US3165586A - Microscope spectrum analyzer - Google Patents

Description

Jan 12, 1965 s. J. CAMPANELLA 3,165,586

MICROSCOPE SPECTRUM ANALYZER Filed Aug. 11. 1960 INV EN TOR. SAMUEL J.CAMPANE\.\ A

AT TolzNEYS very complex manner.

United States Patent O 3,165,586 MICROSCOPE SPECTRUM ANALYZER Samuel J. Campanella, Springfield, Va., assiguor to Melpar, Ine., Falls Church, Va., a corporation of Delaware Filed Aug. 11, 1960, Ser. No. 48,987 11 Claims. (Cl. 179-1) The present invention relates generally to spectrum analyzers, and more particularly to spectrum analyzers capable of analyzing any predetermined small portion of a relatively long repetitive complex wave form, the portion to be analyzed being selectible at will over the entire complex wave form.

Analysis of speech and other like complex signals existing in the time domain has acquired considerable significance because of its utility in facilitating development of speech compression systems. Speech cannot be properly analyzed by means of conventional scanning spectrum analyzers, because such analyzers require a long time to scan over the audio band with low resolution, while the frequency content of the speech varies rapidly and in a Moreover, the spectrum analysis of a signal of considerable duration, representative of speech, may be virtually devoid of significance, even if accomplished, precisely because the speech contains such a wide range of frequencies and because the speech is so rapidly varying in time. It may, however, be of significance to provide an analysis of the frequency content of a syllable, or of a single sound, such as a particular vowel sound, included in a long speech sample. It is the latter problem to which the present invention addresses itself.

In accordance with the invention, a sample of speech signal of considerable duration is recorded, as on a magnetic tape, so that it may be periodically and repetitively reproduced. A signal indicative of zero time is also recorded on the tape. The tape contents are then reproduced repetitively and periodically, at a relatively slow rate, i.e. at a rate such that a complete reproduction of the tape requires a period of about lone second. The output of the tape as reproduced is applied to a bank of analyzing filters, in accordance with known practice in the art. Such filters constitute an ordered array of band pass filters, which overlap only very slightly, soi as to leave no gaps between filters. The selectivities and number of filters are so chosen as to encompass the entire frequency band of interest. The selectivities can be so chosen that the responses of the filters at every instant of time represents substantially the Fourier sepctrum of the reproduced speech signal. The outputs of the separate filters are detected, and the detectors are connected to the stationary contacts of a `scanning switch, which samples the responses of the filters in sequence, repetitively. The repetitive sampling may be relatively rapid. For example, one complete scan of all the filters may occupy a time of 1&0 second. This time is particularly convenient because it represents a common power line frequency, so that the switch may be electrically driven directly from the power lines by means of a synchronous motor. The output of the switch is applied to the vertical deflection electrodes of a cathode ray tube indicator.

The output of the reproducer is applied to a channel containing `a device for separating out the zero time pulse signal, sometimes hereinafter called sync pulse. The sync pulse is passed through a variable delay network, so that it may be delayed over any time interval equal to or less than the period of scanning of the speech signal by the reproducer. If the reproducer requires one second t complete a scan then the variable time delay network must have a maximum :capacity of one second. The output of the delay device generates an on gating wave for application to a normally closed gate. 'Ihe gating wave 3,165,586 Patented Jan. 12, 1965 ICC is of duration equal to the total sampling time of the scanning switch, i.e. %0 second in the exemplary modification.

A s-ignal is also derived from the power line at an adjustable time with respect to the phase of the scanning switch, this signal being a synchronizing pulse for a sawtooth generator. The synchronizing pulse is passed through the gate when the latter is opened by the gating wave, and initiates lscan in a sawtooth generator, the output of which is applied to the horizontal deflection electrode of the cathode ray tube indicator.

The cathode ray tube is normally not scanning, but provides one horizontal scan for each synchronizing pulse gated thereto, and accordingly provides one horizontal scan for that 1%,0 of a second time interval, in the course of the one second reproduction time, which had been selected by the variable time delay device. Since the time at which the scanning switch selects the first one of the filters has been in no sense synchronized with the gating wave, the adjustment of timing of the sawtooth synchronizing pulse enables the zero time of the scanning switch, i.e. the time when the scanning switch selects the first filter of the filter bank, to coincide with the initiation of the gating wave.

It then follows that the filter bank and the scanning switch provide an ordered frequency analysis of a sample of speech signal occurring at any selected point of the speech representative signal, and enduring for 1&0 second. This sample may be taken at any point in the recorded speech, and regardless of where the sample is taken the display generated by the cathode ray tube will represent the frequency content of the sample in correct order of frequencies.

