Brüel & Kjær 7001 User guide

Tape Recorder Type 7001
The Tape Recorder has been designed as a two-
channel precision laboratory recorder with particular
emphasis on frequency transformation . It utilizes the
frequency modulation (FM) techniques and the fre-
quency response is DC to 20 , 000 Hz. Four tape speeds
are available with frequency transformation ratios of
4, 10 and 40. A direct recording channel is included
for tape identification and data reclaim . To allow for
sequential analysis of nonstationary signals a variable
length loop adaptor is suppiied with the instrument.
The instrument itself is of a compact solid-state
design with plug-in card construction. Both recording
and reproduce heads are of the long life ferrite type.
The control panel is removable for remote operation .
BRUEL& K.'-'AC.A
N~RUM, DENMARK. Teleph.: 800500. Cable: BRUKJA, Telex: 5316 . COPENHAGEN
B B 7001 . 0300059

Tape Recorder
Type 7001
NOVEMBER 1967

Contents Page
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
General Introduction ......................................... :. . . . 3
The Principles of Magnetic Tape Recording . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Description ..................................................... ·.· 7
General . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Description of the Measurement Channels
.......................... 12
Description of the Voice Channel ................ . ................. 18
Description of the Marking Arrangement . . . . . . . . . . . . . . . . . . . . . . . . . . . .
20
Description of the Power Supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Description of the Tape Transport System . . . . . . . . . . . . . . . . . . . . . . . . . . 22
3. Control Knobs, Terminals etc. ...................................... 29
4. Tape Loading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
Reel Mounting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
Tape Threading .................................................. 36
Mounting of the Loop Adaptor UD 0011 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
Tape Threading of the Loop Adaptor ..................... . ........ 38
5. Operation .......... ... .......................................... 41
Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
Procedure ....................................................... 42
Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
Monitoring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
Voice Track . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
Remote Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
Frequency Analysis on Uncalibrated Paper . . . . . . . . . . . . . . . . . . . . . . . . . . 46
Frequency Analysis on Precalibrated Paper . . . . . . . . . . . . . . . . . . . . . . . . . 47
Record and Playback Times . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
6. Combined Units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
7. Applications ..................................................... 51
Measurement and Analysis of Acoustic Noise . . . . . . . . . . . . . . . . . . . . . . 51
Analysis of Low Frequency Vibrations .............................. 53
Frequency Analysis of Dynamic Strain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
Graphic Recording of the Vibration Waveform . , ....... .. ........... 57
Room Acoustic Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
Sound Insulation Measurements ................................... 59
8. Appendices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
Appendix A ........................... ... ........................ 60
Frequency Modulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
Appendix B .................. . ............... .. ............ ...... 62
Maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
Appendix C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
DC Balance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
9. Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

1. Introduction
General Introduction
Magnetic tape recording has proved to be of immense value in modern society.
From its early beginning in 1898, when the danish
scientist and inventor
Valdemar Poulsen demonstrated his
first "telegraphone", and up to modern
instrumentation tape devices a tremendous amount of scientific effort and
ingeniuity has been laid down in modifying and improving the magnetic
recording and
reproducing technique.
Even though there are many
limiting factors in modern tape recording the
ability of such systems to store information for later analysis, to expand and
compress time scales, and, by multichannel recording technique,
to preserve
time
coincidence between events has made the magnetic tape recorder a
key instrument in today's instrumentation systems.
The continuous improvement
of magnetic recording technique over the last
decades has resulted in the development of various recording principles, such
as
direct recording, frequency modulation, pulse coding, pulse width modula-
tion, amplitude modulation etc.
All of these types of recording techniques have their advantages and dis-
advantages,
but the most widespread recording principles used for general
purposes and analog measurements today seem
to be the direct recording
(with
high frequency bias) and the frequency modulation techniques.
If the recorded (analog) data are to be stored for later ordinary spectrum
analysis
of single samples the direct recording technique is the simplest
and most economical type of data preservation.
On the other hand, if very high amplitude stability is required, or if the stored
data are very low frequency vibrations or contain necessary DC (static) in-
formation the frequency modulation (FM) technique is
far superior to direct
record:ng.
Due
to this superiority of FM recording in analog measurement systems the
Tape Recorder Type
7001 utilizes the FM technique and great care has been
taken in the development
of the Recorder to achieve optimum performance
and reliability of its various components.
The Principles of FM Magnetic Tape Recording
In most practical FM *) magnetic tape recording systems the input signal
frequency modulates a carrier frequency oscillator of frequency fa to a
*) For a brief explanation of the concept of frequency modulation the reader is referred
to Appendix A.
3

maximum frequency deviation, . M, (frequency "swing") of ± 40 % of the carrier.
