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Патент USA US2403983

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July 16, 1946. '
Filed April 3, 1945
2 Sheets-Sheet l
' FIG.‘ I’
/2 A
July-16, 1946.
-, 2,403,983
Filed April 3, 1945
. I //
2 Sheets-Sheet 2
Patented July 16, 1946
5Walterlioenig, J12, Clifton,'-N. J., assignor to
Bell vTelephone Laboratories, Incorporated,
'New York, N..Y.,.a corporation of New York
Application April 3, 1945, Serial No.-586,308
10 Claims. (Cl; 1'79-—-1)
This invention relates'to the analysis of com
plex Waves and morev particularly to the produc
Figs. 8 to 12 illustrate various types of‘ spectro
gram ‘produced in accordance‘with the invention;
tion of complexavave spectrograms.
It has been proposed heretofore to record com
plex waves, such as speech‘waves'for typical ex
ample, in the‘form of a spectrogram or ‘pattern
the dimensions of which have the sense of‘ coor
dinate axes representing frequency and time re
The character of the spectrogram is
such that the effective intensity, or envelope am
plitude, of ‘the wave component or components
‘appearing in'anyparticular‘frequency band at
any particular time is indicated in the pattern at
the coordinate‘ position“ respective to the particu
‘Fig, 13 illustrates an optional auxiliary for cer
tain of the modi?ed systems.
‘Referring more particularly‘now‘to'the system
illustrated in‘ Fig. 1, there is included ama'gnetic
recordcnreproducer ‘ shown diagrammatically 'as
comprising‘an endless magnetic’ tape I ‘which is
passedpontinuously at asuitable'constant speed
between the pole-pieces‘ of an" electromagnet com.
prising a coil 3. The 'complex'waves of 'which'a
pattern is to be forme'd'may' be‘speech ‘bearing
waves. for example, received‘ from a microphone
circuitsv ‘which can be connected at will to coil 3
byi'means of a switch‘i. ‘With the latter in its
be indicated, for example; by the density or black
upper operating position the'incoming waves are
ness of thepattern‘ at the respective coordinate
recorded on the'magnetic tape I. ‘The‘switch. is
position; as disclosed in my‘copending application
then opened and therecorded waves'are there
SerialNo. 568 88G, ?led December 19, 1944.
One‘ of the objects of the present invention is to 20' upon reproduced electrically or'played'back, re
peatedly, once for each complete revolution, or
produce complex-wave spectrograms of certain
‘cycle of movement, of tape‘ 1.
novel types disclosed ‘herein. Another object is
With switch 5.121 its 'lowerposition'as shown,
to provide simpli?ed and improved equipment for
the WaVGS’SO‘I‘EDI‘OdUCBd are ‘passed to'a fre
producing spectrograms of types known hereto
fore. including spectrograms of the contour type 25 quencyianalyzer ‘or scanner ‘of'the heterodyne
type comprisinga modulator 1,‘W1'li01’l is supplied
disclosed in the copending' application'of R. K.
with beating oscillations'from" an oscillator 8, and
Potter, Serial No; 569‘,557;‘?led December 23, 1944.
bandepass or scanning‘ ?lter '9 ‘which is con
iA'further-object is. to'facilitate accurate de
nected to receive‘ the wave output‘ from modulator
termination‘ 'of ' the' envelope amplitude indicated
"a. vModulator ‘l e?fectively translates they applied
' i-n-a complex-wave‘ spectrogram,
band of speech bearing ‘waves’ to a'hi‘gherjposi
YPrincipal features of the ‘invention lie in means
tion in the frequency range depending on the
provided‘ for translating the variationsin enve
‘frequency of the beating oscillations, and the lat
lope amplitude ‘detected 'in any’ particular fre
‘ter frequency is varied ‘continuously from one
quency band into variations in recording'current.
limiting value to another such that the translated
In accordance ‘with embodiments of the'inven
‘band progressively shifts in‘frequencyposition,
tion hereinafter to be described in detail,‘ a beam
The total'shift of‘ the'band is comparablev with
v'o’fil-ight‘ is de?ected across a multiply-apertured
lar frequency ‘band and time. ' The‘ amplitude may
its ‘width. Scanning ?lter il'has a relatively nar
rowjpass band the mean frequency of‘ which is
ponent,-' and * the‘- light intermittently transmitted 40 such that as the translated'band wave shifts in
screen‘ or'mask' more or less in ‘proportion to the
-varying envelope amplitude of a’selected com
through the’apertures is utilized to‘control the
application of "marking "current 7to a recording
frequency. ?lter ‘9 selects progressively different
component frequency bands therefrom. .In eifect,
the ‘pass band of ?lter 9 moves gradually ‘across
the frequency range occupied by the speech bear
features; objects and advantages will appear more 45 ing' waves and. admits the wave components ap
pearing in the. different bands. The waves select
‘fully on‘ consideration of the embodiments illus
ed by ?lter 9 are passed to an ampli?er-recti?er
tratedin the: accompanying 'drawings‘and the
it, the constants of which are so chosen that
;' f‘c-llowingdescription ‘thereof.
