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Claims  |
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I claim
1. In a frequency spectrum analyzer having means for repetitively providing
a plurality of successive signals, each corresponding to the energy in a
different frequency range of a successive frequency spectrum of an input
signal to be analyzed, apparatus for providing a real time spectrogram of
said input signal consisting of a plurality of interval spectrograms each
for a different one of said successive frequency spectrums, which
spectrogram displays variations with respect to time in the energy thereof
in each of said different frequency ranges, said apparatus comprising:
means for generating a plurality of display control signals which are
repetitive, the first of said plurality of control signals being
repetitive at a rate corresponding to the repetition rate of said
successive signals, the second of said plurality of control signals being
repetitive at a rate corresponding to the repetition rate of said
plurality of successive signals and having a period equal to the period of
each of said interval spectrograms of a submultiple thereof, the third of
said plurality of control signals being repetitive at a rate of a
plurality of said successive spectrums, and
means operated by said control signals for visibly displaying said
successive signals along a raster which is defined in one direction by
said first and third control signals and in a second direction, transverse
to said first direction, by said second control signals whereby to produce
a plurality of concatenated interval spectrograms which display the
spectrogram of the input signal in real time.
2. The invention as set forth in claim 1 wherein said analyzer includes
timing signal generating means for producing first timing signals
concurrently with each of said successive signals, said display control
signal generating means including:
a. first sweep generator means operated by said first timing signals to
produce, as said first control signals, first sweep signals concurrently
with said first timing signals;
b. second sweep generator means operated by said timing signals generating
means after occurrence of a plurality of said first timing signals equal
in number to the plurality of said successive signals which constitute an
interval spectrogram for producing, as said second control signals, second
sweep signals, and
c. third sweep generator means operated by said timing signal generating
means after occurrence of a plurality of said pluralities of said first
timing signals for producing, as said third control signals, third sweep
signals, and
d. wherein said displaying means includes cathode ray tube means for
providing said raster vertically in said second direction in response to
said second sweep signals and horizontally in said first direction in
response to said first and third sweep signals.
3. The invention as set forth in claim 2 including means for applying said
second sweep signals to said cathode ray tube means to provide vertical
deflection, means for applying the sum of said first and third sweep
signals to said cathode ray tube means to provide horizontal deflection,
said vertical and horizontal deflection providing said raster, and means
operated by said first timing signals for applying said successive signals
to said cathode ray tube means during said first timing signal to control
the intensity of the display.
4. The invention as set forth in claim 3 wherein said cathode ray tube
means consists of a scan converter unit and a video monitor operated by
said scan converter unit, said scan converter unit having fast and slow
deflection inputs for storage of signals in vertical and horizontally
displaced locations therein, said fast and slow deflection inputs being
connected respectively to said means for applying the sum of said first
and third sweep signals and to said means for applying said second sweep
signals, and said scan converter also having video input means connected
to said means operated by said first timing signal for applying said
successive signals.
5. The invention as set forth in claim 2 wherein said analyzer has time
compression circuits including a recirculating memory for said input
signal, said timing signal generator including a source of clock pulses
for circulating said input signal through said memory upon occurrence of a
certain number of said clock pulses, a first counter for counting said
clock pulses and having capacity to count at least said certain number,
means responsive to the count in said first counter for producing first
and second timing pulses respectively when said counter counts said
certain number of clock pulses and when said counter counts a number of
clock pulses less than said certain number, means operated by said first
and second timing pulses for providing as said timing signals, levels
commencing with said second timing pulses and terminating with said first
timing pulses, a second counter for counting said first timing pulses,
means responsive to the count in said counter providing said second
control signals each time said plurality of first timing pulses is
counted, a third counter for counting said second control signals and
providing said third control signals.
6. The invention as set forth in claim 5 wherein said analyzer also has a
heterodyne detector, said apparatus including a first switch operated by
said first timing signals for applying the output of said heterodyne
detector to said cathode ray tube means only for the duration of said
first timing signals to control the intensity of the display thereon
whereby said display is blanked when said first timing signal is absent.
