Phase detector for a recorder/player using a conducting loop driven by a winding strand of the head drum motor

In an electronically commutated head drum motor in combination with a motor control circuit, commutation control and tacho signals are generated from the commutation of winding strands. One of the winding strands is used for the phase detection of the read/write heads arranged on the perimeter of the rotating head drum. A commutation signal is derived from the commutation of this winding strand and the commutation signal is set in relation to the reference signal of a signal track to be recorded or scanned.

BACKGROUND 
With commercially available recorders which have read/write heads, 
hereinafter called heads, arranged on the perimeter of a rotating head 
drum (cylinder) for recording and/or reproduction of signals according to 
the helical track method, such as with video recorders, it is known to 
utilize electronically commutable motors for driving the head drum and for 
the drive of a tape-type recording medium wound around the head drum. The 
motor driving the recording medium has a motor shaft formed as a capstan. 
The other motor forms, together with the head drum, one mechanical unit, 
whereby its motor shaft makes a phaselocked link between the motor rotor 
and the head wheel supporting the heads. Both motors are each a part of 
rotational speed and/or phase regulation loops which couple both drives 
together with the aid of a reference signal and produce, according to the 
respective recorder standard, optimum reference position conditions for 
the heads, signals to be recorded and/or signal tracks to be scanned. 
Essential reference signals for a video recorder are the change-of-frame 
impulses to which all further auxiliary synchronizing signals generated in 
the recorder are aligned. 
In the VHS standard it is laid down, for example, that the recording medium 
winds around a head drum with a first and a second read/write head by a 
little more than 180 degrees. Each read/write head which is in contact 
with the recording medium records or scans one field in one track within a 
contact length of 180 degrees whereby the first head is allocated to the 
first fields and the second head to the second fields respectively. Both 
head gaps have on both sides of the middle axis in a mirror image way an 
azimuth angle of approximately 6 degrees. The length of contact of 
approximately 7 degrees which extends beyond the 180 degrees is provided 
for the following lines of the next field so that an overlap zone of two 
neighboring tracks results. The position of this overlap zone is fixed as, 
on average, 6.5.+-.1.5 lines before the appearance of a vertical 
synchronizing impulse. It is also the switching position for the 
switchover of the heads upon playback. 
It is also known that in the recording mode the phase of the head drum 
drive is regulated, while the r.p.m. of the capstan drive is regulated. In 
the playback mode the reverse is true. It is normal to use microprocessors 
for such regulating procedures and for the control of both drives as 
determined by the mode of operation. To detect the phase position for the 
purpose of obtaining command and regulating variables for the phase 
regulation of one or the other drives respectively, at least one position 
indicator sensor is disposed in the vicinity of the head drum. The command 
variable for the phase regulation of both drives is obtained in a known 
way with the help of a reference tape and stored in the recorder so that 
compatibility with recorders of the same system and tapes not recorded on 
the device is created. The command variable can, in this case, also be 
stored in a microprocessor provided for regulating and controling the 
drive, as is described, for example, in German patent P 35 28 452.8. 
Integrated motor circuits (model designation TDA 514x) for the rotational 
speed regulation of electronically commutable motor for driving such head 
drums are known from the TECHNICAL PUBLICATION published by PHILIPS in 
1990. The regulating variable required for regulating the rotational speed 
is derived from the tacho signals which are obtained from the commutation 
through detection and evaluation of the zero crossings of the 
countervoltages induced upon disconnecting the winding strands (phase 
windings). Furthermore, control signals, corresponding to the result of 
the evaluation, for the output amplifier stages working in the switching 
mode of these motor circuits are generated in these motor circuits to 
which the winding strands are connected. However, the accuracy of the 
motor drive is determined by the manufacturing tolerances of the winding 
strands and the rotor. Therefore, for high-precision motor drives in 
recording and playback devices, an additional phase regulation is made use 
of, derived from markings of the rotating parts. 
At the present time, forked photoelectric barriers coupled with the rotor 
of the head drum motor or Hall elements or inductive magnetic sensors are 
usually used as position indicator sensors. Such position indicators are 
relatively expensive and apart from that, their arrangement requires 
manual activities. 
It is therefore the object of the invention to reduce the effort for the 
phase detection of the rotating heads with a circuit arrangement for the 
phase regulation of head drum drive and/or tape drive with such a motor 
circuit. 