Furthermore,
to obtain a reasonably large dynamic range, even at the highest
modulating frequency f m••· this is normally chosen to be approximately 1/4 of
the total frequency deviation (i.e. 20 % of the carrier). All the sidebands
necessary for a faithful reproduction of the input signal will then be recorded
(see also Appendix A) .
0
l
E" -
<!
dB
- 10 v
20
v
v
-30 /
- r- -
~ -
1-
-- 1----
_,0
W+-
I :
~ :
j I
I -2l>f "•2Af r\\
1-- --- -
r- -
fo =Car rie r
Frequency
.o. f= Ma x. Freq.
Deviat io n
f, m ox= Mo x. M od.
Frequency
~- r-
Am plitud e
Lim i ting
Level
v
0.01 0. 02 0. 05 0.1 0.2
0. 5
10
R el at iv e Frequency------..
Fig. 1.1. Typical frequency response characteristic of the reproduce head in a
magnetic tape recorder.
With constant frequency independent magnetization of the tape and constant
tape speed the output voltage from the head will , at low frequencies, increase
with frequency because e o"' "' dd~ . At higher frequencies where the wave-
length
of the signal present on the tape becomes of the order of magnitude
of the head gap width averaging effects take place which cause the output
signal e o"' to decrease with increasing frequency. When the signal wavelength
equals the
effective gab width of the head e out should theoretiCally become
zero. At still higher frequencies the averaging effects become predominant
and th1s part of the frequency response curve is not normally used in magnetic
tape systems.
The actual
carrier frequency chosen depends basically upon the tape speed
and the
characteristics of the magnetic head, see Fig. 1.1. In the Tape Recorder
Type 7001 the highest carrier frequency used is 108 kHz and consequently
the highest input signal frequency component that can be recorded with full
dynamic range is approximately 20 kHz.
A maximum
tape speed of 1.524 m/sec (60 inches per second) is used.
The
choice of carrier frequency , tape speed and input signal frequency range
might be regarded as the basic factors in the design of an FM magnetic tape
recorder . From these factors , and a careful development of the tape transport
mechanism as well as the circuitry used in the record and reproduce elec-
tronics , then follows the optimum achieveable dynamic range. The upper limit
of this range is set by the so- called deviation ratio*) and the phase non-
* )Deviation Rotation = Maximum Frequency Deviation/ Modulating Frequency, see also
Appendix A.
4

linearity in the circuitry while the lower limit is normally determined by the
wow and flutter of the tape transport as well as spurious (random} noises
inherent in the recorder.
As can be seen from Fig.
1.1 the characteristics of the reproducing process
cause the
amplitude to vary considerably with frequency. This variation, which
in direct recording / reproducing systems is of great importance , is in the case
of frequency modulation systems, of practically no importance at all because
the amplitude is limited (clipped} anyway before detection of the modulating
signal takes
place (Fig. 1. 1} , and the magnetic tape can be magnetically
saturated.
Also
the amplitude vs. frequency nonlinearity produced by the tape itself due
to demagnetizing effects of neighbouring magnetic areas is unimportant in
FM systems for the same reason. As stated above, however, phase non-
linearities are very important and great care must be taken to minimize
and/
or compensate for their existence .
The lower limit of the dynamic range is, in single track recording systems.
normally determined by wow , flutter and spurious noises.
t
1.0
0.9 f--- ---t----+----+----+----f-----t-- -1
o.a I--- --t- 'Now and Flutte r -+-----+----t----t--- ---4
;II.
0.7 ~-- -+---t-------+- - --t----+-- ----1
~ 06 "White Noise::......,..../
~ 0.5 f-- --t---
c; 0.4 f--- --+---+~-----t----t _,.,. / "---- ./ --t----+- ---i
~ 0.3 r--- Me as ured at 1.5 ips +- -- -1----c. ~,/ =---+ f-' ----1----+--- 1
0.2 Recording · '---. ~---
--
0.1 r- -
10 20 50
100 200 iOO 1000
High Cut o ff Frequency f(H>) - _ ._
/66!? 0 !2-
2000
Fig. 1. 2. Typical cumulative noise and flutter characteristic measured at low
tape speeds and indicating the dependency of the noise level upon reproduce
bandwidth .
In multi-track recording systems, on the other hand, crosstalk between
channels, and a
further effect commonly called skew or yaw also enters the
picture.
Wow and flutter are affected by many factors: Possible small excentricities
in the capstan or pinch rollers, tension variations in the tape, variations in
power supply frequency and phase, mechanical vibrations in the recorder, and
finally friction effects and tape roughness.
Some of these factors cause periodic flutter components while others are of
a more random nature. If the flutter was of a completely random nature and
no resonance
effects were present in the tape system a more or less white
5

inherent noise would be expected. The output voltage caused by such a
noise would increase with the square root
of the measurement bandwidth as
indicated by the dashed line in Fig. 1.2.