7. The invention as set forth in claim 6 wherein said cathode ray tube
means consists of a scan converter unit and a video monitor, triggerable
flip-flip means operated by said second timing signal for providing
control signals synchronous with the write/read period of said converter
unit, and means for combining said last-named control signal and said
first timing signals and operating said switch therewith such that the
video input to said converter unit from said heterodyne detector is
blanked during the read periods of said converter unit.
8. Apparatus for producing a spectrogram of speech and the like input
signals in real time which comprises
a. means for compressing said input signals such that successive segments
thereof containing redundant portions of said input signals occur
repetitively for a given period of time,
b. heterodyne detection means responsive to said successive segments of
time compressed input signals from said compressing means for providing
successive video signals which vary in amplitude for the interval of said
segments in accordance with the energy in a plurality of successively
higher frequency ranges over the frequency band of said input signals so
as to provide successive interval spectrogram signals,
c. visual display means responsive to said video signals for providing a
frequency-time-intensity spectrograph pattern of said input signals as
they occur consisting of concatenated interval spectrograms, and
d. display control means for generating a plurality of sweep signals
synchronously with said segments to form a raster on said display means
for displaying said interval spectrograms so as to provide a flicker free
display commencing without appreciable interval delay, including means for
generating fast horizontal sweep signals, each concurrent with and of the
same duration as a successive one of said segments, means for generating
vertical sweep signals, each concurrent with and over the same interval as
a plurality of said successive segments equal in number to said plurality
of frequency ranges, means for generating slow horizontal sweep signals
each concurrent with and over the interval as occupied by a plurality of
successive ones of said vertical sweep signals, and means for applying
said vertical sweep signals and the sum of said horizontal sweep signals
to said visual display means.
9. The invention as set forth in claim 8 wherein said display means
comprises cathode ray tube means having intensity control inputs for said
video signal and vertical and horizontal sweep inputs for said vertical
and horizontal sweep signals.
10. The invention as set forth in claim 9 wherein said cathode ray tube
means includes a scan converter and a video monitor coupled to said
converter for receiving the composite video output signal therefrom, said
intensity input being said scan converter video input, and said vertical
and horizontal sweep inputs respectively, being the slow and fast
deflection inputs of said converter.
11. The invention as set forth in either claim 9 including a switch means
connected between the outputs of said heterodyne means and said intensity
inputs, and means for generating a control signal for enabling said switch
means only during the intervals of said segments.
12. The invention as set forth in claim 8 further comprising timing and
control signal generating means including a clock oscillator connected to
said compressing means for establishing the factor by which said input
signals are compressed in time, at least three counters, means for
connecting said counters to each other in succession such that the first
of said counters counts said clock and provides first and second pulses at
the beginning and end of said segments respectively, said second counter
counts said second pulses and provides a third pulse after counting a
number of said second pulses equal to said plurality of frequency ranges
to said third counter, and means for applying said first and second pulses
to said fast horizontal sweep generating means, the output of said second
counter to said vertical sweep generating means and the output of said
third counter to said slow horizontal sweep generating means.
13. The invention as set forth in claim 12 wherein said heterodyne detector
includes a variable frequency oscillator, and means for applying said
vertical sweep signal to said oscillator for changing the frequency
thereof in synchronism with said segments.
14. The invention as set forth in claim 12 including switch means connected
between said heterodyne detector means and said display means for enabling
and inhibiting the application of said video signals thereto, means
operated by said first and second pulses for providing a control signal
level which enables said switch means in the interval between said first
and second pulses and inhibits said switch means in the interval between
said second and first pulses whereby to blank said display in the latter
interval.
15. The invention as set forth in claim 14 wherein said vertical sweep
generator includes a first digital to analog converter connected to said
second counter for providing as a vertical sweep signal a staircase
voltage which increases with increasing counts in said second counter,
said fast horizontal sweep generator comprises circuit means for
generating a ramp wave which increases in amplitude in the interval
between said first and second pulses, and said slow horizontal sweep
generator includes a second digital to analog converter connected to said
third counter.
