GB2213341A

GB2213341A - Transmitting digitally assisted television data signals

Info

Publication number: GB2213341A
Application number: GB8728668A
Authority: GB
Inventors: R Ian Richard Pullen; Charles Peter Sandbank
Original assignee: British Broadcasting Corp
Current assignee: British Broadcasting Corp
Priority date: 1987-12-08
Filing date: 1987-12-08
Publication date: 1989-08-09
Anticipated expiration: 2007-12-08
Also published as: GB8728668D0; GB2213341B

Abstract

A method of transmitting a digitally assisted television (DATV) signal is described in which a received DATV signal is compatible with non-DATV receivers. The number of pulses in the field synchronising sequence may be reduced in conjunction with a reduction in the lengths of the line synchronising pulses, thus providing a free data channel. A DATV data signal is combined with the video waveform such that the DATV data signal occupies at least part of the free data channel. The resultant combined signal is then transmitted. Alternatively the free data channel can be provided by reducing the lengths of the colour bursts. This reduction in length of the colour bursts may necessitate an increase in their amplitude so as to maintain compatibility with non-DATV receivers.

Description

METHOD AND APPARATUS FOR TRANSMITTING A DIGITALLY ASSISTED TELEVISION SIGNAL This invention relates to digitally assisted television (DATV) in which DATV data is transmitted alongside the existing analogue video signal. The DATV data is used to enhance the conventional UHF PAL (or other) television transmissions to provide improved quality on new receivers. Some of the possible enhancements are: a) increased picture resolution; b) suppression of PAL cross-effects; c) suppression of scanning impairments (line structure, flicker, etc.); and d) extension of aspect ratio.

If DATV data is to be added to broadcast transmissions, it is essential that the transmitted signal be compatible with existing receivers.

Currently, a number of lines of the field blanking intervals are used to transmit teletext information. A similar form of coding could be used with DATV. However, the capacity currently available for transmitting additional data in the field blanking intervals is not large enough to accommodate the desired amount of DATV data.

The object of the present invention is to make it possible for a video waveform to carry a large amount of DATV data.

According to one aspect of the present invention there is provided a method of transmitting a digitally assisted television (DATV) data signal comprising the steps of providing a video waveform, removing a portion of the synchronising information in the video waveform to provide a free data channel, the said portion being non-essential to reception of the video waveform on a non-DATV receiver, combining a DATV data signal with the video waveform, the DATV data signal occupying at least partially the said free data channel, and transmitting the resultant combined signal.

In a preferred embodiment, the step of removing a portion of the synchronising information comprises reducing the lengths of the line synchronising pulses.

In another embodiment, the step of removing a portion of the synchronising information comprises reducing the number of equalising pulses in the field synchronising pulse sequence. The most advantageous reduction is to remove all the equalising pulses.

The number of broad pulses in the field synchronising pulse sequence may also be reduced to enlarge the free data channel.

In a further embodiment, the line synchronising pulses are reduced in addition to the field synchronising sequence being reduced. This enables the line synchronising pulses to be reduced more than when they are shortened with a conventional field synchronising sequence being transmitted.

In another embodiment of the invention, the step of removing a portion of the synchronising information comprises reducing the lengths of the colour bursts. This preferably also involves increasing the amplitude of the bursts.

According to a second aspect of the invention there is provided apparatus to transmit a digitally assisted television (DATV) data signal comprising, input means to receive a video waveform, signal processing means to remove a portion of the synchronising information in the video waveform to provide a free data channel, the said portion being non-essential to reception of the video waveform on a non-DATV receiver, combining means for so combining the video waveform with the DATV data signal that the DATV data signal occupies at least partially the said free data channel, and transmission means to transmit the resultant combined signal.

In a preferred embodiment the signal processing means may be used to reduce the length of the line synchronising pulses.

The signal processing means may be used to reduce the number of equalising pulses in the field syhnchronising pulse sequence. All the equalising pulses may be removed without affecting reception on a non-DATV receiver.

Additionally or alternatively the signal processing means may reduce the number of broad pulses in the field synchronising pulse sequence.