In the figure the frequency scale
indicates the measurement bandwidth.
Actually at low frequencies periodic
flutter components and tape system resonances cause the noise voltage, and
thus
the lower limit of the dynamic range, to vary with measurement band-
width as the curve
drawn in full, indicating that the noise is here not com-
pletely "white".
The curve shown has been measured on a prototype Tape Recorder Type
7001 with a tape speed of 3. 81 em/sec (1 .5 ips). Similar curves for other tape
speeds are given in Fig.
2.4.
Crosstalk between channels is mainly determined by the shielding between
the sections
of the magnetic heads belonging to different channels and the
separation distance. As the crosstalk process is basically
different for FM and
direct recording systems the two FM channels in the Tape Recorder Type 7001
have been separated on the tape by a direct recording track . (The direct re-
cording channel is, however, only meant as a voice channel for marking and
identification of special parts of the tape when desired, and is not intended
to be used
for measurement purposes.) This together with careful shielding
of
the heads ensures the crosstalk between the two FM channels to be
negligible (below the overall inherent noise level) even when one channel is
modulated by a DC
producing maximum frequency deviation.
Skew (or yaw) is a measure of the phase (or time) errors between the
channels caused
by the skewing or yawing of the tape as it passes over the
heads. This effect is smallest when the tape is recorded and played back
on the same Recorder and can normally be ignored. When different Type 7001
Recorders are used for recording and play back the timing error between the
channels due to
skew may be roughly estimated to be of the order of one
tenth of a step pulse rise time, see also "Specifications".
While wow , flutter and crosstalk cause errors and set the lower limit of the
dynamic range for one data channel, skew introduces relative errors between
two channels and will thus only be of any importance when the channels are
used
to determine exact phase relationships between two recorded signals.
6

2. Description
General
The Tape
Recorder Type 7001 is designed basically as a two-channel laboratory
recorder for frequency transformation purposes. The recording principles
utilized in the two measurement channels is based on frequency modulation
(FM) technique while an extra voice channel, which is included for marking
and tape
ident ification purposes, employs ordinary direct recording.
A loop adaptor allows detailed analysis of special parts of a recording and
four different speeds make it possible either to bring very low frequency
signals up into the analysis range of modern analog frequency analyzers,
I
I
I
I
I\\lice
,channel
I
Play Bee
Head L-------l
'----~- - --~p~if_ier_ -- - - -- - __ ___ ---- ___ ----~'
:1 662 06
Fig. 2.1 . Block diagram of the signal converter and amplifier sections of the
Tape
Recorder .
7

or to bring more high frequency signals down to the range of direct graphic
pen recording. Typical fields of application are shock, vibration and noise
measurements as well as room acoustic model experiments. Also the recording,
storing and analysis
of two time-interdependent phenomena are possible due
to the two measurement channels.
To avoid overloading, both measurement channels are equipped with peak
responding meters
for signal level control, and special overload indicators
are triggered if the maximum recording level has been exceeded, see block
diagram, Fig. 2.1. In the block diagram only one FM-channel is drawn up
together with the voice channel. The second FM-channel, however, is identical
to
the one shown.
Fig. 2.2 shows the frequency response
of a single FM-channel while Fig. 2.3
indicates typical differences in the phase response*)
of the two channels
contained in one Tape Recorder Type 7001. Finally, Fig. 2.4 shows typical
inherent noise and
flutter curves measured as a function of the reproduce
bandwidth. The wide band dynamic range is of the order of 48 dB. If the
recorded signal is to be frequency analysed the dynamic range will, to a
certain extent, depend upon the analysis bandwidth. Measurements indicate
l •~l~• • •0
0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
};'p,:R:.nrdu • •
111"'700 ~ • •
--- ··
:: J:LM_
.... , .B.M. s____
~~~"'
,..__5Q_d8-
.... '"' _20Q _ mm._
'- '"' JD..mm/sec-
;;;~; ~ ' -O · 1o
10000
Fig. 2.2. Typical frequency characteristics of the Tape Recorder Type 7001
valid for various tape speeds.
*) The lines shown in Fig. 2.3 represent limit cases for the phase difference to be expected
between the two measurement channels . To measure the actual phase difference the
inputs to the two channels should be connected in parallel and the frequency of the
input signal swept from 0 Hz (DC) to the upper limiting frequency corresponding to the
tape speed used. The phase difference between the two outputs should then be measured,
for instance by means of a double beam oscilloscope , and stay within the two lines in
Fig.
2.3 (0 ° phase difference at 0 Hz and ± 24 ° phase difference at the upper limiting
frequency. 24 ° phase " error " corresponds to approximately 1/ 10 of the pulse rise time).