16. The invention as set forth in claim 15 wherein said visual display is a
scan converter unit having a composite video signal output and a video
monitor responsive to said composite video signal output for producing
said spectrogram, said scan converter having a video input connected to
said switch means and having slow and fast deflection inputs, means for
applying said vertical sweep signals and the sum of said fast and slow
horizontal sweep signals respectively to said slow and fast deflection
inputs of said converter, means responsive to said third pulses for
providing a square wave repetitive at the same rate as said third pulses,
means connecting said square wave to said switch means for blanking said
video signal during one half the period thereof, means for applying said
square wave to said third counter such that said third counter is
incremented synchronously with alternate ones of said third pulses, and
means for controlling the amplitude of said slow horizontal sweep signals
with respect to said fast horizontal sweep signals such that the rasters
of successive interval spectrograms are written adjacent to each other in
said scan converter. |
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Claims |
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Description |
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The present invention relates to apparatus for the spectrum analysis of
signals such as speech and the like and which displays a spectrogram of
such input signals, and particularly to apparatus for generating and
displaying a spectrogram of complex signals such as speech and the like as
they are uttered or as they occur, in real time.
The invention is especially suitable for use in the training of speech and
elocution to deaf persons. The invention however is also applicable for
the analysis and observation of the frequency-time-intensity
characteristics of sound signals, such as music and the outputs of various
transducers such as hydrophones, geophones, vibration transducers and
etc., for providing a spectrographic display thereof.
In an article appearing in Nature Magazine, Jan. 14, 1961, pps. 117 through
119, there is described a real time spectrum analyzer utilizing a digital
time compressor and a heterodyne detector. The heterodyne detector output
is blanked during the initial portion of the analysis of each frequency
range which makes up a spectrum so as to suppress the effects of unwanted
transients. The heterodyne detector output is averaged over each short
time interval of window during which successive frequency steps of the
signal are scanned by the heterodyne detector. The spectrums are
concatenated. The display is generated with an oscilloscope on a film.
However, the resolution or fine detail of the distribution of signal
energy in the time domain is not displayed.
The distribution of signal energy with respect to both frequency and time
contains significant information. In speech, the distribution with respect
to time depicts the pitch characteristics and linguistic information of
the speech utterance. In speech training, especially the training of deaf
persons to speak, the fine detail of the distribution in the time domain
is of special significance. It is a feature of this invention to provide
spectrograph apparatus which visually displays not only the frequency
characteristics of the speech or other input signal being analyzed, but
also, continuously and in fine detail, the time variations in intensity of
the spectrum, as the speech is uttered in real time.
The invention affords the means for spectrographic analysis which has
heretofore been obtainable only with a tape loop spectrograph apparatus
which is not capable of operating in real time, as for example has been
described in W. Koenig, H. K. Dunn, and L. Y. Lacy, "The Sound
Spectrograph," Volume 18, No. 1, July 1946, pps. 19-49. Various types of
spectrographic analysis apparatus have also been heretofore disclosed. For
a general background of the field, reference may be had to the following
U.S. Pats. Nos.: 2,403,986; 2,938,079; 2,998,568; 3,243,703; 3,344,349;
3,548,305; 3,566,035; 3,581,192; 3,634,759; and 3,728,623.
The conventional sound spectrograph plots the rectified but unaveraged
waveform from the bandpass analysis filter, there being a continuous time
history of this waveform along the time axis at each frequency for which
an analysis is performed. The time resolution is therefore limited only by
the impulse response characteristic of the bandpass filter. In the present
invention the waveform from the bandpass analysis filter is likewise
rectified but unaveraged in deriving the sound spectrogram for each
interval. When the intervals are concatenated, continuity of time over the
interval junction is preserved. Thus, pitch periodicity or randomness of a
speech signal can be readily observed as is the case with a conventional
broad band spectrogram.
It is an object of this invention to provide improved apparatus for
producing spectrograms of signals which may vary rapidly in time, such as
speech and the like, which display the spectrums of such signals with fine
resolution, without loss of detail, of the distribution of the signal
energy thereof in the time domain as well as in the frequency domain.
It is another object of the invention to provide an improved system for the
analysis of the spectrum of signals which have complex frequency
distribution and which vary rapidly with respect to time, such as speech
and the like, which apparatus derives and displays continuously and
without loss of detail, the spectrogram of the signal without loss of
resolution in each of the frequency, intensity and time domains.
