US2476445A

US2476445A - Analysis and display for complex waves

Info

Publication number: US2476445A
Application number: US620823A
Authority: US
Inventors: Lester Y Lacy
Original assignee: Bell Telephone Laboratories Inc
Current assignee: AT&T Corp
Priority date: 1945-10-06
Filing date: 1945-10-06
Publication date: 1949-07-19
Anticipated expiration: 1966-07-19

Description

July 19, 1949. 1 Y. LACY ANALYSIS AND DISPLAY FOR COMPLEX WAVES Filed Oct. 6, 1945 JNJOHJ INVENTOR [..KL Cy ATTORN Patented July 19, 1949 ANALYSIS AND DISPLAY FOR COMIPLEX WAVES Lester Y. Lacy, Madison, N. J., assignor to Bell Telephone Laboratories,

Incorporated, New

York, N. Y., a corporation of New York Application October 6, 1945, Serial No. 620,823

9 Claims.

This invention relates to the visual representation of complex waves and more particularly to methods and means for displaying the frequency composition of such complex waves as speech waves. Y

One ofthe objects of the present invention is to provide improved methods and means for deriving from complex Waves a substantially contemporaneous visual representation thereof and more especially a representation of the spectrographic form disclosed in my copending application, Serial No. 534,669, filed May 8, 1944, which issued as U. S. Patent No. 2,403,986, July 16, 1946.

In an embodiment of the invention hereinafter to be described in detail, a spectrographic representation of received complex waves is formed on the luminescent screen of a cathode ray oscilloscope, the representation appearing to iiow uninterruptedly across the screen at a steady rate. The received Waves are continuously subjected to frequency analysis and the results of the analysis are continually recorded on a magnetic tape. The latter is advanced into a reproducer which operates repeatedly over a fixed length of the tape and continually presents to the oscilloscope information as to the frequency composition observed at various times throughout a preceding time interval which may be of the order of a second in duration, for speciiic example. The information so presented is translated into the form of a luminous picture or spectrogram which at any given instant portrays how the various frequency components varied in strength throughout the aforementioned time interval.

The nature of the present invention and its various features, objects and advantages will appear more fully upon consideration of the embodiment illustrated in the accompanying drawing and the following description thereof.

In the drawing:

Fig. 1 illustrates a system embodying the invention; and

Fig. 2 illustrates a portion of a typical speech spectrogram.

Referring to Fig. 1, the complex waves that are to be translated into visual form may be assumed to be, for specic example, speech bearing waves received over a microphone circuit I. The latter is connected through a wave translator 2, described in the next paragraph, to a lter bank 3 comprising band-pass filters FI, F2, F8, .which separate the received waves into component frequency bands. The transmission bands of the lters may overlap slightly and they may be either of equal or unequal width depending on what frequency resolution and frequency scale is desired in the spectrogram. Although eight l- .ters are shown in the filter bank 3, improved frequency resolution can be obtained by employing a considerably larger number of filters, and thirty, for example, is an entirely practical number. The wave components delivered by the several n1- ters are applied to individual detectors or rectii'lers 4 which are followed in circuit sequence by individual smoothing devices such as low-pass lters 5. The latter are designed to deliver unidirectional voltages each of which varies in substantial conformity with the varying strength or envelope amplitude of the corresponding selected wave components, and for this purpose each may have a cut-off frequency of about 25 cycles per second.

The wave translator 2 may include wave ampliiying and equalizing means to put the received Waves in better condition for the frequency analyzing operation that follows, and it may include also a frequency translating device for shifting the received waves to a substantially higher p0- sition in the frequency range where the operating characteristics required of the filters in bank 3 can be more readily attained.

The smoothing lters 5 are connected on their output sides to respective segments of a rotary switch or commutator 8 which has a contactor 9 that makes contact with the several segments in succession. The contacter is connected through a low-pass filter I3 to the recording element I5 of a magnetic recorder, the latter including also a wire or tape I6 of magnetic material which passes at a steady rate from a supply reel or pulley I'I to a take-up reel or pulley I8. One of the commutator segments, segment I0, is relatively short, and it is flanked by a pair of unconnected segment portions Il, It is connected to an electric source, such as a battery I2, so that once during each complete rotation of contacter 9 a strong pulse suitable for synchronizing purposes is recorded on the magnetic tape I6. Contactor 9 is rotated rapidly enough to give thel desired time resolution, that is, to allow each of the filters 5 to operate on the magnetic recorder a sufficient number of times per second to dene, as accurately as desired, the variations in envelope amplitude that appear in the corresponding fre" quency band. It may be assumed, for specic example, that the commutator 8 operates at 60 revolutions per second. .The cut-off frequency of lter I3 is selected or adjusted accordingly with a view to suppressing superfluous high frequency variations in the current supplied to the recording element I5.

