CA1063178A - Modulation and filtering device - Google Patents
Modulation and filtering deviceInfo
- Publication number
- CA1063178A CA1063178A CA234,814A CA234814A CA1063178A CA 1063178 A CA1063178 A CA 1063178A CA 234814 A CA234814 A CA 234814A CA 1063178 A CA1063178 A CA 1063178A
- Authority
- CA
- Canada
- Prior art keywords
- filter
- digital
- frequency
- signal
- modulation
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Classifications
-
- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03C—MODULATION
- H03C1/00—Amplitude modulation
-
- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03H—IMPEDANCE NETWORKS, e.g. RESONANT CIRCUITS; RESONATORS
- H03H17/00—Networks using digital techniques
- H03H17/02—Frequency selective networks
- H03H17/06—Non-recursive filters
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L27/00—Modulated-carrier systems
- H04L27/02—Amplitude-modulated carrier systems, e.g. using on-off keying; Single sideband or vestigial sideband modulation
- H04L27/04—Modulator circuits; Transmitter circuits
Abstract
ABSTRACT:
A digital data transmitter is described , whose line carrier frequency need not be an intefral multiple of one half the clock frequency of the digital input signal. Use is made of a digital filter for increasing the sampling frequency to a value which is twice the modulator carrier wave frequency.
A digital data transmitter is described , whose line carrier frequency need not be an intefral multiple of one half the clock frequency of the digital input signal. Use is made of a digital filter for increasing the sampling frequency to a value which is twice the modulator carrier wave frequency.
Description
~i3~7~ PHN. 77~7.
Mbdulation and filtering device , (A) Backgro~md of the invention _ tl) Field of the invention The inventicn relates to the modulation and filter-ing of digital signals, in particular of signals which are S usually called binary data signals.
Data signals, which consist of a succession of signal elements of uniform duration whose amplitude represents a binary "o"
or binary "l", are not simply suitable for transmission via, for ~;
example, a telephone channel. It has therefore beo~me normal prac-tice to use so-called "modems" in which the data signals before transmission m~dulate a carrier wave and in which after reception the modulated signal is demLdulated.
Any of the well-known mDdulation techniques such as FSK, PSK, AM~DSB, AM~SSB, AM~VSB can be used for the realization of a frequen~y spectrum suitable for transmission via a telephone channel.
m e invention results fram investigations in the field of a 2400 Baud AM~VSB modem, but is not restricted to this field alone, as the same principles can be used for other data speeds and for other linear modulation methods such as AM~DSB, AM~SSB, FSK (modulation-index l/2) and PSK. Although in what follows hereafter, reference is made to a 2400 bit/sec AM~VSB
mod~m and its extension to a 4800 bit/sec AM~VSB modem this should not be interpreted as a restriction of the possibilities of application of the principles according to the invention.
Mbdulation and filtering device , (A) Backgro~md of the invention _ tl) Field of the invention The inventicn relates to the modulation and filter-ing of digital signals, in particular of signals which are S usually called binary data signals.
Data signals, which consist of a succession of signal elements of uniform duration whose amplitude represents a binary "o"
or binary "l", are not simply suitable for transmission via, for ~;
example, a telephone channel. It has therefore beo~me normal prac-tice to use so-called "modems" in which the data signals before transmission m~dulate a carrier wave and in which after reception the modulated signal is demLdulated.
Any of the well-known mDdulation techniques such as FSK, PSK, AM~DSB, AM~SSB, AM~VSB can be used for the realization of a frequen~y spectrum suitable for transmission via a telephone channel.
m e invention results fram investigations in the field of a 2400 Baud AM~VSB modem, but is not restricted to this field alone, as the same principles can be used for other data speeds and for other linear modulation methods such as AM~DSB, AM~SSB, FSK (modulation-index l/2) and PSK. Although in what follows hereafter, reference is made to a 2400 bit/sec AM~VSB
mod~m and its extension to a 4800 bit/sec AM~VSB modem this should not be interpreted as a restriction of the possibilities of application of the principles according to the invention.
(2) Descripkion of the state of the art.
l]lN. 77 7 23.5.197~
~63~78 According to a frequently usecl method :in conventional AM-VSB modems the data s:ignals are first filtered ; in a low-pass filter. The filtered signals modulate a carrier wave in amplitude and, of the siclebands of the modulated signal, one of the sidebands plus a part of the other sidebancl is selected by a vestigial sideband filter whose output si.gnal is transmitted.