The phase
difference due to wow and flutter will be of the order of 10 °. By manupulating
with the mechanical position of the reproduce head it is possible to reduce the phase
" error" to the above mentioned 10 ° and to ensure that the " error " be symmetrical
around zero at the frequency of the signal used for adjustment , see also Appendix B.
8

..
30
25
20
15
10
5
-5
-10
-15
-20
-25
-30°
f/f.
f. =Upper limiting
frequency
¢ = Phase difference
in · degrees
/ 67:?o9
Fig. 2.3. Limit lines tor phase differences between the two measurement
channels
contained in one Tape Recorder.
that the dynamic range for one third octave frequency analysis will be as
shown in Fig. 2.5a. By still
further reducing the analysis bandwidth a dynamic
range as given in Fig. 2.5b may be obtained. The analysis was here
of the
constant bandwidth type, the - 3 dB bandwidth used being 8 Hz.
To
allow for convenient operation of the tape transport controls, Record··
Playback-Stop-Rewind-Fast-Forward and Speed Changes they are all located
'¥ 1.0
~ 0.9
J: .
~ 0.8
. I I . I
- Typ1col curves of cumulative flutter
_Recorded and played bock at speeds indicated
/
'\s\\Js...,........v
---r /
~ 0.7
:u 0.6
~ 0.5
g! 0.4
~ 0.3
~ 0.2
u
,~
-6
ipS ----=:::::
:::.---
~
0.
10 20 50
100
200 500
1000
15.!£.5
1----
60~
2000 f(Hz) 5000 10000
High cut off Frequency
t66!ZO/
Fig. 2.4. Typical cumulative noise and flutter curves measured on the Tape
Recorder Type 7001 at various tape speeds.
9

a.
c.
-.-
!
•• ~ . ~:. :[:~ 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0'
~ ~
~ure m er(
00
l" ~ 1~- ~ ; :~
.. ~~~ + •0 aD 0 ~~.: 0 0 0 0 0 0 0 0 0 0 0 ~~: 0 0 0 0 D 0 0 D 0 D 0 0 0 ~~: D D D 0 0 0 =-
-- -- ,.,. : ..
~
l!ll!!!L_ ••
Anal)'sis
,_p _ emL _
I JI!S ... ~ , •
I - • .
l=R6__
.liz__.'
mm_
1 000
""""~clo-Aitl,oo.
~ .. _ -- - .....
1 00
10000 ~ ... Hil l A I C /i.s/ 98
~~ -
§_j_p_j_
.__ .... ..~10
-~
.._ _H______
~JlMS____
~:.:.20JiL_ ool-!
~ 5Q.d£L_____
~ ~2QQmm__
-~.J.ilJnln/SK.
_.__., · +---···.
OP1123 10
I
1 00
1000
Fig. 2.5a. Spectrograms showing the results of one third octave frequency
analysis
of the inherent noise and flutter in the Tape Recorder. The level is
here measured in terms
of RMS values, re. maximum input signal (1 % distor-
tion level) and level lines
of - 60 dB are indicated.
a) Tape speed 1.5 ips. c) Tape speed 15 ips.
b) Tape speed 6 ips. d) Tape speed
60 ips.
· -• r=- -' u'
10000 ( •• ;" .. :o!'"': ~p;
b.
d.
0

......
......
a.
c.
I
a.u.t&X i c.• • ~Obtect" -- a. ~
OOOOOO DDO DDODODOODOODDOOOOOO OD OOOOOO OOOOO ODOODOOOOO
~
0:, IPS:-.._ . ~ · ~~ """ •- - '-'-' •-
1--i.LL..:.J=:-
Bondw<lth ~8H z . _ _ _ ~ j 1-:::::::: i J=tt+!
. - IMJIIW'· . I I II' I
.: T --J= J = ~~ - ~lf =:= .lf ~ .· = Reflml 1mV"{ . 60d8</n
:~ Ji' =FF r=;=-;: :_: ,,,
~ ~;: - :''
;; ,,
i-i -ii E
'J-=:--
~ 1
J_,, I
l ~=
~- ±·~: ~ I
I td " ~ +- . .= ~ ~ ~ - ~ .- - - -i==i==rr.=r. ==rl l
t=
0:
'
~
Ma- _ .. t..ailiw"
Ho ......... Sp.od
~ ~ - ~~ ---- Brii•l& ~
l i!'r - f ~~" ""=".:.: =""" ""
~ ~ ~8Hz . ,06i; , ~,-+ H
r Et -=J~:,
Ref . level 1mV"" 60d8 s/n
f j
-1 ~·1 ·· -
-F-: i' .. ==
i··i. -t:::
- l lUlt
~
-
-==
-~ =
......... .,..,..,._,.7 ...- I
C8 :'a & ~~: a o a a a ~~ o o a a a a a a o o a o c a o a o o o a a a a a a o :·~•' ~ ~
01
~ a o aJ
.. lt.coool~<>eN& : __ ..,. , ___ _ ___ ·- ·-- '-""' ---
' I
J:=1 tPs~s -~·
§Bandwidth= 8Hz
...J=tE!:l:l.-t~·:HJ..
