 |
Description  |
|
|
BACKGROUND OF INVENTION
This invention relates generally to video systems for the storage and
retrieval of documentary information, and more particularly to a high
resolution information storage and retrieval system which makes use of a
conventional low-resolution video tape recorder in conjunction with an
electronic refresh buffer.
Video filing systems are known (see U.S. Pat. Nos. 3,594,729 and 3,514,537)
which are adapted to record and store documentary information whereby a
large body of information may be concentrated in a compact bank from which
it can readily be retrieved on demand. In one such video filing system,
paper documents are converted by a high-resolution video camera into
corresponding high-resolution video image signals. These signals, together
with identifying addresses, are automatically filed and stored on magnetic
tape reels.
In such known forms of video filing system, any individual document page
can automatically be retrieved, looked at in its original size, purged,
reorganized with other images or shifted to various locations. An image of
a recalled document is presented for viewing on a high-resolution
television screen from which it can be reproduced as a hard copy. Since
the document images are electronic in nature, filing and retrieval can be
carried out remotely from a central file.
The crucial cost factor in a video filing system of this type lies in its
means to store the documents as compact video images on magnetic tape
which, when played back, are as readable as the original documents. The
fact that commercial television systems afford clear images does not mean
that such systems are suitable for document storage and retrieval. There
is a vast difference between being able to read on a T-V screen an 8-1/2
.times. 11 inch document having more than a thousand characters printed on
the page, and being able to see on the screen a picture of a house; for in
the latter instance, gross detail is sufficient to give one a clear
impression of a house, whereas in the former, small printed characters
cannot be deciphered.
Thus a video filing system must employ high-resolution video means for
recording and displaying documents. An electronic image of a document is
created by scanning an optical image of the document focused on the
photo-sensitive surface of the video camera tube. Scanning is effected by
sweeping an electron beam across the sensitive surface, each sweep being a
scan line. By the time the bean has sequentially scanned across the entire
picture area from top to bottom, it has created an electronic image of the
original document to complete an image frame.
The number of scan lines in an image frame determines one dimension of its
resolution or readability. Resolution is a measure of how readable a
document is when retrieved from the video picture. Commercial television
in the United States has an established standard of 525 scan lines per
frame. The resultant resolution is altogether inadequate for normal
printed matter. Hence is one known video filing system, use is made of a
high-resolution camera and a display tube having 1,280 scan lines in each
frame. The other dimension involved in resolution is normally determined
by the upper frequency limit of the system.
With a conventional high-resolution video filing system, the magnetic tape
storage components and all other functioning elements of the system are
designed to operate with the high-resolution scan line number. As a
consequence of this requirement, use cannot be made of
commercially-available video recording components designed to operate with
the standard 525 scan line number per frame.
Another drawback of the known system which discourages its adoption is that
when video signals from the storage bank are to be transmitted over common
video carrier lines to a remote user terminal, one cannot use standard T-V
transmission facilities for this purpose, for such facilities are
incapable of conveying the frequencies of a high-resolution video signal.
For example, even if a common video carrier line is capable of carrying a
7 megacycle high-resolution video signal as well as the standard 4
megacycle video signal, the associated synchronization system which is
designed of the existing standard will not function with the high
resolution signal. Hence, special carrier lines are called for, and this
fact adds considerably to installation and operating costs.
The practical consequences of these restrictions are serious and have
discouraged the adoption of video filing systems; for while standard
components are mass-produced, high-resolution recording equipment is not
an off-the-shelf item. High resolution devices must be custom-manufactured
and inevitably are far more expensive than standard equipment.
With a view to overcoming the practical limitations of a video filing
system of the above-described type, the Goldberger U.S. Pat. No. 3,803,352
discloses a hybrid high-resolution/low-resolution video information
storage and retrieval system. The entire disclosure of this patent is
incorporated herein by reference. In the Goldberger system, the video
camera tube for converting the documents into video signals and the video
display tube on whose screen the stored documents are reproduced both
function as high-resolution devices with a scan line number per frame that
is a predetermined multiple of the standard T-V low resolution number and
with a frame repetition rate that is a complementary sub-multiple of the
standard rate.