In the operation of the portion of the Fig. 1 system that has been described in detail, the commutator 8 virtually samples the voltage output of all of the filters 5 in succession during each sixtieth of a second and delivers to the recorder the information thus obtained regarding the frequency composition of the received waves at each instant. The set of effects delivered to the recorder during each sixtieth of a second vary in relatively strong recorded effects orf pulses which, f

as previously indicated, are used for synchroniz-V ing purposes.

In the Fig. 1 system, furthera looped section Y 20 is formed in the magnetic tapel 'abouta Ycircular guide 2|, and a magnetic pick-up or reproducer element 22 is arr-anged to rotate within the looped section 2i) and to scan that section of the magnetic tapeY Hi,` or the greater part of it, repeatedly, many times per second. The period of rotation of element 22 is correlated with the period of persistence of human vision, as will presently appear; in an illustrative case the element 22 may .be rotated at 'about 25 revolutions per second. The length of the'looped section 20 that is in reproducing Yrelation with element 22 controls the time dimension of the visual representation that is to be formed, or in other words it determines how many seconds Worth of speech waves will be displayed at any instant. In an illustrative case sixty of the aforementioned sets of recorded effects may be containedin this length of the tape, corresponding to one secondsworth of the received speech waves. In the same case, it will be noted, the element 22 travels relative to and lalong the tape 59 or 61 times as fast as the recording element I5, depending on the direction chosen for the rotation of element 22. Y

The Fig. 1 system comprises also an oscilloscope for displaying the received Waves in visual form. Cathode ray tube 3U is of a conventional design, having a luminescent screen 3|, Va pair of plates 32 for deflecting the cathode ray ina vertical plane, a pair of plates 33 for deflecting th-e ray in the horizontal plane, and a control grid 34 for regulating the intensity of the cathode ray and the brightness of the luminous spot formed thereby on screen 3|. A saw-toothed voltage wave generator or sweep circuit 35 which is connected to deflecting plates 33 operates in synchronism with the rotation of pick-up element 22 and causes the cathoderay to progress from, say, left to right across the screen 3| from `one side to the other of the rectangular picture area 4!! while the pick-up 22 moves from one extremity to the other of the section 20. Sweep circuit 35 may, therefore, have an operating frequency `of about 25 cycles `per second,V corresponding toV the periodicity of element 22.v Synchronism may be maintained by any suitable means. For ex-V ample, a contactor 24 may be rotatably driven from the same shaft as pick-up 22 and arranged to make Contact momentarily once during each cycle with a segment 25 to which an electric source such as battery 2,5 is connected. Synchronizing pulsesl thereby generated are transmitted over a conductor 21 from contactor 24 to sweep circuit 35.

A second saw-toothed voltage Wave vgenerator or sweep circuit 36 which is connected to deecting plates 32 may be the same as sweep circuit 35 excepting for its operating frequency. The latter is so chosen that the cathode ray progresses substantiallyV vertically across the screen 3|, from,-say, bottom to top of the picturev area 40, while pick-up element 22 progresses from one synchronizing pulse to the next on tape i3. It will be evident, therefore, that the operating frequency of the Vsweep circuit v36 maybe of the or- 4 der of 60 times 25 or 1500 cycles per second. Synchronization may be effected by applying the pulses that are reproduced by element 22 over conductor 29 to sweep circuit 36. Pick-up element 22 is connected also through an amplier 38 to the control-grid 34. For simplicity'there h-avebeen omitted from this disclosure numerous circuit details and possible refinements which one skilled in theart would undoubtedly supply in eblanking-out the cathode r-ay during retrace and pulsing periods are among the omitted details.

In the course of operation of the Fig. 1 system, pick-up element 22 makes one complete transit ofthe looped section 20 while the cathode rair follows a zig-zag path across the screen 3| .cdv-1- ering in succession substantiallyevery point in the rectangular area 40 as indicated qualitatively in Fig. 1. The intensity of the cathode ray'iss concurrently varied by the reproduced effects applied to grid 34. If the portion .of tapeV 'I6 in the looped section be halted, an unchanging picture of a one-second fragment of the received speech waves will appear on screen 3|. VThe gradual change that takes place in the visual representa tio'n when the magnetic tape advances -at normal rate is such that the representation appears to ilow from left Vto right across the areal. It is to be noted that the brightness of the luminous spot formed by the ray at any point in the rectangular area 40 is correlated with the strength of the Wave components found in a par'- ticular frequency band, the frequency position of the particular band being indicated by the position of the spot relative to the upper or lower boundary of.V the area 40. Furthermore, the variation in brightness observed at a given instant along different verticalrstrip-like portions of the picture area portrays the frequency 'anaiy sis or approximate Wave energy distribution as of corresponding different time intervals.

For certain purposes it will be desirable to re duce to a minimum the length of magnetic'tape IB between the `recorder i5 :and the point where the tape is rst brought into operativerelation with pick-up 22.