Preferably the field synchronising sequence is reduced at the same time as the line synchronising pulses are reduced in length thus enabling a greater reduction in line synchronising pulse length to be achieved.

In another preferred embodiment the signal processing means reduces the lengths of the colour bursts. Preferably, the colour bursts are also increased in amplitude.

The invention will now be described in more detail by way of example with reference to the drawings, in which: Figure 1 shows an experimental arrangement to test the invention; Figure 2 shows the effect of over-shortening the line sync pulses in a video waveform; and Figure 3 shows normal and shortened field sync sequences.

The synchronising information transmitted with video waveforms is of three types, the line synchronising pulses, the field synchronising pulses, and the colour burst pulses. The removal or shortening of some of these pulses will enable additional data signals to be transmitted with the video signal within the same bandwidth.

The experimental arrangement of Figure 1 has been used to test the compatability of several domestic receivers with a video signal in which some of the sync pulses have been removed or shortened.

The circuit comprises a sync pulse generator 2. This produces normal mixed syncs and mixed blanking signals which are the inputs to an R.G.B. pattern generator 4. The R.G.B. pattern generator 4 simulates R, G and B camera signals to form the inputs to a PAL coder 6.

The sync pulse generator 2 provides a V-axis switch signal and a subcarrier signal for the PAL coder 6. Additionally the sync pulse generator produces Line Drive, Field Drive, and Mixed syncs signals as well as a colour Burst Gate signal. These signals form the inputs to a sync processor 8. The outputs of the sync pulse processor 8 are a Modified Mixed Sync signal and a Shortened Burst Gate signal. A switch 10 is provided to bypass the shortened burst gate signal. These two signals are also inputs to the PAL coder 6.

The output of the PAL coder 6 is available as a baseband signal 12 or as a UHF PAL signal 14 after modulation in a UHF TV modulstor unit 16.

The sync pulse processor 8 in Figure 1 is a purpose built unit for the experimental arrangement. It can shorten and remove selected line sync pulses, remove field sync pulses, and can shorten and reposition colour bursts. The processor is programmable by means of DIL switches.

A number of domestic receivers have been tested. The models are: a) Ferguson TX10 receiver b) JVC portable receiver/monitor model CX6 10GB c) Decca receiver/monitor model CS2645/AL d) Philips VHS recorder model VR6860/05.

These are capable of being fed with either a UHF or a Baseband signal. Results obtained with the UHF signal are of prime interest but the results with the baseband signal give clues as to how the receivers operate with a reduced amount of synchronising information.

The Philips video recorder has been tested with its output connected to the Ferguson TX10 receiver using the dedicated VCR channel at UHF.

The UHF signals have been fed to the receivers at levels of -45 dBm. Several tests have however been conducted at -75 dBm, this level corresponding to a fairly weak signal.

A first modification to the video waveform is to shorten the lengths of the line sync pulses. The effect of this on the receivers has been tested by using the sync processor to gradually shorten the pulse lengths and thus find the minimum pulse length at which each receiver can operate. The results obtained are shown below.

Mode Receiver Minimum Impairment pulse pulse length ~~~~~~~~~~~ Baseband TX10 O.8,s Tearing verticals JVC Monitor 1.2,s s Tearing verticals Decca monitor 2.7pus Incorrect Colour UHF TX10 3.3)As Bent Verticals JVC 1.4us Bent Verticals Decca 3.O)4s Incorrect Colour Philips VHS 3.0J4s Bent Verticals TX10 -75 dBm 3.084s Bent Verticals The minimum sync pulse width for correct operation with any of the receivers is 3.3s s for a UHF signal.This is a saving of 1.4fez on the 4.7/4s pulse width conventionally transmitted with a 625 line PAL signal and thus a saving of 438pAs per video field. The tolerance of all the receivers to shortened line sync pulses is less when fed at UHF than at base band. This is probably due to incorrect operation of the clamping circuits and automatic gain control circuits in the receivers. The effect of this is illustrated in Figure 2 which shows the video waveform out from the TX10 receiver for 4.7tis and 3.0yis line sync pulse lengths at (a) and (b) respectively. In (b) the waveform is not being correctly clamped and is leading to bent verticals in the received picture.

