Sept. 9. 1959
`
`R.V'W,CEAiE.FEE ETAL’
`
`3.466.392
`
`VESTIGIAL SIDEBAND FREQUENCY SHIFT KEYING MODEM
`
`Filed March 3, 1966
`
`.
`
`‘
`
`‘
`
`'
`
`‘ e Sheets-Sheet 1
`
`Lu
`
`©:
`
`3 :_
`
`J
`:1.
`2
`
`“
`U-’>
`
`E._J
`3;
`
`éjféjm’
`
`‘
`
`._&/c—;;&/ we
`FREQUENCY
`
`-.
`4/C4-aw
`
`~
`
`‘
`
`FIG 1
`'
`
`.
`
`.
`
`E
`
`.5 $
`
`3 ca
`
`CHANNELRESPONSE
`
`*5oo1*1'ooo
`.
`E
`
`$1500’
`raaoueucv
`
`2000
`
`2500
`
`
`
`‘
`
`E
`
`«
`
`.
`
`—
`
`t
`_
`E
`«
`V
`ASOLIDALINE 3 MODULATING SIGNAL AND NSTANTANEOUS
`'
`AFREOUENCYECHANNEL INPUT
`_
`DA_SHED,‘LlNEi
`INSTANTANEOUS FREQUENCY CHANNEL
`— ouwur
`
`'
`2500
`V B
`
`V
`
`.
`
`. INVENTORS
`RICHARD w. CALFEE
`' EMIL HOPNER
`.
`NORMAN F. MEYER‘-
`LYNN P- WEST
`‘
`'
`7 r4u.‘1w/L iecém
`.
`' ATTORNEY
`
`_
`
`SIMPLEAIR EXHIBIT 2029
`Google v. Simp|eAir
`v|PR2015—OO18O
`V
`
`§+2m ;.+2
`§
`.3 +Af
`‘<2/3+1
`g 03.
`‘ $231.0 g cc
`3 Ao_.
`51.2‘ E -Ar E’-1
`<
`ca:
`-
`. 3
`:3 1 4
`U, “ZAE :2 "2
`5
`E
`E‘;
`-SM -3
`
`16
`
`-
`
`HG 3 ,
`-
`A
`
`_
`
`-
`
`2 ‘
`
`V
`
`0
`
`-6 I
`
`
`
`GAIN(DECIBELS)
`
`-12
`
`'18
`-24’
`
`
`W »_
`-30
`»
`E» —,
`-
`1oo
`N
`500
`‘ me 1500
`FREOUENCY(cps)
`-
`FIG 4
`’
`'
`-
`
`I
`
`_ _'
`
`‘
`
`
`
`

`
`%5¢Pt- 9. 1969
`CALFEE ETAL
`VESTIGIAL SIDEBAND FREQUENCY SHIFT KEYING MODEM
`.6 Sheets-Sheet 2
`
`. 3,466,392
`
`Filed March 3 . 1966
`
`3,
`
`$5..._
`
`_
`
`53:2$:_EA\._...%.
`
`
`.
`.
`
` _.3...
`
`
`
`IIIIIIlLf|Ae% % %
`
`
`
`
`
`
`7.4::uuuuu:+....1_.%..__.11:IIII
`._:.2_.:=.sE1.44._._._.~m...__>_._.§..m..4§_._._..
`
`
`
`
`
`
`
`
`..__._.___;s=.o§_...._._......2..:§_s_.
`
`
`
`:_._._..._..a.HM“"_ww.unmn..4u“7.1¢.I._IlwitwwlwII_H.1+I+.NHHHHHHHHMHIJ
`
`_.___._I____rill...IIIL___J_..”32.2.5"E3_"2.Ex.1:
`e_..HHH:IHH|I>um:wnlI!:IIh_..“_Fn{Lu".95‘6H§:zoQi._~4om,w_m_:=<L.1?W:%s§m-E1ruuuuuuuuuuuIINHHHIIIILLFIIIIIIIIN_I$m__.w_EI
`
`
`__1__:-:-..-.... _ .: “__.“T:._£.A__m
`
`
`__fl.,.1_.4rJ_..~_____3_H___:‘i9‘z:mug.”
`
`
`
`

`
`3,466,392.
`%
`a.