The low-pass filter causes a llmitation of the frequency spectrum Or the da-ta signals at the side o~ the high signal frequencies. A-t the, with rega:rd to the highest data signal frequency, relatively low carrler ~requency o-f`
data moclelll.s ~or telephone chanlle:Ls~ the low-pass f`ilter a:Lso preven~s the phenorn~non of ro:Lclovor wh:Lcl1 occurs wh~n h:igh signal Irequcnc:ies are superimposed, after modula-tion on a ` 15 carrier wave, in the area which is occupied by the lower signal frequencies after modulation.
The low-pass filter is usually called the pre-modulation filter and the vestigia1 sideband filter the post-~oduIation filter. Modulation takes place between -these two filters.
The output s:ignal o~ the pre-modulat:ion filter .Ls an analog signal, even when the input signal is a binary signal, so that an analog modulator must be used.
It has been proposed to replace the analog modulator by a simple logic circuit and to interchange the ! order of the steps of pre-modulation filtering and modulation .:. . . .
in order to profit from the binary form of the input signal.
It has been proved that it is only poss:ible to interchange these steps when the carrier fre1uency amounts to an integral . . .
~6;~1 7~
~ .
multiple of one half the clock frequency of the digital input signal. Only in this case the resulting foldover distortion can be compensated by a linear network.
For practical purposes this is of limited importance, as may appear from a provisional proposal by the Comité Consultatif International Télégraphique et Téle~phonique (CCITT) for a 4800 bit/sec AM-VSB modem, that this modem must have a carrier frequency of 2100 Hz, which is not in the said proportion to the clock frequency ~in this case 2400 Hz at four-level coding).
Interchange of the steps of pre-modulation, filtering and modulation, which is only permissible in a limited number of cases, results in that only one filter is still needed, namely a post-modulatlon filter. This filter lS can be designed in such a way that the foldover distortion is compensated by it.
By realising the post-modulation filter as a binary transversal filter, a data modem can be realized in the 'l proposed manner from digital integrated circuits.
According to another line of development and with the purpose of mitigating the requirements the post-modulation ~ -fllter must satisfy it has been suggested to realise the pre-modulation filter as a band filter. This proposal is based on the fact that by the use of short pulses for the represent-ation of the binary information the signal energies at the higher frequencies, which also fully represent the binary information, are amplified. By selecting these higher-fre-quency signal components, a signal is obtained such that after modulation the sidebands thus created can be separated in a simple manner.
':
- 4 ~
~'.`;' "'':
: . . .
.
1'111\1 ~ / /-. /
~.5.1975 ~3178 (B) Summary o:L ll-~e :i.nvent:ion The inventi.oIl has for .its obJect to provide a dlgital. data -transmitter, in which the line carrier frequellcy neecl not be an integral multiple of one llalr the eloelc f`requency.
The inventi.on has for its ~ur-ther object to realise such a digital data transmitter w:ith the least possible technical means and, in particular, to mi.tigate the requirements for the postmodulation filter to such a degree 10 that a simple RC network will do.
The modula-t:ing and f`:i.ltoring dev:iee according to the :Lnven-tioll i.s eharacterl~ed in that the s:Lgnal salnples of the ~I:Lgital lnput s:Lgna:L whLeh oceur at a I`lrst sampl:l.ll~
frequeney are eonverted in a digital f`ilter to signal samples : 15 at a seeond sampling frequeney, whieh is twice the modulator earrier ~requeney and the signal samples at the seeond ; sampling frequency are modulated in a modulator by signal samples of the modulator earrier wave which oeeur at the same seeond sampling frequeney, the digital filter being 20 ada~t~d to have a a bandpass filter eharaeteristie.
In this clig:Lta:L transrll:Ltter the rmodulator earrier wave is sampled at a freqlleney which is twiee the modulator earrier frequeney, so that the samples of the earrier wave ean be represented in numerleal ~orm by the numbers ~1, -1, ~1, -1,:
... The modulation proeess then limits itself to an alternatlng.... :
inversion of the signal samples whieh have been derived fxom the digital input signal by the digital bandpass ~il~r.
In its simplicity this dig:Ltal rnodulation can be compared to the modulation whlch is used in the system : .. . . .
~63178 mentioned above in the chapter: "state of the art" in which the steps of modulation and filtering are interchanged. How-ever, in the digital transmitter according to the invention no special requirements are imposed upon the carrier wave S in relation to the clock frequency or sampling frequency of the digital input signal.