The idea behind the invention is to create a relation between the rotating 
heads, signals to be recorded, signal tracks to be scanned and a reference 
signal, such as the change-of-frame impulse in a video recorder, with the 
help of the commutation cycles of the electronically commutable, 
stationary winding strands of the head drum motor, whereby command and 
regulating variables for a phase regulation of head drum drive and/or 
capstan drive can be derived from said reference in order to create the 
above-mentioned reference position conditions, according to the standard, 
for the rotating heads, signals to be recorded or signal tracks to be 
scanned. 
Instead of a separate position indicator sensor, one of the winding strands 
of the head drum motor is, in principle, used for the phase detection of 
the head drum drive. In doing this, a signal, hereinafter called 
commutation signal, is derived from the commutation of the winding strand 
and said signal is set uP in relation to the reference signal in order to 
obtain the command variable for the phase regulation of head drum drive 
and/or capstan drive from this. For this, the difference between the 
current rotor position and a zero phase position is measured which the 
rotor is to assume relative to the reference signal. Thereby, an 
unambiguous phase determination is guaranteed which not only takes into 
account the current rotor position but also the tolerances of the 
mechanical system of the motor. Thereby, the commutation signal detected 
by at least one winding strand can also be used in an advantageous way for 
the phase regulation of precision drives with a motor circuit like the TDA 
514x mentioned. 
The following facts and findings are exploited by the invention. A 
commutation is always carried out only at that place where the rotor, or 
rather its field magnets, exhibits a position which is favorable for the 
torque generation with the winding strands. However, the number of 
commutations per winding strand and rotor revolution, and thereby the 
frequency of occurence of such a commutation signal with reference to one 
rotor revolution and one winding strand, depends on how many poles a 
winding strand has or, respectively, how many individual coils, 
hereinafter called stator coils, are always wired together to form one 
winding strand. This means that certain phase positions of the rotor, and 
thereby several phase positions of the heads phaselocked coupled with the 
rotor, are allocated to the commutation signal derived from a winding 
strand. 
The frequency of the appearance of such a commutation signal per rotor 
revolution leads to the problem of classifying, by means of such a 
commutation signal, the rotating heads, the signals to be recorded or the 
signal tracks to be scanned respectively according to the recorder 
standard. 
The problem is solved by an initialization provided for the recorder which 
is always carried out at the start of each recording and playback 
operation. Doing this, the command variable for the combined operation of 
both drives specified by the recorder standard is determined from the 
commutation and reference signals respectively with the signals to be 
recorded or signal tracks to be scanned respectively. 
With the initialization for the recording mode, according to the invention, 
with a stationary recording medium at first a modulated so-called 
initialization signal is fed to one of the rotating heads during a certain 
head drum swing angle; the signal track of said signal left behind on the 
recording medium will be read subsequently. The length of this signal 
track depends on which of the rotor positions detectable with the 
commutation signal at the start of the recording formed the zero phase 
with respect to the reference signal. The length of the signal track as 
well as its phase-wise position with respect to the reference signal can 
be evaluated by, for example, a microprocessor, in order to generate both 
the standard head/signal allocation through a corresponding phase 
regulation of the head drum drive, and also to determine the command 
variable for the combined operation of both drives with the signals to be 
recorded. In order to avoid that, upon scanning, a track remnant, for 
example, from another recording, which follows coincidentally the 
initialization signal track leads to an incorrect interpretation, the 
reading duration is limited in time. 
Because the initialization signal feeding and reading duratio can be 
matched to the angle of tape contact on the head drum, the invention can 
be applied to differing tape contact angles. 
In the recording mode, the difference between the current rotor position 
and the zero phase is always established just once at the start and can 
then be used as a constant command variable throughout the entire duration 
of recording. 
For the initialization for the playback mode of the recorder, the existing 
recording is used because the head/signal allocation and the command 
variable can be obtained from its signal tracks. Determining the 
head/signal track allocation and determining the command variable are 
carried out here by detecting a complete signal track using one of the 
rotating heads. For recognizing, or rather detecting, a complete signal 
track, the tape is driven at a speed deviating from the normal playback 
speed during the initialization. 
