Note: Descriptions are shown in the official language in which they were submitted.
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"Improvements in and relatin~ to picture transmission syste~-
! This invention relates to picture transmission
¦ systems and to display units for use in the recei~ers
¦ of such systems~
~ The object o~ the invention is to utilise the
5 I magneto-optic properties of a magnetic bubble domain
device iIl a lo-~-cost display unit capable of showing
picture information transmitted over a communication
channel, for example a low bandwidth telephone or
mobile radio channel.
According to the invention there i~ provid-
ed a picture transmission system in which a transrr.it-
ter includes means adapted to scan a picture in a
.
boustrophedral pattern to produce an analogue video
signal, in which a receiver connectable to the trans-
mitter by a co3~nunication channel includes a displ~y
unit, said display unit including a visible light
source, polarising and analysing means 9 and a body
- capable of supporting maglletic bubble domains which
has a tran~parent display area and a bubble shift
register channel on`the display area whose path cor-
.
responds to the complote boustrophedral scan pattern
at the transmitter, and in which an analoguo-to-
digital convcrt,or at the transrnitter or recoiver is
, ~ i adaptod to produce, from the analogue video signal,
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sorial binary information to bo fed into the bubble
shift rogister channel.
According to the invention there is also pro-
vided a display unit suitab~e for use in the abo~
picture tr~rlslnission system, said display unit includ-
ing a visible li~ht source, polarising and analysing
means, and a body capable of supporting magnetic bubble
dolnains which has a transparent display area and a
bubble shift register channel on the display area
,whose path corresponds to'the complete boustrophedral
scan pattern at the transmitter.
- The invention will now be described in more
detail with reference to the accompanying drawings, "-
in which:
Figure 1 shows a schematic outline in pers-
pective ~iew of a display ~lit, and
Figure 2 shows a block schematic circuit dia-
gram of a picture transmission system including the
' display unit of Figure 1.
Referring now to Figure 1, the display unit
includes a visible light source in the form of a lamp
1, a pair of cross¢d optical polarising elements com-
prising a polariser Z and an analyser 3, and a body
4 capable of supporting magnetic bubble domains which
has a transparent display aroa between the polarising
elom~nts alld a single bubblo shirt register chanllel
5 whofie path on the display area rollows a boustrophedral
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1055147
(left to right to left) pattern. A coil 6 is also shown
for providing a bubbl~ supporting magnetic bias field
normal to the body 4.
The bubbles 7 present in the shift register
5 each have a direction of magnetisation opppsite to
that o~ the surroundin~ material and so effect an op-
posite Earaday rotation on the light from the polariser
2 which gives a corresponding difference in the in~en-
` sity of light transmitted by the analyser 3. An ex~ple
of a suitable materlal for the body l~ is the monocrystal-
line bismuth substituted garnet Bio 8Sm2 2Ga1 QFe4 012-
~` This material has a magneto-optic figure Or merit of
- 5~dB corresponding to a ratio of Faraday rotation
per cnl./ Absorption coefficient, ~/C~ = 22 degrees
J5 at 560 nm. If operated between crossed polarising ele-
~j ments so that there is zero light transmission with
this material saturation magnetised in one direction,
then a light transmission of 19% can be achieved when
the magnetisation of the maberial is reversed; this
~ figure being uncorrected for polarisation and reflec-
tion losses. Experimental contrast ratios of 50 to 1
have been observed. The body 4 can be a plate cut ~ -
from a crystal, polished to the desired thickness,
~,
and supported e.g. on a gla~s plate. An alternative
is for the body 4 to bo a layer epitaxially grown on
!' a transpar~nt non-magnetic crystal substratc, e.g.
rare-¢arth ~alliu~n garnet, hicil acts as a a-~pport.