The ratio between the second and the first - sampling frequencies can be a whole number in which case the digital filter can be realized as an interpolating filter.
If the ratio is not integral in which case it can be ex~
pressed by: M/L, where M and L are mutually indivisible integers, the digital filter can be realized as a cascade oF an interpolating filter with interpolation factor M and an extrapolating filter having the extrapolation factor L.
By way of alternative an interpolating digital filter, hav-ing an interpolation factor M/L, as described in applicant's ~-copending patent application 235,394 (PHN 7729) filed Sept-ember 11, 1975, may be used.
(C) References "Digital filtering of band-limited signals: i Interpolation, extrapolation and distortions due to various truncations. Reduction of Computation speed in digital filters"
by M. Bellanger, J.L. Daquet, G. Lepagnol, ICC June 11-13 1973, pages 23-11 to 23-15.
"Terminology in digital signal processing".
IEEE Transactions on Audio and Electroacoustics, Vol. AU-20, No. 5, December 1972, pages 322-337.
"A digital signal processing approach to inter-polation" by R.W. Schafer, L.R. Rabiner. Proceedings of the ~ -IEEE, Vol. 61, No. 6, June 1973, pages 692-702. ~
'~ ~
r~
~63~715 ~
United States patent specification 3,611,143.
(D) List of abbrev;ations FSK - "frequency shift keying"
PSK - "phase shift keying"
AM - "Amplitude modulation" ;
DSB - "double sideband"
VSB - "vestigial sideband"
SSB - "single sideband"
(E) nescription of the embodiments Figure 1 shows a digital data transmitter according to the invention, and Figure 2 shows waveforms helpful in explalning the op-eration thereo~.
(1) General description Fig. 1 shows the digital data transmitter, comprising a `~
source of binary data signals 1. Purely for illustrative purposes, the speed of the data source 1 is set at 2400 bit/sec. This data source supplies 2400 binary signal elements per second, each signal element having a duration Tl = lt2400 sec. ;
The data source 1 is controlled by a clock 2 which sup- ~ ~
plies a clock signal of a clock frequency fl = 2400 Hz, to data source 1. ' ;
By 3 is indicated a digital filter. This filter may be constructed in known manner. The output sampling frequency of the digital filter is equal to f2. Hereinafter, this frequency will be -further determined. As is known, a digital filter has a transfer function with a periodic variation, whose period is ~qual to the sampling frequency9 in this case equal to f2. The part of the fre-quency characteristic between -1/2 f2 and +1/2 f2 is repeated from nf2 ~ 1/2 f2 to nf2 ~ 1/2 f2, where n is any integer. By a suitable choice of the filter coefficients, it may be achieved that the part of .. . . ... . . .
~ . 7727 23.5.1975 , ~63178 the -transfer fullc-tion be-tween --2-f2 and ~ f2 obta.ins a certain des:ired shape, I`or examp.le corresporlding to that of a VSB
filter.
~ sampling device l~ admits samples of the data sigrnal to the digital f.ilter 3 at t1le sampling frequency f1 =
2400 I-lz. Each signal elernent is sampled once in order to ascertain whether the signal element represents a binary "0" or a binary "1" and the corresponding value is entered in the digital filter 3.
Re:t`erence 5 des:ignates a modulator and reference 6 a carr:ier wa-ve .source. This carr:ier wave source del:ivers~
at the samp:L:Lng ~requenc~ f2, s~.gnal s~mp:Les of a carrier wave of the carr:ic:r frequency -l-f2. These carrier wavo samples ~' can be represented in numerical form by the nwllbers +1, -1, +1, -1, .... Moduhtor 5 multiplies each signal sample of the digital filter 3 by a carrier wave sample and feeds the r0sults to the digi-tal-to-analog converter 7.
In the digital filter 3 and in the digital modu-lator 5 signal samples are tr~ated in the form of code words which represent numbers. These code words are converted to a corre~ponding amplitude value of a current or voltage by d:;gital-to-analog converter 7 and are normally kept at *his value during the sampling period. The quantised signa}
obtained in this manner is fed to a low~pass filter 8, which .
passes the lower sideband of the quantised signal and sup-presses the other sideband. The output of the fil-ter is connected to a transmission line 9.
Fig. 2a shows a diagrammatical representa-tion of a part of the frequency spectrurn of a digital signal P~ . '77~7 23.5.'1975 .
~631'78 composed of` discrete time pulse~s at mutua] distances T1 = 1/f1' as this signaL appears at the output of sampler 4.
