GB2238202A

GB2238202A - Method for transmitting HDTV signal

Info

Publication number: GB2238202A
Application number: GB9000709A
Authority: GB
Inventors: Arthur Howard Jones; John Oliver Drewery
Original assignee: British Broadcasting Corp
Current assignee: British Broadcasting Corp
Priority date: 1989-11-17
Filing date: 1990-01-12
Publication date: 1991-05-22
Anticipated expiration: 2010-01-12
Also published as: GB2238202B; GB9000709D0; GB8925996D0

Abstract

In order to transmit an HDTV signal having a first plurality of lines by means of a channel capable of transmitting a signal having fewer lines, the HDTV signal is down-converted to an intermediate signal having a number of lines suitable for transmission through the channel. The intermediate signal is split into two signals, one having a larger proportion of the intermediate signal lines and the other having a complementary portion. These two signals are combined for transmission in such a manner that the signal having the larger proportion of lines occupies one area, preferably the central portions 10 of the vertical dimension of the picture conveyed by the channel. The signal having the complementary proportion of lines occupies the remaining area or areas, preferably, at the borders 12 and 14 of the picture. <IMAGE>

Description

METHOD FOR TRANSMITTING HV SIGNN~ The present invention relates to a method for transmitting a highdefinition television (HYMN) signal through a conventional television channel.

It is, on occasion, necessary to transmit an HDTV signal, which typically has 1000 lines per picture and an aspect ratio of 16:9, using a conventional 652/50 television channel. At the receiver, the signal is up-converted, possibly using information fram a subsidiary channel, for display on HDTV apparatus. It is, however, desirable that it be possible to display the intermediate 625/50 signal on conventional television apparatus using a 625/50 display with an aspect ratio of 4:3.

Previous proposals have achieved this by using a 'letterbox' presentation, as shown in Figure 1, in which the signal occupies only about 432, that is three-quarters or (4/3)/(16/9), of the 576 active lines, centrally disposed. This permits the reproduction of a full picture at an aspect ratio of 16:9 but has the disadvantage that less information can be carried about the HDTV picture than if all 576 active lines were used. The remaining lines, usually 144, could, however, be used as a subsidiary channel to transmit information other than picture lines.

In accordance with the invention, there is provided a method of transmitting an HDTV signal having a first plurality of lines by means of a channel capable of transmitting a channel signal having a second plurality of lines less than the first plurality, the method comprising down-converting the HDTV signal to an intermediate signal having the said second plurality of lines, splitting the intermediate signal into two signals, one having a larger proportion and the other having a complementary proportion of the said second plurality of lines, and combining the two signals into a channel signal having the said second plurality of lines so that the signal having a larger proportion of lines occupies one area of the vertical dimension of the picture conveyed by the channel and the signal having a complementary proportion of lines occupies the remaining border area or areas of the picture.

Preferably, the method also comprises receiving the channel signal, splitting the channel signal into two signals, combining the two signals to form an intermediate signal in which the order of the lines is the same as the order of the lines in the intermediate signal at the transmission end, and uproonverting the intermediate signal at the receiving end to give an HDTV signal having the said first plurality of lines.

This method has the advantage that it permits the display of the signal received fram the channel as an acceptable ("compatible") picture, having an aspect ratio similar to that of the original HDTV picture, whilst also permitting all of the lines conveyed through the channel to contribute towards the vertical resolution of the HDTV signal.

The invention further comprises apparatus and a system for transmitting an HMN signal using the method of the invention and for receiving a signal so transmitted.

A preferred method in accordance with the invention will now be described in detail, with reference to the drawings in which: Figure 1 illustrates 'letter-box' presentation of a downconverted HDEV signal on a conventional 625 (576 active) line display; Figure 2 is a block diagram illustrating a method in accordance with the invention; and Figure 3 shows the sequence of operations involved in both the vertical and spectral domains in the method of Figure 2.

As mentioned above, previous proposals have used a 'letter-box' display in which an HDTV signal down-converted to 432 lines is displayed on the central area 10 of a 625 line display, as shown in Figure 1. The remainder of the 576 active lines form blank areas at the upper and lower margins 12 and 14 of the display. This arrangement provides a full picture display with a 16:9 aspect ratio over the central portion of a 4:3 aspect ratio area.

The method of the invention, however, proposes that the HDTV signal be down-converted to 576 lines rather than 432 as before. If these lines were simply displayed on a conventional television screen, the resulting image would have a 4:3 aspect ratio but would be distorted so that objects appearing on the screen would be unnaturally tall and thin.