`V s§p:.9,196§" ’
`VESTIGIAL sxugaakb FREQUENCY SHIFT xmvxuc MODEM
`med March 5. 1966
`A‘
`'
`A
`'
`—
`C
`' e Sheet%—Sheet3
`
`VEOUTIEm(DECIBELS)
`
`FIG. 6
`
`10
`
`'
`
`"I
`
`so
`
`-
`
`1007%»‘.‘ §§‘oo'_'-%1ooo
`F*:E.¢UE??°¥.~¥=¢P€"-
`
`_
`
`5000. 10,000
`'
`
`’
`
`(‘WITHOUT
`PRE - EMPHASIS)
`
`({Z)§(‘l(l(:AGig(f)g\TED)'-
`
`
`
`
`
`F'G~8 ®—
`
`®
`
`' ~ v< zeno caossmc
`PRESERVATION)
`’
`
`fOUT(cps)
`
`3200
`
`3100
`
`L
`
`3000
`
`2900
`
`"2800
`
`F|¢. 7
`
`

`
`3,466,392
`R. w. CALFEE ETAL
`Sept 9, 1969
`VBSTIGIAL SIDEBAND FREQUENCY SHIFT KEYING MODEM
`
`Filed March 5, 1966
`
`6_ Sheets-Sheet 4
`
`4
`1
`
`Q
`Q
`
`29203
`
`3 :35 2255.2 4
`
`‘Fl.l.|||:| l I l I I I I I I lll?.
`
`
`
`_ _ _ _ _ _ _
`
`

`
`3,466,392
`R.w.cA1.|-':E ETAL
`_ Sept 9, 1969
`VESTIGIAL SIDEBAND FREQUENCY SHIF'I" KEYING MODEM
`
`Filed March 3, 1966
`
`6 Sheets-Sheet 5
`
`0
`
`I
`
`|
`
`l
`
`|
`
`|
`
`l
`
`I
`
`5
`g
`
`L;
`
`E
`‘E
`<
`
`FIG .10
`
`- —600
`
`- ~'soo E’;
`- -400 g
`
`- ~3oo ;
`
`— -2oo g
`z
`—-100 w
`
`“ °
`
`5
`
`l
`
`.
`
`l
`
`I
`
`l
`
`l
`
`l
`
`l
`
`0
`
`800
`
`2400
`1600
`FREQUENCY (cps) ’
`
`3200
`
`.
`
`1
`
`l
`
`l
`
`1
`
`l
`
`l
`
`l
`
`I
`
`I
`
`I
`
`i
`
`l
`
`0
`
`400
`
`1200 1600 2000 2400
`‘800
`FREQUENCY (cps)
`
`. A"""‘l'. lv‘va" L’
`
`@@ @c-aca
`
`FiGgiZ
`
`110001100010100110011
`
`

`
`3,466,392
`'
`R. w. CALFEE ETAL
`5°!“- 9, 1969
`VES'I'IGIAL SIDEBAND FREQUENCY SHIFT KEYING MODEM
`
`Filed March 5, 1966
`
`6 Sheets-Sheet 6
`
`[10
`
`FROM
`SOURCE °_" ' ’
`
`FILTER
`TRAP
`
`+ 72'
`
`'
`
`#3 T0 MV CONTROL
`cmcun 18
`(no.5)
`
`FROM CLOCK
`
`TUNED
`°_'—’ cmcun
`
`/14
`
`Ms
`
`FIG. 13
`
`FROM
`
`TRANSITION - ro- PULSE <>
`CONVERTER (me)
`
`-'
`
`-
`
`,
`
`’
`
`'
`
`_
`
`w W W5“ 44
`' We?"
`
`

`
`United States Patent O?ce
`
`3,466,392
`Patented Sept. 9, 1969
`
`1
`
`3,466,392
`VESTIGIAL SIDEBAND FREQUENCY
`SHIFT KEYIN G MODEM
`Richard W. Calfee, San Jose, Emil Hopner and Orman
`F. Meyer, Los Gatos, and Lynn P. West, San Jose,
`Calif., assignors to International Business Machines
`Corporation, Armonk, N.Y., a corporation of New York
`Filed Mar. 3, 1966, Ser. No. 531,488
`Int. Cl. H041 27/00; H04b 1/00, 1/62
`US. Cl. 178-66
`7 Claims
`
`ABSTRACT OF THE DISCLOSURE
`A binary communication system having an oscillator
`that is frequency modulated by a binary signal that is
`twice the frequency of the output signal. The output of
`the oscillator is connected to a vestigial sideband ?lter
`to thereby provide the frequency modulated vestigial
`sideband output. At the receiving end, there is a detector
`that detects transitions to provide an output pulse for
`each transition which is then ?ltered to provide an ap
`propriately coded three-level signal.
