»
`3,845,391
`(11)
`United States Patent
`[45] Oct. 29, 1974
`Crosby
`
`
`119
`
`[54] COMMUNICATION INCLUDING
`SUBMERGEDIDENTIFICATION SIGNAL
`
`3,406,344
`3,613,004
`
`10/1968 Hopper... 325/66 X
`10/1971 Wycoff oeeee 325/64 X
`
`Inventor: MurrayG.Crosby» Syosset, Long
`[75]
`oo"
`[73] Assignee: Audicom Corporation, New York,
`N.Y.
`
`Primary Examiner—Benedict V. Safourek
`Attorney, Agent, or Firm—Ryder, McAulay,Fields,
`Fisher & Goldstein
`
`ABSTRACT
`[57]
`July 15, 1971
`Filed:
`(22]
`A techniquefor identifying a program with an identifi-
`No.: 162.7
`Appl.
`21]
`cation code in which the code is modulated onto an
`TT4 -
`Appl. No
`[21]
`audio frequency subcarrier and transmitted with the
`Related U.S. Application Data
`[63] Continuation-in-part of Ser. No. 848,381, July 8,|program. A shorttime period, narrow band width win-
`1969, abandoned, which is a continuation-in-part of
`dow is cut out of the program material to accommo-
`Ser. No. 530,563, Feb. 28, 1966.
`date the code carrying modulated audio subcarrier.
`The amount by which the code modulates the subcar-
`tier is made to track with the audio envelope of the
`[52] US. Che teeeeeee 325/64, 343/225
`[51]
`Int. Chce eseasernecemess H04h 9/00
`program and thus minimizes the listener’s ability to
`{58] Field of Search.......... 179/2, 2 TC, 3, 100.2 R;
`hear the code. The receiver equipment automatically
`325/31, 51, 52, 64, 55, 66, 392, 396, 311,
`responds to the presence of the subcarrier and detects
`37; 178/5.6; 343/225-228
`the code. Unmodulated subcarrier is transmitted im-
`mediately prior to the code modulation to assure that
`there is no ambiguity between the code signal and pro-
`References Cited
`gram material. Automatic frequency control respon-
`UNITED STATES PATENTS
`1,922,627
`sive to the unmodulated subcarrier compensates for
`8/1933) Mathes ccccccccccsccccecscscccssccssees 325/52
`2,513,360
`tape or disc recorder speed variation. The automatic
`7/1950
`Rahmel..............
`325/31
`2,630,525
`3/1953
`Tomberlin et al.
`frequency control is disabled during the actual code
`veces 325/64
`2,671,166
`3/1954 O’Brien...
`transmission to prevent a receiver response that might
`.. 325/64 X
`
`
`3,044,018 7/1962=Wilson........ ... 325/63 X wipe out the code signal.
`
`
`3,387,212
`6/1968
` Kaufman.............
`325/64 X
`.
`9,397,401
`8/1968 Winterbottom... 325/64 X
`17 Claims, 5 Drawing Figures
`
`
`
`[56]
`
`
`
`Sony Exhibit 1014
`Sony Exhibit 1014
`Sony v. MZ Audio
`Sony v. MZ Audio
`
`

`

`PATENTED OCT 29 1974
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`SHEET 1 0F 3
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`3,845,391
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`POLESHAD
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`VS7
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`€42192=>+Ey2162=p
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`41pez=8}
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`

`

`SHEET 2 OF 3
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`FEem a~T
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`B4tANCED
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`PATENTEDOct 29 1974
`
`3,845,391
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`FeoseanFR
`
`MODULATOR
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`

`

`(02987)
`
`YPbIf
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`3,845,391
`
`PATENTED OCT 29 197:
`
`SHEET 30F 3
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`aayyYY00/
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`

`

`3,845,391
`
`1
`COMMUNICATION INCLUDING SUBMERGED
`IDENTIFICATION SIGNAL
`
`CROSS REFERENCE TO RELATED
`APPLICATIONS
`
`5
`
`This is a continuation-in-part of a patent application
`Ser. No. 848,381 filed July 8, 1969, now abandoned,
`which in turn was a continuation-in-part of now aban-
`doned patent application Ser. No. 530,563 filed Feb.
