Endless magnetic tape video recorder/player with head centering means

A device for recording and playing back information comprising an endless tape whereof one part has a translational movement in a direction x and n magnetic heads integral with one another whereof the gaps face that part and are aligned in a transverse direction y. The heads have a slow movement in the direction y and form n continuous tracks forming parallel and equidistant portions in the direction y.

The present invention relates to devices for recording and playing back 
information on a magnetic tape by virtue of at least one magnetic head 
whereof the gap faces the tape, this head having a continuous movement 
relative to the tape. 
Various types of video tape recorders, generally comprising a plurality of 
magnetic heads, are used in order to record signals in the video-frequency 
range. In order to obtain the desired pass-band without too many heads, 
the relative speed between the tape and the heads must be very high. The 
solution generally used resides in imparting a rapid rotary movement to 
the heads combined with a translation of the tape. In video tape recorders 
of the "quadruplex" type, 4 magnetic heads are arranged on a drum rotating 
at high speed with a rotational axis parallel to the direction of 
translation of the tape. Their arrangement on the drum gives rise to 
portions of track parallel to the transverse direction of the tape. In 
video tape recorders of the "helical-scan" type, the tape is arranged so 
as to form a loop round the record-playback member. A magnetic head 
rotates with respect to this loop with an axis of rotation slightly 
off-centre with respect to the centre of the loop, thus forming on the 
tape as it is unwound a helical track having parallel portions slanting 
with respect to the transverse direction. In both cases the track obtained 
is discontinuous, and forms parallel portions. These two types of 
video-tape recorders require complex, heavy and expensive mechanical 
elements. The price of the "quadruplex" video tape recorder limits it to 
professional applications. Video tape recorders of the "helical-scan" type 
are less expensive, but require complicated servo-controls since centring 
is critical. 
An object of the present invention is to provide a simpler, less fragile 
and less expensive record-playback device capable of forming part of a 
reporting unit which is easily transportable by virtue of its low weight. 
For this purpose, the device according to the invention enables rotary 
movement of the head to be avoided by using, instead of a magnetic carrier 
in the form of a ribbon transferred from a feed spool to a take-up spool, 
a loop known as an "endless tape" such as that usually used for sound 
recording. In order to use all the available width of the tape, a slow 
movement is imparted to the magnetic heads in a direction transverse to 
the tape, while the tape moves past indefinitely in the longitudinal 
direction. The slow movement is easily obtained by simple mechanical 
means, and may be regulated above all during playback by a likewise simple 
servo-control loop in order to ensure the tracking. The tracking of the 
tape by the magnetic heads is continuous, which eliminates the problems of 
synchronisation. The device is thus appreciably less sensitive to faults 
in the tape due to stretching for example, and to any possible vibration. 
Finally, a variant of embodiment enables crosstalk between tracks to be 
considerably reduced, and the tracking servo-control to be simplified by 
virtue of angular offset of the record-playback gaps. 
According to the present invention, there is provided a video tape 
recording and playback device comprising an endless magnetic tape, driving 
means for continuously moving said tape, at least one magnetic head having 
a gap facing a part of said tape and two terminals for the input and 
output and electrical voltage, and mechanical means for moving said head 
in front of said part in a direction y; said driving means causing a 
displacement of said part in a direction x perpendicular to y.

FIG. 1 illustrates the interior of a case containing an endless tape. The 
magnetic tape 6 is wound round a spool 13 having a rotational axis z. Its 
two ends have been joined in order to form an endless loop: the part 61 
emerging from the interior of the winding is guided by a series of rollers 
9, 11, 12, and rejoins the part 62 situated outside the winding. A roller 
8 connected to a driving motor causes the tape to move past, and the tape 
winds on outside at the same time as it unwinds from the inside, which is 
made possible by the fact that the wound-on turns slide with respect to 
one another and that the feed zone is situated inside the winding. 
Recording-playback means 5 are provided in front of that part 63 of the 
fraction of tape 6 which is moving past between the rollers 11 and 12. 
