GB2137844A - Scan Conversion Circuit - Google Patents

Abstract

A method of converting a conventional ruster scan into a boustrophedral scan in which the input video signal representing successive lines of the display scanned in a forward direction is applied to an addressable digital store 10, 12 capable of storing at least one such line. The electron beam of the cathode ray tube on which the image is to be reproduced travels so as to scan alternate lines in the forward and reverse directions respectively and the stored video signal is applied to the display such that the signal displayed on alternate lines is time-reversed relative to the input video signal. <IMAGE>

Description

SPECIFICATION Improvements in Raster Scanning The present invention relates to the raster scanning of an image display device as in, for example, the cathode ray tube (CRT) of a video display, especially when in a television receiver (or monitor).

In a conventional television system, the image which is to be transmitted is converted by the camera into an electrical signal by scanning a transducer across the image line-by-line to produce a raster scan.

The signal produced by the transducer represents the characteristics of the image across a first line from left to right, whereupon the trace returns to the left hand side of the image before representing the image characteristics across the next subsequent line from left to right. In this specification such a signal will be termed a "conventional signal".

In known television receivers, the electron beam of the CRT naturally enough follows the same raster scan pattern as that used to derive the conventional video signal at the camera.

Between the lines, the electron beam travels back from the end of one line to the beginning of the next more or less horizontally during a short period of time known as the fly-back period.

The invention is defined in the claims appended hereto, to which reference should now be made.

As the scanning rate of a conventional raster scan is increased to obtain better image definition, it becomes increasingly difficult to achieve horizontal fly-back within a reasonable fraction of the line period, that is, the time taken to scan the beam across the active part of a line.

Using a boustrophedonic scan in accordance with this invention the beam travels only a very short vertical distance between adjacent lines rather than the whole horizontal width of the screen and, consequently, the scan rate can be much faster.

With high CRT beam currents which are now commonplace it becomes more difficult to avoid the fly-back trace breaking into the displayed image. Such problems are avoided in accordance with this invention and "fly-back" blanking can possibly be eliminated altogether as the scanning spot can be held-outside the field of view for the whole of the line blanking period.

Furthermore, the perturbation to the magnetic field controlling the movement of the electron beam is greatly reduced, enabling a reduction in the complexity of the scanning system.

We have appreciated therefore that the natural symmetry between the raster scans at the transmitter and the receiver of a television receiver can be broken in this way to provide one or more of these advantages.

Two arrangements for putting the method according to the invention into effect will now be described in detail, by way of example, with reference to the drawings, in which: Figure 1 is a block diagram of a first circuit for converting a conventional video signal into a form suitable for boustrophedonic scanning; Figure 2 is a block diagram of a second circuit for converting a conventional television signal for boustrophedonic scanning; Figure 3 is a diagram showing the address sequence used in the circuit of Figure 2; Figure 4 is a block diagram of a first circuit for generating a suitable vertical scan for display of a non-interlaced image; and Figure 5 is a block diagram of a modified vertical scan generating circuit for use with an interlaced scan.

In order to convert a conventional television signal to a form suitable for boustrophedonic or zig-zag video scan, it is necessary to reverse the order of the image data for every alternate line of the scan. This is achieved by reading the data into a store and reading it out again before applying it to the electron beam of the CRT so that the order of the data representing each alternate line is reversed, either at the input or the output stage.

Two circuits for achieving the necessary reordering are shown in Figures 1 and 2.

The circuit of Figure 1 includes two addressable, digital stores 10 and 1 2 each having a capacity equal to the number of samples, N, required adequately to represent a single line of the image being transmitted. Input signal data is commutated to one or other store by means of an input switch 1 4 and the output data is taken by an output switch 1 6 from whichever store is not writing in input data. Since it is usual for the transmitted signal to be in analogue form the input data is passed through an analogue-todigital converter 1 8 and the output signal is returned to analogue form by a digital-toanalogue converter 20.

Each of the stores 10 and 12 is capable of either reading or writing a single sample value from a given address in one sampling period. The stores 10 and 1 2 are switched beween read and write modes in antiphase at the end of each line so that during a given line, the input data is written into store 10 and the output data read from store 1 2. During the next subsequent line, these functions are reversed and the input is written into store 1 2 while the output is read from store 10.

Addresses are presented by means of an address generator 22 to both stores 10 and 12, when they are in the write mode, in the form of a count from 0 to (N-1). When store 10 is in the read mode, the addresses are again in the form of a count from 0 to (N-1), but when store 12 is in the read mode the count runs from (N-1 ) to 0, so that the order of the output data from store 12 is reversed relative to the input signal. Thus, as the data representing line 1 is read out of store 10, store 12 is writing in the data which represents line 2. The data for line 2 is then read out of store 1 2 in reverse order while line 3 is written into store 10 and so on.

As no fly-back time is needed with a boustrophedonic scan pattern, line blanking can be eliminated. This can be achieved either by suspending the write operation during line blanking or by not addressing those store locations which contain line blanking data during the read period.

As the reading and writing operations are separate, they can take place at different rates if desired. For example, the output reading rate may be reduced so that the output data relating to the active line expands to fill the whole line period.

The circuit shown in Figure 2 has only a single addressable digital store 30 of a capacity equal to that of one of the stores 10 or 12 employed in the circuit of Figure 1.

The store 30 is such that it can read from a given address and write a new piece of input data into the same address within one sample period.