`28, 1966. Both of these patent applications were enti-
`tled: Communication Including Submerged Identifi-
`cation Signal.
`
`10
`
`BACKGROUND OF THE INVENTION
`This invention relates in general to a communication
`system and moreparticularly to a technique for provid-
`ing a uniqueidentification code for any broadcast pro-
`gram material, and in particular for advertising, so that
`an appropriate receiver can detect the code and iden-
`tify that the program has beensent.
`There are a number of systems that have been devel-
`oped and proposed for transmitting auxiliary informa-
`tion along with the main program being broadcast. Su-
`per-audible and sub-audible subcarrier transmission
`has been used in the.prior art for achieving such multi-
`plexing of an allocated broadcast channel. Some idea
`of the scope of techniques employed can be obtained
`from a review of U.S. Pat. Nos. 2,766,374; 3,061,783
`and 3,391,340. These known techniquesare not partic-
`ularly well adapted to the transmission of unobtrusive
`coding signals for identifying and verifying the trans-
`mission of particular programs.
`.
`In general, the known and proposed techniques em-
`ploy an unacceptably large portion of the program
`channel. In particular, there is too much interference
`with the program material.
`Accordingly, it is a major purpose ofthis invention to
`provide a coding technique for identifying a program,
`wherein the coding technique occupies a minimum
`amount of program space.
`In particular, it is an important purpose ofthis inven-
`tion to provide a program identification technique that
`is unnoticed by the listener.
`One current
`technique for monitoring advertise-
`ments on television is to hire individuals around the
`country who look at television and make a record of
`the time, nature and duration of various advertise-
`ments. This technique is expensive, subject to some de-
`gree of error and cost considerations greatly limit its
`use.
`
`15
`
`20
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`25
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`30
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`40
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`45
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`50
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`2
`tising and recorded music. A binary identification code
`is modulated onto an audio frequency subcarrier to
`provide a narrow band modulated subcarrier requiring
`a channel of one hundred Hertz (Hz) in width.
`The audio subcarrier is transmitted for about three
`seconds at the beginning, and for about three seconds
`at the end of the program materialbeing identified. The
`audio subcarrieris frequency shift modulated with the
`binary code signal for the latter part of that three sec-
`ond time period. During the three second period when
`the audio subcarrier is added to the program material,
`a bandstopfilter is switchedin to filter out the program
`material over the one hundred Hz subcarrier channel
`width. The band stopfilter is switched out at the end
`of the three second time period. Thus a three second
`long, one hundred Hz wide window is provided in the
`program material to accommodate the code.
`signal
`The magnitude of
`the audio subcarrier
`(whether or not modulated by the code) is made to
`track with the audio level of the program so that the
`amplitude of the audio subcarrier (that is, the modu-
`lated audio subcarrier) can be as low as possible to pro-
`vide accurate code detection at the receiver while re-
`maining unnoticed by the listener.
`In one embodiment, when program audio levelis nil,
`the subcarrieris fifty-five decibels (db) down from the
`audio level that provide 100 percent carrier modula-
`tion. When program audiois at a level that will modu-
`late the carrier 100 percent, then the audio subcarrier
`is forty db down from that program audio level.
`A band passfilter in the receiver passes only the
`modulated subcarrier, which subcarrier is then de-
`modulated to provide the binary identification code for
`the program involved.
`The audio frequency subcarrier is run unmodulated
`for 1.5 secondsprior to being modulated by the 1.1 sec-
`ond duration binary identification code. The relatively
`long (1.5 second in duration) continuous tone, which
`is the unmodulated subcarrier, provides a condition
`that enables the code receiver to distinguish between
`the immediately following code modulated audio sub-
`carrier and other audio signals that might be present,
`particularly when music is played.