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Propagation of bubbles alon~ the shift regis-
ter channel 5 can be achieved by drivc means, kno~ per
se, which create magnetic field gradients at selected
. ¦ points on a surface of the body 4. For example, the
I drive means can comprise current-carryin~ conductors
¦ which also define the channel 5 or external means for
¦ producing magnetic fields in conjunction with the
channol 5 being defined by a magnetic overlay pattern
on the body 4. The magnetic over].ays can be produced
directly by deposition of ma~netic material, or by
modifying the surface magnetic properties of the body
4 usin~ ion implantation or etchil~g so as to maintain
the transparency of the body 4. An example is the so-
call0d ~angel fish~ overlay pattern which requires
15. only an oscillating bias magnetic field for propa-
gation. The shift register drive means will be adapt-
ed to be controlled by clock pulses. A bubble genera-
tor will be provided adapted to feed bubbles into the
. .:
. channel 5 in response to picture information in serial
binary form and under the control of the clock pulses,
. and a bubble annihilator will be provided at the out-
,
. . put end of the channel 5. The shift register drive
means can also be adapted to bo turned on and off by
marker pul~es before and after the serial binary pic-
turo information, and in this case there is no need
to provide ~ub~lc dotection means,
i On~ of tll~ factors which influcnce the con-
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1055147
trast ratio of thc display and hence its optical effi-
ciency is the ratio of the bubble si7.e to the bubblo
spacing. That is to say that large bubbles close to-
gether will give a high contrast ratio. For a given
body 4, the bubble size is determined by the bias mag-
notic field, and the bubble spacing is determined by
tho structure of the shift register channel 5 which
will require a certain bias magnetic field. It may be
found that a bias field which is suitably low for
la~e`bubble size is lower than that required by a
particular structure of the channel 5 giving small
bubble spacing. A possible solution to this problem
would be to use a high bias field for propagation of
- bubbles through ~he ~hift register, followed by re-
duction of the bias field to expand the bubbles when
a stationary pattern of bubbles which is to be d~ 9-
played has been formed.
:~ -
It should be pointed out that the display
has the advantage of possessing memory, that is to
say that once a particular pattern of bubbles has been
formed by propagation through the shift register chan-
nel 5, it can be viewed indefirlitely as long as the
magnetic bias field is present to support the bubbles.
Thus permanent magnetic means could be provided, instead
; 25 of the coil 6 shown in Figure 1,~to provide the bias
field and thus be sufficient alone to maintain a par-
ticular bubble pattorn. The permanont magnetic means
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migllt, for exan~ple, be in the form of a layer of perma-
nent magnetic matcrial on the body 4. -
As described above, the display unit is
j adapted to be f0d via an input to the single shift
I register channel 5 with information in serial binar~;
¦ form, and this is the form in which information can be
¦ easily transmitted over a communication channcl, ~or
, examp]e a low band-~idth telephone or mobile radio
¦ channel. The present state of the art of magnetic
- bubble shift registers is that such shift registers
having a capacity Or 104 bits with an operating speed
of 10 ~z are attainable, and this performance is -,
sufficient for, a display unit having a single shift - -
register channel as described above to be provided ~ -
in'a receiver at one end Or a low bandwidth telephone - '
or mobile radio channel. For example, a still picture
having 10 elements could be fed into the shift re-
gister from a 3 KHz bandwidth t,elephone channel in
approximately three seconds,'an alpha-numeric message
20 , of 100 characters and 50 elements per character could
be fed into the shift register from a 5 ~Iz bandwidth
radio channel in one second, and an alpha-numeric
, message of 12 chari~cters and 50 elements per charac-
.: .
,` ter could be fed into the shift register from a 1.2
' 25 -- KHz bandwidth radio channel in half a seoond. In all ''
these oxarnples the capacity of the shift register
relative to the bandwidth Or tho ColNnUniOatiOn
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1055147
channel is such that the bubble pattern for a ocmplete
still picture is built up in the shift register during
a time longer than the integration time of the eye.
If required, illumination control means can be pro-
vided in the display unit whereby the display area is
not illuminated during operation of the shift register.
It will be appreciated that if the bandwidth
of the ocmmunication channel used is sufficiently
large, then information can be received at such a speed
as to enable the display unit to show mDving pictures.