This distortion is overcome by splitting the channel signal into two signals, one having, say, three-quarters of the number of lines and carrying the bottom three quarters of the spectrum of the vertical detail in the picture and the other having, say, one quarter of the number of lines and carrying the remaining top quarter of the spectrum of the vertical detail.

Thus, the first signal represents the picture further downconverted, whereas the second signal conveys high-frequency vertical detail which has, in general, a low level and integrates to zero.

These signals are combined to form a "conventional" channel signal in such a way that the first signal occupies the central area of the picture and the second signal occupies the borders. At the receiver, the first signal can, if the receiver is a conventional 625 line receiver, be displayed in 'letterbox' presentation to give a compatible image. The lines forming the second signal are then displayed in the border areas 12 and 14 as shown in Figure 1.

Because the upper and lower margins 12 and 14 of the display are no longer blank but contain picture information, it is preferred that steps be taken to inirse the distraction caused to viewers by changes or movement in the areas 12 and 14 above and below the picture. A number of proposals for dealing with this have been put forward, as follows: (a) physical masking by placing an opaque mask over the display around the central picture area 10; (b) transmission of the lines forming the upper and lower margins 12 and 14 at reduced amplitude. This would, however, entail a noise penalty in the HDTV chain. The attenuation value could be picture dependent and could be sent as a digital signal in, say, the field blanking period; (c) transmission of the lines forming the upper and lower margins 12 and 14 with a mean value equal to that which just avoids clipping in the negative direction. Such a value would be picture dependent and could be sent as a digital signal in, say, the field blanking period; (d) scrambling of the lines forming the upper and lower margins 12 and 14 using reversible techniques, for example, active line rotation, to produce a random effect in the areas above and below the picture area 10;; (e) the signal could be accoopanied by a teletext page containing full-width 'empty boxes' for typically three or four teletext rows at the top and bottom of the page so that teletext receivers could electronically mask the greater part of the disturbance in the areas above and below the picture area 10. One or both of these boxes could, of course, carry subtitles or other material when required; (f) lines could be selectively inverted or have colour phasing selectively varied to minimise large-area effects.

Alternatively, if the receiver is capable of displaying an HDTV picture, the reverse processes of selection, up conversion and catination are carried out to produce an HDTV signal down converted to the number of channel lines. This can then be up converted to the original number of lines in the HDrV signal.

This sequence of operations is shown diagrammatically in Figure 2 in the case where the compatible picture occupies three-quarters of the active lines conveyed through the channel. As shown in Figure 2, the HDTV signal at the transmission end 20 is subject to vertical down-conversion at the circuit 22 to provide the intermediate signal.

As can be seen from Figure 2, the intermediate signal is split into two. The first signal, which ultimately has the larger proportion of lines is subject to vertical supersampling by 3 at circuit 28 before being passed through a low-pass filter LI. The effect of supersampling is to introduce blank lines into the signal.

In a digital system the 'blank' level is that which corresponds to the number zero. In an analogue realisation the 'blank' level is that which is unaffected by an amplifier. The filtered signal is then subject to vertical subsampling by 4 at circuit 30.

The second signal formed by splitting the intermediate signal is passed through vertical high pass filter H1 and is then subsampled by 4 at circuit 32. The subsampling at the circuit 32 is in antiphase to the subsampling operation on the first (lowfrequency) signal at circuit 30. The two signals formed by splitting the intermediate signal are then combined to form the channel signal which is transmitted over the conventional channel.

At the receiving end, the channel signal is split to form two signals which are subject to supersampling by 4 at circuits 50 and 52. Again the supersampling operations at circuits 50 and 52 are in antiphase to one another.

The first signal formed by splitting the channel signal is then filtered by means of vertical low-pass filter L2 and subsampled by 3 at circuit 48 so as to recreate the low-frequency part of the intermediate signal at the transmission end. The second signal, after supersampling at circuit 52 is passed through vertical highpass filter H2. This supersampling and post-filtering recreates the high-frequency part of the intermediate signal at the transmission end. This recreation of a 576 line signal fram 144 lines of transmitted signal is possible due to the bandwidth of the signal.

Post-filtering of the two signals derived from the channel signal takes place simultaneously and the signals output by the subsampling circuit 48 and the high-pass filter H2 are then combined to form an intermediate signal which can then be vertically upconverted at circuit 42.

