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Description  |
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FIELD OF THE INVENTION
This invention relates to an apparatus for recording video and audio
signals in separate areas of slant tracks and more particularly is
directed to an 8 mm video tape recorder in which video signals and digital
audio signals are recorded in separate sections of the slant tracks formed
on a magnetic tape by rotary magnetic heads.
BACKGROUND OF THE INVENTION
Video tape recorders having two rotary magnetic heads separated by
180.degree. which record a video signal in alternate successive slant
tracks on a magnetic tape are well known in the art. Recently, these video
tape recorders (VTRs) have been provided in an 8 mm format in which the
rotary heads record both the video signal and an accompanying digital
audio pulse code modulated (PCM) signal in separate record sections of
each of the successive slant tracks. One such VTR is disclosed in U.S.
Pat. No. 4,551,771, issued Nov. 5, 1985 and assigned in common with the
present application. As disclosed therein, the tape is wrapped around a
rotary drum mounting the rotary heads in an arc greater than 180.degree.,
so that each track scanned by a head includes an overscan section
subtending a small angle and then a main section subtending 180.degree..
During recording, the audio signal associated with each field of the video
signal is converted into a block of digital audio PCM words, error encoded
and time base compressed so that it may be recorded within the overscan
section of one track. The time required for the conversion, encoding and
time base compression generally corresponds to one field interval of the
accompanying video signal, so that the block of audio PCM data is
available for recording only after the corresponding field of the video
signal has been recorded, and is therefore recorded in the overscan
section of a following track. The audio PCM signal is supplied to the
rotary heads while they trace the overscan sections and the video signal
is supplied to the rotary heads while they trace the main sections so that
the audio and video signal are recorded in the overscan and main sections,
respectively.
During reproduction, the slant tracks are alternately traced by the two
rotary heads to produce output signals. The digital audio PCM signals are
time base expanded, decoded and converted into analog form, a process
which also requires a time corresponding to one field interval of the
video signal. Consequently, a total delay between the audio signal and the
corresponding video signal created by recording and subsequent
reproduction amounts to two field intervals.
One highly advantageous method of error correction encoding which may be
applied to the audio PCM data recorded by an 8 mm VTR is the cross
interleaving method disclosed in U.S. Pat. No. 4,562,578, issued Dec. 31,
1985 and assigned in common with the present application, in which index
words are annexed to blocks of digital audio words obtained by digitizing
fields of stereo audio signals, and the combined words are then scrambled,
parity and error correction codes are added thereto and the resulting data
is converted into serial data for transmission. This method provides the
ability to correct errors in the recorded digital audio signals to a very
high degree of accuracy.
However, the cross interleaving method requires that the index words, error
correction words etc. be thoroughly intermixed with the audio data words,
with the result that it is impossible to edit or dub a limited area within
each block recorded in each overscan section while leaving the remaining
areas unchanged. In fact, in order to edit any of the audio signal, it is
necessary to reproduce the entire block, mix or dub the audio data,
recompute the error correction codes and rerecord the entire new block.
Since the time base compression and expansion, conversion and error
correction processing require a total delay of two field intervals, that
is, one field interval during reproduction and one field interval during
recording, each dubbing operation will cause each block of audio signal to
drop two more field intervals, i.e. two more tracks, behind its
corresponding field of video signal. As a result, the timing control
required for presenting the audio signal simultaneously with the video
signal becomes extremely complicated.
OBJECTS AND SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an apparatus
for recording video and audio signals which removes the above-described
difficulties of the prior art.
It is a further object of the present invention to provide apparatus for
recording video and audio signals in which a dubbing operation may be
performed while maintaining an unchanged correspondence between the
recorded video and audio PCM signals.
It is yet a further object of the present invention to provide an apparatus
for recording video and audio signals in which a limited portion of the
block of digital audio data recorded in each overscan section may be
edited or dubbed in a simplified operation.