The . nature ‘of the invention 1' and its various
‘Fig. 1 illustrates an analyzing and recording accordance with the, present‘ invention;
Figs." Z‘to rZillustrate rotatable screens that are
.utilized; in‘ modi?cations of the Fig. 1 system, in
theplace of'a .stationaryscreen and light modu
lating disc shown‘in'Eig.v 1;
the unidirectional output voltage is more-0r’ less
proportional to the envelopeamplitude or e?ec
tive intensity of the selected waves. This voltage
may be taken also as an approximate measure of
the power content of the selected wave band.
The ,_ pattern recorder is illustrated diagram
55 matically in Fig. 1 as comprising an endless mov
ing belt l2 of sensitized paper and a rotating
threaded shaft IS on which rides a traveling nut
I4 that carries the recording stylus [5 across
the strip of paper. The latter may be dry fac
verse slits 30 spaced apart at different distances
from the axis of rotation of screen 28. The lat
ter is disposed in the focal line of lens ,2"! and it
serves to chop or interrupt intermittently any
simile paper, preferably one with a titanium
_ light that passes through the openings 30 to an
oxide coating and carbon backing, such as the
“Teledeltos Grade H” facsimile paper developed
elongated photoelectric cell 3i. Screen 28 may
comprise a multiplicity of transparent and opaque
sectoral portions disposed in alternation, in which
by the Western Union Telegraph Company. Belt
I2 is driven at constant speed in synchronism
case it is disposed between screen 23 and photo
with tape I by means of a motor or the like (not 10 electric cell 3i as shown, although alternatively
shown) connected to belt drum l6. Stylus I5,
which may be a stainless steel wire 10 mils in
diameter for speci?c example, bears lightly
against the recording surface of the facsimile
it may comprise alternate light re?ecting and
light absorbing portions, the photoelectric cell 3|
in such case being so disposed as to receive the
interrupted re?ected light. In either case the
paper and in the course of production of a spectro 15 varying light emanating from the screen 28 and
gram it is driven slowly across the paper, that
reaching the photoelectric cell 3i gives rise to
is, longitudinally of the drum l6. Whenever and
electric currents of corresponding varying inten
so long as electric current of sufficient intensity
sity in the photoelectric cell circuit 32. The cur
is supplied to stylus IS, the current passes
rents are intensi?ed by an ampli?er 33 and then
through the stylus and the facsimile paper to 20 transmitted to the stylus l5.
drum it‘, which has a grounded conductive sur
In the operation of the Fig. 1 system the beam
face, and by virtue of the current passing through
26 sweeps back and forth along the screen 29,
the point of contact of stylus l5 a mark is made
following the variations in envelope amplitude of
on the paper.
the wave components selected by the frequency
The progressive change in the operating fre 25 analyzer, The several slits 30 are respective to
quency of oscillator 8 is electrically or mechani
different discrete values of envelope amplitude.
cally geared with the progressive change in the
for beam 26 is incident on a given slit only when
position of stylus 15 so that as the stylus moves
and so long as the envelope amplitude has a pre
once across the facsimile paper, the oscillator fre
determined particular value. The light momen
quency progresses from its one limiting value to
tarily passing through a .particular slit 3!} tends
the other. This operation is completed only after
to produce in circuit 32 a unidirectional current
many revolutions of the belt l2 or in other words
pulse of corresponding duration. The screen 28.