In such a sequence of operations the form of the filtering in circuits L1, L2, H1 and H2 and the vertical phasing of the sampling is crucial to ensure that the operations can be reversed without causing any degradation, i.e. the whole process, between the initial down-conversion and the final up-conversion, is transparent. In general, any filter will have-a finite frequency region over which it changes from passing to stopping frequencies, known as the transition band. In this region the spectral components travel -in both low and high pass channels. Nevertheless, if the low and high pass filter characteristics obey a certain condition, the overall gain of the system can be maintained at unity for all frequencies.

In addition, alias components in the transition band are generated by the renewed sampling in both low and high pass channels but if the sampling in one channel is antiphase to that in the other and the filter characteristics obey a further condition the aliasing in one channel can be cancelled by that in the other.

It can be shown that the first condition is Ll(f) L2(f) + Hl(f) H2(f) = 1 where Ll(f) is the (generally camplex, i.e. having non-zero phase) amplitude/frequency characteristic of the filter etc. If the receiver filters have the same number of terms as the transmitter filters then this can be satisfied by L2(f) = Ll*(f) H2(f) = Hl*(f) where * denotes the complex conjugate. (The conjugate of a transversal filter is obtained by taking the mirror image of the coefficient pattern.) One form of the second condition is Hl(f) = 12*(f2 - f) H2(f) = Ll*(f2 - f) where f2 = (3/4)fl and fl is the vertical sampling frequency of the channel.If the above relationships between L1 and L2 and between H1 and H2 are assumed then these conditions become Hl(f) = Ll(f2 - f) H2(f) = L2(f2 -f) = Ll*(f2 - f) This shows that the frequency characteristics of the high pass filters are mirror images of the corresponding low pass filters about the frequency f2/2 and so the system is characterised by only one filter.

Figure 3 relates to the case where the compatible picture occupies three-quarters of the active lines conveyed through the channel and shows the sequence of operations involved in both the vertical and spectral doarains, assuming a filter transition band characteristic which is the square root of a ramp. At (a) is shown the original signal, sampled at a frequency fl. At (b) is shown the same signal augmented by zero samples (blank lines) so as to increase the sampling frequency to 3fl and at (c) is shown the same signal after low pass filtering.At (d) is shown the result of subsampling by a factor of 4 so as to decrease the sampling frequency to (3/4)fl. At (e) is shown the original signal after high pass filtering and at (f) is shown the same signal subsampled by a factor of 4 in the opposite phase to that in the low pass part so as to decrease the sampling frequency to (1/4)fl. Note that the alias components in the transition band in (f) are inverted with respect to those in (d). At (g) is shown the low pass part, augmented by zero samples so as to increase the sampling frequency to 3fl and at (h) is shown the same signal after low pass filtering.

At (i) is shown the result of subsampling by a factor of 3 so as to restore the sampling frequency to fl. At (j) is shown the high pass part, augmented by zeros in the opposite phase to that in the low pass part so as to increase the samping frequency to fl and at (k) is shown the result after high pass filtering. Addition of the signals at (i) and (k) allows the alias components to cancel whilst the wanted components sum to unity.

The filters L1, L2, H1 and H2, being transversal filters, operate by cambining lines of a picture so as to shape the vertical spectrum It will be appreciated that where the channel signal is 2:1 interlaced, there is a conflict between adequacy of motion portrayal and munimisation of the high pass signal. If good motion portrayal is required then the filters should combine the lines of only one field at a time so that the band split occurs at 3/4 of the resolution limit for the field. Alternatively, if motion portrayal can be sacrificed, then the filters may combine the lines of both fields so that the band split occurs at double the vertical frequency with a considerably-reduced high pass signal.In such a case it is preferable that the filter ccmbination algorithms be reset after each picture so that the process is transparent. If the HDTV signal is derived from film then no motion degradation will result, provided that the resetting is "frame phased", i.e. reset after two fields belonging to the same film frame.

The principles described above can be applied to signals in the PAL, NTSC, SECAM or MAC formats, although the colour coding of lines may need to be related to their position in the display rather than their position in the 'original' HDTV signal.

The method and principles described above may be applied to both luminance and chrominance signals. However, we have appreciated that it is not necessary to use the high frequency signal derived from the intermediate chrominance signal to provide an acceptable image at the receiver end. The high frequency part of the chrominance signal could therefore be sacrificed so that the space (144 lines in the arrangement described above) can be used for the transmission of other information such as data rather than picture information.

Claims

1. A method of transmitting an HDTV signal having a first plurality of lines by means of a channel capable of transmitting a channel signal having a second plurality of lines less than the first plurality, the method comprising down-converting the HDTV signal to an intermediate signal having the said second plurality of lines, splitting the intermediate signal into two signals, one having a larger proportion and the other having a complementary proportion of the said second plurality of lines, and combining the two signals into a channel signal having the second said plurality of lines so that the signal having a larger proportion of lines occupies one area of the vertical dimension of the picture conveyed by the channel and the signal having a complementary proportion of lines occupies the remaining border area or areas of the picture.