`
`10
`
`15
`
`25
`
`35
`
`45
`
`2
`be combined in a practicable and economical communi
`cations system.
`The foregoing and other objects, features and advan
`tages of the invention will be apparent from the follow
`ing more particular description of a preferred embodi
`ment of the invention, as illustrated in the accompanying
`drawings:
`FIGURE 1 is a graph of the theoretical pass band of a
`VSB ?lter;
`FIGURE 2 is a graph of the theoretical vpass band of a
`2400 baud VSB-FM system based on FIGURE 1 con
`straints;
`FIGURE 3 is a graph showing a modulating signal and
`instantaneous frequency of the resulting FM signal after
`passing through a VSB—FM system having a. pass band as
`depicted in FIGURE 2;
`>FIGURE 4 is a graph of the overall response of a
`VSB-FM modem;
`FIGURE 5 is a circuit diagram of the modulator of
`the invention;
`FIGURE 6 shows the amplitude response of the input
`circuit of the modulator of FIGURE 5;
`FIGURE 7 is a graph of the transfer characteristic of
`the modulator of FIGURE 5;
`FIGURE 8 shows typical waveforms present at various
`points of the modulator of FIGURE 5;
`FIGURE 9 is a circuit diagram of the demodulator
`of the invention;
`FIGURE 10 shows the amplitude and phase response
`of ‘the ?lter of the demodulator of FIGURE 9;
`FIGURE 11 shows the gain characteristic of the de
`emphasis circuit of the demodulator of FIGURE 9;
`FIGURE 12 shows typical waveforms present at vari
`ous points of the demodulator of FIGURE 9; and
`FIGURES 13 and 14 are block and circuit diagrams
`of clock circuits which may be included in the system of
`the invention, the former in the modulator of FIGURE
`5 and the latter in the demodulator of FIGURE 9'.
`The present invention comprises a VSB-FM modem
`developed in accordance with the following considera
`tions.
`An FM signal may be represented as
`(1)
`S(t) =cos (wet-H3 sin wmt)
`=cos wet cos (/3 sin wmt) —sin wct sin (5 sin wmt)
`where:
`S(t)=instantaneous signal amplitude
`t=time
`W=21rf
`fc=center frequency
`fm=sine wave modulating frequency
`B=modulation index
`For B<0.5,
`
`This invention relates to a communications modem
`(modulator-demodulator) and, more particularly, to a
`modem capable of unusually high speed communication
`of binary information over readily available lines, through
`utilization of a combination of vestigial-sideband fre
`quency modulation and three-level coding techniques.
`Conventional double-sideband (DSB) modulation re
`quires a transmission bandwidth of at least twice that
`of the modulation signal. For amplitude modulation
`(AM), the requirement is exactly twice the signal band
`width; for frequency (FM) and phase modulation (PM),
`it may be even higher, depending on the permissible dis
`tortion and the modulation index ,8 (the ratio of the
`maximum deviation from the center frequency, f0, to the
`modulating frequency fm). To utilize the bandwidth e?i
`ciently for FM and PM, 5 should be small (<05) so that
`the bandwidth is as close as possible to that for AM.
`In order to transmit binary information at high speed,
`i.e., 4800 bauds, over a channel bandwidth of no more
`than that available on leased telephone lines, i.e., 2300
`c.p.s., e?icient bandwidth utilization techniques must be
`chosen. For transmission, FM is attractive because it has
`been suggested as a standard for low speeds and appears
`adaptable for high speed operation. Also, since, for DSB,
`the same information is contained in both sidebands,
`vestigial sideband (VSB) can nearly double the speed for
`a given bandwidth, and thus is an attractive choice for
`modulation. In addition, binary coding in three voltage
`levels (duobinary) is, as well known, an e?‘icient band
`width compression technique and thus also comprises a
`good choice.
`It is an object of this invention to provide a communi
`cations system capable of handling binary data at speeds
`in excess of those which previously characterized com
`munications of high reliability and limited bandwidth.
`Associated with this object, it is a further object to pro
`vide for the requirements normally associated with binary
`communications such as synchronization and clocking.
`It is another object of this invention to accomplish the
`foregoing utilizing VSB communications and FM modu
`lation in a combination which absorbs the advantages of
`both of these systems.
`It is still another object of this invention to provide a
`modern capable of communicating the analog signals of
`facsimile documentation and similar applications.
