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Патент USA US2125879

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Aug. 9, 1938.
'
I'
H. BELAR
‘
2,125,379
SIGNAL RESPONSIVE DEVICE
Filed Sept. 25, 1955
2 Sheets-Sheet 1
//
INVENTOR
HEBBET A-Z/IR
BY.
1/)"
‘
Aug. 9, 1938-.
v
H. BELAR
‘
SIGNAL RESPONS'IVE DEVICE
Filed Sept. 25, 1955
2,125,379
‘ 2 Sheéts-Sheet 2
‘E? I
INVENTOR
Patented Aug. 9, 1938
2,125,879»
UNITEDSTATS
PATENT orrics
2,125,879
‘
SIGNAL RESPONSIVE nnvron
‘
Herbert Belar, Philadelphia,‘ Pa., assignor to
Radio Corporation of America, “a corporation
of Delaware
.Aprpiication September 25,‘ 1935, Serial No. 41,987
.1‘ 6‘ Claims. “(01. 179-1003)
This invention relates to signal responsive de~
vices suitable for‘indicating or recording a signal
of audio frequency or ‘the like, and has for its
details of the connections of the apparatus of
Fig.
1.
.
l
'
The apparatus of Fig. 1 includes a sound pick~
principal object the provision of an improved up device or microphone Ill from which signal
i; apparatus and method of operation for produc
current is supplied through an ampli?er H and
ing‘a resultant effect dependent on a signal com
leads [2 to operating coil I?» of a galvancmeter
ponent and a signal envelope component;
.forming part of a sound recording head. The
It has heretofore been proposed in the photo
graphic recording of sound to reduce the light
10? transmitting area of the record to a minimum by
varying the recording beam in response both to
the signal and the envelope of the signal. It has‘
been considered necessary to provide in the re
cording apparatus the . usual signal recording
15,; channel and, in addition to this . channel,
a channel including a recti?er, an ampli?er
and a ?lter or the like for producing an en
velope current which may be supplied either
to the signal recording element or to a shutter
which independently varies the width of the re
cording beam. Such apparatus has not been al
together satisfactory, especially from the view
point of cost,‘ for the reason that the apparatus is
complicated and requires a considerable number
of parts. In accordance with the present inven
vention, this dif?culty is avoided by the provision
of means including an ampli?er wherein the con~
trol circuit is subjected to a potential which biases
it substantially to'cut-off when ‘no sound is re
. corded and during the recording of ‘sound is sub
jected to a potential which varies in response to
the amplitude or volume of the signal to be am’
pli?ed. >As hereinafter explained, this type of am
pli?er combines the advantages of class A and
l class B operation, reduces the number of ampli?er
tubes required, and makes possible the ‘use of
smaller ampli?er tubes for the reason that the
tube losses at zero signal are greatly diminished.
Further objects of the invention are to provide
an improved amplifying apparatus which com
bines the advantages of class A and class B oper»
ation, to provide an ampli?er wherein the control
grid bias potential is responsive to the amplitude
of the signal to be ampli?ed, and to reduce the
expense and complexity of sound recording ap
paratus.
The invention will be better understood from
the following description when considered in con
nection with the accompanying drawings, and its
scope will be pointed out in the'appended claims.
Referring to the drawings:
>
Fig. 1 illustrates a sound recorder constructed
in accordance with the invention, and.
55
Fig. 2 is a wiring diagram illustrating certain
ampli?er also supplies through leads M to a gal
vanometer operating coil l5, a current which
varies as the envelope of the signal current. As
will be readily understood, a galvanometer re
corder or light valve recorder, properly adjusted
and supplied with these component currents, will
record sound with a minimum of transparent
area in the positive record, thus avoiding the 15
background noise due to irregularities in the
transparent‘part of the record.
As shown in Fig. 2, the ampli?er Il may in.
clude single tube stages l6 and l T and a push-pull
stage including tubes [8 and 19. Connected in
the common plate return lead of the tubes 58-! 9
is a self-bias resistor 23, and a network compris
ing a resistor 2| and condenser 22 and the plate
circuit of a diode triode tube 23. As is well un
derstood, this type of tube combines a recti?er
and amplifier tube in one envelope. It will be
noted that this tube is coupled to the output cir
cuit of the tube I1 through a transformer winding
24 and has interposed between its cathode and
gridta grid bias potential source 25 and a grid
leak element including a resistor 26 and con
denser 21.