=f Ref. level1mV ...... 60dB!Oin
t=.l :~____.t:::._ l ~ . I trf---l-1==1 I ,__ . ·_·. I
, c;-:-~
±±:~ b
l=::::l::-:t::= ·' ==R=t"1=t==== •
l I
.,., -- .- RittE£ 2 ~ ~
I ''"'
'' im• "-~"- -
0 a a o a o a a a a a a a a a o o a a a a o o a a o a a o o o o_o o o a a
.. lt.c""""'o ... .: __ Si,ft.: Oooo ___ ,_
8r ii ei &I( JOif
I 0
!Ia I I i!j t f i f f ,j. l lf I I I
. -
t--
I ~
n.' ! ITfltr-!
d.
i l ~
-E ::: H_
l-
1-
mr:J,
-H
.r--=
1 :::-i
Fig. 2.5b. Similar to Fig. 2.5a. However the measurement bandwidth was in
this case 8 Hz.
a) Tape
speed 1.5 ips. c) Tape
speed 15 ips.
b) Tape
speed 6 ips. d) Tape
speed 60 ips.

in a small control box which can either be inserted into the front of the in-
strument,
or can be taken out for remote control of the Recorder. The tape
reel size to be used is a 26.5 em (1 0.5 inches) reel with NARTB hub, or
17.8 em (7 inches) reel with Cine type hub.
The Tape
Recorder is designed to operate in an environmental temperature
range
of 0 to + 40 °C (32 to 105°F).
Description of the Measurement Channels
The two measurement channels are identical and utilize the standard FM
technique. Each channel consists of a recording section and a reproduce
section, see Fig. 2.1 . Separate magnetic heads are used for the recording and
reproduction.
The heads are
of special construction of glass bonded ferrite material for
+ 1 ( . + 40)
low wear with gap widths of 3 ,u m ---=-D 120 ,u mches -----=-D for the record
head and 1.5
,u rn ± 0.3 (60 ,u inches ± 12) for the reproduce head.
One should, however, be careful not to damage the polished contact surface
of the heads by scratching or by chipping.
In the following a brief description of the various blocks shown in the block
diagram, Fig. 2.1 is given, starting with the recording section:
From the INPUT terminal , which is a standard B & K 14 mm coaxial socket, the
input voltage is led to a
variable attenuator via a record switch.
The RECORD
switch should be set to position "Ref.", " AC", "DC" or "Off"
depending upon the purpose of the recording, see also " Operation" (or
"Control Knobs and Terminals").
The variable attenuator is built as a ladder type attenuator and is independent
of frequency to within ± 0.1 dB in the frequency range from DC to 20 kHz. Its
input impedance is 20 kQ, which is thus also the recorder input impedance,
and a total attenuation range
of the input signal of 28 dB is available in the
form
of fourteen 2 dB steps. The variable attenuator is followed by a Calibration
Selector.
The
Calibration Selector has five positions of which two (pas. 1 and 2)
serve for the zero and gain adjustment of the reproduce DC amplifier. A
further position (3) allows the exact setting of the carrier frequency in the
modulator of the recording section while position 4 is used for modulator gain
setting.
Finally, position 0 connects the wide band DC amplifier in the recording
section to the output of the variable attenuator , i.e. the Recorder is ready for
operation when the calibration selector is in position 0.
When the calibration selector is in any position other than 0 the tape transport
mechanism is locked.
12

The wide-band DC-amplifier is used for voltage amplification and ensures
that a very low output impedance is presented to the succeeding low pass
filter and meter circuit. The amplification of the stage is approximately 10 dB
and to safeguard the amplifier , input signals higher than 15 volts are automa-
tically clipped. Within the prescribed frequency and dynamic ranges of the
Tape Recorder , however , the harmonic distortion in the amplifier is less than
0.1 %. As the amplifier is built as a difference amplifier with a large amount
of negative feedback a very high stability (DC drift and temperature dependency
less than 0.5% in 24 hours) has been achieved .
The
low pass filter, which is connected to the output of the DC wide band
amplifier , ensures that only that part of the input signal frequency range
which can be correctly recorded on the tape, with the chosen tape speed is
passed on
to the FM modulator . If a too high modulating frequency was
allowed to pass it would result in undesired beat frequencies and would
also influence the dynamic range of the recording, see chapter "Introduction"
and Appendix A. The low pass filter has been designed as a normal LC-RC
filter and the ripple in the pass band is less than 0.2 dB . It is automatically
switched to give the correct frequency response by means of the tape transport
speed selector . Typical frequency response curves for the low pass filter are
shown in Fig. 2.6.