`It is an additional object of this invention to teach how
`the techniques of VSB, FM and three-level coding may
`
`50
`
`55
`
`60
`
`Therefore, for p<0.5, Equation 1 becomes
`S(t) ECOS wct—/3 sin wmt sin wct
`ECOS wct— (13/2) cos (wc—wm)t+ (18/2) cos
`(w\=+wm)t (2)
`In a VSB system in which the upper sideband is chosen
`for deletion, Equation 2 becomes
`‘
`
`65
`
`70
`
`In an FM system, the amplitude variations are removed
`by limiting; Equation 4 becomes
`
`

`
`3,466,892
`
`(5)
`
`10
`
`20
`
`3
`SLL(t)§cos [wJ-i-(B/Z) sin wmt]
`Comparing Equations 2 and 5,
`SLL(t)=cos wct—(B/4) cos (wc—wm)t+ (6)
`(5/4) cos (wC-i-wm)t
`Equation 6 represents the original FM waveform with
`the sidebands down 6 db.
`But a binary data waveform has low-frequency compo
`nents as well as high-frequency components. When these
`components modulate the FM signal, the above analysis,
`which contemplates sine wave modulation, is no longer
`valid. However, for large ,8, most of the energy of the
`PM waveform lies between we: (?+l)wm.
`Further, to prevent distortion, the transmitter’s VSB
`?lter must pass, without attenuation, the band of fre
`quencies wctdw (Aw is the deviation). FIGURE 1 shows,
`in graph form, the total pass band theoretically required
`for the VSB ?lter.
`.
`From the above analysis, it may be expected that high
`modulating frequencies would be 6 db lower than low
`modulating frequencies and, to correct this distortion, ‘it
`would appear that, in the receiver, a complex ?lter would
`be required between the limiter and the discriminator. But
`a reduction to practice according to these principles dem
`onstrated that, at a carrier (fc) of 2700 c.p.s. and a
`deviation (A1‘) of :75 c.p.s., shown in FIG. 2, the wave
`form was reproduced without distortion, and no correc- '
`tion was required. This result was completely unexpected
`and indicated a need for further and more complete analy
`sis, involving the following considerations.
`If the channel is such that the carrier is decreased 6 db
`and the upper sideband eliminated, Equation 3 above be
`comes
`
`25
`
`30
`
`where:
`
`0(t) = tall-1
`
`,8 sin wmt
`l—B cos wmi
`When amplitude variations are removed by limiting (in
`the demodulator),
`
`40
`
`Further, it was shown experimentally in the above
`reduction to practice that, in the receiver, when the wave
`form is limited before being detected, low modulating
`frequencies (large )8) are passed through a ?lter with a
`characteristic as shown in FIG. 2 without distortion. Thus,
`limiting not only restores the upper sideband for high
`modulating frequencies, but corrects the “tilt distortion”
`of the deviation range that affects low modulating frequen
`cies. For instance, if the modulating waveform is that
`shown in FIG. 3, then this waveform represents the in
`stantaneous frequency of the resulting FM waveform and
`also the amplitude characteristics of the waveform after it
`has passed through the ?lter of FIG. 2. For DC signals
`(all ones or all zeroes), such as may characterize binary
`data, the output of the frequency modulator is a steady
`frequency, which must be detected at the receiver, even
`though it may be attenuated somewhat by the channel.
`Thus, for low data rates, the deviation is ?xed by the
`upper frequency cutoff of the channel, corresponding to
`+1Af (where A)‘ is the DC deviation) of FIG. 3; but at
`high modulating frequencies, it is advantageous to in
`crease the maximum deviation to as much as :4Af.
`As indicated, the instantaneous frequency output of the
`modulator is shown in FIG. 3. Since the channel cannot
`pass the high frequencies, the instantaneous frequency out
`put of the channel is distorted (dashed waveshape), with
`the result that the deviation becomes asymmetric. For
`negative excursions, the instantaneous frequency devia
`tion is not affected; for positive excursions, distortion
`of instantaneous frequencies by the channel results in
`lower deviation. If the resulting waveform is then dis
`
`50
`
`60
`
`70
`
`75
`
`4
`criminated, the combination of amplitude variations in
`the input signal and the asymmetric deviation caused by
`the loss of high frequencies in the channel produces out
`put distortion.