The plate current of tube 23 has its maximum
value when no sound is received at the micro
phone lll. Under these conditions, the potential
drop of resistor 2| operates to bias the grid con
trol circuits of push-pull tubes I8 and l 9 substan
tially to cut-off and the output current of these
tubes is maintained at a very low value. When
sound is received, however, the tube 23 functions
to rectify the audio current delivered to it through
the winding 24, this recti?ed current is transmitted
through the resistor 26 and produces a potential
in the direction of source 25 thus decreasing the
grid potential of the tube and correspondingly 45
varying the potential drop across the resistor 2 i.
Since the resistor 2| is connected -in the grid
control circuit of push-pull stage l8—l$l, the bias
potential of this stage is varied in accordance with d
the amplitude or volume of the signal. By proper
adjustment of these various elements the grid
potential of the push-pull stage is caused to vary
in accordance with the signal amplitude until the
average plate current is approximately half of
2,125,879
2
the peak or maximum value and does not increase
beyond this point.
The grid bias is prevented from becoming more
positive than that required for normal class'A
operation by the self bias resistor 20, and the
choice of circuit constants of tube 23, which
is made to bias itself to “cut-01f” before the
grids of tubes I8 and I9 can be fully modulated.
If- the common plate supply circuit of tubes I8
10 and I9 is continuous, the device will act as an
audio ampli?er-and if used, as such only, re
I sistor 29 and reactor 30 may be omitted.
The
output from this ampli?er will be undistorted if
the variations in bias are limited to the linear
15 range of the tube characteristic and if the bias
variation with envelope of signal, is' always great
enough to permit the signal modulation to’ re
main substantially within the linear range. Inv
the transient state, the rate of change of aver
'20 age bias with time is also important, as it must
be slow enough not to cause audible signals in
the output lines 12 and at the same time fast
enough to prevent audible distortion of suddenly
built up louder signals.
The timing of the en
25 velope bias potential is determined by resistor
26 and capacitor 21, and also by resistor 2| and
capacitor 22. For instance, if capacitor 21 is
increased in size, the time required to charge it
through the diode portion of tube 23 is increased
30. because this increases the product of RxC,
where R in this case is made up chie?y of the re- .
?ected resistance in winding 24, the equivalent
resistance of the diode in series with it and the
resistor 26 in parallel with the combination.
35 Formulae for the solutions of such circuits being
well known, thus the rate at which the bias of
tube 23 follows increases in signal strength can
be chosen. Moreover, the rate at which the bias
returns to the original value after a signal has
40 ceased can be adjusted by varying the value of
resistance 26 with respect to capacitor 21. Here,
as is also well known, the charge on capacitor
21 will leak off slower if the value of resistance
26 is increased.
The rate of change of envelope potential on
the grids of the push-pull stage IB and I9 is
further reduced by the action of capacitor 22
in conjunction with. resistor 2| and the plate
resistance of tube 23. This additional ?ltering
50 of the envelope potential is particularly impor—
tant if the output stage is single ended and not
push-pull as shown as well as when the ampli
?er is used as a recording ampli?er supplying
also biasing current to a galvanometer, for in
stance, for “noiseless” recordings. In this case,
the modulation coil of the galvanometer is
connected to the output of tubes l8 and I9
through leads I2, whereas current varying in
accordance with the envelope of the signal
60 current is derived from the common lead of
the anode circuits of push-pull stage [8-49
through leads l4 through reactor 30 to the bias
ing coil I5 of the recording galvanometer. Re
actor 30 serves to isolate the bias circuit of the
galvanometer from the plate circuit at audio fre
quencies. This is particularly important, if the
modulation coil and bias coil of the galvanometer
combines the advantages of both class B and
class A output stages. A variable bias output
stage requires no more power in the grid circuit
than a class A stage but operates on the average
at a lower plate dissipation and a higher plate
supply efficiency. When used in connection with
noiseless recording of sound, it has the additional
advantage of avoiding the plate battery drain
and the use of output tubes for supplying bias
potential. Attention is called to the fact that 10
with the connections described, the galvanometer
is always de?ected in the same direction and pro
vision for this fact should be made in the
galvanometer adjustment.