For example, in the Goldberger system, a preferred high-resolution frame
line number is 1,575, which is exactly three times the standard 525-line
number, in which event the sub-multiple is 10 frames per second, which is
exactly one-third the standard frame repetition rate of 30 frames per
second. Thus if the standard scan line number is multiplexed by three, the
frame rate is divided by the same factor.
The video signals generated by the high-resolution camera in the Goldberger
system are stored in a standard low-resolution magnetic tape recorder
functioning as a storage device. In order to reconcile this low-resolution
apparatus with the high-resolution camera and with a high-resolution
display tube, means are provided that function to divide the video signals
representing a single image frame into distinct signal fields, each having
a scan line number equal to the standard line number per frame within a
time period equal to the full frame period of the standard frame
repetition rate.
For this purpose, Goldberger makes use of a buffer to temporarily record
and store a single frame, the buffer being in the form of a disc-type
recorder whose operation is controlled by a switching circuit. The buffer
is adapted to accept the 1,575 line per frame video signal (10 frames per
second) from the camera and to divide this signal equally among three
parallel continuous tracks on the disc recorder, whereby the first 525
lines of the full frame, which appear during a one-thirtieth of a second
interval, go to the first continuous track, the second 525 lines, which
appear in the next one-thirtieth of a second, go to the second continuous
track, and the final 525 lines of the same frame, which appear in the last
one-thirtieth of a second, go to the third continuous track. Thus the
high-resolution video signal is divided into six equal signal fields, each
of which has the standard scan line number and frame repetition ratio.
However, each field represents only one-sixth of the total high-resolution
frame.
In playback, the buffer disc acts as a "refresh memory" and is continuously
rotated to repeat the image frames to provide a stationary image on the
T-V screen for as long as the user requires the image. With a
low-resolution disc recorder in which the course of a full disc revolution
taking place in one-thirtieth of a second, a recording of a single
high-resolution frame from the video camera takes place sequentially on
three tracks in the course of three revolutions. Thus, a full
high-resolution frame composed of three signal fields is recorded in
one-tenth of a second.
If one were to play back any one continuous track on the disc on a standard
T-V display tube, one would see only a third of the original document,
which would appear at the standard 525 line --30-frame-per-second rate. In
order, therefore, to reconstruct the document, all three tracks must be
played back in sequence.
The buffer disc recorder is provided with a unitary assembly of three
recording heads, each associated with one of the three tracks in a disc
having a large number of concentric tracks. The switching circuit, which
is controlled by the T-V camera, acts to render the first head operative
for the first signal section of 525 lines, the second head being rendered
operative for the next 525 lines and the third head for the final 525
lines.
Thus the three continuous tracks on the buffer disc are recorded in
sequence. When another document is to be recorded, the tri-head assembly
is automatically mechanically indexed to the next set of three tracks on
the disc. As the buffer disc serves only for temporary storage, means are
provided to erase the recordings after the buffer has performed its
required function. Since a document frame appears in a set of three
continuous tracks on the buffer disc, it may be transferred to the
magnetic tape recorder which is in the standard format.
Hence in the Goldberger system, in order to store the high-resolution video
signals produced by the high-resolution camera, the signals must be
recorded temporarily on the separate track of a rotating buffer disc and
then transferred from the buffer disc to magnetic tape for permanent
storage therein, whereas in the playback mode, the fields recorded on the
magnetic tape must be returned to the disc and from there fed to the
high-resolution display tube.
Goldberger's use of a mechanical video disc buffer to temporarily store the
output of the video recorder functions as a "refresh memory" for the high
resolution display tube and requires exact mechanical synchronization
between two mechanical systems; that is, between the video tape recorder
and the rotating disc buffer. To attain exact synchronization between two
such mechanical systems is difficult, particularly with respect to the
phase relationship even if the mechanical velocities are matched.
Furthermore, a relatively long period of time is necessarily entailed to
acquire synchronization, this time being "wasted" in the sense that use of
the same period for any other useful purpose is precluded.