only after many reproductions of the recorded
however, is rotated at a speed such that the light
waves. In one instance in practice, for exam
reaching photoelectric cell 3| is interrupted re
ple, in which a 3500-cycle band of speech waves
peatedly at a high rate such as 10,000 times per
was to be recorded the parts were so arranged
second for example. The result is that the cur
that the complex waves were reproduced 200
rent supplied to stylus I5 is alternating in char
times while stylus I5 moved once across the fac
acter, which is substantially more effective in
simile paper. Disregarding the slight change in
marking the facsimile paper than a unidirectional
the frequency of oscillator 8 that takes place 40 current. While the frequency analyzer selects
during each reproduction of the speech waves
the waves in a particular component frequency
and the corresponding slight change in the fre
band, therefore, the stylus l5 traverses the cor
quency band selected by ?lter 9, it will be under
responding longitudinal path on facsimile paper
stood that during each reproduction of the re
[2 and produces a mark whenever the envelope
corded waves the ?lter 9 selects a de?nite prede
amplitude of the selected wave passes through
termined frequency band while the stylus l5
one of the predetermined values.
traverses a respectively corresponding path sub
Fig. 8 illustrates the type of spectrogram that
stantially longitudinally of the facsimile paper,
1. e., lengthwise of the belt. The mechanism for
is produced by the Fig. 1 system, assuming that
the width of the slits 30 is small in comparison
supplying current to stylus I5, which will be 50 with the distance between them. The marks
described presently, is such that along each path
made in the course of successive reproductions
on the paper the stylus leaves a trace that is
align themselves, it will be observed, to form
interrupted in a variable manner depending on
contour lines such as 31, 38, 39, in Fig. 8. Each
the varying envelope amplitude of the respectively
contour line is identi?ed with a particular value
corresponding band of waves selected by ?lter 9. 55 of envelope amplitude. The envelope amplitude
The Fig. 1 system includes also a mirror gal
pertaining to any particular frequency band and
vanometer 20 comprising a mirror 2| and a driv~
time may be determined by noting what contour
ing coil 22 both mounted for limited rotary move
line, if any, passes through the corresponding co
ment about a vertical axis. Coil 22 is connected
ordinate point On the spectrogram, or by inter
to the output terminals of ampli?er-recti?er l0, 60 polating with reference to the adjacent contour
and the mirror 2| is therefore de?ected from its
lines. One will observe in Fig. 8 several regions
rest position more or less in proportion to the
envelope amplitude of the wave components that
where the beam 26 repeatedly dwelt on a particu
lar slit 30 long enough to produce horizontal
lines of appreciable length. In such regions the
are selected by ?lter 9 at any time. Light from
the vertical ?lament of an incandescent lamp 23 65 contour line takes on the appearance of a band.
is directed through an optical system symbolized
The width, number and spacing of the slits 30
by lens 24 to mirror 2| from which the light is
can be adjusted to provide a variety of di?erent
re?ected in a beam 26 that varies in horizontal
effects. If the width of the slits 30 is made equal
direction according to the varying de?ection of
mirror 2|. Disposed horizontally across the path 7 O
traversed by beam 26 is a semicylindrical lens 2'!
which extends radially along the face of a rotary
screen 28. Interposed between lens 21 and screen
28 is a stationary screen 29 in the form of an
elongated mask that has a multiplicity of tlfans
to the spacing between them, for speci?c example,
all of the contour lines will appear as bands or, in
other words, successive contour zones in the spec
trogram will appear alternately light and shaded,
respectively. If the Spacing between slits is small
compared with the width of the slits the resulting
spectrogram comprises light contour lines 011 a
:dark::or..:sha‘ded background. .Againgthe slits (.30
may The spaced :approximately :equal :distances
ED311550 ithat ithe :contour ‘lines: representeequal
"amplitudestepsmrthey: maybe idi?erentlysp aced
distinctly ” .di?erent :interedot spacings, ' and ?ve
‘ to provide :lo'garithmic ::amplitude ;steps.
.di?ferent logarithmically related 1ranges :of ‘en
velope amplitude earegindicated in :thespectro
gram. The general appearance {of the: spectro
5 ggramlis illustrated in Fig. 9, which pictures "a
In lieu of the:screensi28:and 29zagsinglerrotary
screen may‘i-be :employed. .that :combines 2th alight
.7 fragment 10f -~ a speech spectrogram.
The Joand
width of the scanning ?lter .79 employed inz-ob
staining 1 this. spectrogram ,Was approximately ,300
chopping! function 'ofcscreen r28randxtheoamplitude
-' cycles,’ or .wide g. enough to suppress detail of the
"discriminating ciunctioniofiscreen <29. rIn such-a
‘1 screen the transparent portions may be: con?ned 10 harmonicv structure. rThe . marks‘ may be. caused
to so align lines
to concentric.» annular band'sLspaced from the "axis
.of variousslopes. Thus, .by: maintaining ,theppe
.'.of;rotation ias the .slits 53!! in Fig.1 1 :arevspaced
riod- .of ' rotation of . the .screen -, in:- accurate .har
TThis I will ’ become :more “evident : on
.monic-.relat-ionr.yto the period of movement'of .belt
-.l.2,._all.of~ these shadinglinesmay he made vertical.