Br i.ie l & Kjc:e r . M
easu ring Object : __ _ BrUel &
Kjmr
ODODODDODDODDODODODDDDOOOOODODDDDDODDOOOOOODDOODOOO
dB Reco
rd ing No.: Sign
.·
50
·-
--
1--1 -
1-- t---
~ t---1-
2 Hz S
OP1141 Pot . R onoe:
· ~
1- ---1-
-I-
1-
1 -f-
-
-
1-
1--
I-
I-
1-
1-
'--
-
1-
1- 1---1-
1 -+--
1--1- I-
I-- 1 -t---f-
t- 1- t -
20 so 100 200
dB Rectifier .: low.r lim. Freq. :
, ___.___
= =
-I--
- 1 --
r----1---
~='-'-'-
\--
-c---f-
- \-
-\ -
- \- -
1--
1- 1 -t---
-t---
. ;i -
1. -'
1-c-c-: t---
I
1 ---'\
1 -1---
1\
t--- 1-
1 -1--- 1---
2000
Ht Writing Speed:
mm tsec.
16 71 10
Fig. 2.6. Typical frequency characteristics for the low pass filters contained
in the recording section of the Tape Recorder Type 7001 .
The FM modulator is connected to the low pass filter and consists of an
integrating network, a wide band DC amplifier , a signal mixing network , a
voltage controlled oscillator , a discriminator drive amplifier and precision
limiter , and a frequency discriminator circuit.
In the input to the FM modulator the data signal is mixed with a negative
13

feedback signal derived from the frequency discriminator output of the wide
band DC amplifier. This amplifier, which is of the differential type to ensure
low drift characteristics, drives a control current through the cores in a free-
running inductive multivibrator. In
this way a square wave output signal is
produced the frequency
of which is proportional to the data signal voltage,
i.e.
the circuit operates, in essence as a voltage controlled oscillator.
A feedback signal is derived from the output of the voltage controlled oscillator
by means of an FM discriminator similar to that used in the FM reproduce
unit. This
feedback signal is as mentioned above mixed with the data input
signal
providing an exceptionally stable overall performance and extremely
good linearity
characteristic for the voltage to frequency convertion.
The
center frequency of the FM modulator is 216 kHz and is calibrated against
a reference generator as described
under "Calibration". Both the carrier fre-
quency and the gain of the FM modulator are adjusted by means of screw-
driver operated potentiometers. One of the potentiometers (frequency adjust-
ment} controls the zero-bias current in the magnetic cores of the modulation
multivibrator via the bias currents in the DC wide band amplifier stage. The
second adjustment potentiometer controls the negative feedback in the ampli-
fier and thereby the gain.
To
allow as simple and stable circuitry as possible to be used in the modulator
only one carrier frequency (216 kHz} is employed, independent of tape speed.
The actual
carrier frequency used on the tape is then obtained by frequency
division after
the modulation process has taken place.
The frequency
divider used in the Tape Recorder 7001 consists of seven
binary frequency
dividers dividing the frequency of the "original" FM signal
by factors of 2, 8, 20 or 80.
Two of the binary dividers are connected in a special arrangement to achieve
a 1 :2.5 division while the remaining operate as ordinary binary circuits. The
last flip-flop also supplies the driving power to the record head. To obtain
optimum operating
conditions for the tape, the magnetic field produced by
the head is adjusted so that it just saturates the tape. If the tape had been
"over" saturated it would have affected the crosstalk between the channels,
which, in the described arrangement
is negligible.
Solid state switches, controlled from the tape speed selector, are used to
activate the
different flip-flop circuits to ensure correct frequency division.
An OVERLOAD
indicator has been introduced to avoid overloading of the
modulator. It has been adjusted to light up when the input signal peak value
exceeds 1.5 Volts. However, as the
indicating circuit needs a certain (very
small} amount of energy before it will light it will react differently to different
kinds of input signals, somewhat dependent upon the signal crest factor*).
14
*)Crest factor = Signal peak value
Signal RMS value.

If the input signal consists of rectangular pulses with crest factors lower than
5
the overload indicator will indicate the true peak value of the signal. In
the case of wide band (20- 20,000 Hz) gaussian random noise with a flat
power spectrum, on the other hand, the indicator will light up for a voltage
around 2.5 X RMS value of the noise, and this is then not the highest value
of the input signal.
Should it be desired to record noise signals for later signal analysis where
peaks higher than 2.5 .a (a = RMS value) are regarded essential for the analysis
this should thus be kept in mind.
The overload indicator will when once lit, remain lit until it is released by
pressing the reset button (which covers the lamp).