`If the channel has linear phase characteristics over the
`DC deviation range, the zero crossings are preserved, and
`the tilt distortion of the FM signal is removed. The PM
`waveform can then be accurately reconstructed by limit
`ing. When larger deviations are allowed, the reconstruc
`tion of the upper sideband by limiting makes it possible
`to detect the higher frequencies, even though the cor
`responding instantaneous frequencies are not passed by
`the channel.
`An optimum system is one that maximizes the signal
`to-noise ratio while minimizing the necessary bandwidth
`and the distortion due to spectral foldover, improper ?lter
`ing, etc. In the present system, VSB operation makes
`optimum use of the available bandwidth and distortion is
`minimized by keeping the deviation (and therefore ,8)
`small. But to maximize the signal-to-noise ratio, it is nec
`essary to operate with a high 6. The fact that the noise out
`put of a discriminator with white noise input is not ?at,
`but a function of frequency, suggested a possible solution
`to this problem: increasing the signal-to-noise ratio by
`the technique of pre-emphasis=de-emphasis.
`For the channel described, a carrier of 2700 c.p.s. and
`deviation of :75 c.p.s. appears to give optimum results.
`However, since {3 is equal to Af/fm, the ratio decreases
`with increased modulating frequency. Operation for small
`,8 is, as shown, similar to that of a VSB-AM system,
`and the resulting frequency components are independent
`of 13. Therefore, to operate at a maximum signal-to-noise
`ratio, pre-emphasis of the modulating waveform by 6
`db per octave above the point where 5:0.4 was incor
`porated. Thus, 5 remains constant above this frequency.
`For the system under discussion, {3:04 at f=l90
`c.p.s. Therefore, a pre-emphasis of 6 db per octave was
`added to the modulating signal at 200 c.p.s., and the Out
`put of the discriminator was equivalently de-emphasized;
`this technique decreased by 14 db the signal-to-noise ratio
`necessary to maintain a given error rate; i.e., reliability
`is considered excellent and acceptable up to a 3600 baud
`data rate.
`In addition to the above considerations, the present sys
`tem employs another bandwidth compression technique,
`three-level coding. The resulting modem, embodying all
`three techniques, SSB, FM and three-level coding, was
`found to be able to transmit 4800 bands as reliably as
`3600 bands could be transmitted with binary detection
`methods. A brief discussion of this type of coding may
`now be appropriate.
`Multilevel coding has long been of interest as a method
`of increasing the speed of transmission over a band-limited
`channel, mainly because of the possibility of a considerable
`increase in data rate, a lowering of the signal-to-noise
`ratio of only 6 db, and an ease of implementation.
`The form of this coding that was developed for the sys
`tem of this invention was effected as follows:
`(1) In the transmitter, the binary non-return-to-zero
`(NRZ) waveform is converted to an inverted non-return
`to-zero (NRZI) code in which, for a bit period, a binary
`zero is characterized by a transition in level and a binary
`one is characterized by a constant level.
`(2) The NRZI waveform is then passed through the
`band-limited channel, which results in a smoothed output
`waveform with three distinct levels.
`(3) In the receiver, the three-level waveform is inter
`preted such that an up or down level is read as a binary
`one and the middle level as a binary zero.
`The advantage of this coding technique is that the three
`level signal will remain at any level as long as required
`by the data pattern, and therefore offers DC transmission
`capability.
`A channel described by the graphs of FIGURES 3 and
`4, which is capable of 3600-baud transmission in a binary
`
`

`
`mode, has thus been found able, in a three-level mode,
`to transmit v4800 bands with no decrease in reliability or
`economy. It has been found that, for zero interference at
`4800 bits per second, 1200 c.p.s. must be passed without
`attenuation and 1600 c.p.s. should be attenuated 6 db, as
`shown in FIGURE 4.
`Thus, by combining VSB-FM transmission and a three
`level signaling technique, 4800-baud service can be pro
`vided over a leased telephone line whose bandwidth is
`nominally 400 to 2700 c.p.s.
`FIGURE 5 presents the circuit diagram of the trans
`mitter modulator contemplating the above principles, as
`well as the following considerations.
`For simplicity, a voltage controlled multivibrator with
`center frequency (f,,) at the desired center frequency of
`the transmitter output is preferred; however, this type
`of oscillator would require a complex low-pass input ?lter
`with linear phase shift and a sharp cutoff. If the high fre
`quency components of the binary data, near or above the
`modulator frequency, are allowed to modulate the multi
`vibrator, intermodulation distortion would result. On the
`other hand, to ?lter out these high frequency components
`and still allow the data to be correctly interpreted at the
`receiver demodulator, the ?lter must be exceptionally
`complex. However, it has been found that this complexity
`can be reduced if the modulator operates at a high cen
`ter frequency and is divided down to the desired fc. Ac
`cordingly, the oscillator embodied in the invention operates
`at twice fc and with twice the desired deviation (A1‘), and
`is coupled to a frequency divider to obtain the desired f0
`and deviation; input ?ltering thus is accomplished by a
`combination of a simple low-pass ?lter and a pre-em
`phasis network.