A rotor position with which the command variable is determined can also be 
determined with the aid of a simple conducting loop which can be a part of 
a feed line to the head drum motor, is disposed outside the area of the 
tape/head contact and, together with one of the rotating heads, forms a 
position indicator sensor. After detecting the rotor position and storing 
of the zero phase allocation of a corresponding commutation signal to the 
reference signal, the conducting loop can be switched off or 
short-circuited respectively.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 shows, schematically, a rotating head drum 1 with two opposing 
read/write heads K1, K2 for the scanning or recording of signals in 
lawnless helical tracks of a tape-type recording medium 2, hereinafter 
called tape, which is wound around the head drum 1 by somewhat more than 
180 degrees. The drive for head drum 1, or rather the head wheel carrying 
heads K1, K2, is performed here by a motor 3 which forms a mechanical unit 
with the head drum and preferably consists of a 12-pole stator and an 
8-pole rotor 31. The direction of rotation of the head wheel coupled with 
the rotor 31 is indicated by the arrow. 
The rotor 31 is formed essentially by eight field magnets which rotate 
around 12 stator coils X1 through X4, Y1 through Y4, Z1 through Z4 
arranged in a ring, whereby always one group of four stator coils X1 
through X4, Y1 through Y4, Z1 through Z4 forms an electronically 
commutable winding strand X, Y, Z (FIGS. 3 and 5). The winding strands X, 
Y, Z are preferably wired in a star connection. The stator coils X1 
through X4, Y1 through Y4, Z1 through Z2, and the magnetic poles north N 
and south S of the rotor are all arranged evenly distributed. However, the 
respective locational accuracy is determined by manufacturing tolerances. 
The rotor position in the playback mode at which the head change-over 
mentioned takes place is indicated by the head K1 making contact with the 
tape loop. 
For obtaining command and regulating variables for the phase regulation of 
head drum and/or tape drives as well as a regulating variable for the 
creation of the standard head/signal or, respectively, head/signal track 
allocation with the aforementioned initialization of the circuit 
arrangement for the regulation of head drum and tape drives, one of the 
winding strands is used, according to the invention, for example, winding 
strand X as shown in FIG. 3 and FIG. 5 with the head drum motor 3. A 
pulse-shaped commutation signal `I` is used as position indicator signal 
which appears at certain rotor positions as can be seen in FIG. 2 from the 
voltage progression at the winding strand X for one rotor revolution. The 
commutation sequence of winding strand X for one revolution of the rotor 
is illustrated in FIG. 2. The voltage progression is predetermined through 
the combination of motor 3 with a motor control circuit 31 of the type of 
motor circuit TDA 514x already mentioned. The respective switch-on 
(connected) periods T1 and switch-off (disconnected) periods T2 for 
winding strand X can be seen. The shape of the repetitive commutation 
signal `I`, alternating with respect to the reference line with positive 
and negative polarity, is caused by reverse current diodes integrated 
within the motor circuit 31 in that these diodes are switched through 
respectively by the reverse-electromotive force which is generated with 
the commutation-dependent disconnections of winding strand X by induction 
in the winding strand X. Each of the winding strands X, Y, Z is allocated 
two such reverse current diodes. For determining the rotor position for 
the standard head/signal or head/signal track allocation respectively, 
preferably only the commutation signals `I` of one polarity are used. Such 
commutation signals `I` are generated in each of the winding strands X, Y, 
Z. 
The manipulated variable for the respective drive regulation of head drum 1 
is determined using the value of the voltage +U or -U respectively, which 
is applied (added) to the winding strands X, Y, Z during their connected 
(switch-on) periods T1. 
FIGS. 3 and 5 show, respectively, the principle of a circuit arrangement 
for regulating head drum and tape drives, whereby in FIG. 5, a second 
embodiment type of the invention is illustrated and the arrangement 
according to FIG. 1 as well as the diagram according to FIG. 2 are the 
basis of the further description. 
FIG. 3 shows a block circuit diagram of a circuit arrangement for 
regulating of head drum and tape drive with the winding strand X used for 
the phase detection of heads K1, K2. The circuit arrangement essentially 
comprises the following electronic circuits: a motor control circuit 41 
for a capstan motor 4 for driving the tape 2, a motor circuit of the type 
TDA 514x used as a motor control circuit 32 for the head drum motor 3, an 
impulse detector 5 connected with winding strand X and used for detecting 
the commutation signals `I` of winding strand X, a detector circuit 6 
which can be connected to head K1 for detecting the initialization signal, 
and a microprocessor uP as well as a switching unit 7 containing a head 
change-over switch 72, which can be controled from microprocessor uP via 
flipflop 73, and also a switch for signal path switchings dependent on 
operating mode and initialization and controled by microprocessor uP via a 
multiwire line 71. A line 9 indicates connections of switching unit 7 with 
the recorder circuits, not illustrated, for processing the recording and 
playback signals. The microprocessor uP is connected via a databus 10 to 
the recorder's operating controls which are not illustrated. The 
microprocessor uP receives instructions via databus 10 and indicates their 
execution like, for example, that the initialization of the circuit 
arrangement has been completed. 