Also the indicating meter measures the peak value of the input signal (at the
output of the input attenuator) when the Function Selector is in position
"Record" or "Stop". Again certain limitations are present for the indication
due to the (small) power consumption of the meter circuit. For periodic signals
the true maximum peak value is measured for signals with crest factors up to
4,5, while in the case of wide band noise the measured value will be of the
order of 2 a.
If the RECORD switch is in position "Off" and the Function Selector in position
"Record" the meter circuit is connected directly to the INPUT terminals by-
passing the
input attenuator. When recording is made on one channel only
the meter circuit of the second channel can thereby be used to monitor the
signal that has actually been recorded on the tape. To do so it is only
necessary to make an external connection between the OUTPUT terminal of
channel in use and the INPUT terminal of the channel not in use and to set
the RECORD
switch of the latter channel in position "Off".
If the Function Selector is set to position "Play-back" the indicating meter is
automatically switched to monitor the output signal from the reproduce section,
see Fig. 2.1.
The reproduce section is actually an "inverted" record section. As can be
seen from the block diagram, the signal from the tape is picked up by a
reproduce head and fed to a preamplifier.
The reproduce preamplifier has been designed to give a very high amplifica-
tion (of the order of 120 dB) and a low noise level. Because the output from
the reproduce head varies with frequency and tape speed and is lowest at
the lowest center frequency (2.7 kHz) the amplification is greatest at low
frequencies and drops off gradually as the frequency increases. Furthermore,
to obtain an efficient amplitude limitation with zero phase shift, and thus a
stable operation of the amplifier at all frequencies, a diode clipper has been
introduced in the negative feedback circuit.
The limiter amplifier following the preamplifier actually contains two amplifiers
utilizing a diode coupled feedback circuit to accomplish the desired limiting
operation. The circuit behaves as a conventional open loop, high gain amplifier
15

until the output voltage exceeds the forward conduction level (knee) of the
feedback diodes. Above this level the negative feedback becomes 100%
and thus sharply limits the output amplitude.
The FM discriminator (demodulator) contains a monostable multivibrator which
is triggered by the pulses developed in the limiter amplifier and differentiating
circuit. In this way the frequency of the multivibrator is determined by the
frequency of the reproduce input signal. To convert the frequency variations
into voltage variation (demodulation of the signal) the output pulses from the
multivibrator are shaped to have equal and very sharply defined widths and
heights , whereafter they are led to a low pass filter. The number of pulses
per second will then determine the magnitude of the output voltage from the
low pass filter, or said in other words: the number of equal pulses per second
determines the (variable) "DC" output from the filter.
BrUel & Kjcer • Measuring Object: __ _ BrUel & Kjcar
OODODDODDDODOODODODDODDDODODODOOOOOOOOODDDODOODODOD
d~ Recording No.: Sign. :
c-- r---r-
r---r-
c-- r---
c-- r--
1;;;;- -- -
F--
1--
+
-
-
-r---
--
- r---
--
--
r--
1-
1-
r ~
1-
1--- 1 +-
- H- 1-
1---
r---- 1 -t -r-
- \- · -c
t-- ~-t-H+_ H _-f-- ~ ~-+~+H+-~ t-- +-t-rTrn +rt---T -t · I --
~
2
OP1141
Pot . Ronoe :
,_ r--
- 1-
-
20
dB R ectifier.:
1-
1-
r----r--
lowerlim.Freq .•
1 === ~-=--
1000 2000
Hz Writing Sp.ed 1
--r-
1-
I -f-
1 -~
r---
- -
-
-r---
=-:
- -
-r---
-r--
-
-r--
r--
-r---
-r--
-r--
-1-
mmf see. Peper Speed:
50
0C:mfsec.'IOOOOO
{6?1?9
Fig. 2.7. Typical frequency characteristics for the low pass filters contained
in the reproduce section of the Tape Recorder .
The
low pass filter is a seven-poled constant-k type filter and the ripple in
the passband is less than
0.1 dB. A very high attenuation is obtained outside
the passband as can be seen from Fig. 2.7. It should be noted in this
connection that the pulses obtained from the multivibrator have a repetition
frequency which is twice that of the "instantaneous" carrier frequency. This
is due to the fact that one pulse is produced per zero-crossing of the carrier,
no matter whether the crossing takes place with a positive or a negative slope .
From the low pass filter the demodulated signal is fed to a DC wideband
amplifier the output of which is connected to the Tape Recorder output
terminal.
