveform at the line frequency of the television signal.
16. A device according to any one of claim 13 to 15, wherein the additional signal represents a data signal and the device includes data sampling means for sampling the output of a sensor at the line frequency of the television signal.
17. A device according to claim 16 when dependent on claim 15, including means for inserting said audio signal and said data signal into alternate lines of the television signal.
18. A device for decoding a television signal containing a video signal and at least one additional signal, wherein said device includes an input port for the combined television signal, extractor means for extracting from the back porch of the television line signal at least one voltage pulse representing an instantaneous value of said additional signal, an output port for the video signal and an additional output port for said additional signal.
19. A method of transmitting a television signal containing a video signal and at least one additional signal, the method being substantially as described herein with reference to and as illustrated by the accompanying drawings.
20. A device for transmitting a television signal containing a video signal and at least one additional signal, the device being substantially as described herein with reference to and as illustrated by Figs. 1 and 2 of the accompanying drawings.
21. A device for decoding a television signal containing a video signal and at least one additional signal, the device being substantially as described herein with reference to and as illustrated by Figs. 1 and 3 of the accompanying drawings.