‘consideration ofFig. ‘2.
In taccordancawith a further. modi?cation . of=
'theIZE'ig. l»:system, the :screens e28 ~and_.z2.9iarel-zre—
.By maintaining a?xedinharmonic relationfhow
ever, .theshading' line maybe made ‘to slopedif
' .placedjoyr asingle.'rotaryscreen-cf the kindrillus
.ferently for each.different.oontourvzonetin the
spectrogram. This is illustrated diagrammati
cally, in. Fig. 1.0. Theamplitude range pertaining
.toiany given. zone-it is evident, can be identi?ed
tratedsin'fFig. 2. The shaded'vareas inJi‘ig. 2 rep
:resent;transparentportions of thescreen. .These
iportions define; in part, .a - multiplicity of marrow
~ concentric:light-chopping bands-4 I. The» several
bands-Al are identi?ed »with:respectively corre~
spondingv different values of v-envelcpe- amplitude
by. the inclination ofwthe shading lines, andbythe
spacing of the shading lines. Therate».of.rota
and beam de?ection, land - their , presence .1 results
-tion of therscreen may beallowed toivary-slightly
1' 1111 the formation’, of corresponding .contour lines 25
inthe spectrogram substantially as illustratedv in
» if . it .be. desired . to: prevent. the . formationrof. such
. shading-lines.
.Fig. > 8. The intervening annular “zones. 42, :which
vFigs. - 4 .and -5 -.illustrate rotary screens 'of .a
are identi?ed with ‘respectively -corresponding
typesuchthat thelength of the markmadeéby
.stylus .l.5 issensibly dependent .on theiamou-nt
' different ranges" of =>enve1ope amplitude-values and
of ‘(beam "de?ection, are opaque-iexceptfor occa
, sional ~ sets/ of -. closely spaced, .ra-di-al transparent
30 of deflection of >beam_.26 .and thereforealso on
.the .envelope amplitude. Eachofthese screens
portions-43. ‘The severallsets of transparent por
tions~43 are uniformly spaced around- each zone
. may .be regardedas -.beingrdivided into an in
~4_2,.~but their angular-separation (1801'885851131'0
."gressiveily from aemaximum in the‘innermost'or
.the . light-chopping striationstllll. extend . from. the
tegral numberzofveq-ual sectors in. each of which
5.leading edge of :thesectorwarcuatelyi towardthe
low-amplitude zone. Whenever and so long 3.313136
.trailingedge to 1an :extent that increases apro
.light beamz? impingesrononecf the zones 42, it
gress-ively .with :progressive increase -.-in distance
. gives; rise in<circuit<32~to¢a succession of .current
. from theaxisof- rotation. .In Fig. .4: theprogres
.sive .increase in -.arcuate extent is substantially
pulses, ' each. of anfalternatingwcharacter adapted
for e?ective marking: of the facsimile paper, .and.
40rc0ntinu'ous whereasin Fig.
several ‘distinct steps. .Wherever .the beam -.-of
-light $26 strikes it .will ,be transmitted through
the‘ timecinterval betweenpulses depends on the
:radius Ofthepart-icular rzone. :Theumarks .pro
duced by the stylus ii in any giveniong-itudinal
. the. screen, modulated. atthechopping.frequency,
path are-spaced.inproportion to-thetime separa - ~at regularly-recurring times,.heginning each time
tion eftthe currentpulses, and;they.-are substan ."45 -as -the ~leading.ecige of .a :sectorpasses through
.,the beam .and .. continuing . for a period ‘that .de
tiallyuniform. in size. Thus reach-l range .of .en
velope amplitude -. is identified vwith :a .particular
.zone- 42,‘.1with .a particular manner of. modulation
:pends .on the (position .of. the .beam. .TI'hermarks
producedbystylus l5 are made-likewisev atreg
ularly recurring. intervalsand their lengths .are
of the .marking circuit, via,‘ a particular-spacing.
~of.the currentipulses, and with aparticular. spac ~50.correspon'dingly variable, cbeing .least _for ‘low
ing 7 of .the dOtz-lik? . marks - that... are produced .by
values of envelope amplitude andprogressively
thecurrent pulses.