Another drawback arising from the use of a mechanical video disc buffer is
that, as a practical matter, one cannot switch from one speed to another,
in that the inertia of the large diameter disc resists sudden changes in
velocity. Since in order to reduce flicker of the screen image it is often
desirable to operate the cathode ray tube monitor at rates faster than
that at which the picture is delivered to the mechanical video disc
buffer, this adversely affects the performance of an information retrieval
system of the Goldberger type; for the persistence of the usual screen is
insufficient to avoid flicker at slow "refresh" rates.
In my copending application, above-identified, there is disclosed an
improvement over a Goldberger-type video filing system, in which improved
system each document to be filed is scanned in the storage mode by a
high-resolution video camera to produce a video signal representing a
single image frame having a predetermined number of scan lines. The frame
signal is electronically divided into a series of fields which are
successively applied to the recording heads of a four-headed magnetic tape
recorder whose heads are vertically offset with respect to each other.
The heads are mounted on a rotating arm and are caused thereby to sweep
across a stationary magnetic tape to transversely record the series of
fields thereon in parallel tracks, the set of tracks representing the
image frame. The tape is indexed to store each document in a distinct set
of tracks. Also recorded on the tape along longitudinal tracks are
position control data for accurate positioning of the tape and address to
facilitate retrieval of the recorded information.
In the retrieval mode, the tape is advanced to present to the heads a
desired track set, and the heads are then rotated to continuously yield a
series of fields which are applied in the proper sequence to a
high-resolution video display monitor to repeatedly reconstitute the image
frame and thereby recreate the document selected from the magnetic tape
file.
As pointed out in this application--whose entire disclosure is incorporated
herein by reference--this four-headed assembly obviates the need for a
separate refresh memory which would be required when using a standard
video tape recorder. However, the application notes the advantages to be
gained by using an electronic refresh memory such as a charge-coupled
device having about a million bits of solid-state memory; for then,
instead of a frame repetition rate of 15 per second which produces a
flicker, one can play back the four-field frame from the tape into the
memory during a period of 1/15th of a second, and operate the refresh
memory to present this image to the cathode ray display monitor at the
normal standard rate of 30 frames per second. On a cathode-ray tube having
standard phosphors, this rate would provide a flicker-free image.
Moreover, a purely electronic buffer has negligible inertia and can be
readily synchronized to any rate and phase.
SUMMARY OF INVENTION
In view of the foregoing, the main object of this invention is to provide a
high-resolution video filing system employing a standard low-resolution
video tape recorder to store and retrieve documentary information, which
system does away with the need for a mechanical buffer or for a special
four-headed tape recorder.
More particularly, it is an object of this invention to provide a video
filing system in which the video tape recorder in the retrieval mode
operates in conjunction with an electronic refresh buffer, the image frame
selected for retrieval being written into the buffer at a relatively slow
rate determined by the limitations of the recorder and being repeatedly
read out from the buffer into a video monitor at the same or higher rate
to produce a flicker-free display.
Also an object of this invention is to provide a video filing system which
makes use of a standard low-resolution tape recorder whose tape is
pre-recorded to establish thereon a control track containing synchronizing
signals for maintaining tape movement in synchronism with the video camera
and other components of the system as well as an address track containing
binary bits to locate the image frames recorded on the tape, whereby once
the tape is prepared, it may be used and reused for many different video
recordings without the need to rewrite these tracks.
Still another object of this invention is to provide a video filing system
of the above type which in the storage mode uses a semi-random address
technique permitting the tape of the recorder to run continuously in
synchronism with a video camera while documents to be filed are
sequentially presented to the camera, the image frames representing the
documents being non-sequentially recorded on the tape.
A significant feature of a semi-random address technique in accordance with
the invention is that while the sequentially-presented documents are not
recorded in sequence, the tape addresses at which they are recorded are
printed on the original document and stored in a computer memory to
facilitate their subsequent retrieval.
Yet another object of this invention is to provide an electronic buffer
constituted by charge-coupled devices in integrated-circuit chip form
which are multiplexed to afford the bit capacity needed for a
high-resolution image display.