Fig. 2b sl1ows a part of tlle transfer function for positive frequencies of the digital filter 3, with an output sampling frequency f2 = 13800 Hz. '~he part for the negrative frequency is the mirror image about the frequency zero of the part for the positive frequencies. The "folding~' frequency -1-f2 is 6900 H2. The transfer function between ~ f2 and ~ f2 has the shape of a VSB filter for a "carrier frequency~' 2f~
4800 Hz~ In Fig. 2b a dashed line 20 indica-tes the part of the spectrum of fig. 2a whioh is encompassecl by the VSB filter.;
Fig. 2c shows t'hc frequency spectrum of a cligital sigllal that conststs of cliscrete t:ime pu:L6es at mutual dis-tances T2 = 1/f2, whlch have alternately the value ~1 and ~
This spectrum consists of spectral lines spaced at f2 = " ' 13800 Hz. This cor,responds to the spectrum of the carrier wave samples at the output of carrier wave source 6.
Fig. 2d shows in the frequency doma:Ln the result , of the multiplication of the'output signal samples o~ digital filter 3 by the carrier wave ~amples of carrier wave source 6 in the ~lodulator 5. The dashed lLne 21 corresponds to lin~ 20 ,' of fig. 2b and illustrates the new position of the relevant '' part of the frequency spectrum of the data signal.
Normally, the digital-to-analog converter 7 25, converts each output signal sample Or modulator 5 to a signal element of duration T2 and constant amp:Litude during this time.
This introduces in the frequency domain a ~ln x ~shape factor whose first zero point is at 13800 ~Iz. Owing to this shape factor the higher-frequency components of the spectrum g_ PIIN. '7 ~7 ` ~L063~8 according to F:ig. 2d are a:Lready attenuated with respeG-t to the lower-frequency compone}lts in the 600-2700 Hz band. A
further supprcss:ion can be rea:l:ized with the aid o~ ~C-filter whose transfer function is,l:Llustrated by tha dashed line 22 in ~ig. 2d.
As will be clear from Fig. 2d, the output signal of filter 8 has the form of a VSB signal wlth a line carrier , '~
frequency of 2100 Hz and, by way of illustration, a sideband ,,'' of 2100 Hz to 600 Hz and a vestigial sideband of 2100-2700 lIz. ~, l~800 bit/sec data transmitter A ~800 bit/sec data transmltter with the same frcquency spectrum as the 2l~00 b:Lt/sec data transmitter accordin~ to ~:ig. 1 can be realized ~y the use of dibit-coding at the inpu-t,of the data t,ransmitter. In accordance with this coding, the, signal elements of the data source are divided in groups of two sig,nal elements and the latter are coded in the dibit-coder according to the following rule:
input dibit ou-tput dibit numerical value 11 00 +3 - ' 01 ~1 ", Digital filter 3 handles the dibits according to the numerical value which is,indicated in the third cclumn.
25, The coding of the output dibits (aOa1) is such tha-t the numerical value is indicated by:
2. (-1) o ~ (-1) 1 The multiplication of a dibit by a filter coefficient Ck can now be performed in two stages, nalnely first ~ .
P}IN. 7'727 23.5.1'~75
l]lN. 77 7 23.5.197~
~63~78 According to a frequently usecl method :in conventional AM-VSB modems the data s:ignals are first filtered ; in a low-pass filter. The filtered signals modulate a carrier wave in amplitude and, of the siclebands of the modulated signal, one of the sidebands plus a part of the other sidebancl is selected by a vestigial sideband filter whose output si.gnal is transmitted.
The low-pass filter causes a llmitation of the frequency spectrum Or the da-ta signals at the side o~ the high signal frequencies. A-t the, with rega:rd to the highest data signal frequency, relatively low carrler ~requency o-f`
data moclelll.s ~or telephone chanlle:Ls~ the low-pass f`ilter a:Lso preven~s the phenorn~non of ro:Lclovor wh:Lcl1 occurs wh~n h:igh signal Irequcnc:ies are superimposed, after modula-tion on a ` 15 carrier wave, in the area which is occupied by the lower signal frequencies after modulation.
The low-pass filter is usually called the pre-modulation filter and the vestigia1 sideband filter the post-~oduIation filter. Modulation takes place between -these two filters.
The output s:ignal o~ the pre-modulat:ion filter .Ls an analog signal, even when the input signal is a binary signal, so that an analog modulator must be used.