It is accordingly a broad object of the present invention to provide a system for sampling a relatively long speech signal for a relatively short sampling time, the time being selectable at will, and for analyzing the Fourier spectrum of the sample.

It is another object of the invention to provide a system capable of fiexible selection at will of any portion of a relatively long speech sample, and the spectrum analysis of that portion to the exclusion of the remainder of the speech sample.

The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following detailed description of one specific embodiment thereof, especially when taken in conjunction with the accompanying drawings, wherein:

The single figure of the drawings is a functional block diagram of a system arranged in accordance with the invention.

Referring now to the single figure of the drawings, the reference numeral 10 denotes a length of magnetic tape on which has been pre-recorded a relatively long speech signal which it is desired to analyze, the speech signal being sufficiently long to encompass perhaps one, two or three seconds of time. The tape 10 is driven by means of a motor 11, and is providedwith a read-out head 12, for reading out periodically and repetitively the signal recorded on the tape 10. Recorded on the tape 10, in addition to the speech signal, is a zero time or synchronizing pulse 13, which is of such nature that it may be readily distinguished from the speech signal recorded elsewhere on the tape. For example, the sync pulse 13 may be constituted of a high frequency pulse, higher in frequency than any speech component, and therefore readily separable therefrom.

The read-out head 12 is connected to a bank of filters in parallel, these being denominated F1, F2, F3, Fn, and each being associated in cascade with a detector, the latter being identified by the reference letters D1, D2, D3 Dn.

The filters F1 to Fn, inclusive, are band pass filters, having each a selectivity adequate for the type of analysis desired, and having, in general, equal pass bands, both higher center frequencies as one proceeds upward in the order of subscripts, adjacent ones of the filters being adjacent or slightly overlapping in frequency response, so that the bank of frequencies completely encompasses the spectrum of interest. The detectors Dl-Dn, inclusive, are connected respectively to stationary contacts Cl-Cn, on a one for one basis, so that there is applied to the contacts Cl-Cn, inclusive, voltages representative of the responses of the filters on a one for one basis. The contacts Cl-Cn, inclusive, are then sampled by a rotating wiper 16, which is driven from a synchronous motor 17. The latter may be energized from a 60 c.p.s. power line represented by the terminal 18. The switch, 19, may be taken to include the contacts Cl-C, the rotating contact wiper 16, and the drive motor 17, and may be considered a sampling device for sampling the filters F1-Fl on a periodic and repetitive basis. Each sampling cycle requires 1/0 of a second, on the basis that the terminal 18 supplies 60 c.p.s. signal and that the drive motor 17 is synchronous.

The moving contact 16 of the switch 19 is coupled via a suitable amplifier 20 to the vertical deflection electrode of the cathode ray tube indicator 22. The terminal 18 is connected through a manually controllable phase shifter 23 to control timing pulse generator 24. The latter is arranged to generate one sync pulse each time that the sixty cycle signal applied thereto through the phase shifter 23 passes through zero in a positively going direction, and applies such pulses to the output lead 25.

The read-out head 12 is connected to a sync pulse separator 26, in cascade with which is a variable time delay network 27. The latter is capable of controlling the delay accorded to the sync pulse 13 for any selected time equal to or less than the entire period of rotation of the tape 10. The output of the time delay device 27 is applied to a gate wave generator 28, which supplies a gate wave having a duration of 1/60 of a second, i.e. the time required for one complete sampling cycle of the switch 19. This gate wave is utilized to open a normally closed gate 29, the output of which is connected to a sawtooth generator, 30, which supplies sawtooth deflection voltage to the defiection electrode 21 of the cathode ray tube indicator 22.

In operation, as the tape rotates, a continuous analysis of the frequency content of the tape takes place, and the filters Fl-Fn may be arranged to have sufficiently rapid rise time to enable accurate analysis of a /o second sample of the tape contents. Read out of the sync pulse 13 signals zero time. The time delay At introduced by device 27, in net effect, transfers the effective position of the sync pulse 13 to a point delayed by a time At from the zero point of the tape, and since At may extend from zero to the time of one complete read out cycle, read out may be obtained anywhere over the entire tape length. The gating wave generator 28 then sets up a gating wave representative of a 1&0 second sample of the contents of the tape 10, commencing at time At.