`An automatic frequency control (AFC) system at the
`receiver overcomes the de-tuning of the audio fre-
`quency subcarrier that occurs due to such factors as
`variations in tape or disc recorder speed. The AFC
`jocks onto the audio subcarrier during the 1.5 second
`period of unmodulated subcarrier transmission prior to
`code transmission. The binary code is modulated onto
`the subcarrier by a frequency shift key (FSK) genera-
`tor. Thus for the condition of “mark™ the subcarrier is
`up thirty-five Hz from the center frequency and for the
`condition of “space” the subcarrier is downthirty-five
`Hz from center frequency. To avoid having the AFC
`wipe out the identification code which is modulated
`onto the audio subcarrier by a frequency shift modula-
`tion, the AFC is frozen to a fixed tuning immediately
`prior to the appearance of the modulation (the identifi-
`cation code) on the subcarrier.
`
`Accordingly, it is another important purpose of this
`invention to provide an identification technique for
`program material that is automatic on the receiving end
`and does not require a human monitor.
`The cost of human monitoring is sufficiently great so
`that it can be used only in connection with television
`and not in connection with radio, and evenat that, only
`on a sampling basis.
`Accordingly, it is another purposeofthis invention to
`provide an automatic program monitoring technique
`that can be employed in both television and radio
`broadcasting.
`
`BRIEF DESCRIPTION OF THE INVENTION
`
`This invention is a techniquefor identifying and veri-
`fying the transmission of and duration of recorded
`radio and television program material including adver-
`
`55
`
`6C>
`
`45
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`In the drawings:
`FIG. 1 is a block diagram of that portion ofthe sys-
`tem of this invention which adds the identifying code
`to the program material so that combined code and
`program can be placed on a record.
`
`

`

`3,845,391
`
`3
`FIG. 1A illustrates a variant of FIG. 1 in which a time
`delay unit is employed to assure that the modulation
`volume for the code is synchronized in time with pro-
`gram volume.
`FIG. 2 is a block and schematic diagram ofthe up-
`ward modulator 30 of FIG. 1.
`FIG. 3 is a block diagram ofthe automatic receiving
`unit
`for detecting and recording the identification
`code.
`FIG. 4 is a block and schematic diagram of the noise
`responsive time delay switch 72 of FIG. 3.
`
`DESCRIPTION OF THE PREFERRED
`EMBODIMENTS
`
`4
`turned on. During the first 1.5 seconds of that 1.7 sec-
`ond time period, the output of the FSK generatoris
`held at the center frequency f, of 2,877 Hz. Thus, the
`FSK generator 18 output has three separate frequency
`values, all in the relatively high audio frequency range
`and covering a shift frequency range substantially 70
`Hz. These three audio frequency values are added to
`the audio program material. The succession of fre-
`quency shifts between the mark frequency f,, and the
`space frequency f, constitute the code that identifies
`the program material. The center frequency /, is used
`to identify the code transmission to the receiver. The
`center frequency plus the code frequencies plus the
`standby mark frequencyare called herein the code sig-
`nal.
`It should be recognized that the FSK generator 18
`has the mark frequency of 2,912 Hz as a standby fre-
`quency and that it is the application of the timer 24
`(described below) output whichshifts the FSK genera-
`tor 18 output down 35 Hz to provide the center fre-
`quency output of 2,877 Hz andthatit is the application
`of the space signal from the reader 16 which shifts the
`generator 18 output down 70 Hz to provide the space
`output frequency of 2,843 Hz. The center frequency of
`2,877 Hzis called a center frequency herein because
`that is the center of the code transmission channel and
`that frequency is halfway between the mark frequency
`and the space frequency.
`The FSK generator 18 and reader 16 are not turned
`on except for the purposeof applying the codingsignal.
`Thus, both ofthese units 16, 18 are normally off. At the
`beginning of the program which is to be encoded, an
`operator closes the switch 20, thereby starting a code
`timer 22, The code timer 22 provides an enablingsignal
`V, at its output for a period of, for example, 3.0 sec-
`onds. This enabling signal V, turns on the FSK genera-
`tor 18.