Fig. 2 represents a fragment of the visual representation or spectrogram that might appear on screen 3| when speech Waves are being received. As previously indicated the vertical and horizontal dimensions of the representation have the sense Iof coordinate axes representing frequency Vand time respectively.

What is claimed is:

1. The method which comprises receiving complex waves, selecting the wave components found in each of a multiplicity of parts of the frequency range occupied by the received waves, deriving from the said selected components during each of successive time intervals a set of effects representing the strength during each 'said time ins energy ray in another direction in timed relation with the reproduction of the effects in each set, and varying said ray continually under the control of the successively reproduced effects to mark the variations in strength of the various components.

3. In combination with a receiver of complex waves, frequency analyzer means operative on the received waves for deriving therefrom a continual succession of effects, the different effects in each of successive sets thereof representing the strength of the Wave content of corresponding different frequency bands, means for recording said effects successively in reproducible form at corresponding diierent points along a record carrier, reproducer means for scanning repeatedly a portion of the said carrier presented thereto to reproduce the effects recorded in said portion in repeated succession, said portion embracing many of said sets of recorded effects, and means responsive to the reproduced effects for displaying, separately and substantially simultaneously, the different successions of reproduced effects that correspond to the different frequency bands.

4. In combination with a receiver of complex waves, frequency analyzer means operative on the Waves being received for deriving therefrom a continual succession of effects each corresponding to a particular frequency band and each containing a measure of the strength of the wave content of the corresponding band, the diierent frequency bands being represented in the same order in each of successive sets of said effects, means for continually recording said effects, means for reproducing a group of said sets of recorded effects over and over again including means for gradually changing the composition of the group being reproduced, and oscilloscopio means responsive to the reproduced effects for forming a luminous spectrographic representation of the Waves being received.

5. A combination in accordance with claim 4 in which said last-mentioned means comprises a cathode ray tube having a luminescent screen, means for deflecting the cathode ray in a first direction repeatedly in timed relation to the reproduction of successive sets, and means for deflecting the ray in another direction repeatedly in timed relation to the reproduction of successive groups.

6. A combination in accordance with claim 4 in which said last-mentioned means comprises a cathode ray tube having a luminescent screen, means for deflecting the cathode ray in a first direction repeatedly in timed relation to the reproduction of successive sets, means for deflecting the ray in another direction repeatedly in timed relation to the reproduction of successive groups, and means for varying the intensity of the ray under the control of the reproduced effects.

7. A system for the visual representation of complex Waves comprising electric wave input means, a multiplicity of distinct paths having respective different transmission frequency bands, said paths being connected at one end to said wave input means, a recorder-reproducer comprising a recording element, a record carrier and a reproducing element, switching means for individually connecting the other ends of said paths in succession, over and over again at a first cycli- 7 cal rate, to said recording element, means for establishing progressive relative movement between said recording element and said record carrier, means for establishing movement of said reproducing element relative to said record carrier such that said reproducing element moves repeatedly, at a second cyclical rate of repetition that is many times said first cyclical rate, over a portion of said carrier that changes with said progressive movement, an oscilloscope including ray producing means and a luminescent screen, means for deflecting the ray in one direction across said screen repeatedly at said first cyclical rate, means for deflecting the ray in another direction across said screen at said second cyclical rate, and means connected to said reproducing means for further varying said ray.

8. A system for the visual representation of complex Waves comprising a multiplicity of electric circuits including individual filters connected to receive the complex waves, said filters having mutually different transmission frequency bands, an electric source, a magnetic recorder comprising a recording element and a tape or the like of magnetic material, commutator means for connecting said circuits and said source in succession to said recording element during each cycle of operation of said commutator means, means for progressively moving said tape relative to said recording element whereby effects derived from the several said bands and a pulse derived from said source are recorded in successive sets along said tape, a reproducer element operative cyclically over a looped section of said tape that embraces many of said successive sets, a pulse generator operative synchronously with the cyclical operation of said reproducer element, a cathode ray oscilloscope, a pair of sweep voltage sources connected to said oscilloscope, a synchronizing connection from said pulse generator to one of said sweep voltage sources, means including said reproducer element for applying to the other of said sweep voltage sources a synchronizing pulse derived from said recorded pulse, and means connected to said reproducer element for varying the intensity of the cathode ray in conformity with variations in the intensity of the effects reproduced thereby.

9. In combination, frequency selective means for deriving from applied complex Waves a multiplicity of effects each of a variable intensity correlated with variations in the strength of the wave components appearing in a respective different part of the Wave frequency range, means for recording said effects on a record carrier in continually repeated succession, means for repeatedly reproducing in succession the effects recorded in a predetermined length of said record carrier, means for progressively changing the section of said carrier over which said reproducing means is operative, and oscilloscopic means controlled by the reproduced effects for visually representing the composition o f said waves.

LESTER. Y. LACY.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number