Method and apparatus for measuring the electroacoustic properties of magnetic tapes

A method and apparatus for measuring the electroacoustic properties of magnetic tapes by means of recorded test signals of varying frequency and amplitude, wherein measured values of the reproduced signals which depend on the biassing current and correspond to predetermined parameters are obtained; for each of the relevant values of the biassing current the amplitude of the test signal to the recorded is varied until the predetermined value of the reproduced signal is attained, and the measured value of the reproduced signal obtained in this way is registered, an amplitude-modulated test signal is recorded and, by means of a test programming unit and upon the attainment of a parameter value which controls this programming unit, the relevant measured value of the reproduced signal to be determined is transmitted to an intermediate storage device for further evaluation and the biassing current is varied in order to determine the next value to be measured.

This invention concerns a method of measuring the electroacoustic 
properties of magnetic tapes by means of recorded test signals of varying 
frequency and amplitude. It also relates to measuring apparatus for 
carrying out this method. 
For this assessment of the electroacoustic properties of magnetic tapes 
which are to be employed as audio tapes, usually the following tape 
properties are used: 
E.sub.T =sensitivity at long wavelengths 
E.sub.H =sensitivity at short wavelengths 
A.sub.T =maximum output level at long wavelengths 
A.sub.H=maximum output level at short wavelengths 
K.sub.3 =distortion ratio at long wavelengths 
D.sub.3 =difference tone distortion at short wavelengths 
R.sub.GA =dynamic range, related to a reference level. 
Up to now, these properties were ascertained in dependence on the bias of 
the magnetic tape for different parameters, such as distortion ratio and 
saturation magnetization, by the manual adjustment of the test conditions 
resulting in this way. After the individual measurements, mathematical 
calculations are necessary in many cases, in order to obtain the numerical 
value of a parameter, e.g. the distortion ratio, or the numerical value of 
the quantity to be ascertained which is suitable for graphic 
representation, for example in relation to a reference tape. 
For example, in order to ascertain the quantity A.sub.T, the recording 
level is raised until the reproduction voltage of the test signal on the 
magnetic tape appears with a third-harmonic distortion of 5%. For this 
purpose, the ratio of the third harmonic to the entire composite signal 
must be continuously calculated, in order to be able to read the 
reproduction voltage corresponding to 5%. Usually, this voltage is then 
referred to the value of a reference tape. This test program is repeated 
for various values of the biassing current if A.sub.T is to be graphically 
plotted in dependence on the bias. The other quantities are ascertained in 
a similar manner. 
It is readily apparent that this test method is complex and takes a lot of 
time. Moreover, correct test results can only be obtained after a large 
number of measurements and the intermediate calculations associated 
therewith have been made. This applies particularly to the measurements of 
quantities A.sub.T, A.sub.H, K.sub.3 and D.sub.3. 
Accordingly, an object of the present invention is to provide a method and 
an apparatus with which the quantities which are characteristic of the 
electroacoustic properties of a magnetic tape can be ascertained and 
displayed much faster, more reliably and more accurately than hitherto. 
The method and apparatus according to the invention make possible automatic 
measurement of the quantities which are characteristic of the tape 
properties as well as the graphic recording of the test results. The 
required values can now be ascertained much faster, more reliably and more 
accurately than hitherto, even though the apparatus is much simpler to 
operate. 
This means that the new method and apparatus are suitable not only for 
routine tests, e.g. quality control in production, but also for 
controlling the manufacture of magnetic tape with respect to the tape 
parameters.