In accordance with an aspect of the present invention, apparatus for
recording video and audio signals in a plurality of successive tracks on a
recording medium comprises first audio signal processing means receiving
an original audio signal for converting the same to an original encoded
digital audio signal, video signal processing means for providing a video
signal, means for transporting the record medium in a transporting
direction, at least first and second main rotary heads for recording
signals supplied thereto in respective ones of the successive tracks on
the recording medium, first switch means for selectively supplying the
video signal and a recordable digital audio signal to the main rotary
heads, control means for controlling the first switch means so that the
video signal is supplied to the main rotary heads for recording in a main
section of each of the tracks and the recordable digital audio signal is
supplied to the main rotary heads for recording in an overscan section of
each of the tracks, at least first and second auxiliary rotary heads
disposed for reproducing the digital audio signal recorded in respective
tracks which are positioned upstream, considered in respect to the
transporting direction, relative to tracks to be next scanned by the first
and second main rotary heads, respectively, second audio signal processing
means for at least converting the signals reproduced by the auxiliary
rotary heads to a decoded reproduced audio signal, audio signal inserting
means having inputs for receiving a new decoded audio signal and the
decoded reproduced audio signal and selectively combining the same to
provide a decoded composite audio signal, third audio signal processing
means for encoding the composite audio signal to a new encoded digital
audio signal, and second switch means operable by the control means for
supplying a selected one of the new encoded digital audio signal and the
original encoded digital audio signal to the first switch means for
supplying by the latter as the recordable digital audio signal to the main
rotary heads.
The above, and other objects, features and advantages of the present
invention will become readily apparent from the following detailed
description of illustrative embodiments of the present invention which is
to be read in connection with the accompanying drawings, throughout which
like reference numerals denote like elements and parts.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic view of a rotary magnetic head unit in a conventional
8 mm VTR;
FIG. 2 is a schematic diagram showing a pattern of slant tracks in which
signals are recorded on a magnetic tape by the VTR having the rotary head
unit of FIG. 1;
FIG. 3 is a timing chart illustrating the operation of the VTR having the
rotary head unit of FIG. 1 in a recording mode;
FIG. 4 is a timing chart illustrating the operation in the VTR having the
rotary head unit of FIG. 1 in a playback mode;
FIG. 5 is a block diagram of a preferred embodiment of the present
invention advantageously applied to an 8 mm VTR;
FIG. 6 is a schematic diagram showing a pattern of slant tracks in which
signals are recorded on a magnetic tape by the VTR of FIG. 5 together with
an indicated disposition of the rotary magnetic heads during a recording
operation thereof;
FIG. 7 is a timing chart illustrating a PCM signal dubbing operation
performed by the VTR of FIG. 5;
FIG. 8 is a schematic diagram showing the pattern of slant tracks in which
signals are recorded by the VTR of FIG. 5 together with an indicated
disposition of the rotary magnetic heads during an editing operation
thereof;
FIG. 9 is a schematic diagram showing the pattern of slant tracks in which
signals are recorded by an 8 mm VTR to which a second preferred embodiment
of the present invention is advantageously applied; and
FIG. 10 is a block diagram of the second preferred embodiment of to which
the present invention advantageously applied to an 8 mm VTR.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to the drawings and initially to FIG. 1 thereof, it will be
seen that the rotary magnetic head unit of a conventional 8 mm VTR, such
as disclosed in the above-cited U.S. Pat. No. 4,551,771, includes a rotary
drum 51 having a magnetic tape 52 wrapped around the peripheral surface
thereof to subtend an angle of 221.degree.. First and second rotary
magnetic heads 53A and 53B having different azimuth angles are mounted on
drum 51 with an angular interval of 180.degree. therebetween, that is, at
diametrically opposed positions. Tape 52 is transported at a selected
constant speed in the direction of arrow X, while rotary magnetic heads
53A, 53B are rotated at a constant speed in the direction of arrow Y at a
frame frequency of 30 Hz, or 30 rotations per second. Because tape 52 is
wrapped at an angle relative to rotary drum 51, rotary heads 53A, 53B
trace across tape 52 at a slant angle to produce a pattern of successive
slant tracks as represented in FIG. 2. In FIG. 2, arrow X denotes the tape
transporting direction as in FIG. 1, while arrow Y indicates the direction
of travel of rotary heads 53A, 54B as they successively trace along the
slant tracks on tape 52. As is conventional in such two head VTRs, rotary
head 53A successively traces alternate tracks 54A, while rotary head 53B
traces the remaining tracks 54B.