`In the ?gure, the source of binary data is coupled to
`input circuit 10 which includes ampli?er 12, connected to
`pre-emphasis network 14; the latter in turn is ‘connected
`to ?lter 16. As shown, pre-emphasis network 14 preferably
`consists of a parallel resistance-capacitance combination
`well known to operate as above outlined. Filter 16 com
`prises a modi?ed ?ve-pole Butterworth low-pass ?lter hav
`ing a raised cosine impulse response. The overall fre
`quency response of input circuit 10 is shown in FIGURE
`
`Input circuit 10 connects to multivibrator control cir
`cuit 18, which includes ampli?er 20 feeding voltage con
`trol circuit 22. The latter contains a pair of potentiometers
`24 and 26, adjustment to which varies the center fre
`quency fc and frequency deviation A)‘ of multivibrator
`28 to which it connects. Multivibrator 28 is seen to be
`an astable type and is set to 5400 c.p.s. with a deviation
`of :150 c.p.s. Frequency divider 30 receives the output
`of multivibrator 28 and, since it comprises a binary ?ip
`?op (T element), divides by two to provide an output fc
`at 2700 c.p.s. with a Af of :75 c.p.s. to emitter follower
`stage 32.
`It is to be noted that the circuitry is characterized by a
`long charging time constant, thereby insuring good lin
`earity, as shown in FIGURE 7, which presents the modu
`lator transfer characteristic (solid line) and, for com
`parison, a linear trace (dashed line).
`The output of emitter follower 32 feeds the transmitter
`VSB ?lter 34 and thence the communications channel.
`FIGURE 8 contains line drawings of signals at various
`points, drawings A through E, of FIGURE 5, resulting
`from the NRZI binary data input signal, drawing A. It
`should be observed, however, that these drawings do not
`represent the corresponding signals exactly, since, if an
`exact representation were attempted, crowding of pulses
`Would reduce clarity. Thus, drawing B, as indicated, does
`not consider the action of pre-emphasis network 14 where
`as drawing C exaggerates the frequency deviation A)‘ in
`order to show the effect of modulation more clearly.
`Drawing D points up the operation of frequency divider
`30 and drawing E shows the communications channel sig
`
`45
`
`50
`
`55
`
`60
`
`65
`
`75.
`
`10
`
`25
`
`30
`
`35
`
`40
`
`20
`
`3,466,392
`6.
`nal, for which it is apparent that the transitions (crossings
`of the zero reference level) are preserved.
`FIGURE 9 presents the circuit diagram of the receiver
`demodulator embodying the following considerations.
`Since the received waveform is a narrow band (VSB)
`FM signal whose transitions (zero crossings) contain the
`coded binary information, the upper sideband compo
`nents need to be restored and amplitude variations caused
`by noise or other interference in the communications
`channel need to be removed. Then the transitions are con
`verted to pulses from which the data may :be recovered
`by ?ltering; de-emphasis is required to compensate for
`the pre-emphasis introduced by the modulator. The data
`then comprises a three-level signal which is translated to
`a binary code.
`In the ?gure, the communications channel provides
`input to limiter 40 which, as is known in the art, removes
`any amplitude variations and restores the upper sideband.
`Limiter 40 feeds transition-to-pulse converter 42, which
`provides squaring and sharpening for the transitions and
`converts them to pulses, each transition in the FM Wave
`form being replaced by a positive pulse of speci?ed height
`and width. Filter 44 comprises two sections: a six-pole
`Butterworth low-pass ?lter cutting off at 1600 c.p.s. and
`a similar ?lter, having a cutoff at 2450 c.p.s. which fur
`ther attenuates the center frequencies of the signal with
`out introducing distortion; reference to FIGURE 10 will
`divulge the characteristics of ?lter 44. De-emphasis cir
`cuit 46 operates in a fashion complementary to pre
`emphasis circuit 14 of FIGURE 5; this circuit includes a
`feedback network to provide the gain characteristic shown
`in FIGURE 11. The three-level data signal is converted
`to a binary signal by decision circuit 48, which translates
`the “up” and “down” levels to binary one and translates
`the center level to binary zero.