`This enabling signal V,. also starts a timer 24 operat-
`ing. This timer 24 is called herein a center frequency
`timer because the output of the timer 24 shifts the FSK
`generator 18 to its center frequency (2,877 Hz) state
`and holds it in that state for a period, in this embodi-
`ment, of 1.5 seconds. During this 1.5 second period,
`the FSK generator 18 will not be receiving reader 16
`output. More importantly, during this 1.5 second pe-
`riod, generator 18 output is exactly at the center fre-
`quency of 2,877 Hz. The value of having the FSK gen-
`erator 18 output exactly on the center frequency for a
`short pericd of time prior to application of the reader
`16 output will become clear in connection with the de-
`tailed description of the receiver. At this point, let it
`suffice to be said that this 1.5 second duration of a pre-
`determined center frequency output assures that the
`decoding receiver (FIG. 3) has a basis on which to dis-
`tinguish between code signal and program signal.
`This enabling signal V,. also switches the state of the
`switch 12 to the state shownso that the signal recorded
`on the recorder 14 is the program, plus the encoding.
`
`Finally, this enabling signal V, turns on a delay timer
`26, which delay timer 26, after a period of 1.7 seconds,
`turns on the reader timer 28. During the 0.2 seconds
`between turn off of the timer 24 and turn onofthe tiner
`28, the generator 18 puts outits standby signal, which
`is a mark signal. The reader timer 28, once turned on,
`causes the reader 16 to start generating the code to be
`applied to the program material. The reader timer is on
`
`20
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`25
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`30
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`35
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`4(
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`45
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`50
`
`One of the most important contemplated applica-
`tions for this inventionis in the encoding of advertising
`that is being sent on either television or radio. Accord-
`ingly, in order to give some focus to the description of
`an embodiment of this invention, the embodimentin-
`volved will be one that is:adapted to be employed for
`encoding recorded advertising and the description will
`assume such an application.
`The block and electrical schematic diagrams of
`FIGS. | and 2 illustrate the equipment required to add
`the code to the advertising When recording the adver-
`tising on a dise or tape.
`The Basic Encoder (FIG. 1)
`The advertising message, which may be picked up
`live by microphone 10 is normally transmitted directly
`through a switch 12 to a recorder 14, such as a disc or
`tape recorder. Under this normal operation, the state
`of the switch 12 is not as shown in FIG. 1 but rather the -
`movable arm 12a will be connected to the terminal 126 ‘
`so that the program will be directly passed through the
`switch 12 to the recorder 14. However, for the short
`time the code is being added to the advertising, the
`state of the switch 12 will be as shown with the movable
`arm 12a connected to the terminal £2c.
`The reader [6 generates the identifying code thatis
`added to the recorded program. In one embodiment,
`the code is an eight character code, each characterre-
`quiring an eleven-bit binary code. In that embodiment.
`employing a 7.5 character per second transmission
`rate, the total duration of the eight character (88 bit)
`identifying code is 1.1 seconds. A code is applied at the
`beginning of the advertising message and again at the
`end of the advertising message. The receiver thus can
`determine not only that the advertising message has
`been sent and thatit is the right advertising message.
`but also that the message has been sent from beginning
`to end and, further by means of a clock in the receiver,
`the receiver can determine the duration of the advertis-
`ing Message us recorded and as transmitted.
`The output of the reader 16 is applied as a binary
`code to modulate the frequency shift key (FSK) gener- ”
`ator 18. The relationship between the reader 16 and
`FSK generator 18 is such that when the reader code
`output is a one bit, the generator 18 output is its mark
`frequencyf,, and when the reader 16 outputis a zero
`bit, the generator 18 output is shifted to its space fre-
`quency f;. The mark frequency of generator 18 is 35 Hz
`above center frequency and thus is, 2,912 Hz. The
`space frequency of the FSK generator 18 is 70 Hz down
`from the frequencyat the mark state and thus is 2,843
`Hz.