Such a case is usually intended for recording sound. It is then used at a 
speed of 10 cm/s. The length L of the tape is equal to 150 m, its width 1 
is equal to 6.3 mm for a "quarter-inch" tape, 12.6 mm for a "half-inch" 
tape, etc. . . In altogether unexpected fashion, experience shows that, in 
spite of friction, a speed much higher than 10 cm/s may be reached without 
either breaking the tape or spoiling the recorded signal. For example, the 
tape retains good quality of reproduction at the end of about a hundred 
hours of operation at a speed of 2.5 m/s. The invention provides for using 
this tape at the speed of 2.5 m/s for the purpose of recording signals 
having a wide frequency-band, such as video signals. At such a speed, a 
signal having a pass-band of 2 MHz may be recorded with a signal-to-noise 
ratio of about 35 dB, using a magnetic head whereof the gap-height is 
about 50.mu.m. The use of two heads enables a video signal of moderate 
quality to be recorded on two contiguous tracks, one of the tracks being 
reserved for luminance, and the other for chrominance and sound. The use 
of a stack of eight heads makes it possible to record on eight separate 
tracks a television signal having a pass-band of 16 MHz split up into 
eight components by a coding device. These two examples do not limit the 
invention. In the absence of any indication to the contrary, in the 
remainder of the description the recording-playback means 5 will consist 
of a stack of 4 magnetic heads describing 4 separate tracks. 
Referring to FIG. 2, a group of four magnetic heads 100, 200, 300, 400 are 
stacked in the direction of the height of their respective gaps 1, 2, 3, 
4. Each head comprises a winding, one only, 101, being visible in the 
Figure. For recording, a signal emanating, as will be seen later, from a 
coding device having 4 output channels is supplied between the terminals 
of each winding. For playback, signals S.sub.1, S.sub.2, S.sub.3, S.sub.4 
respectively characterising the items of information written in onto the 4 
tracks and read out by the 4 heads 100, 200, 300, 400 are picked up at the 
terminals of their winding. The distance between the tracks recorded or 
read out depends on the height h of the heads, which may be slightly 
greater than the height .delta. of the gaps, which itself determines the 
width of the tracks. The height .delta. of the gaps may be around 30 to 50 
.mu.m and their a width .lambda. about 0.5 .mu.m. It is nevertheless 
critical to obtain total heights h small enough. If it is desired that the 
tracks described by the 4 heads be very close together (about 50 .mu.m 
apart), it will therefore be preferable to use integrated magnetic heads 
such as that illustrated in FIG. 3. Conventional masking techniques are 
used in order to deposit on a substrate 102 made of a non-conductive and 
non-magnetic material a layer of magnetic material 105 in the form of a 
ring intersected by a gap 1. The winding 101 of the head is obtained by 
depositing conductive turns, for example of copper, before and after 
depositing the layer 105. The two ends of the winding are joined to two 
output terminals 103 and 104 at which the signal s.sub.1 is supplied or 
picked up. The desired gap-heights are thus very easily obtained. Similar 
heads having the desired total height h may be stacked. 
FIG. 4 diagrammatically shows one way of using the stack of heads 5 
according to the invention. For better understanding, the casing 
containing the stack of heads 5 has illustrated on it the respective gaps 
1, 2, 3, 4 of the heads 100, 200, 300, 400, these latter being embodied in 
accordance with one of the two forms of FIGS. 2 and 3. As the tape 6 is 
unwound in the direction x, each head tracks a track having a width 
.lambda. which will be considered to be slightly equal to h. The portions 
10, 20, 30, 40 respectively tracked by the gaps 1, 2, 3, 4 are therefore 
contiguous, and occupy a width H=4.times.h of the tape 6. The value of H 
is much less than the total width 1 of the track. This is why the 
invention provides for imparting to the stack of heads 5 a transverse 
translational movement, in a direction y perpendicular to x in the plane 
of the tape 6, so that, if the total length of the tape were unwound and 
the tape were placed round a cylinder, four overlapping helical tracks 
would be obtained. For this purpose, the stack of heads 5 is mechanically 
connected to a motor 7, and its translational speed is such that, at the 
end of the unwinding of a complete length of tape, the stack 5 is in a 
position 5a at a distance from the initial position which is greater than 
or equal to H. The tracked portions 10a, 20a, 30a, 40a follow in the 
direction y the portions 10, 20, 30, 40 tracked on the preceding 
revolution. It is desirable to make maximum use of the width of the tape, 
and it is therefore preferable to choose a helical pitch exactly equal to 
H=4.times.h. Under these conditions, the number of parallel portions of 
tracks following one another with a pitch h in the width 1 of the tape is 
(1/h), which is obtained at the end of (1/4 h) times the complete 
unwinding of the tape. V being the speed at which the tape moves in the 
direction x, and v the transverse speed of the stack 5 in the direction y, 
with, by way of example h=50 .mu.m, L=150 m, V=2.5 m/s, each complete 
unwinding of the tape lasts 1 mm, and v=0.2 mm/mn. Signals having a 
pass-band of 2 MHz may be recorded on a tape having a width 1 of 12.6 mm 
along 240 parallel portiions of track by making it run over its whole 
length 60 times if there are 4 heads, and 30 times if there are 8 heads, 
the recording then lasting for half an hour. The product: "duration x 
pass-band" of the device according to the invention is 8 hours .times. MHz 
for the chosen values of L, 1, V. This results in a device of high 
performance with respect to existing magnetic video tape recorders with 
simple, cheap and strong mechanical means: cases containing a loop of 
magnetic tape which winds in endless fashion, a stack of heads 5, a motor 
8 equipped with a roller associated with the cartridge and a motor 7 
providing very slow feed and associated with the stack 5. 