`Referring speci?cally to decision circuit 48, the three
`level signal output from de-emphasis circuit 46 is fed to
`emitter follower 50 and thence to a paralleled paid of
`detectors, high level detector 52 and low level detector
`54, which respond to levels higher and lower, respective
`ly, than the median. Detectors 52 and 54 connect to OR
`circuit 56. Thus, if the ‘signal input to decision circuit 48
`is higher or lower than, nominally, the zero level, the
`demodulator output to the utilization device (not shown),
`which may be an indicator, other receiver circuitry, a
`computer, etc., comprises a relatively high voltage level
`generated by detectors 52 and 54 and transmitted through
`OR circuit 56, whereas, if the signal input is, nominally,
`at the zero level, the demodulator output comprises a
`relatively low voltage level generated by detector 54 and
`transmitted through OR circuit 56.
`The operation of the demodulator is exempli?ed by
`the line drawings of FIGURE 12 in which the Waveshapes
`correspond to the points of FIGURE 9 similarly lettered.
`For the same reason as given in connection with FIG
`URE 8, the drawings in this ?gure are not exactly repre
`sentative of the signals. Correspondence of drawing E of
`FIGURE 12 with drawing A of FIGURE 8 is apparent.
`FIGURE-S l3 and 14 are concerned with circuits which
`permit signi?cance to be attached to the binary signals,
`i.e., “clocking” circuits required by any binary communi
`cations system. Where the code structure of the system
`is restricted, such as in the transfer of computer data
`which is characterized by at least one binary reversal
`during a speci?ed period of time (usually provided as a
`parity bit each word period), a “soft” clock, i.e., one
`derived from the demodulated signal itself, may be used.
`However, the present system is characterized by inde
`pendent data and carrier rates and is adaptable to appli
`cations wherein no restriction constrains the code struc~
`ture, such as in document scanning (facsimile). There
`fore, it is necessary to employ an independently derived
`“hard” clock.
`
`

`
`3,466,392
`
`10
`
`3O
`
`7
`FIGURES 13 and 14 show circuits for such a clock,
`the former for the modulator of FIGURE 5 and the lat
`ter for the demodulator of FIGURE 9. With regard to
`FIGURE 13, ?lter 16 of input circuit 10 (FIGURE 5)
`is replaced by ?lter-trap combination 70, the ?lter of
`which comprises a duobinary low-pass network and the
`trap of which comprises a network responsive to half the
`repetition rate (bit period) of the binary data. The out
`put of ?lter-trap 70 provides one input to OR circuit 72,
`the other input to which is energized by a clock signal
`source operating at a frequency corresponding to half
`the bit period of the data, through tuned circuit 74, which
`resonates at this frequency. The amplitude of the clock
`signal fed to OR gate 72 is controlled ‘by potentiometer
`76. As should be apparent, the action of ?lter-trap 70 is
`to preclude data components from interfering with the
`clock signal at the same rate injected by the clock source.
`The output of OR gate 72 supplies energization to multi
`vibrator control circuit 18 (FIGURE 5).
`With regard to FIGURE 14, in the demodulator (FIG
`URE 9), all that is required to recover the injected clock
`signal is tuned circuit 78 having a very high Q at the bit
`period rate.
`The 4800-baud speed of this system has not previously
`been achieved with an FM modem. Yet the system is
`basically simple, and it is quite simple to adapt the mo
`dem for standard low-speed transmission over switched
`networks. A further advantage of this system is that the
`modem is also capable of transmitting the analog signals
`required for facsimile document scanning and other appli
`cations. With this unusual ?exibility, the system is very
`attractive for international data transmission.
`While the invention has been particularly shown and
`described with reference to a preferred embodiment there
`of, it will be understood by those skilled in the art that
`the foregoing and other changes in the form and details
`may be made therein without departing from the spirit
`and scope of the invention.