`The FSK generator 18 is turned on for a short period
`of time (1.7 seconds) prior to the reader 16 being
`
`60
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`6S
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`

`20
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`25
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`30
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`5
`for a period of 1.1 seconds whichis sufficient time for
`the reader 16 to apply eight characters, each requiring
`an eleven bit binary code of mark and space signals to
`the input of the FSK generator 18.
`Thus, it may be seen, by virtue of the timers 22, 24,
`26 and 28, arrangedin the fashion shown, that thefol-
`lowing sequencing takes place after the switch 20is ac-
`tuated by an operator:
`1. The switch 12 is switched into the encoding state
`as shown.
`2. Simultaneously, the FSK generator 18 is turned on
`and its output is held at its predetermined center fre-
`quency for 1.5 seconds.
`3, Then, for 0.2 seconds, the generator 18 output is
`the standby marksignal.
`4, Then the reader 16 is turned on for 1.1 seconds
`and generates its pre-programmed mark and space
`code, which code has been programmed to uniquely
`identify the particular program input.
`5. After the reader 16 is turnedoff, there is a 0.2 sec-
`ond time period before the code timer 22 turns off.
`During this last 0.2 second period, the generator 18 is
`in its standby mark frequency output state (2,912 Hz).
`
`6. Then the code timer 22 turns off and the enabling
`signal V, turns off so that (a) the switch 12 switches
`backto its normal state connecting the terminal 12b to
`the recorder 14 and (b) the generator 18 turnsoff.
`The output of the FSK generator 18 is, as can be seen
`from the above description,initially 1.5 seconds after
`frequency followed by 0.2 seconds of mark frequency,
`followed by 1.1 seconds of reader output predeter-
`mined mark and space frequencies.
`,
`The outputof this generator 18 is applied to upward
`modulater 30, The function of this upward modulator
`30, (the structure of which is described in more detail
`in connection with FIG. 2) is to increase the amplitude
`of the FSK generator 18 output audio signal as a func-
`tion of the program audio level. Accordingly, the out-
`put of the modulator 36 is the sameas the input, except
`that the level of the output is increased by an amount
`that directly relates to the magnitude of an envelope of
`the program audio signal.
`The attenuator 32 serves as an isolating amplitude. It
`attenuates because the modulator 30 output is bound
`to be at a much higher volume level than is desirable
`to be added to the programmed material. This attenua-
`tor assures that the FSK generator 18 output frequen-
`cies fm. f and f, are added to program material at a
`level which is between forty and fifty-five decibels
`down from the level of program material that will pro-
`duce 100 percent modulation on the carrier.
`The band stop filter 34 performs a very important
`function of cutting out a narrow frequency band from
`the program material when the recording apparatus ts
`in the state shown in FIG. 1. With the switch 12 shown
`as in FIG. Lt, the program input is applied to the band
`stop filter 34, The filter 34 cuts out all frequencies in
`a one hundred Hz band from 2,827 Hz to 2,927 Hz.
`The adder 36 simply adds the program material with
`the frequency window cut outofit by the filter 34 and
`the properly attenuated modulated subcarrier signal
`from the attenuator 32 to provide the audio input for
`the recorder 14,
`It should be noted that the switch 12 is in the encod-
`ing state shown for only three seconds at a time and
`that it is only during this three second time period that
`
`3,845,391
`
`6
`the bandstopfilter 34 functions to cut out the narrow
`one hundred Hz band from the program material. Thus,
`a frequency window of one hundred Hzwith this three
`second duration is provided.It is, so to speak, through
`this window that the encoded information passes as a
`frequencyshift key type of modulation on an audiofre-
`quencysignal. Thus, the amount of detraction from
`program material is minimal.
`It should be notedthatthe forty to fifty-five db down
`range is a range foundsatisfactory in one embodiment.
`It is expected that the techniqueofthis invention will
`permit the low end of the range to be as low as 60 db
`down from 100 percent program audio modulation.
`Upward Modulator (FIG. 2)
`FIG. 2 illustrates in greater detail the structure of the
`upward modulator 30 shown and described in connec-
`tion with FIG. 1. Thefirst unit in the upward modulator
`30 is a doubly balanced modulator 40 of a known type.