It is a further object of the invention to provide improved apparatus for
the analysis and display of the spectrum of complex signals, such as
speech, in real time to provide a spectrogram wherein the distribution of
energy of the signal in frequency is displayed continuously with respect
to time so that the fine detail present in the spectrum, such as pitch and
other periodic variations, will be shown in the spectrogram.
It is a still further object of the present invention to provide improved
spectrograph display apparatus which is especially suitable for speech
training purposes, particularly in the training of deaf students to speak.
Briefly described a spectrograph system embodying the invention produces a
real time sound spectrogram of signals such as speech which are complex in
their frequency content and also vary rapidly in time, and which displays
the sound spectrogram on a visual display device. The signal is divided
into short time intervals and a sound spectrogram of each interval is
produced, there being only a short delay before the result appears. The
intervals are concatenated such that the perceptual effect is that of a
continuous sound spectrogram which unfolds in real time. Specifically, in
accordance with an embodiment of the invention, a time compressor and a
heterodyne detector are provided wherein successive segments of the
signals to be analyzed, which contain redundant portions of such signals,
are analyzed in a plurality of successively higher frequency ranges over
the frequency band of interest. A visual display device, preferably a scan
converter and a video monitor which receives signals from the scan
converter, displays the spectrogram. A display control generates a
plurality of sweep signals synchronously with the time compressed segments
as they are analyzed in the heterodyne detector. The sweep signals consist
of fast horizontal sweep signals which are concurrent with and of the same
duration as the signal segments being analyzed; vertical sweep signals
concurrent with and over the same interval as the plurality of segments
which are analyzed to provide the spectrum over the frequency band of
interest; and slow horizontal sweep signals which are concurrent with and
have a duration of the interval occupied by a plurality of successive
vertical sweep signals. In order to provide a spectrogram which
perceptually "unfolds" in real time on the visual display, the time
duration of each interval spectrogram and the delay before that interval
spectrogram appears on the display must be selected. This is accomplished
by making the rate of fast sweep signals consonant with the interval
spectrogram such that complete intervals are scanned and displayed with
minimal delay and without flicker. If the duration of the interval
spectrogram is too long, (on the order of 100 ms) then as the interval
spectrograms appear there will be an annoying flicker effect due to the
low frame rate (10 per second) at which each new interval spectrogram
appears. Likewise if the delay due to processing time of the spectrum
analyzer is too long (on the order of 1/2 sec.) then a person speaking
into a microphone to produce the signals may be annoyed or missled by the
delay between microphone signal and appearance of the transformed signal
on the display. On the other hand, if the interval chosen or the
processing delay is very short (on the order of 5 ms) the perceptual
effect may be satisfactory but the spectrum analyzer circuitry will be
operating very inefficiently (viz., at higher speed than is necessary for
producing the desired display). The choice of suitable parameters (e.g.,
16.7 ms interval, 25.5 ms average delay) is enabled by the invention.
These sweep signals define a raster on the scan converter unit which
stores the output of the heterodyne detector so as to preserve the
intensity variations with time in each frequency segment making up the
spectrogram. Reading and writing are alternately carried out by the scan
converter unit such that the video monitor continuously displays the
spectrogram in real time while preserving the fine detail and resolution
of the energy distribution of the signal, not only in the frequency
domain, but also in the time domain.
The foregoing and other features, objects and advantages of the invention
as well as a preferred embodiment thereof will become more apparent from a
reading of the following description in connection with the accompanying
drawings in which:
FIg. 1 is a block diagram of a real time sound spectrograph for the
analysis and display of speech input signals, in accordance with a
preferred embodiment of the invention;
FIg. 2 is a block diagram of the timing and control signal generator used
in the system shown in FIG. 1;
FIG. 3 is a series of waveforms which illustrates the operation of the
system during the analysis of a segment thereof in one frequency range of
one spectrum;
FIG. 4 is a waveform diagram on a much greatly expanded scale than the
diagram shown in FIG. 3, the diagram showing the operation of the system
illustrated in FIGS. 1 and 2 in analyzing the spectrums which are used to
make up a spectrograph of an interval of the input signal approximately
25.5 ms in duration; and
FIG. 5 illustrates a spectrogram of the speech signal as displayed on the
video monitor of the system shown in FIG. 1, only the first, second and
last spectrums which make up the spectrogram being shown to simplify the
illustration.
