Video recording and display equipment

In the recording of broadcast television text, the text is separated from the video signal and formed into blocks at slicer and then is fed into shift register. It is then taken out at a data rate lower than that when broadcast and coding is assigned to at least some of the blocks prior to recording onto a video cassette tape recorder. Retrieval and display incorporate similar functions in reverse order. In another embodiment data in block form is stored on a video disk for similar retrieval and display.

FIELD OF THE INVENTION 
The present invention relates to video recording and display, and more 
particularly to the handling of digital data transmitted in a television 
signal additional to the television programme information carried by the 
same signal. 
In the television broadcast system presently used within the United 
Kingdom, additional information consisting of digitally coded signals 
(known as teletext data) is inserted into the otherwise unused parts of a 
television signal; these parts consist of one or more of the lines 
preceding those lines containing video information in each frame of the 
scan. While the invention is hereafter described with reference to 
teletext data, it is not limited to use solely with the television system 
within the United Kingdom, but can be applicable to any television system 
in which additional information of digital format is inserted within the 
signals of the standard video information. 
DESCRIPTION OF THE PRIOR ART 
British Patent Specification No. 1478695 discloses the recording of 
broadcast teletext signals onto an audio frequency recorder (or similar 
recorder) by reducing the data digit rate of the broadcast teletext signal 
from about 4 MHz to about 7 KHz (a reduction by a factor of about six 
hundred) and then recording all the data relating to a row of teletext 
information as a continuous stream of data bits. However, only part of the 
broadcast teletext information can be recorded and reproduced and this 
part must be selected prior to recording. Moreover the recent increase in 
the number of teletext lines per field broadcast in the United Kingdom 
(namely from two to four lines per field) results in a doubling of the 
time required for storing the teletext information on the recorder; it is 
possible that there will be further increase in the number of lines 
broadcast per field. 
An object of the invention is to provide equipment permitting the display 
of video recordings containing both standard video signals and teletext 
data. 
SUMMARY OF THE INVENTION 
The present invention provides video recording equipment comprising: 
means to receive a composite signal having standard video information and 
teletext data, the teletext data occupying a predetermined number A of 
television line periods in the field blanking period of the standard video 
information; 
a signal encoder; 
means in the encoder, to separate out teletext data from a received 
composite signal; 
means, in the encoder, to reduce the rate of the teletext data from its 
value at the reception means; 
mixing means, in the encoder, to insert the reduced rate teletext data in a 
number, corresponding to N.times.A (where N&gt;1), of line periods in the 
field blanking period of the standard video information; 
means to record the combined signal output from the mixing means. 
The present invention also provides video display equipment comprising: 
means to derive, from a recording, a combined signal having standard video 
information and teletext data, the teletext data occupying a predetermined 
number A of television line periods in the field blanking period of the 
standard video information and the teletext data being at a rate reduced 
in comparison to the rate of broadcast-teletext; 
a signal decoder; 
means, in the decoder, to separate out the teletext data from the combined 
signal; 
means, in the decoder, to increase the rate of the teletext data to a value 
substantially corresponding to that of broadcast-teletext; 
mixing means, in the decoder, to insert the output from the rate-increasing 
means in a number, corresponding to N.times.A (where N&gt;1), of line periods 
in the field blanking period of the standard video information, thereby to 
form a composite signal; and 
means to display the composite signal. 
In one form of the invention, the video equipment may be a video disc 
player for use with a video disc containing, inter alia, video data and 
teletext data; preferably all the signals are recorded onto the video disc 
during its manufacture, but some may be recorded thereon later. 
Alternatively the video equipment may be a video tape cassette recorder for 
use with a pre-recorded (whether commercially or privately) tape. 
Thus broadcast teletext information with television pictures (e.g. when 
teletext is used for providing foreign language sub-titles or captions for 
the deaf) can be recorded onto video cassette tape for subsequent 
replaying. 
The rate-reducing means may include a first-in/first-out data store with a 
clock-in inhibit control; also the rate-increasing means may include a 
first-in/first-out data store with an output-rate control. 