An example of the size of the display area
of the body 4 shown in Figure 1 is 2 mm square. ~his
area would accommodate a shift register of capacity 104
bits æranged as 100 lines with 100 elements per line,
in which the elements, i.e. bubbles, are 10 /u diameter
with 20 /u centre-to~centre spacing. As another
example, an æea 10 mm square would acocmmDdate a shift
register also of 104 bits capacity but with bubbles of
50 /u dia~e~Pr. m ese display areas oould be directly
viewed with a magnification of x 10 prcduced by a lens
æ ranged`between the analyser 3 and the viewer. In the
tw~ examples just given, this wculd produce picture of
2 cm square and 10 cm respectively. me display unit
giving a 2 cm square pichlre could be ocmpact enough to be
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incorporated in a hand-held portable radio receiver.
One possible variation of the viewin~ ar-
~ rangcment of the display unit shown in ~iguro 1 is to
¦ havo the analyser 3 on tho same side of the body 4 as
¦ the polariser 2, ~ritll a mirror behind thc channel 5
¦ so that the picture is seen by reflection rather than
¦ transmission. In this case it may be possible to com-
bine the polariser 2 and the analyser 3 in a single
polariser which is traversed by both the incident
` 10 and the reflected light. It is to be noted that for
; a given magneto-optic effect, half the thickness of
the body 4 is required when used in the reflection
mode compared with the thickness required when used
- in the transmission mode.
- Referring now to Figure 2, a complete picture
transmission system is sho~n includlng the display
unit of Figure 1. At the transmitter there is a te-
levision camera 10 constituting means to scan a pic-
ture, an analogue-to-digital converter 11 connected
between the output of the television camera 10 and a
data transmitter 12 adapted to be coupled to a com-
munication channel 13, and a clock unit 14 for con-
trolling tho camera, converter~ and data transmitter.
The camera 10 is adapted to scan a picture
in a bouskrophedral (left to right to lcft) pattern,
i.o. in general t~rms the scan is triangular rather
than ~awtooth. In more prrcisc terJns, the complete
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boustrophedral scan pattcrn of the camera 10 and the
boustrophedral path of the bubble shift register chan-
nel in the display Ullit sllould correspond as closely
as possib].e. Tl~c analoguc video signal output 101 of
the camera lO is converted by the ana].ogue-to-digitsl
con~erter 11 into seri.al binary information 111 which
is ~ed to the data transmitter 12. The clock unit 14
provides synchronisation pulses 141 to conl:rol the
scan of t~le camera 10, cloclc pulses 142 to control
the converter 11, marker pulses 143 ~hich are trans-
mitted by the data transmitter 12 at the beginning
and at the end of the binary information 111 derived
from each complete scan of the camera iO, and clo'ck
pulses 144 to control the data transmitter 12.
At the receiver there is a data receiver 15
adapted to be coupled to th~ col~unication channel
13, a unit 16 to extract from the rcceived data 151
the serial binary picture information 161, the clock
pulses 162 and the marker pulses 163, and a gating ~ -
unit 17 which in response to the inputs 161, 162 and
163 provides clocked serial binary information 171
to the bubble generator of the display unit 18 and ~ -
also pulses 172 (i.e. clock pulses preceded and
followed by marker pulses) to th~ shift register drive
means of the display unit 18.
One possiblc variation of the above-describ-
ed system is to trarlsmit only rrame s.ynchronisation
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pulses at the beginning of each picture, instead of
marker pulses at the beginning and end of each picture.
However, this would require the display unit to have a
bubble detector at the output of the shift register
which would respond to the passage of the frame
synchron;sation pulses to determine when the bubble
pattern for a complete picture is in the shift re-
gister.
If required, the analogue-to-digital conver-
ter 11 can include means for coding the serial binary
information so as to enable the display unit 18 to
show a grey scale picture. For example, the serial
binary information can be coded to produce a pattern
of bubbles in the shift register whose spacing is
modulated so as to effectively produce a grey scale
picture.
The picture transmission system described
above with reference to Figure 2 is particularly
suitable for inclusion in an existing communication -
system in which a transmitter and a receiver are
adapted for communicating digital information. How-
ever, by having the analogue-to-digital converter
at the receiver end this picture transmission system
could be made better suited for inclusion in an exist-
ing communication system in which a transmitter and a
receiver are adapted for communicating analogue infor-
mation.
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