The sync pulse generator 24 supplies sync pulses to gate 29 periodically, once in each sixtieth of a second, but these pulses do not normally pass through the gate, and accordingly the sawtooth generator 30 remains quiescent. Eventually, during a read-out cycle, a sync pulse supplied by the sync pulse generator 24 arrives at the gate 29 while the lattter is opened by the gate Wave generated by the gate wave generator 28. At this time the sawtooth generator 30 generates the horizontal or frequency scan of the face of the cathode ray tube indicator 22.

It is unlikely that the first filter F1 of the filter bank will be sampled precisely at the time when the sawtooth generator 30 initiates a scan. Should this not be the case, the phase shifter 23 can be adjusted to bring about phase coincidence of the position of the rotary contacts 16 and the initiation of the gate generated by the gating wave generator 28. When the phase shifter 23 has been properly adjusted, the presentation provided on the face of the cathode ray tube indicator 21, denominated 32, represents in correct order the frequencies present in the 1,60 cycle sample selected from the tape 10. By adjusting the time delay device 27 other samples may betaken, and for each sample taken the phase shifter 23 may be so adjusted as to cause the filter selection process to proceed in proper order, i.e. to initiate with the filter F1 at the instant the gate opens, and to terminate with filter Fn just before the gate re-closes.

While I have described and illustrated one specific embodiment of the present invention, it will become apparent that variations of the specific details of construction may be resorted to without departing from the true spirit and scope of the invention as defined in the appended claims.

What I claim is:

1. A spectrum analyzer comprising a bank of filters, each of said filters being a relatively narrow band pass filter compared to the spectrum width, the band pass frequencies of said filters being arranged in an ordered array, said filters together encompassing a relatively wide frequency band signal subject to analysis by said spectrum analyzer, a periodically scanning commutator for sequentially sampling the responses of said filters, a repetitively operative reproducer of a time varying signal, said reproducer including structure for imparting a relatively long operating period per repetition, said reproducer including means to provide a first synchronizing pulse at the initiation of each of its operating periods, means coupling said reproducer to said filters in parallel, a visual plotter having provision for defiecting a visual indication in two coordinate directions, means coordinated with said commutator and operative at a predetermined time in each cycle of scanning thereof for generating a second synchronizing pulse, means responsive to said first synchronizing pulse for generating a gate wave of relatively short duration at any time in each of the periods of said reproducer, said gate waves having durations approximately equal to the scanning period of said periodic commutator, a gate circuit for passing said second synchronizing signals only in response to said gate waves, means responsive to each passage of said second synchronizing signals through said gate circuit for effecting a scan of said visual indication in one of said two coordinate directions, and means for deflecting said visual indication in the other of said coordinate directions in response to sampled responses of said filters as supplied by said commutator.

2. The combination according to claim 1, wherein is provided means for lat will establishing the times of said gate wave and the times of said second synchronizing signals.

3. The combination according to claim 2 wherein said visual plotter is a cathode ray tube indicator, and said means for effecting a scan of said visual indication is a sawtooth generator responsive to each of said second synchronizing signals to generate a sawtooth wave form.

4. A spectrum analyzer system for relatively long time period speech signals, said speech signals being reproducibly recorded, comprising means for repetitively reproducing said recorded speech signals, a spectrum analyzer comprising an ordered band of contiguous analyzing band-pass filters continuously connected to said reproducer for continuously analyzing the spectrum of the reproduced speech signals, and means for at will visually representing the ordered responses of all said filters for a selected short time interval compared to said time period only during each reproduction of said speech signals.

5. The combination according to claim 4 wherein is provided means for manually selecting said short time interval.

6. A spectrum analyzer for speech signals, said speech signal being reproducibly recorded on a re-entrant recording medium, means for continuously repetitively repro ducing the record content of said recording medium at a slow rate, means for continuously rapidly analyzing the spectrum of the reproduced signal, and means synchronized with operation of said means for analyzing for visually presenting a plot of the spectrum content of only any preselected small portion of said speech signals.

7. A spectrum analyzer system for real time signals pre-recorded in a recorder having relatively long repetitive readouts of said signals, comprising a visual indicator having provision for generating a two dimensional display, a spectrum analyzer responsive to said signals, said spectrum analyzer being arranged to provide a series of spectrum signals representative of all the frequency components of said signals during time intervals equal to a relatively small traction of the repetition period of said readouts, and means for actuating said visual indicator to generate said two dimensional display only during any selected small fraction of each of said repetitive period.

S. A spectrum analyzer system for a band of signals recorded on a re-entrant medium, means for repetitively reproducing said signals from said medium in relatively long time periods, a spectrum analyzer responsive to the reproduced signals for continuously analyzing in sequence relatively small portions of the reproduced signals, all said small portions taken in said sequence being said reproduced signals, and means included in said spectrum analyzer for at will visually displaying the spectrum of only any one of said small portions.