2. A method according to claim 1 in which the intermediate signal is subject to filtering such that the signal having the larger proportion of the second plurality of lines carries a lower part of the spectrum of vertical detail of the said signal and the signal having the complementary proportion of lines carries the remaining upper part of the said spectrum.

3. A method according to claim 2 further comprising receiving the channel signal, splitting the channel signal into two signals, filtering the said two signals and combining the said two filtered signals.

4. A method according to claim 3 in which the filtering operations prior to transmission of the channel signal and subsequent to receiving the channel signal are such that the filtering operations in combination are transparent.

5. A method according to any preceding claim in which the channel signal comprises the signal having the larger proportion of lines transmitted with the lines in the same order as they were in the HDTV signal and the signal with the cooplementary proportion of lines, said signal being transmitted before and/or after the signal with the larger proportion of lines, whereby, when the channel signal is displayed on a receiver display, the signal having the larger proportion of lines forms an image on the display and the signal having the coMplementary plurality of lines is displayed above and/or below the image on the display.

6. A method according to claim 5 in which the signal having the complementary proportion of lines is transmitted at an amplitude less than the amplitude at which the signal having a larger proportion of lines is transmitted.

7. A method according to claim 5 in which the signal having the complementary plurality of lines is scrambled prior to display.

8. A method according to any of claims 1 to 4, further comprising receiving the channel signal, splitting the channel signal into two signals, combining the two signals to form an intermediate signal in which the order of the lines is the same as the order of the lines in the intermediate signal at the transmission end, and converting the intermediate signal at the receiving end to give an HDTV signal having the said first plurality of lines.

9. A method for transmitting an HDTV signal having a first plurality of lines by means of a channel capable of transmitting a channel signal having a second plurality of signals less than the first plurality, the method being substantially as hereinbefore described with reference to the drawings.

10. Apparatus for transmitting an HDTV signal having a first plurality of lines by means of a channel capable of transmitting a channel signal having a second plurality of lines less than the first plurality, the apparatus comprising means for down-converting the HDTV signal to an intermediate signal having the said second plurality of lines, means for splitting the intermediate signal into two signals, one having a larger proportion and the other having a complementary proportion of the said second plurality of lines and means for combining the two signals into a channel signal having the second said plurality of lines so that the signal having a larger proportion of the lines occupies the central area of the vertical dimension of the picture conveyed by the channel and the signal having a carplenentary proportion of lines occupies the remaining borders of the picture.

11. Apparatus according to claim 1 including filtering means for filtering the intermediate signal such that the signal having the larger proportion of the second plurality of lines carries a lower part of the spectrum of vertical detail of the said signal and the signal having the complementary proportion of lines carries the remaining upper part of the said spectrum.

12. A transmission system for transmitting an HDTV signal having a first plurality of lines by means of a channel capable of transmitting a channel signal having a second plurality of lines less than the first plurality, the system comprising apparatus according to claim 11 and receiver means including means for splitting the channel signal into two signals, filtering means for filtering the said two signals and means for combining the two filtered signals.

13. A system according to claim 12 in which the filtering means at the transmission apparatus and at the receiver means are such that the filtering operations in combination are transparent.

14. Apparatus according to any of claims 10 to 13 in which the signal having the larger proportion of lines is transmitted with the lines in the same order as they were in the HDTV signal and the signal with the complementary proportion of lines is transmitted before and/or after the signal with the larger proportion of lines, whereby, when the channel signal is displayed on a receiver display, the signal having the larger proportion of lines forms an image on the display and the signal having the complementary plurality of lines is displayed above and/or below the image on the display.

15. Apparatus according to claim 14 in which the signal having the carplenentary proportion of lines is transmitted at an amplitude less than the amplitude at which the signal having a larger proportion of lines is transmitted.

16. Apparatus according to claim 14 in which the receiver includes means for scrambling the signal having the oomplementary plurality of lines prior to display.

17. A transmission system according to claim 12 or 13 including means for upoonverting the signal formed by the means for combining to give an HDTV signal having the first plurality of lines.

18. Apparatus for transmitting an HDTV signal having a first plurality of lines by means of a channel capable of transmitting a channel signal having a second plurality of signals less than the first plurality, the apparatus being substantially as hereinbefore described with reference to the drawings.