Briefly stated, these objects are attained in a video filing system which
in the storage mode scans each document to be filed with a high-resolution
video camera to produce a video signal representing a single image frame
having a predetermined number of scan lines that is a multiple of the
standard low-resolution scan line number.
The frame signal is electronically divided into a series of fields, each
having a like number of scan lines. These fields are successively applied
to the recording heads of a standard low-resolution video tape recorder
whose recording heads are mounted on a rotating arm and are caused to
sweep across an advancing magnetic tape to transversely record the fields
thereon in a set of parallel tracks, each recorded set on the tape
representing a distinct image frame.
The tape is pre-recorded to define a control track having synchronizing
signals thereon to maintain tape movement in synchronism with the video
camera and an address track having binary bits thereon to locate the
recorded image frames, each image frame being stored at a distinct
address.
In the retrieval mode, the tape is advanced to present the set of tracks
representing an image frame at the selected address to the recording heads
of the recorder and to thereby sequentially reproduce the tracks, the
track signals being written into an electronic buffer. The buffer is then
operated to function as a refresh memory to repeatedly read out the track
signals in sequence into a high-resolution video monitor at a rate
sufficient to produce a flicker-free image.
It is to be noted that in the prior Goldberger system, when the number of
fields per frame is doubled, this provides twice the resolution
capability. In the present invention, in which an electronic buffer is
used, such doubling must be accompanied by doubling the buffer size.
OUTLINE OF DRAWINGS
For a better understanding of the invention as well as other objects and
further features thereof, reference is made to the following detailed
description to be read in conjunction with the accompanying drawings,
wherein:
FIG. 1 is a simplified block diagram showing the arrangement of a
high-resolution video filing system as it operates in the storage mode;
FIG. 2 illustrates the tape of the low-resolution video tape recorder
included in the system, the tape having pre-recorded control and address
tracks thereon;
FIG. 3 illustrates the relationship between the tape recorder and the
control unit which supplies signals for producing the pre-recorded tracks;
FIG. 4 is a block diagram showing the details of a preferred embodiment of
the control unit;
FIG. 5 is a block diagram showing the arrangement for imprinting document
numbers and address numbers on the document being recorded;
FIG. 6 is a block diagram showing the system arrangement in the document
retrieval mode;
FIG. 7 is a block diagram showing the manner in which the electronic buffer
of the digital type is operated in the retrieval mode; and
FIG. 8 is a block diagram of a preferred embodiment of an electronic buffer
of the digital type.
DESCRIPTION OF INVENTION
Tape Recording and Preparation
In a video filing system in accordance with the invention, use may be made
of a conventional, commercially-available, two-headed magnetic tape video
recorder which is capable of recording the full frequency range (4.3
megaHerz) of standard low resolution TV signals. One such recorder is the
Panasonic NV 120 as modified by an Electronic Services Corporation 9475-2
kit to extend the frequency response to encompass the full range. Also
suitable are the two-headed video tape recorders manufactured by Sony
(i.e., AV 3600, AV 3400, AV 3200).
A conventional two-headed magnetic tape video recorder adapted to record
and reproduce video signals includes a supply reel, a take-up reel and a
cylindrical video head drum located between the supply and take-up reels.
A rotor is provided within the drum to turn on an axis colinear with the
drum axis, a pair of video heads being mounted on the rotor on opposite
sides thereof so that the heads are displaced 180 degrees. The video head
drum includes a slot in the vicinity of the heads to enable the heads to
scan the tape which is threaded about the drum to define a half helix.
In conventional operation, the tape is advanced at a relatively slow rate
along the drum and the rotor is turned at a faster rate, the rates being
correlated so that each head is successively in operative relationship to
a field of video scan, each head scanning the same track in succession to
produce two fields of video.
Referring now to FIG. 1, there is shown in simplified form, the basic
components of a high-resolution video system in accordance with the
invention for storing documentary information on a conventional two-headed
video tape recorder. The retrieval or playback of this information will be
considered later.
Information, represented by a printed document 10, is placed on an
illuminated platen. An optical image thereof is focused by a suitable lens
assembly 11 onto the photosensitive surface of a high-resolution T-V
camera tube 12. In the camera tube, an electron beam sweeps across the
photosensitive surface to generate a video signal representing the varying
brightness of the picture elements constituting the document image.