`In one embodiment a four quadrant multiplier inte-
`grated circuit, Type No. MC 1494, manufactured by
`Motorola or by Fairchild, was employed.
`A doubly balanced modulator provides amplitude
`modulation of a carrier with suppression of the carrier
`frequency so that only the side bands are provided. In
`this invention, one of the two inputs to the doubly bal-
`anced modulator 40 is the relatively high audio fre-
`quencyoutputs of the FSK generator 18. The otherin-
`put, on line 40a, is a signal of only a few Hertz because
`it is developed as an envelope of the program audio.
`Thus, when there is a signal on the line 40a, the side
`bands of the generator 18 output frequency that are
`provided as the outputof the modulator 40 are within
`a few Hertz of the generator 18 output frequency.
`From the point of view of the code channel and of the
`5 overall system, this few Hertz displacement of genera-
`tor 18 frequency can be ignored. But from the pointof
`view of the operation of the doubly balanced modula-
`tor 40, this side band generation means that the ampli-
`tude of the output from the doubly balanced modulator
`40 is a function of the amplitude of an envelopesignal
`on the line 40a.
`The modulator 40 is unbalanced slightly so that when
`the input to the modulator 40 online 40a is zero, there
`will be a modulator 40 output having the frequency of
`the FSK generator 18 output. This modulator 40 output
`when program audio levelis zero is set to have a rela-
`tively low predetermined amplitude such that the am-
`plitude of the code signal provided at the adder 36 is
`fifty-five db down from the audio level that provides
`100 percent carrier modulation. As the magnitude of
`the signal on the input line 40a to the modulator 40 in-
`creases above zero volts, then the modulator 40 output
`amplitude increases since increasing amplitude side
`bands are generated.
`The values for the various components in FIG. 2 are
`selected such that when an audio signal from the pro-
`gram material is supplied that has an amplitude equal
`or greater than that which will provide 100 percent car-
`rier modulation, then the magnitude of the signalat the
`line 40a is at a maximum. This maximum amplitude
`audio envelope generates a modulator 40 output which
`is fifteen db above the modulator 40 output when pro-
`gram audio amplitudeis zero. Thus, the maximum am-
`plitude of code signal added by the adder 36 is 40 deci-
`bels below the audio level which provides 100 percent
`modulation. To achieve this result at the line 40a there
`is employed a high pass audio filter 42, an amplifier 43,
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`50
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`7
`a full wave rectifier 44, an envelope following (or rip-
`ple smoothing) circuit 45 and a DC limiter circuit 46.
`
`3,845,391
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`connection with the audio receiver for automatically
`recording the code transmitted and for indicating the
`time at which the code was received. In one preferred
`embodiment,
`this automatic receiver end record is
`maintained on a punchedpapertape. Obviously, other
`recording media could be used.
`As shownin FIG. 3, the audio channel output of the
`receiveris applied to a pre-selector band passfilter 50.
`This bandpassfilter 50 has a 150 Hz band width (2,802
`Hz to 2,952 Hz). The band width of this filter 50 is
`greater than the 100 Hz code channel because of the
`necessity to accommodate for shifts in the frequency
`position of the channel due primarily to disc or tape re-
`cord speed variations at the transmitter end.
`Because the decodercircuit responds to the mark
`frequency and the space frequency to provide an ap-
`propriate binary input for the paper tape perforator,it
`is important that the frequency which represents the
`markcondition be constant and repeatable and thatthe
`frequency which represents the space condition also be
`constantandrepeatable.If speed errors in the transmit-
`ting record are not compensated in the decoder, there
`is a risk that the detector will respond to these signals
`incorrectly and produce a false reading on the paper
`tape perforator. A preferred form of compensating for
`this frequency deviation has been found to be the use
`of an automatic frequency control technique. In order
`to make possible this automatic frequency control, the
`outputofthe pre-selectorfilter 50 is heterodyned with
`the output from a voltage controlled oscillator (VCO)
`52 through a mixer 54. In one embodiment, the center
`frequency of the VCO 52is 5,002 Hz. The mixer $4
`provides the difference frequency as an input to a 100
`Hz wide band passfilter 56. With the VCO $2 center
`frequency being 5,002 Hz andthe pre-selectorfilter 50
`center frequency being 2,877 Hz, the center frequency
`of the 100 Hz wide bandpassfilter 56 is therefore de-
`signed to be 2,125 Hz. Asa consequence, during detec-
`tion of the codesignal, the only substantial input to the
`FSK detector 58 is the contents of the 100 Hz wide
`code channel.