Although 1.4sus is saved in the line blanking intervals, it may not in practice be possible to use all this additional space in the signal. In particular when the transition between a sync pulse and the DATV data requires a change in signal level between sync level and white level. It would probably be necessary to allow a finite time for the channel to settle down and for the data decoder to lock up before valid data is sent.

A second modification to the video waveform is the removal of part of the field sync pulse sequence. The conventional sync pulse sequence used in a 625 line video signal is shown in Figure 3 at (a). The sync pulse sequences consist of five broad pulses, and ten equalising pulses, five preceding the broad pulses and five following the broad pulses.

The sync processor has been used to successively remove the equalising pulses followed by the broad pulses. The minimum field sync sequences which do not cause any significant impairment of the received signal are shown below for each receiver.

Mode Receiver Minimum Field sync Baseband TX10 2 Broad pulses, no equalising pulses JVC As above Decca 1 Broad pulse, no equalising pulses UHF TX10 1 Broad pulse, no equalising pulses JVC 2 broad pulses, no equalising pulses Decca 1 broad pulse, no equalising pulses Philips VHS 2 broad pulses, no equalising pulses TX10 -i5 dBm 1 broad pulse, no equalising pulses It can be seen from these results that modern receivers do not require the entire field sync sequence of the 625 line system.

The use of just two broad pulses is satisfactory for reception on any of the above receivers. With the equalising pulses also removed, as shown at (b) in Figure 3, the result is a field sync sequence which occupies just one line per field instead of the normal 7.5 lines. This gives a time saving of 6.5 lines or 416 s per video field.

The third modification which can be made is the shortening of the colour burst. The standard length for colour bursts is in a 625 line signal. This has been tested by using the sync processor to progressively shorten the colour burst length. The results for the various receivers are shown below.

Mode Receiver Minimum burst length Baseband TX10 1.2+s JVC l.2ks Decca 1.2 s UHF TX10 1.2)s JVC 1.2*s Decca 1.2s Decca -75 dBm 1.2pus For all the receivers the colour saturation increased for any decrease in colour burst length. This is due to the performance of the automatic gain control in the decoder. The above results are for burst lengths which do not cause severe oversaturation or decoder malfunctions.

1.2)as is a suitable length for a shortened colour burst, thus giving a saving of 359 s per video field. To compensate for the increase in saturation, the saturation control on the receiver can be adjusted. Alternatively, the amplitude of the colour burst can be increased. This will cause the automatic gain control in the decoder to automatically reduce the colour saturation. As is the case with the line sync pulses, a finite time before and after the bursts free from data signals is needed.

The above modifications to the video signals may be combined to give greater savings in time available for data transmission. For example, tests have been made on the receivers with the field sync sequence reduced to just two broad pulses and then the line sync pulses gradually reduced in length. The results for the minimum line sync pulse length for two of the receivers are shown below.

Mode Receiver Minimum Pulse Length UHF TX10 2.8 ts TX10 -75 dBm 2.8).is JVC 1.8 s JVC -75 dBm 2.2 cos Therefore the line sync pulses can be reduced to 2.8 gas giving a further reduction of lS6J.is per video field. This is in addition to the normal saving with line sync. pulses i.e. the reduced field syncs. increase the tolerance of receivers to reduced line syncs. The total time saving per video field is 10109us per video field with the combination of the reduced field sync sequence and the reduced line sync pulse length.

The increased tolerance of the receivers to shortened line pulses is due to the fact that the reduced field syncs cause less d.c. shifting of the waveform of the type shown in Figure 2.

Other combinations of the modifications described are also possible and these will similarly increase the capacity for DATV data transmission or for additional analogue video components.

Of the above modifications, the reduction in field syncs causes the least problems. The receivers give no change in picture quality when the field sync sequence is reduced to just two broad pulses.

The above approach to modifying a PAL video signal leads to a substantial increase in free signal time. This gives a suitable data channel for DATV data signals to be transmitted on. The modifications will happen at the transmitter in a dedicated unit and are such as to still enable non-DATV receivers to receive and display the video signal without significant distortion of the picture.