It has been proposed to replace the analog modulator by a simple logic circuit and to interchange the ! order of the steps of pre-modulation filtering and modulation .:. . . .
in order to profit from the binary form of the input signal.
It has been proved that it is only poss:ible to interchange these steps when the carrier fre1uency amounts to an integral . . .
~6;~1 7~
~ .
multiple of one half the clock frequency of the digital input signal. Only in this case the resulting foldover distortion can be compensated by a linear network.
For practical purposes this is of limited importance, as may appear from a provisional proposal by the Comité Consultatif International Télégraphique et Téle~phonique (CCITT) for a 4800 bit/sec AM-VSB modem, that this modem must have a carrier frequency of 2100 Hz, which is not in the said proportion to the clock frequency ~in this case 2400 Hz at four-level coding).
Interchange of the steps of pre-modulation, filtering and modulation, which is only permissible in a limited number of cases, results in that only one filter is still needed, namely a post-modulatlon filter. This filter lS can be designed in such a way that the foldover distortion is compensated by it.
By realising the post-modulation filter as a binary transversal filter, a data modem can be realized in the 'l proposed manner from digital integrated circuits.
According to another line of development and with the purpose of mitigating the requirements the post-modulation ~ -fllter must satisfy it has been suggested to realise the pre-modulation filter as a band filter. This proposal is based on the fact that by the use of short pulses for the represent-ation of the binary information the signal energies at the higher frequencies, which also fully represent the binary information, are amplified. By selecting these higher-fre-quency signal components, a signal is obtained such that after modulation the sidebands thus created can be separated in a simple manner.
':
- 4 ~
~'.`;' "'':
: . . .
.
1'111\1 ~ / /-. /
~.5.1975 ~3178 (B) Summary o:L ll-~e :i.nvent:ion The inventi.oIl has for .its obJect to provide a dlgital. data -transmitter, in which the line carrier frequellcy neecl not be an integral multiple of one llalr the eloelc f`requency.
The inventi.on has for its ~ur-ther object to realise such a digital data transmitter w:ith the least possible technical means and, in particular, to mi.tigate the requirements for the postmodulation filter to such a degree 10 that a simple RC network will do.
The modula-t:ing and f`:i.ltoring dev:iee according to the :Lnven-tioll i.s eharacterl~ed in that the s:Lgnal salnples of the ~I:Lgital lnput s:Lgna:L whLeh oceur at a I`lrst sampl:l.ll~
frequeney are eonverted in a digital f`ilter to signal samples : 15 at a seeond sampling frequeney, whieh is twice the modulator earrier ~requeney and the signal samples at the seeond ; sampling frequency are modulated in a modulator by signal samples of the modulator earrier wave which oeeur at the same seeond sampling frequeney, the digital filter being 20 ada~t~d to have a a bandpass filter eharaeteristie.
In this clig:Lta:L transrll:Ltter the rmodulator earrier wave is sampled at a freqlleney which is twiee the modulator earrier frequeney, so that the samples of the earrier wave ean be represented in numerleal ~orm by the numbers ~1, -1, ~1, -1,:
... The modulation proeess then limits itself to an alternatlng.... :
inversion of the signal samples whieh have been derived fxom the digital input signal by the digital bandpass ~il~r.
In its simplicity this dig:Ltal rnodulation can be compared to the modulation whlch is used in the system : .. . . .
~63178 mentioned above in the chapter: "state of the art" in which the steps of modulation and filtering are interchanged. How-ever, in the digital transmitter according to the invention no special requirements are imposed upon the carrier wave S in relation to the clock frequency or sampling frequency of the digital input signal.
The ratio between the second and the first - sampling frequencies can be a whole number in which case the digital filter can be realized as an interpolating filter.
If the ratio is not integral in which case it can be ex~
pressed by: M/L, where M and L are mutually indivisible integers, the digital filter can be realized as a cascade oF an interpolating filter with interpolation factor M and an extrapolating filter having the extrapolation factor L.
By way of alternative an interpolating digital filter, hav-ing an interpolation factor M/L, as described in applicant's ~-copending patent application 235,394 (PHN 7729) filed Sept-ember 11, 1975, may be used.
(C) References "Digital filtering of band-limited signals: i Interpolation, extrapolation and distortions due to various truncations. Reduction of Computation speed in digital filters"
by M. Bellanger, J.L. Daquet, G. Lepagnol, ICC June 11-13 1973, pages 23-11 to 23-15.
"Terminology in digital signal processing".
IEEE Transactions on Audio and Electroacoustics, Vol. AU-20, No. 5, December 1972, pages 322-337.