Referring more particularly to FIG. 1, the speech input signals as may be
obtained from a microphone are amplified in an amplifier 10 and applied to
time compressor circuits 12. These circuits 12 include an input low pass
filter (L.P.F.)14 which band limits the speech signal to the band of
interest (viz., 0 to 5 KHz). A sample and hold and analog to digital
converter (SHADC) 16 digitizes the speech and applies the samples, which
are in the form of multi-bit digital words, eight bits being suitable, to
a recirculating memory 18. The memory 18 may suitably consist of eight
binary shift registers, one register for each bit, with gates for
recirculating the bits from the output of the registers back to the inputs
thereof. The registers are of sufficient length to hold a time compressed
segment of the signal in memory for a period longer than the time interval
during which a frequency range of the spectrum is analyzed. This latter
time interval is hereafter called the measurement interval.
The length of the memory 18 may suitably be 256 bits and the bits may be
shifted through the memory at a 2.622 MHz rate. The speech signals are
converted by the SHADC once each recirculation of the memory and shortly
after conversion sampled and inputted to the memory. The new eight-bit
digital signal sample displacing the older sample in memory. The memory
length, recirculation rate, sampling and conversion rates, are a function
of the number of frequency ranges which it is desired to analyze in real
time as well as the rate of operation of the display device which is used
in the system. It will be appreciated therefore that the rates and memory
lengths and other specific numerical values are exemplary only and that
other rates, lengths and numerical values may be used, if desired.
The digital signal in the memory 18 is reconverted into analog form by a
digital to analog converter (DAC) 20. A low pass filter 22 smooths the
analog signal from the digital to analog converter 20. Inasmuch as each
digital signal sample is circulated through the memory 256 times as fast
as it is produced, the frequency spectrum is expanded by a factor of 256.
The highest frequency range of interest is 5 KHz. Accordingly, the highest
frequency of interest will be approximately 1.3 MHz which may be the
cut-off frequency of the low pass filter 22.
The reconverted, time compressed analog waveform from the time compressor
circuits 12 is applied to a heterodyne detector 24. The heterodyne
detector includes a variable frequency oscillator which is indicated as a
voltage controlled oscillator (V.C.O.) 26. A different, successively
higher injection frequency for each of the steps of the frequency range in
the spectrum to be analyzed and displayed are provided by the oscillator
26 to a balanced mixer 28 where these injections are mixed with the time
compressed, frequency expanded input signal segments from the time
compressor circuits 12. A bandpass filter (B.P.F.) 30 passes each of the
successive frequency ranges of the spectrum. For example, the V.C.O. 26 is
stepped in 85 steps, from 1.55 MHz to 2.85 MHz. The bandpass filter 30 has
a bandpass of .+-.2.5% about a center frequency of 1.55 MHz, or a total
bandpass of 77.5 KHz. This corresponds to a real time frequency of
approximately 300 Hz (viz., 77.5 JHz + 256). The bandpass filter 30 thus
extracts the lower sideband mixer products produced by the balanced mixer
28, the carrier being suppressed in the balanced mixer. The lowest step
corresponds to a range from zero to approximately 150 Hz in frequency. The
next step is approximately 60 Hz higher. The adjacent frequency ranges
thus overlap, providing redundancy in the frequency dimension. It will
also be observed, as the description proceeds, that the invention provides
redundancy in the spectrum also in the time dimension, since adjacent time
segments have a large number of redundant samples, while continuity of
time over the interval junctions in each frequency range is preserved.
The output of the filter 30 may be applied to a detector 32 such as a
rectifier which provides a unipolar video signal. The detector 32 is
optional and the output signal provided directly from the filter 30 may be
used as the video signal to plot the spectrum. The video signal varies in
amplitude during the time segment over which each frequency range is
analyzed. Thus the variations in amplitude in the video signal from the
heterodyne detector 24 contain the information resulting from the spectral
analysis along the time dimension or axis. It will be noted that this
information is presented continuously and not averaged by utilizing as the
video signal either the output of the bandpass filter 30 directly or the
rectified filter output if the detector 32 is used.