The spectrogramproducedvwith the-aid .oithe
. and appearance -. of the spectrogram depends on
greater for‘ higher values thereof. ‘The. character
"howrfa'stithe .screenlis rotated and onlthe .con
screen shoWninFig. Zmaytake on the-general
appearance. of a-half tonepbut with superposed ;55 stancy offitsjspeedin. relationto that of-belt. l2.
-If ~. an exact -harmonic espeed relation .is ,main
‘contour lines. The Value of envelope amplitude
represented .by : any particular contour .line - can
.tained one obtains a spectrogram .of .the ‘kind
be established readilyby observing l-whatranges
represented in Fig. 11, in which the-marksper
of. envelope amplitude arerecorded on either side
of. it, that:is, by measuring on :each-side the dis
tance,.parallel~with the time axis, between suc
cessive .dots. The outermostzonerof-the. rotary
taining to successive time intervals-are . aligned
in corresponding successive frames. If ‘the, speed
relationis allowed to vary, the alignment of the
‘le'ftehan'd extremities of the. marks shown in'Fig.
11 will.not bev maintained. 'Thelength ofea’ch
line, however, will remaincameasure of.the en
. screen‘ may ,..provide for continuous. chopping. of
the light'beam, as showninFig. .72, in which case
the marksappearingin the maximum amplitude 2‘
.regions of .the spectrogram .' are =longitudinally
7' continuous.
The-rotary screen illustrate-d . in Fig. 3 z-is- sub
.stantiallyithe. sameas that , shown in .Fig. 2,. ex
cepting for :omission of the light chopping-rings
.4 I and inequality of thew-idth of the several. zones
42. ‘The latter are graded‘ logarithmically from
.minimumwidth in the innermost maxi
.velope amplitude pertaining to the particular
Q frequency band. .andtime indicated .by .itscoordi
.nate, position. .Dependingon the relation of .the
various factors noted above..Fig. .lLmayhe taken
as an .unmagni?ed representation .of. the spectro
g'm ,gram ..or - as .a _ greatly . magni?ed . representation.
.In the .latter case.~ thev spectrogram, .held at.- nor
mal viewing distance, .wouldiappearas a half
‘tone picture.
Thev rotary screenillustrated inEig. 6. is vclosely
mum width in the outermost: zone. There being
r?ve ofthe. zones 42, thespectrogram-exhibits "?ve r'l?related to thatdescribed .withreference .to Fig.
4. The light-chopping striations extend radially
inward from the periphery and vary in length
continuously at logarithmic rate in both circum
to the varying value of each said effect, and
means responsive to the light received succes
sively by said aperture means for marking the
variation in effective intensity appearing in each
ferential directions from a minimum length at
the left to a maximum length at the diametri ill of said bands on said record surface in the co
cally opposite point. The central opaque section
of the screen is roughly heart-shaped. The spec
trogram obtained by using the Fig. 6 screen is
I ordinate position respective to each.
2. A system of the kind described comprising
means for repeatedly reproducing a complex
wave, frequency analyzer means for selecting
similar to that illustrated in Fig. 11 except that
the horizontal lines in each frame along the time 10 during each successive reproduction the wave
components appearing in a respective different
axis are symmetrical about a vertical axis.
component frequency band, a rotating screen,
Each horizontal line in a spectrogram of‘ the
means for directing a beam of light to said
type described with reference to Fig. 11, it will
screen, means for displacing said beam relative
be understood, is produced by a continuous series
of light-chopping striations of correspondingr
to and across said screen under the control of
the selected components to different positions
arcuate extent. By breaking the continuous
respective to each different effective intensity of
series into several spaced sets of striations, each
the selected components, a photoelectric device
horizontal line in the spectrogram can be made
exposed to the light incident on and emanating
to appear as a dotted line, the number of dots
from said screen, means for modulating the light
to be counted in any line being a measure of the
emanating from any of a multiplicity of an
envelope amplitude. Fig. 12 illustrates such a
spectrogram and Fig. 7 shows an appropriate
nular regions concentric with the rotational axis
of said screen comprising screen portions of dis
type of screen.