9. A system for presenting an indication of the spectrum of only a small time portion of a repetitive signal having a relatively long time period, comprising a two dimensional orthogonal display of frequency versus amplitude, means responsive to said signal for deriving another signal indicative of the amplitude variations of said signal for the frequencies in a predetermined frequency spectrum, means for coupling said another signal to the amplitude coordinate of said display, and means for varying the frequency coordinate of said display only during the same small portion of each of said time periods.

10. A system for presenting an indication of the spectrum of only a small time portion of a repetitive signal having a relatively long time period, comprising means responsive to said signal for providing a two dimension orthogonal display of the energy distribution of said signal in a predetermined frequency spectrum versus frequency variations within said spectrum, and gating means for activating said display means only during the same small portion of each of said time periods.

11. A system for presenting an indication of the spectrum of only a small time portion of a repetitive signal having a relatively long time period comprising means responsive to said signal for indicating the energy distribution of said signal in a predetermined multiple, continuous frequency spectrum, and gating means for activating said indicating means to derive said distribution over said spectrum completely only during the same small portion of each of said time periods.

References Cited bythe Examiner UNITED STATES PATENTS 2,465,355 3/49 Cook 179-1 2,530,693 11/50 Green 324-77 2,629,778 2/53 Potter 179-1 2,705,742 4/55 Miller 179-1 2,859,343 11/58 Langford et al 178-6 2,971,152 2/62 Ranky 324-77 2,998,568 8/61 Schlessel 179-1 ROBERT H. ROSE, Primary Examiner.

L. MILLER ANDRUS, Examiner.

Claims (1)

1. A SPECTRUM ANALYZER COMPRISING A BANK OF FILTERS, EACH OF SAID FILTERS BEING A RELATIVELY NARROW BAND PASS FILTER COMPARED TO THE SPECTRUM WIDTH, THE BAND PASS FREQUENCIES OF SAID FILTERS BEING ARRANGED IN AN ORDERED ARRAY, SAID FILTERS TOGETHER ENCOMPASSING A RELATIVELY WIDE FREQUENCY BAND SIGNAL SUBJECT TO ANALYSIS BY SAID SPECTRUM ANALYZER, A PERIODICALLY SCANNING COMMUTATOR FOR SEQUENTIALLY SAMPLING THE RESPONSES OF SAID FILTERS, A REPETITIVELY OPERATIVE REPRODUCER OF A TIME VARYING SIGNAL, SAID REPRODUCER INCLUDING STRUCTURE FOR IMPARTING A RELATIVELY LONG OPERATING PERIOD PER REPETITION, SAID REPRODUCER INCLUDING MEANS TO PROVIDE A FIRST SYNCHRONIZING PULSE AT THE INITIATION OF EACH OF ITS OPERATING PERIODS, MEANS COUPLING SAID REPRODUCER TO SAID FILTERS IN PARALLEL, A VISUAL PLOTTER HAVING PROVISION FOR DEFLECTING A VISUAL INDICATION IN TWO COORDINATE DIRECTIONS, MEANS COORDINATED WITH SAID COMMUTATOR AND OPERATIVE AT A PREDETERMINED TIME IN EACH CYCLE OF SCANNING THERETO FOR GENERATING A SECOND SYNCHRONIZING PULSE, MEANS RESPONSIVE TO SAID FIRST SYNCHRONIZING PULSE FOR GENERATING A GATE WAVE OF RELATIVELY SHORT DURATIONS AT ANY TIME IN EACH OF THE PERIODS OF SAID REPRODUCER, SAID GATE WAVES HAVING DURATIONS APPROXIMATELY EQUAL TO THE SCANNING PERIOD OF SAID PERIODIC COMMUTATOR, A GATE CIRCUIT FOR PASSING SAID SECOND SYNCHRONIZING SIGNALS ONLY IN RESPONSE TO SAID GATE WAVES, MEANS RESPONSIVE TO EACH PASSAGE OF SAID SECOND SYNCHRONIZING SIGNALS THROUGH SAID GATE CIRCUIT FOR EFFECTING A SCAN OF SAID VISUAL INDICATION IN ONE OF SAID TWO COORDINATE DIRECTIONS, AND MEANS FOR DEFLECTING SAID VISUAL INDICATION IN THE OTHER OF SAID COORDINATE DIRECTIONS IN RESPONSE TO SAMPLE RESPONSES OF SAID FILTERS AS SUPPLIED BY SAID COMMUTATOR.

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