A video switch 34 (such as an analog gate circuit) is operated by a
switching signal in the form of a level which causes the switch 34 to be
either enabled so as to pass the video signals or to be inhibited so as to
block the video signals. In this manner only the time segments of the
input signal which are free of transients, due to the impulse response of
the filter 30, are used to provide the spectrograph display. In addition,
the video switch 34 enables the video input to be blanked over certain
intervals, such as a scan converter read interval, as will be hereinafter
explained.
The video input is applied to a visual display device which is illustrated
as a cathode ray tube device, specifically a scan converter unit 36 with
its associated video (TV) monitor 38. A cathode ray oscilloscope,
preferably of the type using a storage or long persistence cathode ray
tube may also be used. The scan converter and video monitor are especially
desirable since the display is continuously refreshed and may remain on
the screen as long as desired. also spectrograms of the same utterance by
the same or different speakers which are successfully produced may be
plotted and displayed on the screen of the monitor 38 in side-by-side
relationship. This is especially desirable for speech training purposes.
The student can repeat the same utterance and visualize his mistakes or
changes in his speech. Spectrograms of the teacher's voice followed by the
student's voice can also be displayed and held on the screen. The scan
converter unit may suitably be the model MSC-1 Scan Converter Unit
manufactured by the Hughes Aircraft Company, Industrial Products Division,
Oceanside, California. The video monitor may be a conventional television
monitor which receives the composite video signal (including vertical and
horizontal sync components) as the scan converter unit 36 is read out and
generates a television picture on the screen of its cathode ray tube of
the spectrograph.
Display control circuits 40 are used to generate a raster to write the
spectrogram on the scan converter unit or to display the spectrogram on
the screen of a cathode ray oscilloscope, if the oscilloscope is used
instead of the scan converter unit 36 and video monitor 38. A timing and
control signal generator 42 provides timing signals to the time compressor
circuit as well as the timing and control signals to the display control
circuits 40. The generator 42 also generates the video switching signal. A
video switching signal may be obtained which inhibits the video switch
during the read interval of the scan converter unit. A delay circuit
(e.g., a delay line or one shot) may be used, if desired, to delay the
video switching signal to accommodate for delays in the B.P.F.30.
The display control circuits 40 contain a vertical sweep generator 46 in
the form of a digital to analog converter (DAC). Vertical sweep digital
signals from the timing and control signal generator 42 are translated by
the DAC 46 into a staircase waveform each step of which corresponds to a
successive one of the 85 frequency ranges mentioned above. This vertical
sweep signal is applied to step the voltage controlled oscillator 26 in
the heterodyne detector 24 and is also applied to the slow deflection
input of the scan converter unit 36. In the event that an oscilloscope
display is used, the vertical sweep is connected to the Y axis or vertical
sweep input of the oscilloscope.
A fast horizontal sweep generator 48, suitably a ramp or sawtooth wave
generator of the triggerable type, provides a fast horizontal sweep signal
during the measurement interval (i.e., during each time segment that a
frequency range of a spectrum is being analyzed). Since the duration of
this sweep is equal to the interval of the time compressed signals, it is
the highest frequency sweep signal generated by the display control
circuits 40 and therefore is referred to as the fast horizontal sweep.
A slow horizontal sweep generator 50 in the display control circuit
includes a second digital to analog converter (DAC) 52 which receives
horizontal sweep digital signals from the timing and control signal
generator 42. These horizontal sweep digital signals are incremented every
alternate vertical sweep whether the scan converter unit 36 or an
oscilloscope display is used. The output of the second digital to analog
converter (DAC) 52 is a staircase waveform which is repetitive after a
large number of spectrums are produced, so as to permit a plotting of a
spectrogram several seconds in duration such that an entire speech
utterance, e.g., a syllable or even a complete word or phrase, can be
displayed.