`The FSK detector 58 includesa limiter to remove any
`amplitude modulation that might exist. The detector
`function itself may be performed by a gate FM detector
`of the type described in U.S. Pat. No. 2,470,240. Inte-
`grated circuits that perform both the limiting and gate
`detection functions are manufactured by Sprague Elec-
`uric Co., of Worcester, Mass. under the Type No. UL-
`N-2111 and also by Motorola of Chicago, Ill. under
`Type No. MC 1351P.
`The FSK detector 58 provides a pulse train output
`that is duty cycle modulated as a function of the fre-
`quency of the input signal to the FSK detector 58. In
`one embodiment,the repetition rate of the FSK output
`pulse train is 4,250 pulses per second, essentially dou-
`ble the expected center frequencyof the inputsignal to
`the detector 58. In this embodiment, the duty cycle of
`the output pulses is 50 percent when the input fre-
`quencyto the detector $8 is 2,125 Hz. As the inputfre-
`quency increases, the duty cycle of the output pulses
`increases and as the input frequency decreases,
`the
`duty cycle of the output pulses decreases. The pulse
`train output from the detector 58 is fed to an integra-
`tion circuit 60 (such as an RC circuit) in order to pro-
`vide a code voltage V,. This code voltage V, has a volt-
`age amplitude value which is a function of the duty
`cycle of the FSK detector 58 output and thusis a func-
`
`For the embodiment described, the resistor and ca-
`pacitor in the high pass audio filter 42 are selected to
`start significantly cutting out at frequencies below one-
`half of the space frequency of 2,843 Hz. Thus low
`audio program frequencies which are substantially re-
`moved from code channelfrequencies do notaffect the
`degree or extent of upward modulation. This is because
`the inputfilters at the decoder in the receiver end of
`the system will so completely cut out the lower audio
`frequencies that there is no need to increase the modu-
`lation ofthe codesignals except in response to program
`frequencies that are closer to code channel frequen-
`cies.
`The amplifier 43 provides isolation and assures that
`the transformer T is driven properly.
`The full wave rectifier 44 rectifies the filtered pro-
`gram audiosignal and the resistor and capacitorripple
`smoothing network 45 provide an envelope following
`function on therectified audio.
`The time constant of the RC network 45 should be
`as brief as possible in order to obtain minimum delay
`in response to program audio amplitude so that the
`magnitude of the code signal at the adder 36is in fact
`an accurate function of the program amplitude at the
`adder 36. However,it is also important that the time
`constant of the RC network 45 be long enough to cut
`out the ripple from the rectification of the program. A
`time constant in the order of one to five milliseconds
`has been found satisfactory to meet both of these ob-
`jections. The optimum time constantis in part a func-
`-
`tion of the bit rate from the FSK generator 18.
`35
`The limiter circuit 46 assures that there is a maxi-
`mum modulating signal applied to the modulator 40 so
`that the codesignal transmitted never has a greater am-
`plitude than 40 db down from maximum program au-
`dio. If program audio to the upward modulator 30 is
`otherwise properly limited, this limiter 46 may not be
`needed.
`Asindicated above, the ripple smoothing network 45
`introduces a time constant which in turn provides a
`delay in the response of the modulator 40 to the ampli-
`tude of the program audio envelope. Asa consequence
`of this delay, the amplitude of the code signal provided
`at the adder 36 maylag behind the optimum ordesired
`amplitude which is called for by the amplitude of the
`program signal provided at the adder 36. As shownin
`FIG. 1A. a time delay unit 48 may be employedto pro-
`vide a compensating delay for the program signal. In
`such a case, the undelayed program signalis applied to
`the upward modulator 30 and the delayed program sig-
`nal is applied to the band stop filter 34. If employed,
`the time delay unit 48 is maintained in the circuit dur-
`ing the time when code is not being added because to
`switch the time delay unit in and out ofthe flow of pro-
`gram signal would create a disturbing gap equalto the
`amount oftime delay in the program material.