16

The output DC wideband amplifier is a differential type amplifier with one input
connected to the output of the low pass filter, while the second input is
connected to a DC signal with a magnitude corresponding to that obtained
from the filter with zero modulation of the carrier. To obtain the desired
amount of negative feedback in the amplifier part of the output signal is also
fed
to this second input. The DC drift is less than 0. 5% per 24 hours, the
harmonic distortion is less than 0.1 % and the overall amplification is
approximately 10 dB. An output impedance of less than 5 Q has been achieved
and
the output may be short circuited without damaging the amplifier. How-
ever, even though the output impedance is very low the output should not be
loaded with an impedance smaller than 1 kQ to ensure low distortion (the
output amplifier is no power amplifier). A maximum output voltage of 1 Volt
RMS (sinewave) is the basis for the distortion characteristics given above
The
necessary zero and gain adjustments of the amplifier are made by means
of screwdriver operated potentiometers. Zero adjustment is carried out by
adjusting the base-bias of the input transistor while the gain adlustment takes
place in the amplifier negative feedback circuit.
Calibration of the FM measurement channels is based on two built-in calibra-
tion generators, one of which is used to calibrate the reproduce section
(Calibration Frequency Generator) and a second (440 Hz 1 Volt Ref. Generator)
which is used for gain adjustment in the FM modulator circuit of the recording
section, see also Fig. 2.1.
The
calibration frequency generator supplies a signal of 108 kHz to the
reproduce section when the calibration selector is in position 1. This fre-
frequency corresponds to the highest unmodulated center frequency (tape
speed 60 ips) and is used
for zero setting of the recording section. With
the calibration selector in position 2 a frequency of 64.8 kHz is supplied
to the reproduce section which corresponds to a -40% frequency deviation,
i.e.
the maximum amount of allowable frequency modulation. By adjusting the
voltage level at the output terminal, see Fig. 2.1, to 1.4 Volts the nominal gain
of the output DC wideband amplifier is then set. The stability of both the
calibration frequencies is better than 3 X 10 -s ;oc in the normal operating
temperature range of the recorder (0-40°C).
The stability of the calibration frequency generator allows the reproduce
section of the recorder, when properly calibrated, to be used as a frequency
meter for adjusting the main center frequency in the FM modulator of the
record section (216 kHz). This is carried out by setting the calibration selector
to position 3, whereby the output from the frequency divider (108 kHz) is
connected to the reproduce section , and adjusting the screwdriver operated
potentiometer marked "DC Bal.-Record" until the monitoring meter is on
zero.
To
set the gain control of the FM modulator the adjustment switch is set to
position 5 whereby the "440Hz, 1 Volt Ref. Generator", Fig. 2.1, is connected
to the input of the record section .
17

The 440 Hz, 1 Volt reference generator Is an RC-coupled {double T-circuit)
oscillator which is tuned to 440 Hz {musical pitch A). A differential type
amplifier is used as amplifying element and a special voltage doubling and
limiting circuit ensures constant amplitude of the reference signal as well
as a certain desired limitation in the
driving power of the oscillator. The output
voltage is sinusoidal and has an RMS value of 1 Volt {1.4 Volts peak value).
Its amplitude stability, within the 0perating temperature range of the recorder,
is better than 0.02% per °C, and the frequency drift is less than 0.1 %
per °C.
Apart from its use as reference signal in the
calibration of the Tape Recorder
the
generator also supplies the control signal to the marker unit {see next
section "Description of the Voice Channel").
Description of the Voice Channel
The voice channel has been included in the Tape Recorder to allow for
marking and identification of the tape. It utilizes direct recording technique
with HF-bias but no special arrangements have been made to obtain high
quality recording and
reproduction over a wide frequency range. Because the
channel is
only intended to be used for identification purposes a minimum
amount
of frequency equalization technique has been utilized and the fre-
quency response is
"flat" only from 300 Hz to 3 kHz, the band limits here
being the - 6 dB points on the frequency characteristic.
A small
dynamic type microphone is supplied with the Recorder, which gives
an output voltage of approximately 1 mV for an input sound pressure level
of around
95 dB {normal close talking level). The normal operating input
level
of the channel should thus be in the order of 1 mV ± 10 dB. The input
impedance is approximately 1 kQ. An
output voltage of around 1 Volt RMS
is available on the monitor terminals, and the minimum value of the load
impedance should be 2 kQ.
The voice channel consists
of the microphone, an AVC {Automatic Volume
Control) unit, an
output amplifier for driving of the recording head, a record
head, a bias and erase generator, an erase head, a reproduce head and a
reproduce preamplifier.
To ensure
correct marking of the tape the record and reproduce heads for
the voice channel have been built together with the corresponding heads
of the measurement channels. The record/reproduce
switching of the channel
IS maae by means of relays controlled from a pushbutton mounted on the
microphone. If the button is pushed the channel is operated in Its recording
mode, while, whenever the button is released, it operates in its reproduce
mode.
When operated in its voice
recording mode the signal from the microphone
is led via a relay contact to the input of the AVC unit, see Fig. 2.1 . This unit
consists of a set of prebiased diodes followed by a two-stage linear amplifier.
The bias on
the diodes determines the operating characteristic of the unit.
18