The staircase waveform the DAC 52 may be integrated in an integrator
circuit 54 such as an operational amplifier having a very long time
constant capacitor feedback circuit. The integrator 54 is optional and the
staircase waveform from the second converter 52 may be used directly as
the slow horizontal sweep signal. The fast horizontal sweep signal and the
slow horizontal sweep signal are summed together in a linear adder circuit
56. Preferably the amplitude of the fast horizontal sweep is adjusted
relative to the amplitude of each step of the slow horizontal sweep so
that they are equal to each other. The spectrum for successive time
segments of the input signal will then be concatenated (plotted in
side-by-side relationship) as will be explained more fully hereinafter in
connection with FIG. 5. The composite horizontal sweep signal is adjusted
in amplitude in a variable gain amplifier 58 and then applied to the fast
deflection input of the scan converter unit 36. In the event that an
oscilloscope display is used instead of the scan converter unit, the
horizontal sweep is applied to the X axis or horizontal sweep input of the
oscilloscope.
When the visual display consists of the scan converter unit 36 and its
associated video monitor 38, it is desirable to synchronize the timing and
control signal generator 42 and the timing circuits in the scan converter
unit 36 with each other. The scan converter operates at the standard
television frame sync rate of 60 Hz and provides a frame sync output
repetitive at 60 Hz. This frame sync output is applied to the timing and
control signal generator 42 and used in the generator 42 to synchronize
the analyzer and display portions of the system.
The timing and control signal generator 42 is illustrated in FIG. 2. A
voltage controlled clock oscillator 60 provides clock pulses repetitive at
2.622MHz. These pulses are used directly as shift pulses for the shift
registers of the recirculating memory 18 and provide the shift output of
the timing and control signal generator. The clock oscillator 60 is phase
locked to the frame sync output of the scan converter unit 36. A vertical
sync stripper circuit 62 of the type conventionally used in television
systems extracts the vertical sync pulses which are repetitive at 60 Hz.
These sync pulses are applied to a phase comparator and filter circuit 64
and are compared therein in phase with 60 Hz pulses derived from the clock
oscillator pulses. An error voltge is obtained from the phase comparator
and filter circuit 64 and locks the oscillator 60 at a frequency of 2.622
MHz, in phase with the vertical sync pulses from the frame sync output of
the scan converter unit 36. The clock oscillator 60 and the phase
comparator and filter 64 may suitably be an integrated circuit phase
locked oscillator device of the type which is commercially available.
Alternatively, they may be fabricated from discrete circuit components in
accordance with phase locked loop design techniques which are presently
conventional.
A counter 66 counts the clock pulses. This counter has the capacity to
count a number of pulses at least equal to the bit length of the
recirculating memory. The bit length of the memory is 256 bits. Thus, an
eight-bit binary counter may suitably be used as a counter 66. 257 pulses
are counted during the recirculating time of the memory, which is
illustrated in waveform (a) of FIG. 3, is 98 us. The convert pulses (viz.,
the convert output of the timing and control signal generator 42) are
obtained by a decoder 68 which may include gates connected to the counter
66 which are enabled to provide an output pulse when the counter counts 87
clock pulses and upon the 87th of these pulses.
Another decoder 70 which may also consist of gates connected to the stages
of the counter 66 detects when 170 pulses are counted by the counter and
provides an output on the 170th pulse. The sample pulse is provided by the
decoder 170. This pulse is applied to the sampling gates of the
recirculating memory and enables these gates to pass a sample digital
signal from the SHADC to the memory 18. Otherwise, these sampling gates
are inhibited and the memory recirculates at the shift pulse rate (2.622
MHz). The convert pulse from the decoder 68 is illustrated in waveform (b)
and the sample pulses and their intervening recirculation period of 98 us
is illustrated in waveform (c) also in FIG. 3.
A flip-flop latch 72 and an OR gate 74 which is connected to the reset
input of the counter 66 establish the transient interval and measurement
interval or time segment during which each frequency of each spectrum is
analyzed. The flip-flop 72 also produces the horizontal sync signal at its
"0" output.
Initially, the entire system is reset. This may be accomplished
automatically whenever the erase control of the scan converter unit is
operated; the scan converter providing an erase level which resets the
flip-flop 72, the counter 66, as well as the other counters in the system.