`It is this ripple smoothing network 45 which assures
`that the modulator 40 tracks with an envelope ofthe
`program audio signal. The time constant of the network
`45 will determine what envelope is employed with the
`signal with which the modulator 40tracks.
`The Basic Decoder (FIG. 3)
`At the receiving end of the transmitted encoded pro-
`gram, there is a decoder mechanism that operates in
`
`40
`
`25
`
`30
`
`45
`
`60
`
`65
`
`

`

`3,845,391
`
`9
`tion of the frequency of the received code channelsig-
`nal. In one embodiment, the value of the voltage V,is
`six volts when a center frequency signal
`is received,
`nine volts when a mark frequency signalis received and
`three volts when a space frequencysignal is received.
`
`During thefirst 1.5 seconds of the three seconds dur-
`ing which thecode channelis transmitted, the center
`frequency from the FSK generator 18 is received by the
`FIG. 3 decoder unit. If the center frequency is received
`exactly on frequency (that is, at 2,877 Hz), the output
`of the band passfilter 56 will be 2,125 Hz thereby pro-
`viding a 50 percent duty cycle detector 58 output and
`a six volt value for the code voltage V,. The AFC hold
`switch 62 is normally closed and thusthe six volt V, sig-
`nal is applied to the VCO 52 to hold the VCO 52atits
`center frequency of 5,002 Hz. During this initial time
`period, deviation ofthe received signal frequency from
`the 2,877 Hz center frequency value results in devia-
`tion of the code voltage V, value and thus of the VCO
`§2 output frequency in a direction that tends to bring
`the frequencyof the signal applied to the bandpassfil-
`ter 56 toward the center frequency value of 2,125 Hz.
`By the AFC technique,
`the FIG. 3 decoder tends to
`compensate for frequency deviations in the transmitted
`signals on the code channel.
`The code voltage V, is also applied to a voltage com-
`parator 64. This comparator 64 is adjusted to a voltage
`tripping level to provide a steady state output voltage
`of, for example, 2.5 volts when the input value to the
`voltage comparator 64 is above the tripping level. In
`this embodiment, the tripping level is selected to be 6.0
`volts. Thus, when the input to the FIG. 3 decoderis
`space frequency, the output of the comparator 64 will
`be essentially zero. However, when a mark frequency
`signal is received, the outputof the comparator64 will
`be the 2.5 volt level. Providing that the AND gate 66
`is enabled, this 2.5 volt signal will be passed through to
`the paper tape perforator 68 to provide an appropriate
`papertape record of received signal. The voltage com-
`parator 64 is of a known type and maybe a Fairchild
`UL 710 device or a Motorola MC 1710 device.
`As described below, this AND gate 66is enabled only
`when the code mark and space frequencies are re-
`ceived. Thus, the perforator 68 receives only 2.5 volt
`inputs when a mark frequencyis received, and zero volt
`inputs when a space frequency is received.
`The code voltage V, is further applied to a second
`voltage comparator 70. In this embodiment, the com-
`parator70 is adjusted to a tripping voltage ofeither 4.5
`or 7.5 volts so that it will provide a steady state output
`signal in response to the receipt at the FIG. 3 decoder
`of the center frequency signal. Otherwise, the voltage
`comparator 70 is the same type of unit as the compara-
`tor 64. Prior to the receipt of the 1.5 second centerfre-
`quencysignal, the noise in the system and from the pro-
`gram will result in the comparator70 output beinga se-
`ries of pulses that can be considered noise. The noise
`responsive time delay switch 72 is turned off and held
`in an off state by noise or by any rapidly varying signal.
`When the code channelis opened, as at the b