FIG. 5 illustrates a variant of embodiment in which the heads are not 
juxtaposed with one another as in FIG. 2, but are separated by a distance 
h.sub.1, while remaining integral with one another. This arrangement 
enables non-integrated heads to be used while preserving the same 
track-pitch as before, the movement of the stack of heads being the same. 
Only the arrangement of the tracks on the tape is modified, and it may be 
seen in the FIG. that the distance h.sub.1 separating the gaps 1, 2, 3, 4 
is about equal to a quarter of the width 1 of the tape 6. The 
translational speed v of the stack 5 is such that, at the end of a 
revolution, each portion of track 10, 20, 30, 40 is shifted with a value 
at least equal to the head height h. The speed v is therefore 4 times less 
great than before if the same track-pitch h is maintained. 4 helices of a 
pitch h are thus obtained, being independent instead of overlapping one 
another. Recording stops when the last portion of track followed by a gap 
(1 for example) is at a distance h from the first portion of track 
followed by the following gap (that is to say the gap 2). The stack of 
heads 5 will then have moved forwards by a distance h.sub.1, while in the 
foregoing case it had moved forwards 4 times more quickly by a distance 
1=4h.sub.1. Playback and recording must naturally be carried out with the 
same stack of heads in order to respect the arrangement of the tracks. The 
total number of portions of track is identical with the foregoing case, as 
is the duration of the recording. 
A device such as that shown diagrammatically in FIG. 1 and one of FIGS. 4 
and 5 could be used without any other element for recording and then 
playing back information via a magnetic tape. However, because of 
instabilities of various types due either to the mechanical means or to 
the tape oscillating, it proves in practice to be necessary to supplement 
it with servo-control loops, enabling the conditions set out above 
relating to the speeds v and V to be respected on recording, and the 
tracks written in to be satisfactorily followed at the desired speed on 
playback in order to reconstitute the information with as few errors as 
possible. 
Referring to FIG. 6, an electrical signal S characterising the information 
to be recorded is split up into n components (n=4 in the FIG. ) by means 
of a coder 15 which will be detailed hereinafter. This coder makes 
available 4 signals S.sub.1, S.sub.2, S.sub.3, S.sub.4 whereof the 
pass-band is compatible with the possibilities of the device (dimensions 
of recording heads 5 and speed of the tape 6). Each of these signals may 
according to circumstances consist of a carrier frequency-modulated by the 
information or of a numerical signal. The 4 heads which make up the stack 
of heads 5 are respectively supplied by the 4 signals for the recording on 
4 tracks on the tape 6 in the form of modifications to the magnetic 
characteristics of the tape. The motor 8 driving the tape 6 at the speed V 
and the motor 7 translating the stack of heads 5 at the speed v have 
respectively regulators 38 and 37 controlled by a synchronising signals 
R.sub.8 and R.sub.7. The signal R.sub.8 emanates from an oscillator 34 
whereof the frequency f is so chosen as to obtain the desired speed V. The 
signal R.sub.7 emanates from a frequency-divider 36 receiving the signal 
R.sub.8 and having a division ratio so chosen as to obtain the desired 
speed v. The speed v must be very accurate in order to avoid any track 
overlaps. 