Also the ratio of data rate during storage to the line frequency during 
display is preferably integral, thereby providing "jitter-free" locking 
onto the clock at the lower data rate. Preferably the storage means has a 
data rate capacity of least 1 MHz so that the change in data rate is by a 
factor not greater than 6, as there are a limited number of vacant lines 
in the field blanking period on a video tape recorder or disc player; 
taking all these constraints into account, it is particularly advantageous 
for the change in data rate to be by a factor of 3 with current machines. 
The data held in the information store may relate to any suitable 
telesoftware, for example any one or more of the following: script and/or 
images to be displayed either alone or simultaneously with other 
television images derived from the standard, non-teletext, circuitry; 
computer programming software relating to video games; textual data for 
the information store itself. In the last-mentioned situation computer 
software could be sent to peripheral microprocessors or a mainframe 
computer to allow interaction between the displayed information and the 
user; thus, for example, in an interactive learning process the sequence 
of information presented to the pupil is dependent on his progress in 
answering questions on the recently presented material.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Equipment embodying the present invention is shown in FIG. 3. The equipment 
has an aerial 20, an encoder 21 substantially as described hereafter with 
reference to FIG. 1, a signal mixer 22, a video tape recorder 23, a 
decoder 24 substantially as described hereafter with reference to FIG. 2, 
a signal mixer 25, a videodisc player 26 and a television receiver 27. 
The invention will be described as applied to the recording of teletext on 
a domestic video tape recorder (V.T.R.). In the teletext systems in use in 
the United Kingdom, coded digital information is transmitted during 
selected line periods during the field blanking period of the composite 
video signal. At present two line periods are used, lines 17 and 18 in one 
field and 330 and 331 in the other. The data transmitted during each of 
these line periods comprises 45.times.8 bit bytes viz. two clock run-in 
bytes, one framing code byte, two address bytes and 40 character bytes. 
The data is therefore transmitted at a rate of 6.9375 Mbits per second, 
which far exceeds the bandwidth capability of a V.T.R. In order to record 
teletext data on a V.T.R. it is therefore necessary to reduce the bit rate 
to below, say, 3 Mbits per second and at this reduced bit rate more than 
two line periods are required to record 45 bytes. In practice it is 
necessary to record clock run-in and framing code bytes at the beginning 
of each line period. Therefore to record a teletext row over three line 
periods requires 51 bytes i.e. 17 bytes per line period and a suitable bit 
rate is 2.625 M bits per second. This data rate is chosen to be an 
integral multiple (168) of the television line frequency and an integral 
sub-multiple (1/3rd) of the original data rate. This ensures a jitter-free 
lock to line of any waveform divided from the data clock. 
Referring to FIG. 1, the off-air video, comprising video plus teletext, is 
applied to a data slicer and synch separator 1. The composite line and 
frame synch from 1 is applied to a long-tailed pair video mixer 2, to be 
described later, while the data plus video is applied via a data gate 3 
which inhibits the video and passes the data to a first-in/first-out store 
being a 90 byte i.e. 720 bit serial shift register 4. The off-air video is 
applied via 1 to a conventional teletext decoder 5 which recovers the 
6.9375 MHz clock from the data and applies it to a load clock gate 7 via a 
clock regenerator 6. In response to gating pulses from 1 and 5, gate 7 
passes clock pulses from 6 to shift register 4 via load/unload clock mixer 
8 during lines 17 and 18 of the television signal i.e. during line 17 gate 
7 is open and the clock pulses from 6 clock the 45 data bytes from gate 3 
into the shift register 4. Gate 7 is then closed during the line flyback 
period then opened again for the duration of line 18 to allow another 45 
bytes to be clocked into shift register 4. In order to record the stored 
90 bytes on tape it is necessary to read data from the register at the 
reduced bit rate of 2.625 MHz and to segment it so that it can be recorded 
during six line periods, for example lines 8 to 13 of the next field 
blanking period i.e. lines 320 to 325. The read-out clock is derived from 