The circuit arrangement is described in the following by means of the 
initializations for the recording and playback modes. The various 
regulation and control procedures described hereby for the head drum and 
tape drives are carried out by microprocessor uP through appropriate 
control of the motor control circuits 32, 41 via lines 33, 42. 
Recording mode 
With a stationary tape 2 and a head drum motor 3 adjusted to a desired 
rotational speed, microprocessor uP allocates a zero phase to a reference 
signal 20 (FIG. 4) of the video signal to be recorded to a random 
commutation signal `I` with positive polarity, detected by impulse 
detector 5 and fed from the output of impulse detector 5 to microprocessor 
uP. The microprocessor uP derives the regulating variable for regulating 
the rotational speed of motor 3 from tacho signals which are fed to said 
microprocessor via a connection 34 and obtained using motor control 
circuit 32 in the manner mentioned above. The change-of-frame impulse of 
the video signal is used as reference signal 20 which is fed to 
microprocessor uP via a line 8. The zero phase is a command variable and 
exists as a time and/or phase difference, predetermined by the arrangement 
conditions, between the leading edge of the commutation signal `I` and the 
leading edge of reference signal 20. The command variable is stored in the 
microprocessor uP. 
Subsequently, an initialization signal 21 (FIG. 4) is only fed to head K1 
via switching unit 7 for the duration of one-half of a head drum 
revolution. The video signal, or rather its frequency-converted recording 
signal is used as initialization signal 21. After a further 
half-revolution of the head drum, at the start of which feeding of the 
initialization signal is interrupted by means of switching unit 7 and at 
the end of which the same head is connected to the input of detector 
circuit 6 by means of switching unit 7, the signal track recorded with the 
initialization signal 21 is scanned using the same head K1. The signal 22 
(FIG. 4) detected by the detector circuit 6 when doing this is fed to the 
microprocessor uP for evaluation. The duration of the connection between 
detector circuit 6 and head K1 is in this case also one-half of a head 
drum revolution, or rather rotor revolution, whereby the connection is 
also interrupted by means of switching unit 7. 
The length of the detected signal 22, or rather the length of the signal 
track, and its phase-wise position relative to the reference signal 20 are 
a measure of at which rotor position the microprocessor uP had allocated 
the zero phase to the reference signal 20. 
The relation between rotor position, length and position of signal 22 is 
illustrated by means of diagrams `A` through `D` in FIG. 4. Diagram `A` 
shows the zero phase allocation of such a commutation signal `I` to the 
reference signal 20 at the correct rotor position. Diagram `B` shows the 
zero phase allocation at a rotor position of 90 degrees advanced. Diagram 
`C` shows the zero phase allocation at a rotor position of 90 degrees 
retarded. Diagram `D` shows the zero phase allocation at a rotor position 
of 180 degrees with which signal 21 cannot be recorded and, consequently, 
cannot be scanned. The scale E corresponds to one-and-a-half revolutions 
of the rotor. 
The microprocessor uP evaluates the length of the detected signal 22 and 
its phase-wise position relative to the reference signal 20 and, with an 
evaluation result corresponding to one of the diagrams B, C or D, 
allocates the zero phase relative to the reference signal 20 to a 
corresponding commutation signal `I`. In doing this the correct rotor 
position for the recording mode is created by a phase regulation of the 
head drum drive. At the same time, the tape drive is switched on and, 
through appropriate triggering of switching unit 7, the signal paths in 
switching unit 7 for normal recording operation are created and the 
recording begins. Thereby, the tape 2 is driven at a constant speed. The 
regulating variable for the tape drive is obtained by the microprocessor 
uP from tacho signals of the capstan motor 4. The tacho signals are fed to 
the microprocessor uP via a line 43. 