"A digital signal processing approach to inter-polation" by R.W. Schafer, L.R. Rabiner. Proceedings of the ~ -IEEE, Vol. 61, No. 6, June 1973, pages 692-702. ~
'~ ~
r~
~63~715 ~
United States patent specification 3,611,143.
(D) List of abbrev;ations FSK - "frequency shift keying"
PSK - "phase shift keying"
AM - "Amplitude modulation" ;
DSB - "double sideband"
VSB - "vestigial sideband"
SSB - "single sideband"
(E) nescription of the embodiments Figure 1 shows a digital data transmitter according to the invention, and Figure 2 shows waveforms helpful in explalning the op-eration thereo~.
(1) General description Fig. 1 shows the digital data transmitter, comprising a `~
source of binary data signals 1. Purely for illustrative purposes, the speed of the data source 1 is set at 2400 bit/sec. This data source supplies 2400 binary signal elements per second, each signal element having a duration Tl = lt2400 sec. ;
The data source 1 is controlled by a clock 2 which sup- ~ ~
plies a clock signal of a clock frequency fl = 2400 Hz, to data source 1. ' ;
By 3 is indicated a digital filter. This filter may be constructed in known manner. The output sampling frequency of the digital filter is equal to f2. Hereinafter, this frequency will be -further determined. As is known, a digital filter has a transfer function with a periodic variation, whose period is ~qual to the sampling frequency9 in this case equal to f2. The part of the fre-quency characteristic between -1/2 f2 and +1/2 f2 is repeated from nf2 ~ 1/2 f2 to nf2 ~ 1/2 f2, where n is any integer. By a suitable choice of the filter coefficients, it may be achieved that the part of .. . . ... . . .
~ . 7727 23.5.1975 , ~63178 the -transfer fullc-tion be-tween --2-f2 and ~ f2 obta.ins a certain des:ired shape, I`or examp.le corresporlding to that of a VSB
filter.
~ sampling device l~ admits samples of the data sigrnal to the digital f.ilter 3 at t1le sampling frequency f1 =
2400 I-lz. Each signal elernent is sampled once in order to ascertain whether the signal element represents a binary "0" or a binary "1" and the corresponding value is entered in the digital filter 3.
Re:t`erence 5 des:ignates a modulator and reference 6 a carr:ier wa-ve .source. This carr:ier wave source del:ivers~
at the samp:L:Lng ~requenc~ f2, s~.gnal s~mp:Les of a carrier wave of the carr:ic:r frequency -l-f2. These carrier wavo samples ~' can be represented in numerical form by the nwllbers +1, -1, +1, -1, .... Moduhtor 5 multiplies each signal sample of the digital filter 3 by a carrier wave sample and feeds the r0sults to the digi-tal-to-analog converter 7.
In the digital filter 3 and in the digital modu-lator 5 signal samples are tr~ated in the form of code words which represent numbers. These code words are converted to a corre~ponding amplitude value of a current or voltage by d:;gital-to-analog converter 7 and are normally kept at *his value during the sampling period. The quantised signa}
obtained in this manner is fed to a low~pass filter 8, which .
passes the lower sideband of the quantised signal and sup-presses the other sideband. The output of the fil-ter is connected to a transmission line 9.
Fig. 2a shows a diagrammatical representa-tion of a part of the frequency spectrurn of a digital signal P~ . '77~7 23.5.'1975 .
~631'78 composed of` discrete time pulse~s at mutua] distances T1 = 1/f1' as this signaL appears at the output of sampler 4.
Fig. 2b sl1ows a part of tlle transfer function for positive frequencies of the digital filter 3, with an output sampling frequency f2 = 13800 Hz. '~he part for the negrative frequency is the mirror image about the frequency zero of the part for the positive frequencies. The "folding~' frequency -1-f2 is 6900 H2. The transfer function between ~ f2 and ~ f2 has the shape of a VSB filter for a "carrier frequency~' 2f~
4800 Hz~ In Fig. 2b a dashed line 20 indica-tes the part of the spectrum of fig. 2a whioh is encompassecl by the VSB filter.;
Fig. 2c shows t'hc frequency spectrum of a cligital sigllal that conststs of cliscrete t:ime pu:L6es at mutual dis-tances T2 = 1/f2, whlch have alternately the value ~1 and ~
This spectrum consists of spectral lines spaced at f2 = " ' 13800 Hz. This cor,responds to the spectrum of the carrier wave samples at the output of carrier wave source 6.