a 21 MHz oscillator 9, synchronized by line pulses, divided down by 
divider chain 10 to provide 2.625 MHz clock pulses. A second divider chain 
11 is reset by field pulses and divides line pulses to produce a gating 
pulse during lines 320 to 325. Although the duration a television line is 
52 .mu.s, the teletext display is active for only 40 .mu.s to allow a 
margin at each edge of the page and it is preferable to record the 
teletext data during the same part of the line period. To this end 
appropriate gating pulses are derived from the 6 MHz display oscillator of 
the teletext decoder 5 via a divider chain 12 and line position logic 13 
and applied to unload clock logic mixer 14 together with the gating pulses 
from divider chain 11 and the clock pulses from divider chain 10. The 
unload sequence of shift register 4 is as follows. During line 320, 136 
clock pulses from divider chain 10 are gated to the load unload clock 
mixer 8 and applied to the shift register 4 to clock out the first 17 
bytes viz. the clock run-in bytes, forming code byte, address bytes and 12 
character bytes, at a bit rate of 2.625 MHz. The clocked out data is 
applied via a data mixer 15 to the mixer 2. In line 321 mixer 14 is 
initially inhibited for 24 clock periods and during this time two clock 
run-in bytes and a framing code byte are produced in turn by generators 16 
and 17 respectively in response to clock pulses from divider 10. Mixer 14 
is then enabled for 112 clock periods to clock the next 14 bytes viz bytes 
18 to 31 from register 4 to mixer 15. Similarly during line 322 clock 
run-in and address bytes are generated and bytes 32 to 45 clocked out the 
register. The output from data adder 15 is combined with the composite 
synch in the long tailed pair video adder 2 and is then added to the 
original video signals and applied to the video recorder. In this way the 
45 bytes broadcast in line 17 are recorded as 51 bytes, 17 in each of 
lines 320, 321 and 322 with the run-in and framing bytes repeated in lines 
321 and 322. The remaining 45 bytes in register 4, i.e. those broadcast in 
line 18, are clocked out in the same way as the first 45 and recorded as 
17 bytes in each of lines 323, 324 and 325. It should be noted that the 
generated framing code need not be identical to the one broadcast. It may 
in some cases be preferable to use the inverse of the broadcast code to 
enable the teletext decoder to differentiate between direct broadcast and 
recorded teletext. 
Clearly the invention relates to the processing of the teletext data and 
thereby involves the path it follows via components 1, 3, 4, 15 and 2; the 
details of the remaining components and circuitry would be evident to a 
competent television engineer. 
The data replayed from the tape must be converted back to its original form 
viz 45 bytes/6.9375 MHz before it can be applied to a standard teletext 
decoder. The decoder for performing this conversion is shown in FIG. 2 and 
as it is almost identical to the encoder, much of its operation will be 
self evident. The data reproduced from the tape is clocked into the shift 
register at the low data rate i.e. 2.625 MHz during lines 320 to 325 and 
clocked out at the high data rate i.e. 6.9375 MHz, produced by dividing 
down the output from a 55 MHz oscillator. The clocking-in is inhibited 
while the run-in and framing bytes added to lines 321, 322, 324 and 325 
are being read since it is not necessary to store these bytes. The normal 
run-in and framing bytes recorded in lines 320 and 323 may be stored and 
read out or alternatively may be generated in the decoder and mixed with 
the output data from the shift register. 
The described embodiment relates to the recording of television text as 
broadcast according to the present United Kingdom standards. Clearly some 
modification may be necessary when designing an embodiment of the 
invention to be compatible with text broadcasting according to different 
standards; such modification as appropriate would be evident to a 
competent television engineer. 
By providing additional shift register storage it is possible to receive 
and record more than two teletext rows per field e.g. with two 90 byte 
shift registers 4 teletext rows can be received per field and recorded in 
12 lines of the next field. 
The invention is not limited to the recording of broadcast teletext or to 
the use of magnetic recording. It may be used to record any data which is 
required to be reproduced at a rate beyond the capability of the recording 
medium and is applicable to video discs as well as video tape.