FIG. 7 illustrates a device for playing back a magnetic tape 6, recorded by 
the device of FIG. 6. Playback requires much greater accuracy than 
recording. The playback device is provided with a servo-control of the 
radial position of the stack of heads by means of a signal characterising 
the radial tracking error, and a "longitudinal" servo-control by 
monitoring the tape speed with synchronising pulses recorded onto the tape 
itself upon recording. The operation of the device of FIG. 7 implies that 
the information signal S, a television signal for example, comprises 
high-frequency synchronising pulses. The line pulses, which have a 
frequency of 15625 Hz in the European television standard and a frequency 
of 15750 Hz in the U.S. standard, may be used for this purpose, the 
information being coded in the coder 15 in such a manner that these pulses 
may subsist in at least one of the signals S.sub.1 to S.sub.4, S.sub.1 for 
example. Each head of the stack of heads 5 delivers a signal (S.sub.1 to 
S.sub.4) which is a function of the magnetic characteristics of the 
portions of tape tracked by the heads. The 4 signals are processed by a 
decoder 150, which decodes in a manner corresponding to the coding carried 
out by the coder 15, for the purpose of delivering a signal S. A decoder 
22 extracts the line pulses from the signal S.sub.1 amplified by an 
amplifier 18, and a phase-comparator 24 delivers an error signal .DELTA.V 
proportional to the phase-difference found between these line pulses and 
reference pulses delivered by an oscillator 23 at the line-frequency. The 
error signal .DELTA.V enables the oscillator 34 described in reference 
with FIG. 6 to be controlled. This makes the translational speed V of the 
tape on playback equal to the speed on recording. The radial servo-control 
of the stack of heads 5 is based on a wobble process: a low-amplitude 
transverse oscillation is imparted to the stack 5 at a frequency f above 
the frequency-band of the tracking errors which it is desired to correct 
and below the frequency-band of the signals S.sub.1 to S.sub. 4. For this 
purpose, the stack of heads 5 is made integral with the moving coil 171 of 
an electrodynamic motor 17 of the loudspeaker-drive type whereof the base 
172 is mechanically connected to the radial feed motor 7. The moving coil 
171 is moved by a signal .SIGMA. supplying the terminals 173 and 174 of 
the moving coil and resulting from the sum, provided by a summation 
amplifier 25, of a signal .SIGMA. having a frequency f delivered by an 
oscillator 27 and an error signal .DELTA.v characterising the tracking 
error. The periodic excursion and radial correction are thus carried out 
by the same element 17. A group 151 of high-pass filters enables the 
component at the frequency f to be eliminated from the signals S.sub.1 to 
S.sub.4 before they are processed by the decoder 150. The servo-control 
loop comprises: an envelope-detector 19 receiving the signal S.sub.1 after 
it has been amplified by the amplifier 18, the oscillator 27 providing the 
signal .SIGMA. and the signal emanating from the detector 19, a low-pass 
filter 28 eliminating from the product signal the components at 
frequencies greater than or equal to f and supplying the error signal 
.DELTA.v, and the summator 25 referred to above. 
When the tracking made by the magnetic head 100 which provides the signal 
S.sub.1 is satisfactory, the oscillation at the frequency f set up in a 
direction radial to the track produces amplitude modulation of the signal 
S.sub.1 at the frequency 2f. Indeed, the read out signal is maximal at the 
centre of the portion of track, and decreases when this centre is departed 
from. When the mean position of the head 100 departs from this centre by a 
value of .DELTA.h, modulation of the signal S.sub.1 at the frequency f is 
added to the modulation at a frequency of 2f. The modulation level at the 
frequency 2f increases, while the modulation level at the frequency 2f 
decreases, when .DELTA.h increases. Moreover, the phase of the modulation 
(positive or negative) depends on the polarity of the error .DELTA.h. 
After multiplication by the reference signal and filtering of the 
components at frequencies of f, 2f, 3f, the signal .DELTA.v is obtained, 
of suitable amplitude and polarity to control the motor 17 and preserve 
tracking. A low-pass filter 26 extracts the d.c. component from the error 
signal .DELTA.v or from the signal .SIGMA. and controls the motor 7 in 
order to impart regular forward feed to the unit consisting of the motor 
17 and the stack of heads 5. It is clearly understood that the recording 
device illustrated in FIG. 6 and the playback device illustrated in FIG. 7 
are only examples of embodiment which do not limit the invention. The 
latter allows of numerous variants, more particularly of the servo-control 
loops of the playback device. For the recording device, a variant of FIG. 