We shall, for purposes of illustration, provide the necessary high
resolution by using a frame line number of 1050 lines per frame, which is
two times higher than the standard 525 frame line number. The
high-resolution frames have a repetition rate of 15 frames per second,
which is one half the standard repetition rate of 30 frames per second,
there being four fields per frame.
In the storage mode, when documents are to be recorded, the high-resolution
video signal yielded by camera 12 is fed through an electronic switching
circuit 13 to the respective heads of a two-head magnetic recorder 10 of
the above-described type having a pair of heads A and B which are mounted
on one end of a rotating arm. Switching circuit 13 acts effectively to
divide the frame signal of 1050 lines into four distinct fields having a
like number of lines (2621/2), each field being produced within a 1/60th
of a second interval. Thus each field is very nearly identical to a field
in standard low-resolution television.
Heads A and B are caused by the rotating arm to sweep across a magnetic
tape while the tape is being advanced so that the heads traverse two
separate tracks on the tape in the course of one arm rotation. Two
revolutions are therefore necessary to record the four tracks representing
a single image frame on the advancing tape T.
The head positions and switching sequence are such that in the first
revolution, head A, to which the first field is applied, sweeps across the
advancing tape to record the first track T.sub.a on tape T, as shown in
FIG. 2; and when head A departs from the tape, the second head B, to which
the second field is applied, then proceeds to record the second track
T.sub.b in a position parallel to the first track. This action is repeated
in the second revolution to produce tracks T.sub.c and T.sub.d until the
four fields are recorded to form a set of four parallel tracks.
In accordance with an important aspect of the present invention, address
information is pre-recorded along one edge of the tape on track AT to
facilitate the retrieval of the recorded information. A
longitudinally-extending control track CT is pre-recorded on the other
edge of the tape for accurate positioning of the tape.
Referring now to FIG. 3, there is shown the transport mechanism of a
standard two-headed tape recorder as it operates in accordance with the
present invention to effect pre-recording of tracks AT and CT. The
magnetic tape T to be pre-recorded is wound on a supply reel 15, the tape
being typically 1/2 inch wide and 2000 feet long.
Tape T, when it is drawn from reel 15, first passes by an erase head 16
which functions to remove any previous recording. The tape then turns
about an idler 17 from which it is carried around the periphery of a
cylindrical drum 18 and about an idler 19. Next in the tape path we find
an addressing magnetic head 20 followed by a locator magnetic head 21, the
former being used to approximately locate the position of any section of
the tape and the latter for precise location of the desired tape signals.
A pair of pinch rollers 22 and 23 acts to pull tape T through the
mechanism, the tape then being rewound on a take-up spool 24. Mounted for
rotation about a shaft coaxial with drum 18 is the arm 25 of the rotating
head assembly. Supported on one end of arm 25 is magnetic recording head
A, and on the other end thereof is magnetic recording head B. Electrical
connections to these heads are made through concentric slip rings 26. The
arm rotates in a horizontal plane extending through a gap in drum 18.
To prepare a tape for recording four fields per frame, field-synchronizing
signals are pre-recorded upon control track TC of a fresh or erased tape,
and binary address bits are recorded on the address track AT which, in a
conventional recording system, serves as the audio track. These
pre-recording signals are supplied by a control unit, generally designated
by numeral 27. This unit may be a "hard wired" electronic unit or it may
be a software or firmware programmed computer.
As shown in FIG. 2, recording on control track CT is carried out at a
frequency such that South-North transitions occur at the normal field rate
of 60 per second. When played back, this will produce a positive voltage
excursion 60 times a second, this sync signal being used to servo-control
the tape speed, the rotating head velocity and the rotating head phase in
accordance with normal VTR playback synchronization techniques.
The pre-recording on address track AT consists of groups of binary bits.
These groups occur every four fields and represent a complete frame member
within tape T. FIG. 2 shows three groups of four bits each (0000 - 0001 -
0010). The spacing between groups should, in practice, be much larger than
the spacing between bits, but for purposes of simplified illustration,
this spacing relationship is not shown in FIG. 2. If a given tape length
can accommodate 50,000 frames, there must be at least 16 bits in each
group.