Each combined read and write operation is, therefore, associated with a single address value.

The point in the previous input line, from which the current output sample originates, is determined by the position during the previous input line at which the current address occurred.

Since the address sequence can be changed from one line to the next, the desired reordering of the signal data for alternate lines can be achieved by address manipulation.

The store address is generated by an address generator 36. The address cycle repeats with a four line period. During the first two lines the address is presented as a count from 0 to (N-l), a "plus" count, and during the third and fourth lines, the address takes the form of a count from (N-1) to 0, a "minus" count. If, for a particular line, both the current address count, responsible for reading the current output data, and the previous address count, responsible for writing that data, are the same, that is, both plus or both minus, the output signal data order is the same as that of the input signal data. If, however, the current and previous address counts are one plus, one minus, the output signal data order is reversed.Thus the address count sequence shown in Figure 3 achieves the desired reordering, that is, the reversing of the data order for alternate lines.

Since the read and write operations cannot be separated, the input writing rate and output reading rate must be the same.

Again the image signal data must be converted into digital form for storage and then back to analogue form after the read operation. This is achieved by means of conventional analogue-todigital and digital-to-analogue converters 32 and 34.

With either circuit, the input signal may be in coded or component form. If the input is in component form the storage capacity must be approximately doubled. If the input signal is in coded form, a modified decoder is. necessary as the re-ordering of the data and loss of the line blanking period gives rise to difficulties. For example, if a PAL signal is re-ordered in this way the switch in polarity of the V signal is lost. The reordering and removal of the line blanking period may also cause loss of the colour burst and subsequent difficulties in subcarrier regeneration.

The decoder should if possible be modified to take account of these points.

With a boustrophedonic scan of the type described above it is necessary to use a modified vertical or field scan in the form of an equalstepped staircase having M steps where M is the number of active lines in the image. Figure 5 shows a circuit suitable for generating a stepped or incremental vertical scan. The circuit includes a digital counter 40 clocked at line rate and followed by a digital-to-analogue converter 42 and a sample-and-hold circuit 44. The sampleand-hold circuit 44 permits the use of a converter with a slower settling time and, hence, higher accuracy for a given cost. The counter 40 is disabled during field blanking and is reset to zero at the beginning of the even field or after a count of (M-1) so that vertical synchronism is achieved.

It is also possible to use a boustrophedonic scan with a conventional 2:1 interlaced system.

With such a system the field scan takes the form of a staircase having (M/2) steps. The staircase for the odd field must however, be offset by half a step height from that of the even field so as to obtain correct interlacing of the two scans.

Figure 6 shows a scan generatorforgenerating a field scan for interlaced scanning. Again, the generator includes a digital counter 50 followed by a digital-to-analogue converter 52 and a sample-and-hold 54. The converter 50 is reset to zero at the beginning of the even and odd fields or after a count of ((M/2)-1). The least significant bit is connected to a digital signal which is low during an even field and high during an odd field so as to give the required half step height offset.

It is to be noted that with such a scanning system the scan lines are horizontal and the half lines in a conventional video signal are meaningless and should be discarded so as to make M even.

The vertical or field scan signals could alternatively be generated by one of a number of different methods, for instance, using analogue current pumps.

Claims (10)

1. A method of reproducing, from an electrical video signal, an image on a display, in which the input electrical video signal represents successive passes across the display in a forward direction, characterised in that the input electrical video signal is applied to storage means capable of storing at least one pass and the display scans with alternate passes in the forward and reverse directions respectively, the stored video signal being applied to the display such that the signal displayed on alternate passes is time-reversed relative to the input video signal.

2. A method according to claim 1 in which the display scans with an orthogonal scanning raster, the said passes being line scans, and in which the field scan is such that it scans the display in a plurality of discrete increments.

3. A method according to claim 1 or 2 in which the electrical video signal is a broadcast television signal.

4. A method substantially as hereinbefore described with reference to any of the drawings.

5. Apparatus for reproducing, from an electrical video signal, an image on a display, the apparatus comprising a display adapted to scan with alternate passes in forward and reverse directions, storage means capable of storing at least one pass, and control means for applying an input electrical video signal representing successive passes across the display in a forward direction to the storage means and for applying the stored video signal to the display so that the signal displayed on alternate passes is timereversed relative to the input video signal.

6. Apparatus according to claim 5 in which the storage means includes two stores each capable of storing at least one pass, and the control means is adapted to apply the input electrical signal representing successive passes to the stores successively and to apply the signal stored in each of the stores successively to the display; the control device being adapted to apply the signal stored in one store to the display so that it is time-reversed relative to the input video signal.

7. Apparatus according to claim 5 in which the storage means includes a single store of a capacity sufficient to store at least one pass, and the control means is adapted to apply each stored sample to the display in sequential order and to replace it by the next available input video signal sample; the said sequential order being reversed at the end of each alternate pass across the display.

8. Apparatus according to any of claims 5 to 7 in which the display is adapted to scan with an orthogonal scanning raster, the said passes being line scans, and which includes means for generating a field scan such that it scans the display in a plurality of discrete increments.

9. Apparatus according to any of claims 5 to 8 in which control means is adapted to receive a broadcast television signal and to apply it to the storage means.

10. Apparatus for reproducing, from an electrical video signal, an image on a display, the apparatus being substantially as hereinbefore described with reference to any of the drawings.