Fig. 2d shows in the frequency doma:Ln the result , of the multiplication of the'output signal samples o~ digital filter 3 by the carrier wave ~amples of carrier wave source 6 in the ~lodulator 5. The dashed lLne 21 corresponds to lin~ 20 ,' of fig. 2b and illustrates the new position of the relevant '' part of the frequency spectrum of the data signal.
Normally, the digital-to-analog converter 7 25, converts each output signal sample Or modulator 5 to a signal element of duration T2 and constant amp:Litude during this time.
This introduces in the frequency domain a ~ln x ~shape factor whose first zero point is at 13800 ~Iz. Owing to this shape factor the higher-frequency components of the spectrum g_ PIIN. '7 ~7 ` ~L063~8 according to F:ig. 2d are a:Lready attenuated with respeG-t to the lower-frequency compone}lts in the 600-2700 Hz band. A
further supprcss:ion can be rea:l:ized with the aid o~ ~C-filter whose transfer function is,l:Llustrated by tha dashed line 22 in ~ig. 2d.
As will be clear from Fig. 2d, the output signal of filter 8 has the form of a VSB signal wlth a line carrier , '~
frequency of 2100 Hz and, by way of illustration, a sideband ,,'' of 2100 Hz to 600 Hz and a vestigial sideband of 2100-2700 lIz. ~, l~800 bit/sec data transmitter A ~800 bit/sec data transmltter with the same frcquency spectrum as the 2l~00 b:Lt/sec data transmitter accordin~ to ~:ig. 1 can be realized ~y the use of dibit-coding at the inpu-t,of the data t,ransmitter. In accordance with this coding, the, signal elements of the data source are divided in groups of two sig,nal elements and the latter are coded in the dibit-coder according to the following rule:
input dibit ou-tput dibit numerical value 11 00 +3 - ' 01 ~1 ", Digital filter 3 handles the dibits according to the numerical value which is,indicated in the third cclumn.
25, The coding of the output dibits (aOa1) is such tha-t the numerical value is indicated by:
2. (-1) o ~ (-1) 1 The multiplication of a dibit by a filter coefficient Ck can now be performed in two stages, nalnely first ~ .
P}IN. 7'727 23.5.1'~75
3~78 multipl:ication of Ck by 2 and the add.i-tion of a sign corres-pond~ng to (-1) o, the~reaf`ter mult:i.plication by 'I and the addition of a sign, corrcsponding to (-1) 1. The multiplication of a bi.nary number by 2 corresponds to a one-place shif-t of , the decimal point so that, when the above code is used, a simple decimal point shift arrangement can be used instead of a multiplier.
Rela.tion between line carrier frequency and dat~
rate. ..
From Fig. 2 it is clear that the line carrier frequency of 2100 ~Iz is equal to the difference between the :~reqllency of carri~r wave ~ource 6 of 6900 Hz and the frequency of 4800 Hz corresponding to the oentre of one of the Nyquist edges of the VSB f`ilter according to Fig. 2b. ~ .
If now the line carrier frequency is indicated by f , the fr,equenc'y of the carrier wave source 6 by -~f2 and the frequency corresponding to the centre of the Nyquist edge of the f'ilter to Fig. 2b by ~Kf1, then tho equation applies:
c ~f2 ~ ~Kf1 - (1') With f and f1 given the relation between K and f2 is fully d`etermined by equation (1).
When in expression (1) we substitute fc = 2100 Hz and f1.= 2400 l~z, then it will be found with K = 4 that f2 = 13800 Hz.
~ The factor by which digital~ filter 3 increases the sampling frequency then is:
f2/f1 = 23j4 (2) ~ `
The factor K may be an odd number.
, In this case, the spectrum according to Fig. 2a must be shifte~
to the right over an interval 2-f1. This can be reali.zed by 1(lgj3~L~
multiplying the output samples of sampler 4 alternately by +l and ~
Digital filter ~ :
Digital filter 3 according to Fig. l, with an increase of the sampling frequency by a factor L- , which in the example amounts to 24 , can be realized by means of a ~ - n~
cascade of an interpolating filter which ;ncreases the sampling frequency by a factor M and an extrapolating filter which decreases the sampling frequency by a factor L. The transfer function which must be realized can ~hen be divided between the two filters. .
An other possibility would be to replace the ex-trapolat~ng filter by a switch, which is operated at a fre-quency which ~s a factor L lower than the output sampling frequency of the interpolating filter and which supplies only ~ -one of each group of L output signal samples to modulator 5.