6 resides in introducing radial servo-control of the position of the stack 
of heads 5 with respect to a portion of track already recorded. A magnetic 
playback head is provided for this purpose: if n heads are required for 
recording, the stack of heads 5 comprises n+1 heads, one of them being a 
playback head (for example, referring to FIG. 4, the head having the gap 
1). The speed v of radial feed is provided so that the playback head will 
follow previously recorded a track so that the portion of track being 
followed at each instant by the playback head will coincide with the 
portion of track followed on the previous revolution of the winding of 
tape by the recording head situated at the other end of the stack 5, that 
is to say, referring to FIG. 4, the head having the gap 4. This result is 
obtained when the displacement of the stack of heads 5 during a complete 
unwind of the tape is equal to n times the track pitch h. The playback 
head delivers a signal corresponding to a previously recorded information 
component. This signal is used in the same manner as the signal S.sub.1 in 
the playback device of FIG. 7 for the purpose of servo-controlling the 
position of the stack of heads 5 with respect to the last recorded track. 
The servo-control loop may be the same as in FIG. 7, but not necessarily 
so. This variant imparts greater regularity to the tracks. Playback is 
facilitated, and the servo-control loops of the playback device may be 
simplified. In particular, the risk of track overlap upsetting playback is 
avoided. 
Without departing from the scope of the invention it is also possible to 
use a moving-coil electrodynamic motor capable of displacing the heads 
over the whole width of the tape. In this case, it is this motor which 
imparts continuous forward movement to the heads, and provides wobbulation 
and correction of departure in tracking errors. 
A variant in the arrangement of the gaps 1 to 4 of the stack of heads 5 is 
illustrated in FIG. 8. According to this variant, the gaps of the playback 
or recording heads, while remaining aligned on the same axis y, have their 
own axes angularly offset with respect to y at an alternately positive and 
negative angle having a value of .alpha.. Thus two adjacent gaps, 1 and 2 
for example, do not form an extension of one another. This arrangement 
makes it possible to reduce crosstalk between the tracks 10, 20, 30, 40 
respectively described by the gaps 1, 2, 3, 4. Should there be any overlap 
between two adjacent tracks, 10 and 20 for example, the gap 1 which is 
reading out an information element written in at a given instant onto the 
track 10, instead of reading out a part of an information element written 
in at the same instant onto the track 20, which would be the case with the 
arrangement of FIG. 4, reads out some of the information written in onto a 
certain length of the track 20, forming several information elements 
written in at different instants. If this length, which depends on the 
angle .alpha., is sufficient, the obtained mean error tends to cancel out. 
In practice, the problem of crosstalk is solved for small values of 
.alpha.: around 15.degree.. It is therefore thoroughly advantageous to 
adopt differentiated azimuth settings for the different heads of the 
stack. The arrangement of gaps illustrated in FIG. 8 makes it possible to 
embody a tracking servo-control loop on playback which is different from 
and more sensitive than that of FIG. 7. An offset .DELTA.h of each head 
with respect to the track which must be followed produces a phase-shift in 
read out the signal with respect to the information as it was recorded. In 
order to evaluate this phase-shift, it is necessary to have available 
synchronising pulses recorded at the same time as the information on at 
least two of the tracks. In the case of recording a television signal, the 
pulses in question are for example the line synchronising pulses referred 
to above. The speed of the tape 6 having been regulated on recording, the 
pulses are simultaneously written in onto each track at regular intervals. 
FIG. 9 illustrates the location of two of these pulses on the tracks 10 
and 20: 110 and 120 on the track 10, 210 and 220 on the track 20. It has 
been assumed in this FIG. that the radial following error is zero when 
these pulses 110 and 210 are read out. The gaps 1 and 2 then read out the 
pulses 110 and 210 at the same time. It will be seen in the FIG. that when 
the pulses 120 and 220 are being read out the the heads are offset with 
respect to the tracks by a value of .DELTA.h. It will then be found that 
the pulse 120 is read out when the gaps are in position 1a and 2a, and the 
pulse 220 is read out, with a temporal phase-shift (after in the case of 
the FIG.), when the gaps are in position 1b and 2b offset with respect to 
the positions 1a and 2a by a length of track .DELTA.x. The pulses present 
in the signals S.sub.1 and S.sub.2 will therefore be phase-shifted. The 
value of the phase-shift increases with .DELTA.h, and its polarity depends 
on the tracking error .DELTA.h. The loop illustrated in FIG. 10 uses the 
results shown hereinbefore. It comprises two decoders 31 and 32 making it 
possible to extract the line synchronising pulses I.sub.1 and I.sub.2 
respectively present in the signals S.sub.1 and S.sub.2. The pulses 
I.sub.1 and I.sub.2 are phase-compared by a comparator 33, which delivers 
a signal proportional to the phase-shift between I.sub.1 and I.sub.2 
which, after amplification by an amplifier 34, constitutes the error 
signal .DELTA.v applied to the same electrodynamic motors 17 as that of 
FIG. 7. The mean value of the signal .DELTA.v obtained by means of the 
low-pass filter 26, as in FIG. 7, controls the motor 7. The diagram of 
FIG. 9 enables it to be established that, V being the speed of the tape 6, 
the time-interval separating the pulses I.sub.1 and I.sub.2 has a value of 
.DELTA.t=(.DELTA.x/V)=2.DELTA.h sin .alpha./V. For x=15.degree. and V=2.5 
m/s, the result is .DELTA.t=0.2 .DELTA.h. For example, for h=1 .mu.m, 
.DELTA.t=0.2 .mu.s, the interval between two successive line synchronising 
pulses being 64 .mu.s. Such an interval can easily be detected, which 
gives good sensitivity of the loop with an angle .alpha. which is small 
enough to have a negligible effect on the recording pass-band and the 
track width, and therefore on the length of recording and the radial speed 
of displacement of the stack of heads 5. 