In the two head 8 mm VTR, the audio signal accompanying each field of video
signal is converted into a pulse code modulated (PCM) digital signal,
error correction encoded and time base compressed so as to be recordable
by rotary heads 53A, 53B in respective slant tracks in overscan portions
AP thereof. Corresponding fields of the video signal are recorded by the
same rotary heads 53A, 53B in main sections AV of the respective tracks.
Each overscan section AP corresponds to an angular distance of about
36.degree. as traced by the respective rotary head 53A, 53B, while each
main section AV corresponds to an angle of 180.degree.. The total of these
sections, corresponding to 216.degree., is made slightly less than the
tape wrap angle of 221.degree. to ensure that rotary heads 53A, 53B
properly contact the tape during the overscan and main sections. In order
to achieve this recording pattern, the audio PCM signal is supplied to
rotary heads 53A, 53B during the time period when they are respectively
tracing each overscan section AP, while the video signal is supplied to
rotary heads 53A, 53B when they are respectively tracing each main section
AV.
In addition to the audio PCM and video signals, a plurality of pilot
signals to be used as tracking control signals or automatic track finding
(ATF) signals are mixed with the audio PCM and video signals to be
recorded in tracks 54A, 54B. These pilot ATF signals have frequencies
different from those used for recording the audio and video signals and
are switched cyclically from track to track. Thereafter, during
reproduction, as the slant tracks 54A, 54B are alternately traced by
rotary heads 53A, 53B to produce playback output signals, tracking control
is performed on the basis of the pilot ATF signals extracted from the
output signals by appropriate filters.
However, there are two difficulties associated with this conventional 8 mm
VTR. The first is that usually the PCM audio signals are scrambled, error
encoded and cross interleaved prior to recording so as to permit error
correction after reproduction. As disclosed in the above-cited U.S. Pat.
No. 4,562,578, one such encoding system annexes index words ID to a block
of digital audio data words including 1050 words of left and right stereo
audio data corresponding to each field of video data. The audio data and
index words ID are first scrambled, then parity data words Q and P and
cyclically redundant error correction codes CRC are added thereto. The
resulting data block is then converted into serial form for transmission.
Since the error correction codes applicable to the data words are
scattered throughout the entire block, it is impossible to reproduce only
a portion of the data of each block and have all the appropriate error
correction words for that portion of the data. Consequently, the entire
block must be reproduced in order to perform any editing or dubbing
operation so that the data words in any limited area of the block will
receive the corresponding error correction words.
The second problem is one of coordinating each field of the video signal
with the corresponding block of audio PCM data. As shown in FIG. 3, the
fields of the video input V.sub.1 -V.sub.4 are initially aligned field by
field with corresponding "fields" A.sub.1 -A.sub.4 of audio input.
However, during the process of digitizing the fields A.sub.1 and A.sub.2
of the audio input, for example, and encoding and time compressing them to
form the audio PCM blocks P.sub.1 and P.sub.2 respectively, one field
interval is usually required, so that the PCM block P.sub.1 corresponding
to audio input A.sub.1 is not available until the end of the next
following video field V.sub.2. Consequently, when the video and audio
signals are combined and then allocated into separate record signals Ain
and Bin to be supplied to rotary heads 53A and 53B respectively, PCM block
P.sub.1 is available only during field V.sub.2 of the video input, and
practically is available only in time to be inserted just prior to field
V.sub.3 of the video input. Correspondingly, PCM block P.sub.2
corresponding to audio input A.sub.2 is only available for insertion just
before field V.sub.4 of the video input. Consequently, the recording
process requires at least a one field interval delay between the fields of
video input V.sub.1 -V.sub.4 and the corresponding fields of audio inputs
A.sub.1 -A.sub.4.
The reproducing process produces a further one field interval delay. In the
reproducing output Aout produced by rotary head 53A, as shown in FIG. 4,
the PCM block P.sub.2 reproduced during the overscan period just prior to
video field V.sub.4 requires a field interval for time base expansion,
decoding and digital to analog conversion to form the audio field A.sub.2.
Therefore, audio field A.sub.2, which originally was aligned with video
field V.sub.2, is now aligned with video field V.sub.5, and the successive
audio fields are similarly displaced relative to their video fields.
Consequently, when the resultant image is displayed, the video fields must
be stored and produced at a later time to correspond with the proper audio
fields. This requires storage and timing control within the VTR to
compensate for the relative displacement between the fields of video and
audio information. However, when an editing operation on the audio data,
for example block P.sub.1 in FIG. 3, is to be performed by mixing the
already recorded audio signal with a new audio signal, the previously
recorded signal must be reproduced prior to dubbing, and so the dubbing
operation itself requires another two field delay during which block
P.sub.1 is reproduced, expanded, decoded and converted to analog form,
mixed with the new signal, converted back to digital form, compressed,
encoded and then recorded. Therefore, the dubbed areas will have a
different displacement between the respective video and audio fields.
The apparatus according to the present invention removes these difficulties
and provides a simplified system for editing or dubbing an entire block of
audio PCM data or only a portion thereof without disturbing the
correspondence between the respective video and audio fields. Referring
now to FIG. 5, a first embodiment of an 8 mm VTR to which the present
invention is advantageously applied includes a rotary magnetic head unit
10 having a rotary drum 1 about which a magnetic tape 2 is wrapped over an
angular extent of 221.degree. and first and second main rotary magnetic
heads 3A and 3B mounted at an angular separation at 180.degree., that is,
at diametrically opposed positions and having different azimuth angles, as
in the conventional rotary magnetic head unit illustrated in FIG. 1. Tape
2 is held between a capstan 4 and a pinch roller 5. Capstan 4 is driven by
a capstan motor 6 to cause tape 2 to be transported at a selected constant
speed in a transporting direction of tape 2 indicated by arrow X. The
drive current for capstan motor 6 is supplied by a capstan motor drive
circuit 7 subject to a capstan servo control such that, during a recording
mode of operation, capstan motor 6 is driven at a constant speed, while
during a reproducing mode of operation the instantaneous rotational speed
of capstan motor 6, corresponding to the instantaneous running speed of
tape 2, is varied in a known manner in accordance with ATF error signals
supplied from an ATF control circuit 8 responsive to pilot ATF signals
recorded on tape 2, as will be discussed below.
In rotary head unit 10 illustrated in FIG. 5, as opposed to the
conventional rotary head unit illustrated in FIG. 1, first and second
auxiliary rotary heads 3A' and 3B' are further provided with an angular
separation of 180.degree. therebetween. Auxiliary rotary heads 3A', 3B'
are not in the same plane as main rotary heads 3A, 3B, but rather are
axially separated from main rotary heads 3A, 3B in a direction
perpendicular to the plane of FIG. 5 so as to be disposed for reproducing
the audio PCM signal recorded in respective tracks which are positioned
upstream, considered in respect to the transporting direction of tape 2
indicated by arrow X, relative to the tracks to be scanned by main rotary
heads 3A, 3B. Referring to FIG. 6, as tape 2 is transported in the
transporting direction indicated by arrow X, upstream and downstream
directions are defined relative thereto such that video field V.sub.3 is
recorded in a track 2b which is two tracks downstream from track 2b' in
which video field V.sub.5 is recorded. Correspondingly, track 2b' is two
tracks upstream from track 2b. Each rotary head 3A, 3B, 3A', 3B' is driven
in rotation in the direction of arrow Y (FIG. 5) at the frame frequency of
30 Hz by a drum motor, not illustrated, to trace along the respective
tracks. In particular, if rotary head 3B traces along track 2b at one
particular time, it will thereafter trace along track 2b' two field
intervals later after the tape 2 has been moved two tracks to the left in
FIG. 6 in the direction of arrow X. Correspondingly, track 2a' is two
tracks upstream from track 2a, so that if main rotary head 3A traces track
2a at a particular time, it will trace track 2a' at a time two fields
intervals later.
Referring back to FIG. 5, the VTR to which the present invention is applied
is shown to further include a video processing system 17 which receives an
input conventional video signal Vin supplied at a video input terminal 16.
A processed video signal V, which has been processed so as to be
recordable, is supplied from video processing system 17 to each of two
terminals V in head change-over switch 20. A PCM processing system 19
receives left and right channel audio signals Lin, Rin, advantageously in
analog form, at input terminals 18L and 18R, respectively, and produces
output blocks of a pulse code modulated, time compressed original encoded
digital audio signal P, each block corresponding to one field of the input
audio signals Lin, Rin. Digital audio signal P is supplied to a first
input terminal of a switch 33, an output terminal of which is connected to
each of two audio terminals A of head change-over switch 20. Switches 33
and 20, as well as additional switches and elements described below are
controlled in their timing and operation by a system controller 35. In
particular, system controller 35 supplies a control signal S to switch 33
to cause the output terminal thereof to be connected to the first input
terminal to receive the original encoded digital audio signal P in the
normal recording mode of the apparatus. As discussed below, in an editing
or dubbing mode, system controller 35 produces control signal S to cause
switch 33 to assume the condition shown on FIG. 5 in which the output
terminal of switch 33 receives a new digital encoded audio signal P'.
System controller 35 controls the change-over of the moveable contacts of
head change-over switch 20 between the two connected video input terminals
V and the two connected audio input terminals A to occur at each half
revolution of the main rotary heads 3A, 3B. In other words, when each
track is to have one field of the video signal recorded therein, switch 20
is switched at each one field period by a frame-period switching pulse RF
(FIG. 7) which reverses in polarity at each field period, that is, at each
half revolution of rotary head arrangement 10. During the overscan period
of each track, both adders 15A and 15B receive a signal, one adder
receiving a video signal V for one track and the other receiving the
digital audio signal P for the next track, while during the scanning of
the main section of each track only one of the adders 15A and 15B receives
a signal. Thus, when PCM processing system 19 produces digital audio
signal P containing a block of audio data to be recorded in the overscan
section of a track by main rotary head 3A, the moveable contact of switch
20 connected to adder 15A is changed-over to contact its respective audio
input terminal A, while the moveable contact of switch 20 connected to
adder 15B is changed-over to contact its respective video input terminal
V. At the completion of this block of audio data the moveable contacts of
switch 20 are changed-over or returned to the positions shown on FIG. 5 so
that the corresponding field of video data contained in signal V from
video processing system 17 is now supplied through adder 15A. Similarly,
prior to the next video field, adder 15B will receive the corresponding
block of audio data for the next track through the respective terminal A
of switch 20, and, thereafter, switch 20 is changed-over so that adder 15B
receives the corresponding field of video data through the respective
terminal V of switch 20.
Adders 15A and 15B also respectively receive selected ones of four pilot
ATF signals having frequencies f.sub.1,f.sub.2,f.sub.3,f.sub.4 produced by
a pilot signal generator 21. These four pilot ATF signals from pilot
signal generator 21 are supplied to a pilot change-over circuit 22 which
selectively supplies the pilot ATF signals to adders 15A and 15B for
superpositioning on successive fields of the video and digital audio
signals supplied to adders 15A and 15B which thereby output a recording
signal Ain and a recording signal Bin, respectively. The four pilot ATF
signals are also supplied to an ATF control circuit 8 which is operative
during reproduction to supply tracking servo control signals to capstan
motor drive circuit 7.
Recording signals Ain and Bin from adders 15A and 15B are the signals to be
recorded by main rotary heads 3A, 3B, respectively, and are supplied
thereto through respective recording amplifiers 12A and 12B connected to
record terminals R of record/playback switches 11A and 11B, respectively.
The output terminal of switch 11A is connected to main rotary head 3A and,
in the recording mode, switch 11A is controlled by system controller 35 so
that its movable contact contacts the record terminal R and recording
signal Ain is supplied to main rotary head 3A for recording in its
respective tracks. Correspondingly, the movable contact of switch 11B is
connected to main rotary head 3B and, in the recording mode, switch 11B is
controlled by system controller 35 so that its movable contact engages the
respective record terminal R and recording signal Bin is supplied to main
rotary head 3B for recording in its respective tracks.
PCM processing system 19 further includes an input terminal 42I for
receiving input index signals IDin and an output terminal 42O for
outputting index signals IDout. Index signals ID include data words which,
for example, identify the audio signals as stereophonic, monaural or
bilingual, and may further include dubbing protect ID signals which, upon
reproduction, will inhibit duplication of the recorded audio signals. The
index signals may still further include "chapter" or "phrase" codes
depending on the recorded scene, as well as address codes indicating the
record position on tape 2 and additional timing codes. The input index
signals IDin are interleaved and encoded with the data words of the
digital audio signal in order to form audio signal P, for example by using
the method for cross interleaving and encoding disclosed in U.S. Pat. No.
4,562,587.
Thus, in the normal recording mode, the video signal V output from video
processing system 17 to head change-over switch 20 and the digital audio
signal P from PCM processing system 19 supplied through switch 33 to head
change-over switch 20 are alternately supplied at predetermined times to
adders 15A and 15B to be mixed with respective pilot ATF signals from
pilot change-over circuit 22 to form recording signals Ain and Bin.
As head change-over switch 20 switches the received video signals V and the
digital audio signal P at one field periods, the pilot ATF signals are
similarly switched at predetermined periods each corresponding to the
combined duration of an overscan section and a main section of the tracks
by pilot change-over circuit 22 and are supplied to adders 15A and 15B for
superposition upon the received signals V and P to form the recording
signals Ain and Bin. A typical recording pattern of pilot ATF signals is
shown in FIG. 8. Upon reproduction, tracking servo control signals may be
extracted by ATF control circuit 8 in response to the recorded pilot ATF
signals for supply to capstan motor drive circuit 7.
In a normal reproducing mode of operation, the moveable contacts of
switches 11A, 11B are switched by system controller 35 to contact playback
contacts P therein, so that the signals reproduced by main rotary heads 3A
and 3B are respectively supplied therethrough to reproducing amplifiers
13A and 13B. The reproduced signal Aout from reproducing amplifier 13A and
the reproduced signal Bout from reproducing amplifier 13B are respectively
supplied to first and second playback terminals P of a head change-over
switch 23. Head change-over switch 23 has two interconnected output video
terminals V which are respectively adapted to be selectively connected to
the two playback terminals P for providing a single combined output video
signal VPB. Head change-over switch 23 further includes two
inter-connected audio output terminals A which are adapted to be
selectively connected to playback terminals P for providing a single
combined output audio signal. The moveable contacts of head change-over
switch 23 are controlled by switching signal RF from system controller 35
in a manner analogous to the operation of head change-over switch 20 so as
to provide the digital audio signals reproduced from the overscan sections
of the tracks and contained in output signals Aout, Bout as a continued
output signal and to provide the video signals reproduced from the main
sections of the tracks and contained in output signals Aout, Bout as the
continuous video output signal VPB. Thus, when head 3A is about to begin
its scan of a track, for example track 2a in FIG. 6, head 3B is in the
process of scanning the main section AV of track 2b. Therefore, the
playback terminal P receiving Aout will contact its respective audio
terminal A, while the playback terminal P receiving Bout will contact its
respective video terminal. At the point when rotary head 3A finish its
trace of the overscan section AP of track 2a and begins to reproduce the
video signal V.sub.4 in the main section AV thereof, main rotary head 3B
has finished its scan of the main section AV in track 2b, and therefore
the positions of the two moveable contracts are reversed to provide the
video signal reproduced by main rotary head 3A and contained in signal
Aout to the respective video terminal. When head 3B then reaches the
beginning of the overscan section AP of the next following track, the
audio signal recorded therein and reproduced by main rotary head 3B and
contained in output signal Bout will be supplied from the respective
playback terminal P to the audio output terminal A.
The played back video signal VPB alternately supplied from the two output
video terminals V of head change-over switch 23 is supplied to video
processing system 17, wherein it is processed into a conventional output
video signal Vout and output through terminal 40. Similarly, the played
back digital audio signal alternately supplied from the two audio output
terminals A of switch 23 is supplied through a band pass filter 24 and
supplied as signal PPB to PCM processing system 19, wherein it is
de-interleaved, error corrected and transformed into output audio channel
signals Lout and Rout and output through terminals 41L, 41R. The index
signals ID interleaved with the audio signal are recovered and output as
index signals IDout through terminal 420 to be used in other parts of the
apparatus for mode control, timing, etc.
Output audio terminals A of head change-over switch 23 are also connected
to a low pass filter 25 which extracts recorded pilot ATF signals
ATF.sub.1 and supplies them to a first input terminal of a switch 26.
During this normal reproducing mode, the moveable contact of switch 26
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