In certain analytical work it may be desired
to have in addition to the spectrogram a record
of the average power or envelope amplitude, or
of the total energy, associated with each of the
similar optical character disposed in alternation
'. around each said annular region, the several
said annular regions and the said portions there
of being of such size and spacing that the modu
lation of said light is distinctively different for
ifferent displacements of said beam, and stylus
bands selected by the analyzer. If the spectro
graph is so designed that the power contentor
envelope amplitude is represented by a propor
‘ means for marking on a record surface along
tional number of dots the desired record can be
had by recording the total number of marking
current pulses that appear during each repro
duction of the recorded waves. For this purpose
the current pulses may be applied to a counting
circuit or to a storing circuit in which each
pulse contributes to an electrical charge that is
accumulated on a condenser. Then immediately
1.0 L!
after the end of each reproduction and before
the beginning of the next the accumulated‘
charge is caused to operate the amplitude re
cording apparatus. Such an auxiliary to the
spectrograph is illustrated diagrammatically in
'Fig. 13, which shows a pulse accumulator 45
connected to photoelectric cell circuit 32, and a i)
switch 46‘ which may be operated manually or
automatically at the proper time to apply the
accumulated charge to deflecting coil 24. A
mark is thereby made at the end of each line in
the spectrogram, and the character of the mark v50
varies according to the total number of dots
in the line.
It will be understood that the embodiments
herein disclosed are in some respects only il
lustrative of preferred forms and that the in 655
vention is susceptible of embodiment in various
other forms within the spirit and scope of the
appended claims.
successively different collateral paths during re
spective different reproductions of the said com
plex waves, said stylus means being responsive
to electrical currents produced by the exposure
of said photoelectric device to said modulated
3. A system in accordance with claim 2 in
cluding means, comprising said screen, for modu
lating said light in substantially the same man
ner throughout each of different ranges of dis
placement of said beam, the manner of modula
tion differing distinctly from any one of said
ranges to another.
4. A system' in accordance with claim 2 in
cluding modulating means, comprising said
screen, for impressing on the said emanating
light a series of spaced pulsations, the duration
and spacing of the pulsations being different for
different ranges of displacement of said beam.
5. A system of the kind described comprising
means for repeatedly reproducing a complex
wave, frequency analyzer means for selecting
during each successive reproduction the wave
components appearing in a respective different
component frequency band, a rotating screen,
means for directing a beam of light to said
screen, means for displacing said beam relative
to and across said screen under the control of
surface a visual representation of complex waves
in the form of a pattern the dimensions of which
the selected components to different positions
respective to each different effective intensity of
the selected components, a photoelectric device
exposed to the light incident on and emanating
have the sense of a frequency axis and a time
from said screen, means comprising said screen
What is claimed is:
1. In a system for producing on a record‘
axis, respectively, frequency analyzer means op-‘B,5 for varying the intensity of the emanating light
erative on the complex waves for deriving from
each of a multiplicity of component frequency
bands thereof an electrical effect individual to
said band that varies in accordance with the
varying effective intensity of the wave com
ponents therein, means for producing a beam of
light, a screen having a multiplicity of aperture
means respective to different values of said vary
ing electrical effect, means controlled by said
effects in succession for directing said beam of
light to said aperture means selectively according"is
in pulses of alternating character the time sep
aration of which changes progressively with
progressive change in the extent of displacement
of said beam, and stylus means for marking on
a record surface along successively different col
lateral paths during respective different repro
ductions of the said complex wave, said stylus
means being responsive to electrical current pro
duced by the exposure of said photoelectric de
vice to said modulated light.
6. A system in accordance with claim 5 in
a beam of light that is displaced variably in
conformity with the variations in said derived
stantially in discrete steps.
effect, photoelectric means positioned to receive
7. A system in accordance with claim 5 in
the said beam, a screen interposed in the path
which the said time separation progresses sub
stantially continuously.
of said beam, said screen having apertures spaced
apart to pass said beam selectively in dependence
8. A system of the kind described comprising
on the extent of displacement thereof, and means
means for storing complex waves, means for
for applying the currents produced by said
repeatedly reproducing the stored waves, fre
photoelectric means to control the response of
quency selective means for deriving from the
reproduced waves an effect that varies during 10 said current-responsive marking means.
9. A system in accordance with claim 8 in
each successive reproduction in accordance with
which the said time separation progresses sub
the variation in eifective intensity of the wave
content of a different component frequency
band, current-responsive stylus marking means
movable relative to and across a sensitized sur
which said screen is stationary.
10. A system in accordance with claim 8 in
which said screen comprises a rotatable element
15 having a multiplicity of spaced apertures in each
face along a multiplicity of collateral paths in
of a multiplicity of rings concentric with the axis
succession, each of said paths being respective
of rotation, said system including means for
rotating said element at a speed such that the
light incident on each ring is modulated at audio
to a di?erent component frequency band and
each being followed by said marking means while
the said effect is being derived from the said 20 frequency.
respective frequency band, means for producing
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