It is necessary to use a coder 15 (FIG. 6) and a decoder 150 (FIG. 7) in 
order to split up the information signal S to be recorded into n 
components having a pass-band adapted to the possibilities of the magnetic 
heads and to reconstitute this signal from the n signals delivered on 
playback. There are various coding processes, of the analogue or numerical 
type. FIG. 11 illustrates as an example a coder which carries out analogue 
coding for n=4. It comprises 4 field-effect transistors 41, 42, 43, 44 
whereof the sources are connected to ground by way of capacitors C.sub.1, 
C.sub.2, C.sub.3, C.sub.4. Control pulses H.sub.1, H.sub.2, H.sub.3, 
H.sub.4 of very short duration, at the same frequency F and regularly 
offset from one another in time are applied to the grids of the 
transistors 41 to 44 respectively, causing them to pass successively from 
the cut-off state to the conductive state. The information signal S is 
applied to the drains of the 4 transistors. The charge voltages V.sub.1, 
V.sub.2, V.sub.3, V.sub. 4 appearing across the terminals of the 
capacitors C.sub.1, C.sub.2, C.sub.3, C.sub.4 respectively represent 
samples of the signal S during the respective durations of the control 
pulses. These pulses remain constant between two successive memorising 
pulses. The control pulses H.sub.1 to H.sub.4 emanate from a 
shift-register 46 controlled by a clock signal H at a frequency of 4F 
(more generally nF) emanating from a generator 45. The chronograms given 
in FIG. 12 show on an example the mechanism of splitting up a signal S 
into 4 components V.sub.1 to V.sub.4. The pulses H.sub.1 to H.sub.4 occur 
respectively at the instants t.sub.1 to t.sub.4 which are such that 
t.sub.2 -t.sub.1 =t.sub.3 -t.sub.2 =t.sub.4 -t.sub.3 =(1/4F). The signal S 
having a pass-band of B, F is chosen to be of the same order of magnitude 
as (B/4). The signals V.sub.1 to V.sub.4 have a pass-band approximately 
equal to (B/4) (more generally (B/n). They could be supplied directly to 
the 4 magnetic heads of the recording device. In order to improve the 
signal-to-noise ratio, it is preferred to use them in order of 
frequency-modulate a carrier P emanating from an oscillator 55 at a 
frequency Fo higher than F but lower than or equal to the pass-band which 
can be attained with the heads used. This frequency-modulation is carried 
out by 4 similar modulators 51, 52, 53, 54, which deliver the signals 
S.sub.1, S.sub.2, S.sub.3, S.sub.4 respectively intended to be supplied to 
the magnetic heads. By way of example, the recording head according to the 
invention is intended to record video information with a pass-band of 10 
MHz. For this purpose, 8 magnetic heads enabling tracks with a pass-band 
of 2 MHz to be traced are used. Fo=2 MHz and F=1.25 MHz are chosen, so 
that 8F=10 MHz. 
In order to decode the signals S.sub.1, S.sub.2, S.sub.3, S.sub.4 picked up 
across the magnetic heads after demodulation at the instant of playback, 
use may be made of the gates 41, 42, 43, 44 controlled by the same clock 
signals H.sub.1, H.sub.2, H.sub.3, H.sub.4. Each demodulated signal is 
supplied to the input of one of the gates. Since the latter operate one 
after the other, the signal S is obtained by combining the signals picked 
up at their outputs.