A hard-wired control logic to carry out the function of control circuit 27
is shown in FIG. 4 where field sync pulses are applied to control track
head 21 to pre-record on track CT, and frame address pulses are applied to
track head 20 to pre-record on address track AT. In this logic, a 60-cycle
sinusoidal power line signal is applied to a squaring circuit 28, and
these square wave pulses are fed to control track head 21. If the tape is
moving at its normal speed, this will produce the desired magnetic
pattern.
The 60 Hz square wave pulses are applied to a divider 29 which divides the
pulses by 4. The resultant 15 Hz pulses are applied to a 16 bit counter 30
which has been reset to zero at the start of the tape. Counter 30
increases its count by one for each frame period.
The 15 Hz square wave pulse is also applied through delay circuit 31 (10
microsecond delay) to the load input of a 16 bit shift register 32. When
this pulse occurs, all 16 bits of counter 32 are loaded in parallel into
shift register 32. This load pulse is again delayed by a second delay
circuit 33 (10 microseconds) whose output goes to a shift pulse generator
34 which generates 16 shift pulses at a 16 KHz rate.
The shift pulses yielded by pulse generator 34 serially shift out the
contents of shift register 32, and the resultant pulse train drives
address track head 20, thereby producing the desired sequence of counts on
the magnetic head. Once a tape has been pre-recorded in preparation for
video recording, the same tape may be re-used without the need for
rewriting the control and address tracks.
In this arrangement, the camera operator in the storage mode of this system
presents documents to the camera at whatever speed he finds convenient,
this being done while the tape of the video tape recorder is running at
its normal speed. When each new document is ready to be recorded, the
operator signals a counter 36 which supplies a document number to be
printed on the ready document, the document numbers being supplied in
sequence, as shown in FIG. 5.
For this purpose, counter 36 conveys the document number to a document
number and tape-address control device 37. This device is also coupled to
tape recorder 14 from which it receives the next vacant frame address.
Both the vacant frame address and the document number are printed on the
ready document, the address and number at the same time being stored in
computer memory 35.
A sync signal from tape recorder 14 which is fed to control circuit 13 for
camera 12 is also applied to a write-control device 38. Write-control
device 38 generates the exact record time, this device being coupled to
tape recorder 14 so as to record on the video track of the first field, an
indication of whether the frame is blank or not.
A system of the type shown in FIG. 5 will initially produce tapes with
large gaps, the size of the gaps depending on the interval between the
record frame image as determined by the elapsed time between the
presentation to the camera of one document for recording and the
presentation of the next document. However, during subsequent recording
runs, since the system indicates the existence of blanks on the tape, the
tape will in time be filled up to fully utilize the available storage
capacity of the tape.
Semi-Random Access Recording
In recording documents when using the storage mode arrangement shown in
FIG. 1, if the tape of the two-headed video recorder 14 runs continuously
in synchronism with camera 10, the documents will be recorded in sequence
on the tape at a rate determined by the speed at which the camera operator
is able to transfer from one document to the next; or if automatic feeding
is employed for this purpose, at the operating rate of the feeder
mechanism.
In either situation, a blank interval will exist between the presentation
of one document to the camera, and the replacement of this document by the
next, which delay will inevitably be reflected on the tape as an
unrecorded space between successive sets of field tracks representing the
document images.
In order to avoid the need for a mechanical buffer in the storage mode and
yet make full utilization of the storage capacity of the tape, one may, in
accordance with the invention, present the documents in sequence to the
video camera 12 without, however, recording these documents on the tape of
recorder 14 at sequential frame addresses. To this end, as shown in FIG.
5, the tape addresses at which the documents are recorded are printed on
the original document, which addresses are stored in a computer memory 35.
Alternatively, instead of permitting the tape to run without interruption
while the image frames of documents are being recorded at a pace
determined by the camera operator, one may arrest tape movement after
recording each frame, and then resume tape advance when the next document
is ready, without reversing the tape direction. This procedure is
essentially useful if the camera operator is very slow.