`40
`What is claimed is:
`1. In a binary communications system, an FM-VSB
`modem, comprising:
`a modulator, including
`an input circuit at ‘which the binary signal is
`impressed;
`an oscillator operating ‘at a multiple of the de
`sired center frequency of the modulator;
`a control circuit capable of deviating the frequency
`of said oscillator according to the output of said
`input circuit;
`a frequency divider responsive to said oscillator
`to provide the desired modulator center fre
`quency modulated accordingly with the output
`of said oscillator; and
`a vestigial sideband ?lter connected to said fre
`quency divider;
`a communications channel connected to said mod
`ulator; and
`a demodulator, including
`a limiter at which the signal from said channel is
`impressed for providing a corresponding two
`level signal;
`a transition detector connected to said limiter;
`a low pass ?lter responsive to the output from said
`detector for providing a corresponding three
`level signal;
`a decision circuit connected to said ?lter for pro
`providing a binary signal, said input circuit of
`said modulator also includes
`‘a pre-emphasis network effectively increasing the
`relative amplitude of the high frequency com
`ponents of said binary signals;
`a ?lter; and
`said demodulator includes
`
`70
`
`60
`
`8
`a feedback type de-emphasis circuit connected to
`said low pass ?lter and having a characteristic
`complementary to the characteristic of said pre
`emphasis network decreasing the relative ampli
`tude of the high frequency components of the
`output of said ?lter.
`2. In a binary communications system, ‘an FM-VSB
`modern, comprising:
`a modulator, including
`an input circuit at which the binary signal is im
`pressed;
`an oscillator operating at a multiple of the desired
`center frequency of the modulator;
`a control circuit capable of deviating the frequency
`of said oscillator according to the output of said
`input circuit;
`a frequency divider responsive to said oscillator
`to provide the desired modulator center fre
`quency modulated accordingly with the output
`of said oscillator; and
`a vestigial sideband ?lter connected to said fre
`quency divider;
`a communications channel connected to said mod
`ulator; and
`a demodulator, including
`a limiter at which the signal from said channel is
`impressed for providing a corresponding two
`level signal;
`a transition detector connected to said limiter;
`a low pass ?lter responsive to the output from said
`detector for providing a corresponding three
`level signal;
`a decision circuit connected to said ?lter for pro
`viding a binary signal comprising a pair of
`threshold detecting circuits and a gating circuit,
`in which said threshold detecting circuits pass a
`?rst level signal to said gating circuit for input
`signals differing from a reference level by a
`prescribed deviation and pass a second level sig
`nal to said gating circuit for other input signals.
`3. The system of claim 2 wherein said gating circuit
`comprises an OR gate.
`4. In a binary communications system, an FM-VSB
`modern, comprising:
`a modulator, including
`an input circuit at which the binary signal is im
`pressed;
`an oscillator operating at a multiple of the desired
`center frequency of the modulator;
`a control circuit capable of deviating the frequency
`of said oscillator according to the output of said
`input circuit;
`a frequency divider responsive to said oscillator to
`provide the desired modulator center frequency
`modulated ‘accordingly with the output of said
`oscillator; and
`a vestigial sideband ?lter connected to said fre
`quency divider;
`a communications channel connected to said mod
`ulator; and
`a demodulator, including
`a limiter at which the signal from said channel is
`impressed for providing a corresponding two
`level signal;
`a transition detector connected to said limiter;
`a low pass ?lter responsive to the output from said
`detector for providing a corresponding three
`level signal;
`a decision circuit connected to said ?lter for pro~
`viding a binary signal;
`said modulator also including
`a clock signal generator and
`said demodulator also includes
`a clock signal detector.
`
`

`
`3,466,392
`
`5. The system of claim 4 wherein the clock signal gen
`erated by said generator is independent of the binary sig
`nal impressed on said input circuit of said modulator.
`6. The system of claim 4 wherein
`said clock signal generator comprises
`a low pass ?lter connected to said input circuit;
`a clock signal source;
`. a tuned circuit responsive to the output of said clock
`signal source to pass a signal at a subharmonic of the
`frequency of the binary signal and
`a gating circuit connected to said input circuit and said
`tuned circuit to pass a signal to said control circuit.
`7. The system of claim 6 wherein said clock signal de
`tector comprises a tuned circuit highly selective at the
`frequency of the binary signal.
`
`15
`
`10
`References Cited
`UNITED STATES PATENTS
`7/1965
`Hopner et a1 _______ __ 325-136
`6/1968
`Lender __________ __ 178-66 X
`8/1963
`2/1967
`11/1966
`
`Clark ____________ __ 329-—104
`Johnson ________ __ 325—46 X
`
`Bosen ____________ __ 328-27
`
`3,196,352
`3,387,213
`3,102,238
`3,307,112
`3,288,930
`0 ROBERT L. GRIFFIN, Primary Examiner
`W. S. FROMMER, Assistant Examiner
`US. Cl. X.R.
`329—104; 325—27, 320, 163, 136, 46, 30