On a tape recording apparatus, not shown in further detail in the drawings, 
the magnetic tape to be examined is guided past the magnetic heads 1,2 for 
the recording and reproduction of test signals (FIG. 1). The test signal, 
generated by a low-frequency generator 3, is subjected in a low-frequency 
modulator 4 to amplitude modulation, see diagram a in FIG. 2. Via an 
amplifier 5 the modulated signal is taken to the recording head 1 to 
which, for the purpose of biassing the magnetic tape, a high-frequency 
current from a high-frequency generator 6 is supplied. The bias is 
adjusted by a controllable staircase generator 7 by means of which the 
amplitude of the high-frequency voltage can be varied in steps. 
The test signal, scanned from the magnetic tape by means of reproducing 
head 2, is taken to the evaluation circuits 10 for the quantities E.sub.T 
; E.sub.H ; A.sub.T ; A.sub.H ; K.sub.3 ; D.sub.3 ; R.sub.GA (shown in 
FIG. 4 in dependence on the biassing current I.sub.HF) which are connected 
by means of program control unit 11 to digital voltmeters 12 for absolute 
and relative value indication, and to a printer 13 and a coordinate 
plotter 14. Inserted in this connection are a ratio logarithmator 15 and 
an intermediate storage device 16 which consists of a controllable 
scanning unit for the measured values presented by the logarithmator 15 
and of a hold unit. Storage devices of this type are known as "sample and 
hold" devices and are available commercially. The ratio logarithmator 15 
is also a commercial electronic component by means of which the measured 
voltages of the reproduced signal can be converted into decibels (db). 
Such a ratio logarithmator is available as Model 756 from Analog Devices, 
Route 1 Industrial Park; P.O. Box No. 280; Norwood, Mass. 02062. 
The circuit 17 controlling the measuring sequence, which consists of a 
threshold switch and a pulse generator controlled by the latter and 
comprising delay elements, continuously receives from the logarithmator 15 
the values of the test signal only just scanned at the evaluating circuits 
10. The threshold switch which responds to a predetermined value, for 
example a 5% distortion as parameter, which is converted in the 
logarithmator 15 into a voltage value, causes the pulse generator to 
transmit control pulses to the intermediate storage device 16, whereby the 
voltage of the test signal just sampled at this moment in time and 
belonging to a predetermined amplitude of the modulated recording current 
is taken over from the output of the logarithmator 15--which, to this end, 
has meanwhile been switched to the corresponding evaluating circuit--by 
the intermediate storage device. Until the next measured value is 
ascertained, the value being held in the intermediate storage device is 
passed to the indicating device 12 and to the printer 13 and co-ordinate 
plotter 14. The threshold switch of the control circuit 17 also acts upon 
the staircase generator 7 via delay elements, so that the voltage of the 
high-frequency generator is increased for the next measurement, once the 
measuring cycle has terminated. The printer 13 and the co-ordinate plotter 
14 are likewise controlled by control circuit 17 by means of further delay 
elements. 
The program controller 11 is composed of a pulse sequence generator 20, a 
level converter 21, a trigger stage 22, a BCD encoder 23 and a diode 
matrix 24 (FIG. 3). 
The pulse sequence generator 20 consists of a programmable counter and 
exclusive-NOR circuits which switch on the relevant evaluating circuit 10 
via the level converted 21 and the diode matrix 24. To this end, the pulse 
sequence generator 20 receives a counting pulse from control circuit 17 on 
termination of each measurement, and the first incoming counting pulse 
switches on one of the seven evaluating circuits 10 according to the 
program, this evaluating circuit remaining switched on until a 
programmable number of counting pulses has been counted, i.e. the 
measurements have been completed. The next-following counting pulse 
changes over to the next evaluating circuit 10. 
The level converter 21 generates suitable signals for the diode matrix 24 
and the BCD encoder 23 to which the printer 13 is connected, so that when 
the measured values are printed out, the number of the associated 
evaluating circuit is also printed out. Furthermore, the level converter 
21 enables the programmable automatic changeover of the evaluating 
circuits 10 to be replaced by manual operation. A delay unit of the level 
converter ensures that the first test result obtained after a changeover 
will only be printed out when the entire system has attained a stationary 
condition. 
For the purpose of graphic representation of the test results, the level 
converter 21 generates via the trigger stage 22, with every changeover 
between two evaluating circuits 10, a pulse for starting the staircase 
generator 7 which blocks the pulse generator 20, and thus prevents the 
changeover to another evaluating curcuit until the staircase generator 7 
returns to its initial condition after having reached its maximum value. 
The above-described circuits are well known to those skilled in the art and 
have therefore not been illustrated in further detail. 
For a better understanding of the method and apparatus described above in 
general terms, the measuring sequence will now be described with reference 
to the time diagrams illustrated in FIG. 2. 
Let us assume that, when measuring for example quantity A.sub.T, an 
amplitude-modulated test signal (diagram a) having a frequency of 333 Hz 
has, at the instant t.sub.o, the amplitude at which the distortion of the 
scanned test signal attains 5% (diagram b). The control circuit 17 
responding to the attained distortion causes the intermediate storage 
device 16 to be cleared by means of a needle pulse (diagram e) by 
triggering a comparator in the control circuit 17 (diagram c) which at the 
instant t.sub.3 is automatically reset because the distortion has fallen 
below a predetermined value. At the instant t.sub.o, the comparator 
simultaneously starts a delay element which allows a new measurement to be 
made at the instant t.sub.4 after its delay period has lapsed (diagram d). 
Then the test signal voltage corresponding to the distortion value is 
accepted by the intermediate storage device (diagram f) and is printed out 
(diagram g) by a delayed command pulse at the instant t. Progressive 
switching of the biassing voltage (diagram k) is effected by a pulse 
(diagram h) delayed relative to the instant t.sub.o and acting upon the 
staircase generator 7 at the instant t.sub.2 at the end of the print 
command. The next measurement for ascertaining another value can only 
begin at the instant t.sub.4 at the earliest, when the time difference 
between the recording head 1 and the reproducing head 2 is taken into 
account, so that the next measured value can be ascertained when the 
distortion of 5% is reached again at the instant t.sub.5. The interval 
between the mutually corresponding instants t.sub.o -t.sub.5 is equal to 
the time interval between the two magnetic heads 1 and 2 plus the time 
required for the evaluation and the printout of the measured value. The 
modulation frequency is so chosen that, within this time interval, a 
measured value may also be found for the other quantities A.sub.H, 
K.sub.3, D.sub.3, for the ascertainment of which an amplitude-modulated 
test signal is needed. 
K.sub.3 and D.sub.3 are measured in the same way, the size of the parameter 
being a predetermined value of the reproduction voltage of the test 
signal. 
When measuring A.sub.H, the magnetic tape to be examined is magnetized to 
saturation with a signal having a frequency of 10 kHz for example, and, 
when saturation occurs, the value A.sub.H of the reproduction voltage is 
measured and recorded. 
The quantities E.sub.T, E.sub.H and R.sub.GA may also be ascertained with 
the apparatus of the invention using the same measuring sequence, the test 
signal not being modulated in this case. Here, the processing of the 
measured values is not controlled by a test signal parameter to be 
measured at the same time, but by means of a time interval circuit within 
the control circuit. For example, at a tape speed of 4.75 cm/s, 3 seconds 
are needed for every measured value. 
Experiments with the apparatus of the invention have shown that, at a tape 
speed of 4.75 cm/s, a graphic display of a quantity, consisting of 10 
points, can be produced within 30 seconds.