In practice, when stopping the tape after recording each video frame, the
next document is recorded by starting the tape, and after synchronization
is re-attained, recording this document in the next vacant space, the tape
addresses being stored against true document numbers.
As a compromise between a continuous tape advance and a start-and-stop tape
operation for recording each frame image, one may arrange the tape advance
mechanism to cause the tape to run for a predetermined "ready" period,
say, one of 5 seconds duration; and if a new document is ready for
recording in this period, the tape continues to run; otherwise it is
halted until the next document is ready for recording.
Retrieval Mode
Referring now to FIG. 6, there is shown the arrangement in the retrieval
mode for playing back an image frame recording representing a selected
document to be retrieved from the file. The set of tracks constituting a
selected image frame supplied by recorder 14 to an electronic buffer 39 at
a rate determined by the tape speed. This electronic buffer then functions
as a refresh memory to repeatedly read out the tracks in sequencing and
feed them at a higher rate to a high-resolution cathode-ray display tube
monitor 40 to produce a flicker-free image of the selected document. Thus
buffer 39 acts to catch "on the fly" the output of the two-headed recorder
14 and to refresh monitor 40 at a rate higher than the frame rate at which
the recorder is capable of operating.
In the retrieval mode, the operator keys into an address key pad 41 the
tape address of the document to be retrieved or the actual document
number. In actual practice, it may not be feasible to have the document
number match the tape address of the document. In order to reconcile these
numbers, a printed index system may be supplied to all operators, the
index identifying each document and giving its document number as well as
its tape address. Or, in a more sophisticated version of the system,
address control 42 coupled to address key pad 41 may incorporate a tape
look-up register functioning to convert the document number keyed into key
pad 41 into its corresponding tape address.
Address control 42 acts to compare the tape address of the selected
document with the address presented on the tape in tape recorder 14. If
the desired address is far enough ahead of the presented address to allow
the tape to come up to speed, the tape recorder is then instructed to
start forward at its normal speed. But if the selected address is more
than a certain distance away from the presented address, the recorder is
instructed to advance the tape forward at above normal or high speed. When
the presented address reaches a point a given number of addresses away
from the selected tape address, normal speed operation is reinstated to
avoid overshooting the selected address. It is to be noted that addresses
can be read at high speed, but not video recordings.
Electronic buffer 39 is maintained in synchronism with video tape recorder
14, and when the selected address is reached, the four fields of the
selected image frame are sequentially read into the buffer. The speed at
which the fields is written into the buffer from the recorder will
typically be F.sub.max, which is the highest frequency the recorder is
capable of reproducing.
Commercially-available, two-headed video recorders for standard
low-resolution T-V signals are characterized by an F.sub.max that is about
4.3 megaherz. Since in the arrangement disclosed, the frame repetition
rate is 15 per second, then even with interlacing of the four fields which
constitute a frame, a noticeable flicker will be produced with the usual
short persistance phosphor screens found in T-V monitors. To overcome this
drawback, the electronic buffer is read out into the monitor at twice
F.sub.max to provide a flicker-free frame repetition rate of 30 per
second.
Electronic Buffers
The electronic buffer may be of the analog or digital type, the latter
being preferred. Although semiconductor analog memories are available,
such as a bucket-brigade type of charge-coupled device, they tend to
degrade after multiple reads. A scan converter tube is another example of
a large scale analog electronic storage device, but these tubes are
relatively expensive and complicated.
A digital memory suitable as an electronic buffer may be constructed from
digital charge-coupled devices such as those manufactured by Intel,
Fairchild and other manufacturers. A memory suitable for this purpose is
the Intel Corporation IN-65 charge-coupled memory, typically incorporating
about a million bits of solid-state memory. Approximately a million bits
are needed for a good high-resolution picture. However, this number can be
reduced by using data compression techniques.
Where the memory is manufactured in integrated chip form, each chip having,
say, 65,000 bits, with an output rate of 4 mHz, these chips may be
multiplexed to produce a bit rate of 8.6 mHz for write-in and 15.2 mHz for
read-out.
As sho | | |