In ground noise elimination systems as has 15
heretofore been used, it has been customary to
set the galvanometer at its mid position when
no current was ?owing, then to bias it to a
minimum position through the output of the
anti-ground noise ampli?er, and when a signal 20
was applied to reduce this bias in proportion to
the signal until at a certain maximum signal,
the bias reached zero and the galvanometer op
erated in its ordinary manner. Since the bias
current in the arrangement here described in 25
creases instead of decreasing with an increasing
signal, it is necessary to adjust the galvanometer
with no current input so as to be at the zero
position. This adjustment can be accomplished
either magnetically or mechanically. In order 30
to adjust the galvanometer so that it at maxi
mum signal will not be biased beyond a mid
position about which it can oscillate, it may be
desirable to insert a variable resistance 29 in
35
shunt with the galvanometer.
Having thus described my invention, I claim:
1. A sound recording apparatus including a
light source, a light control device, means com
prising an ampli?er for energizing said device,
means for biasing said ampli?er substantially 40
to the lowest plate current on the linear por
tion of its characteristic curve, means for varying
said bias in response to the amplitude of said
sound, and means in the plate supply circuit of
said ampli?er actuated in accordance with the
envelope of signals supplied to said light control
device.
2. A sound recording apparatus including a
light source, a light control device, means com
prising an ampli?er for energizing said device, 50
means for biasing said amplifier substantially to
the lowest plate current on the linear portion
of its characteristic curve, means for decreasing
said bias in response to increase in the amplitude
of said sound, and means in the plate supply cir 55
cuit of said ampli?er actuated in accordance
with the envelope of signals supplied to said light
control device.
3. A signal responsive apparatus including a
light source, a light control device, means com
prising an ampli?er for energizing said device,
means for biasing said ampli?er substantially to
the lowest plate current on the linear portion of
its characteristic curve, means for varying said
bias in response to the amplitude of said signal,
and means in the plate supply circuit of said am
are inductively coupled to each other. In this
case, the use of a reactor of suf?ciently large
value is required in order to prevent the bias coil
from becoming effectively a short circuit across
the modulation coil. The reactor 30 serves also
to further decrease the rate of change of bias
pli?er actuated in accordance with the envelope
of signals suppliedto said light control device.
4. A signal responsive apparatus including a 70
pair of ampli?ers connected in push-pull rela
current with respect to time.
varies in response to the signal to be ampli?ed,
a pair of leads associated with the output circuit
.
A recording ampli?er of thertype disclosed
tion, means common to the control grid circuits
of said ampli?ers for producing a potential which
2,125,879
of said push-pull ampli?ers for delivering (the
ampli?ed signal, and a pair of leads common to
the anode circuits of said ampli?ers for deliver
ing a current which varies as the envelope of
said signal.
'
5. A signal responsive apparatus including a
pair of ampli?ers connected in push-pull rela
tion, means including a grid leak element and a
diode triode common to the control grid circuits
of said ampli?ers for producing a potential which
varies in response to the signal to be ampli?ed,
a. pair of leads associated with the output circuit
of said push-pull ampli?ers for delivering the
ampli?ed signal, a pair of leads common to the
15 anode circuits of said ampli?ers for delivering a
3
current which varies as the envelope of said sig
nal, and a galvanometer provided with actuating
coils connected respectively to said pairs of leads.
6. A signal responsive apparatus including‘ a
pair of ampli?ers connected in push-pull rela
tion, means including a diode triode energized in
response to the signal to be ampli?ed and com
mon to the control grid circuits of said ampli?ers,
a pair of leads associated with the output circuit
of said push-pull ampli?ers for delivering the
ampli?ed signal, and a pair of leads common to
the anode circuits of said ampli?ers for delivering
a. current which varies as the envelope of said
signal.
HERBERT BEL-AR.
15
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