Playback mode 
With the initialization for the playback mode, the prerecorded tape 2 is 
driven at a speed which deviates from the normal speed for the playback 
mode, for example, 0.8 to 0.9 times the normal speed, so that in the most 
unfavorable situation the head K1 can pick up completely a signal track 
allocated to it during a few revolutions of the rotor, something that is 
recognized by the microprocessor uP with the help of detector circuit 6. 
Hereby, the input of detector circuit 6 is permanently connected via 
switching unit 7 to head K1. As the signal track can only be fully 
detected at a corresponding head/signal track allocation, and hence only 
at a correct rotor position, as soon as the signal track has been fully 
detected the zero phase relative to the reference signal 20 of the 
prerecorded tape 2 is allocated to the appropriate commutation signal `I`, 
taking into account the deviating tape drive speed, and switchover to play 
mode occurs. Switchover sequence and switchover time for the half-wise 
switchover of heads K1, K2 are also determined using this commutation 
signal `I` and the signal paths in switching unit 7 are created for the 
playback mode through corresponding triggering of switching unit 7. The 
switching signals for head change-over switch 72 required here for the 
playback mode are generated with the help of the flipflop 73 controled by 
microprocessor uP. The allocation of said switching signals to this 
commutation signal `I` is determined by delay times stored in the 
microprocessor uP resulting with alignment of the recorder, for example, 
with the aid of a tape 2 which contains signals recorded according to the 
standard. The flipflop 73 can be integrated into the microprocessor uP. 
The envelope detector already present in the recorder for special operating 
modes like, for example, still picture, is utilized as a detector circuit 
6. 
The impulse detector 5 can be integrated into the motor control circuit 32. 
The head 1 and the winding strand X making available the respective 
commutation signal (`I`) therefore form a position indicator sensor with 
which corresponding phase positions of the heads K1, K2 can be detected. 
As already mentioned, the invention is in no way limited to recorders in 
which the tape contact angle of the recording medium is 180 degrees. With 
recorders with other contact angles, for example, 270 degrees, the 
procedure may be executed accordingly. 
FIG. 5 shows a circuit arrangement according to FIG. 3 with a conducting 
loop 11 which is disposed in a fixed position outside the tape/head 
contact region on the stationary part of the rotating head drum 1 and 
which can be preferably wired in series with one of the winding strands X, 
Y, Z with a switch 12 controled by microprocessor uP. It represents a 
second embodiment type of the invention. 
The determination of the rotor position, with which the command variable is 
determined, in that the leading edge of the appropriate commutation signal 
`I` is allocated to the reference signal, is carried out in this circuit 
arrangement using a magnetic field which is generated with the conducting 
loop 11 and can be detected at a corresponding rotor position with, for 
example, rotating head K1. For this the conducting loop 11 is switched on 
so that the current of the, for example, winding strand Z provided for 
this, can flow through said loop and can, thereby, generate the magnetic 
field which induces a voltage in head K1 which is then fed to the 
microprocessor uP for the determination of the rotor position. The feeding 
is preferably performed here via a threshold value detector 61 which is 
connected to head K1 via a switch 13 controled by microprocessor uP. After 
detection of the rotor position and storing of the zero pase allocation of 
a corresponding commutation signal `I` relative to the reference signal 
20, the connection between the input of threshold value detector 61 and 
head K1 can be broken and the current in the conducting loop 11 switched 
off. The threshold value detector 61 can be, for example, an operational 
amplifier functioning as a comparator. 
Therefore, in this circuit arrangement the conductor loop 11 and head 1 
form a position indicator sensor with which the phase position of the 
heads K1, K2 can be detected. 
The switching-on and switching-off of the current in the conductor loop 11, 
and the connection of threshold value detector 61 with head K1 can each be 
carried out depending on the motor current, for example, only in the 
revving-up phase of the motor 3. The detection of the rotor position also 
happens during this time. 
The switches 12, 13 and the threshold value detector 61 can be integrated 
into the motor control circuit 32. The conductor loop 11 can be designed 
as a part of the motor feed line and stuck (glued) to the outside of the 
stationary part of head drum 1. 
The conductor loop 11 can also be designed as a part of the neutral 
conductor which can be switched on and off and which connects the star 
point of winding strands X, Y, Z with the motor control circuit 32. 
The invention is especially suitable for video recorders and camcorders. 
The invention is not limited to motors, the winding strands of which form a 
Y (star) connection.