The addition of the DATV data is indicated in Figure 1 by the input DATV DATA to the processor 8 which can be employed to distribute the DATV data to the various "slots" therefor created in association with the various components of the modified mixed syncs.

The above techniques can also be used to transmit additional analogue information. For example, if the extra space created by reducing the synchronising information were used to extend the active line than an extended aspect ratio picture could be transmitted and the additional information would be PAL coded video.

Claims

1. A method of transmitting a digitally assisted television (DATV) data signal comprising the steps of providing a video waveform, reducing the number of pulses in the field synchronising sequence and reducing the lengths of the line synchronising pulses in the video waveform to provide a free data channel, the said reductions being such that the video waveform remains compatible with a non DATV receiver, combining a DATV data signal with the video waveform, the DATV data signal occupying at least partially the free data channel, and transmitting the resultant combined signal.

2. A method according to claim 1 in which the line synchronising pulses are reduced in length to 2.8jim.

3. A method according to claim 1 or 2 in which the step of reducing the number of pulses in the field synchronising sequence comprises reducing the number of equalising pulses.

4. A method according to claim 3, in which all the equalising pulses are removed.

5. A method according to any preceding claim in which the step of reducing the number of pulses in the field synchronising sequence comprises the step of reducing the number of broad pulses.

6. A method according to claim 5 in which the number of broad pulses is reduced to two.

7. A method according to any preceding claim which includes the steps of reducing the length of the colour bursts, and increasing the amplitude of the colour bursts.

8. A method according to claim 9 in which the colour bursts are reduced in length to 1.2ups.

9. A method of transmitting a DATV data signal substantially as herein described with reference to the drawings.

10. Apparatus to transmit a digitally assisted television (DATV) data signal comprising, input means to receive a video waveform, signal processing means to reduce the number of pulses in the field synchronising sequence and to reduce the lengths of the line synchronising pulses in the video waveform to provide a free data channel, the said reductions being such that the video waveform remains compatible with a non-DATV receiver, combining means for so combining the video waveform with the DATV data signal that the DATV data signal occupies at least partially the said free data channel, and transmission means to transmit the resultant combined signal.

11. Apparatus according to claim 10, in which the line synchronising pulses are reduced in length to 2.8pus.

12. Apparatus according to claim 10 or 11, in which the signal processing means reduces the number of equalising pulses in the field synchronising pulse sequence.

13. Apparatus according to claim 12, in which all the equalising pulses are removed.

14. Apparatus according to any of claims 10 to 13, in which the signal processing means reduces the number of broad pulses in the field synchronising pulse sequence.

15. Apparatus according to claim 14, in which the number of broad pulses is reduced to two.

16. Apparatus according to any of claims 10 to 15, in which the signal processing means also reduces the lengths of the colour bursts, and increases the amplitude of the colour bursts.

17. Apparatus according to claim 16, in which the colour bursts are reduced in length to 1.2ps.

18. Apparatus for transmitting a DATV signal substantially as herein described with reference to the drawings.

19. A method of transmitting a digitally assisted television (DATV) data signal comprising the steps of providing a video waveform, reducing the lengths of the colour bursts to provide a free data channel and increasing the amplitude of the colour bursts, the said reduction and said increase being such that the video waveform remains compatible with a non-DATV receiver combining a DATV data signal with the video waveform, the DATV data signal occupying at least partially the free data channel, and transmitting the resultant combined signal.

20. A method according to claim 19, in which the colour bursts are reduced in length to 1.2,us.

21. Apparatus to transmit a digitally assisted television (DATV) data signal, comprising input means to receive a video waveform, signal processing means to reduce the lengths of the colour bursts and to increase the amplitude of the colour bursts, the said reduction and said increase being such that the video waveform remains compatible with a non-DATV receiver, combining means for so combining the DATV data signal with the video waveform that the DATV data signal occupies at least partially the free data channel, and transmission means to transmit the resultant combined signal.

22. Apparatus according to claim 21, in which the colour bursts are reduced in length to 1.2ups.