With the flip-flop 72 reset, an enable input is applied to the gates of
the decoder 68. When 87 clock pulses are counted by the counter 66 the
convert pulse is outputted by the decoder 68. This convert pulse also is
applied to the set input of the flip-flop 72 and to the reset input of the
counter 66 by way of the OR gate 74. The flip-flop 72 is then set and the
counter 66 is reset back to zero. The decoder 68 is then inhibited by the
"0" output of the flip-flop 72. The next ensuing 170 pulses occur. The
decoder 70 then provides an output pulse which is used as the sample pulse
upon occurrence of the 170th clock pulse which follows the 87th clock
pulse. These pulses are illustrated in waveforms (a), (b) and (c) of FIG.
3.
The sample pulse from the decoder 70 again resets the counter 66 and also
resets the flip-flop 72 so that the cycle may commence. The "0" output of
the flip-flop 72 is illustrated in waveform (d) of FIG. 3. The duration of
approximately 33.2 us from the first to the 87th clock pulse of the 257
pulses which constitute the 98 us of the recirculation time of the memory
also constitute a transient interval. This is the interval after the
sample pulse when the transient output of the B.P.F. 30 of the heterodyne
detector is produced. It is desirable to suppress this transient output
and this is done by the horizontal sync signal which is applied to the
video switch 34. The horizontal sync signal is low during the transient
interval and inhibits the switch so that the video input to the scan
converter unit 36 is cut off during the transient interval. When the
flip-flop is again reset, that is during the remaining approximately 64.8
us of the recirculation time of the memory 18, the flip-flop is reset and
provides a positive output level over the measurement interval as shown in
FIG. 3(d). It will be noted that this measurement interval repeats during
successive time segments of the input signal, each corresponding to 170
clock pulse periods. The impulse response of the filter has decayed during
the transient interval and the measurement interval contains the
information as to the frequency and amplitude characteristics of the
speech input signal without any distortion and nevertheless utilizing the
entire bandwidth and broadband response of the filter 30.
It will be noted that a new sample of signal data from the SHADC is
inputted to the memory once each recirculation time (viz., every 98 us or
257 clock pulse periods), 256 recirculations or an interval of
approximately 25.1ms (viz., 98 us .times. 256) being required to enter a
completely new signal lacking any redundancy, into the memory 18. This is
the delay before a new interval spectrogram is displayed. Over a large
number of recirculation times essentially the same information is
available for read-out and is read out of the memory by the digital to
analog converter 20.
In the illustrative system, during the interval of 85 successive
recirculation times, 85 consecutive frequency ranges from approximately
zero to approximately 5000 Hz in real time in approximately 60 Hz steps
are analyzed. A counter 78 having the capacity to count to 85 is used for
counting each of these 85 steps. A seven-bit binary counter which is
capable of counting to 128 is therefore suitable as the counter 78, and is
used in the illustrated system. The counter 78 is connected successively
or in tandem with the counter 66. The decoder 70 provides the sample
pulses which are counted in the counter 78. When the counter 78 counts 85
sample pulses, a decoder 80 which may consist of gates connected to the
stages of the counter 76 provides an output pulse and thereupon resets the
counter 78. When the 85th pulse is decoded by the decoder 80 a triggerable
flip-flop 82 is triggered. The flip-flop divides by two and produces a
square wave signal which is high for 85 sample pulses and low for the next
85 sample pulses. The operation of the counter 78, decoder 80, and
triggerable flip-flop 82 may be observed from waveforms (a), (b) and (c)
of FIG. 4. Waveform (a ) is the same as waveform (d) of FIG. 3, but is
drawn to a greatly expanded scale, and shows the horizontal sync pulses
provided by the flip-flop 72. The sample pulse is concurrent with the
trailing edge of the measurement interval. Thus, at the end of the 85th
horizontal sync pulse cycle the square wave generator flip-flop 82 is
triggered. Each recirculation of the memory 18 and each horizontal sync
period (transient interval plus measurement interval) is 98 us in length.
Eighty-five of such intervals has a duration of approximately 8.33ms. The
period of the square wave is two of such 8.33ms intervals or approximately
16.67ms. This is a 60 Hz rate. The square wave produced by the flip-flop
82 is therefore repetitive at a 60 Hz rate which is identically the
vertical sync period of the vertical frame sync pulses from the scan
converter 36. Accordingly, by comparing the scan converter frame sync
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