Interpolating and extrapolating digital filter are known as such and it may here suffice to refer to the :
references mentioned sub (C). .
An attractive version of an interpolating digital filter has been described in ~o-pending patent application 235,394 (PHN 7729) supra.
Although Applicant has refrained from describing known digital filters, it should be noted that, because in :: :
digital filters output signal samples are calculated from input signal samples on the basis of numbers, it will be clear to one, skilled in the art that the multiplication of the output signal samples of digital filter 3 by the carrler wave .~
samples of carrier wave source 6, can be performed in the ~ :.
digital filter 3.
.
- 12 - ~ .
P}IN. 7727 23.5.1(~75 . .
~L063~7~
~ .
P:ilot si~nal In prac-tice a pilo-t signal is sent wLth the modulated data signa:ls on the line carrier frequency (2100 Hz, ~ig. 2d). Such a pilot signal can he produced in various ways. An attractive method for the production of the pilot signal is the following.
The numerical values of the signal samples which `are supplied by sampler l~ to digital filter 3, are increased by a fixed amount. This causes spectral lines in the fre-quency spectrum to be produced according to Fig. 2a at the frequ~nc~es f1 t 2I1, .,. The spectral l:lne at 2fl (= 1l8oo Hz) shLf`ts by modulation wlth the spectral l:Lne nt 6900 Hz of`
carrier wave source 6 (Flg. 2c), to the line carrier frequency of 2100 Hz (~ig. 2d) and forms here the desired pilot signal.
In the case of ~the 4800 bit/sec data transmitter a conversion of the numerical values +3, +1, ~ 3, to ~4, ~2, 0, -2 results in a pilot signal which is 6 dB below the maximum signal level.
.
Rela.tion between line carrier frequency and dat~
rate. ..
From Fig. 2 it is clear that the line carrier frequency of 2100 ~Iz is equal to the difference between the :~reqllency of carri~r wave ~ource 6 of 6900 Hz and the frequency of 4800 Hz corresponding to the oentre of one of the Nyquist edges of the VSB f`ilter according to Fig. 2b. ~ .
If now the line carrier frequency is indicated by f , the fr,equenc'y of the carrier wave source 6 by -~f2 and the frequency corresponding to the centre of the Nyquist edge of the f'ilter to Fig. 2b by ~Kf1, then tho equation applies:
c ~f2 ~ ~Kf1 - (1') With f and f1 given the relation between K and f2 is fully d`etermined by equation (1).
When in expression (1) we substitute fc = 2100 Hz and f1.= 2400 l~z, then it will be found with K = 4 that f2 = 13800 Hz.
~ The factor by which digital~ filter 3 increases the sampling frequency then is:
f2/f1 = 23j4 (2) ~ `
The factor K may be an odd number.
, In this case, the spectrum according to Fig. 2a must be shifte~
to the right over an interval 2-f1. This can be reali.zed by 1(lgj3~L~
multiplying the output samples of sampler 4 alternately by +l and ~
Digital filter ~ :
Digital filter 3 according to Fig. l, with an increase of the sampling frequency by a factor L- , which in the example amounts to 24 , can be realized by means of a ~ - n~
cascade of an interpolating filter which ;ncreases the sampling frequency by a factor M and an extrapolating filter which decreases the sampling frequency by a factor L. The transfer function which must be realized can ~hen be divided between the two filters. .
An other possibility would be to replace the ex-trapolat~ng filter by a switch, which is operated at a fre-quency which ~s a factor L lower than the output sampling frequency of the interpolating filter and which supplies only ~ -one of each group of L output signal samples to modulator 5.
Interpolating and extrapolating digital filter are known as such and it may here suffice to refer to the :
references mentioned sub (C). .
An attractive version of an interpolating digital filter has been described in ~o-pending patent application 235,394 (PHN 7729) supra.
Although Applicant has refrained from describing known digital filters, it should be noted that, because in :: :
digital filters output signal samples are calculated from input signal samples on the basis of numbers, it will be clear to one, skilled in the art that the multiplication of the output signal samples of digital filter 3 by the carrler wave .~
samples of carrier wave source 6, can be performed in the ~ :.
digital filter 3.
.
- 12 - ~ .
P}IN. 7727 23.5.1(~75 . .
~L063~7~
~ .
P:ilot si~nal In prac-tice a pilo-t signal is sent wLth the modulated data signa:ls on the line carrier frequency (2100 Hz, ~ig. 2d). