Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
- RCA 70,779
10832S7
,.:
Backgroun~ Of The Inv_ntion -~ -
A magnetic recording system widely used for
recording television vi~eo and audio signals and used also
for digital signals, is called the quadraplex system and it ~-
involves a magnetic tape about two inclles wide which is
driven past a transversely rotating headwheel carrying four
equally-spaced magnetic heads. The heads move across the
tape at right angles witll the direction of linear movement
of the tape, and each recorded track or path on the tape
is at an angle ~ on the tape with the directlon of linear
movement of the tape. The angle ~ ~epends on the speed
each head goes transversely across the tape relative to the
linear speed of the tape. The transverse and linear speeds
used for recording are used also for playback to insure that
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the heads will follow the recorded tracks at the angle ~.
; It is sometlmes very desirable,~particularly wlth ` ~-
di~gi~tal~signals, to be able to record the signals at one
speed and play back~the slgnals at anot~herhigher or~lower
speed.;~
~ ; Summary Of The Invention~
A rotating head recording system in which recording
an~d playback can be at different speeds over a limited range -~
of~speeds, such as~two-to-one, by main~taining a constant
ratio between the transverse speed of the heads and the
Iinear speed of the tape. Recording and playback can be at
dlfferent speeds over a large range o~speeds by changing
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the ratio of transverse to linear speeds, and slightly ; ;
altering the angle the headwheel makes with the linear
direction of the tape.
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1 Brief Descri~tion Of 1`he Drawing
FIG. l is an elcvation of a rotating head recorder
constructed according to t]1C teachings of the invention;
FIG. 2 is a vie-~ looking in the dircction 2 at the
headwlleel in the recorder of FIG. l;
IIG. 3 is a diagram illustrating the effect of
transverse head speed and linear tape speed on the angle .
of the tracks on the recording tape;
FIG. 4 is a block diagram of a servo system for con-
trolling the speed of the headwheel and the speed of the
::~ recording tape in the apparatus of FIG. l;
1IG. 5 is a chart from which one can select the con~ditions for recording signals at any desired speed, and
select the condit1ons for playback of the signals at any ~ ;
:15 other desired speed;
FIG. 6 is a tabulation of headwheel tilt angles
for:~the six reg1ons 1n the chart of F1G. 5;
IG. 7 is a series of charts which will be referred
t~o ln describing the operation of the~apparatus:in FIG. l
::20 :: when operated to record signals at a given high speed, and to
playback the recorded signals at selected slower sp~eds;
and
FIG. & is a diagram which will be referred to in :~
:descr1bing the reason for tilting the headwheel when
2S recording or playing back at certain data rates~
: Description:Of The Preferred Embodiment
~ .
-~ Referring now 1n greater detail to FIGS. l and 2,
the rotating head type recording apparatus shown includes
a base lO supporting a magnetic tape supply reel 12 and
motor 13, and a tape take-up reel 14 and motor 15. A .
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magnetic tapc l.6 extending between reels 12 and 14 is about
two inches wide and is ~Iriven by a capstan 18 between idler
rollers 17 and 19. Thc capstan 18 is driven by a capstan
motor 20.
A headwheel platform 22 has an end 23 mounted at
24 a fixed distance above the base 10. The other end 25
of the platform 22 is connected by a link 26 to an eccentric
connection 28 on an adjusting wheel 30. The wheel 30 is
rotatably mounted at 31 on a bracket 32. The wheel 30 has
I0 a knurled edge at 33 to facilitate its manual rotation ~.
for the purpose of moving the link 26 to cause the end
25 of platform 22 to pivot about its end 23 fixed to the
: base 10. The periphery of wheel 30 is provided with notches
34 for engagement by the end of a spring-loaded pawl 36 ~;
to provide detents for six predetermined positions of the
wheel 30 and the platform 22. --~
The platform 22 carries a headwheel motor 40 having
a headwheel 42 on one end of the motor shaft, and having
a tone wheel 44 on the other end of the motor shaft. A ~ -
~: 20 vacuum tape guide 46 ispositioned on the side.of the
magnetic tape 16 opposite from the headwheel 42 to give the
~; ; tape an arcuate shape corresponding with the peripheral
surface of the headwheel. The headwheel carries four
magnetic Ileads 48 which scan across the magnetic tape in
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~: 25 a~direction at right angles to the direction of linear .. ~.. -
~: tape motion. The tonewheel 44 cooperates Wit}I a fixed .~
pick-up 49 in a usual manner to constitute a tachometer
providing a si~nal indicative of the speed of rotation o .
the headwheel 42.- :
A control magnetic head 49'is positioned at one
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edge of the magnetic tape 16 for the purpose of recording
and playbac~ of a tape servo control signal for insuring
in-phase registration of transverse scans over recorded
tracks during data playback.
FIG. 3 shows magnetic tracks 50 made on a tape 16
by the combined rotation of the ~adwheel 42 and the capstan
18. The tracks 50 make an angle ~ with the linear direction
of motion of the tape. The angle ~ is determined by the
ratio of the transverse speed VT of the heads across the
tape and the linear speed VL of the tape. The heads must
follow paths 50 on playback at exactly the same angle
as the paths followed during recording. This condition
is satisfied by the present apparatus even though the
playback speed is greatly different from the recording
speed,
FIG. 4 shows a servo system form~int~ining the
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headwheel motor 40 and the capstan motor 20 at speeds
necessary for recording at a desired data rate and for
playback of the recorded data at a desired data rate.
Operation at a desired data rate, whether for recording or
reproducing, is determined by a scan clock oscillator ~not
shown) having a frequency variable between 20 and 10 ~H7.
~ .
The scan clock signal is applied at 54 to a known sub-
synchronous power servo drive for three phase motor operation
25~ ~of the headwheel motor 40. The headwheel motor 40 is driven
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by a motor driver 56 which is supplied with three-phase
power from a generator 58 producing frequency which causes
the motor to rotate at a rate about 50 per cent higher
than the desired rat~. A frequency and phase detector 60 -
supplies an error slgnal to the motor driver 56 to cause
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the motor to rotate at the desired spee~. The frequency
and phase detcctor generates the error signal by comparing
a reference frequellcy signal divided down from the scan
clock frequcncy with a frequency derived from the tonewheel
44 and representing tlle actual speed of the headwheel. The
headwlleel is thus made to rotate at a speed directly pro-
portional to the scan clock signal applied at 54.
The right-hand side of FIG. 4 shows a servo system
for maintaining the speed of the capstan motor 20 at a
constant, but adjustable, ratio with the speed of the head-
wheel motor 40. The headwheel motor speed indicated by the
frequency signal from the tonewheel 44 is applied to an
adjustable divider 62 which divides the frequency by
2N 1 , 1,2,4,8,16 or 32 in dependence on an N input at 64
of 1,2,3,4,5, or 6, respectively. The six values N represent
six regions of operation which will be described in connection
wlth FIG. 5. Each successive region has a transverse/
near speed ratio double that of the preceding region.
; ~ The capstan servo system on the right-hand side of
.
~ 20 FIG. 4 also includes means operative during recording to ~ `
:
record a control track along one edge of the tape 16 and
means operative durlng playback to insure that the magnetic
heads will sweep paths phased with the recorded transverse
tracks, rather than between the recorded tracks. For this
purpose, the tonewheel frequency signal divided by 2N 3
from divider 62 is applied over line 66 to a control track
.~ .
` recording circuit 68 and through a switch 70 to the cnntrol ~- `
head 49 on the tape 16. This causes the recording of a
capstan servo control signal appropriate to the operating
region N which marks the positions of the transverse scan
.
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tracks on tlle tape 16.
During this timc that data and a servo control
signal are recorded on the tape 16, a switch 72 completes
a loop 74 from and to a frequency and phase detector 76,
whereby to produce at 77 a capstan reference frequency
having a desired subharmonic or harmonic relationship with
the tonewheel frequency representing headwheel speed. The
capstan reference frequency at 77 is applied to a frequency
and phase detector 80 which also receives a signal from a
IO capstan tachometer 82 via a tachometer amplifier 83. The
capstan error signal produced by frequency and phase detector
80 is applied to a motor driver 84 which supplies electric
power to the capstan motor 20. The capstan 20 is driven at
a speed such that the linear speed of the magnetic tape
bears a fixed ratio (determined by operating region N) with
the transverse speed of heads across the tape 16.
When it is desired toplay back the data recorded
on the tape, the switches 70 and 72 are placed in their
playback positions P. Then the servo control signal recorded
~ on an edge of the tape is read from head 49 through switch
70 to a playback servo control clrcuit 86, from which a signal
lS applied through switch 72 to one input of frequency and
phase detector 76. The error output of detector 76 is
translated in frequency to a capstan reference signal at
25 ~ ~ 77 which is applied to the frequency and phase detector 80
where it is compared with the capstan tachometer signal. The
capstan error signal from detector 80 is applied through
motor driver 84 to the capstan motor, The capstan motor is
driven at a speed such that the linear speed of the magnetic
tape bears a fixed ratio ~determined by thecperating region
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N) with thc transverse speed of heads across the tape 16,
and tile transverse scan~ are positionally phased in regis-
tration along the linear ~imension of the tape with the
previously recorded tracks.
FIC. 5 shows the operating conditions of the
described illustrative apparatus for a range of recording
and playback speeds from 40 megabits per second to 0.625
megabits per second. At the highest data rate of 40
megabits per second, the headwheel is operated at 305 ~ ,;
revolutions per secnnd ( a point on the head speed curve 90),
the capstan is operated to provide a linear tape speed of
20 inches ~50.8 cm) per second ~a point on the linear tape
speed curve 92), and the headwheel 42 is positioned by
adjusting wheel 30 so that the~eadwheel rotates in a plane
which is at exactly 90 degrees with the direction of linear
movement of the tape 16. Under these conditions, the
relative speeds of the transverse and linear motions cause
the heads on the headwheel 42 to sweep paths 50 shown in ~ :`
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~ FIG. 7a at an angle Q with the linear dimension of the ~
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tape 16. Data recorded under these speed conditions can,
of course, be played back under the same speed conditions.
If the data recorded at 40 megabits per second is `
desired to be played back at half speed, at 20 Mb/s, the
speed of the headwheel is halved to 152.5 rps, and the ~ `~
linear speed of the tape is halved to 10 ips. The paths `
followed on the tape by the heads are at exactly ~he
same angle ~ during playback ~FIG. 7b) as during recording
(FIG. 7a). It is also true as indicated on FIG. 5, that data
recorded at any speed between 40 and 20 Mb/s can be played
back at any other speed between 40 and 20 Mb/s. It is merely
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necessary to maintain a constant ratio between the headwheel
syeed ~curve 90) and tl~e capstan spee~ (curve 92), i.e.,
the transverse spee~ V.l ~nd the linear speed VL. The
speeds at W}liC]I recording and playback are performed are
determined by proportional frequencies of scan clock signals
applied at 54 to the servo system of FIG. 4, and the constant
ratio between headwheel and capstan speeds is maintained by
the servo system so long as an N=l signal is applied to the
servo system.
loPlayback as described can be at any speed down to
about one-half of the recording speed before the speed of
the heads over the magnetic tape surface gets so slow that ~ -
the induced voltage equal to d~/dt is too low to reliably
reproduce the stored information. If the difference
lS between recording and playback speeds is greater than a
two-to-one ratio, playback can be accomplished by approp-
riately reducing the line~ar~tape speed (curve 90) while
malntaining the headwheel speed sufficiently high Ccurve 94)
to~plck up the recorded lnformation and be at a speed which
20 ~ produces two transverse scans o the tape for every one -"
recorded scan. For example, referring to FIG. S, data; `~
; recorded at 40 Mb/s can be played back at 20 Mb/s by using
a headwheel speed of 305 rps and a linear tape speed of
lO~ips, and an~input N-2 to the servo system o~ FIG.`4.
; 25 ~. :Then, as shown in F~IG. 7c~, there are tw~o pIayback scans
per recording scan, and the angle ~ remains substantially
; the~same a~s during recording. The angle ~ is made exactly
the same as durlng~recording by tilting the plane of the
headwheel from 90 degrees to 89.6594 degrees relative to
30the direction of linear tape motion. This is done by
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1 turning the a~justing ~ eel 30 to a next adjacent detent
position. ;
In ad~ition to the playback described at 20 Mb/s,
the record~d information can be played back at any rate
from 20 Mb/s to lO Mb/s by merely changing the input scan
clock signal applied to at 54 in FIG. 4 so that the head
speed 94 and tape speed 92 have values corresponding with
the desire~ data rate in Mb/s. At a data rate of lO Mb/s
the conditions are as shown in FIG. 7d. ~
If an even greater reduction in playback speed is ; ~'
desired to a rate between lO Mb/s and 5 Mb/s, the scan ` -
,
clock signal is cllanged to a value producing speeds along ~ ~-
curves 96 and 92 corresponding with the desired data rate,
the signal N is changed to N=3, and the headwheel tilt ~ -
angle is changed to 89.4892 degrees. FIGS. 7e and 7f ;- -
show two examples at the ends of this third data speed
range. In this case there are four ~2N l) playback scan
lines for each record scan line. Solely, the signal pickup `~
during the one playback scan going over the recording track
is utilized. In a similar manner, even lower playback
rates can be obtained at speeds where N-4, N-5, and N~.
FIG. 6 tabulates the headwheel tilt angles employed in
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each of the six speed regions N=l through N=6.
Recording may be a* any data rate shown in FIG. 5,
and playback also~may be at any data rate shown. The
headwheel tilt angle used during recording and during
playback must be set to a value appropriate to the rate .
. ~ .
at which data is recorded or played back as indicated in
FIG. 5. The various headwheel tilt angles differ by very
small amounts, but the use of an indicated tilt angle in
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I the exemplary apparatus is necessary for optimum operation
of the apparatus at very high information packing density.
For an explanation of tlle reason why headwheel
tilt angles are important, reference is made to FIG. 8
where line 100 represents a recording scan line across
the magnetic tape 16 at an angle ~. If the headwheel speed
is doubled so that two scan lines are used during playback
without any change in the linear tape speed, the two playback
scan lines may be as shown by lines 102 and 104, which are
necessarily at an angle ~1 different from ~. To make the
playback scan line occur at the same angle ~ as the recorded
tracks, it is necessary to tilt the head~heel an amount
~2 which is approximately equal to one-half of the angle
90-~. When this is done, one of the two playback scans
can be made to exactly follow the one recorded track. The ~-
signal picked up durlng the one registered playback scan
~ is selected to the excluslon of signals occurring during
r~ the unregistered playback scan~by appropriate utllization
; electronic apparatus not shown.
~ ~ While recording at a high data rate and playback
st a low data rate has been described, it will be understood
al$o that recording can be at a low data rate with playback
at a high d~ta rate. ~It~will also be understood that the
data~rates, headwheel speeds~;, number of heads per headwheel,
capstan speeds, spe~ed ratios, and tilt angles herein
described are glven solely by way of~example, and that
other suitable values may be selected and em~loyed by
those skilled in the art.
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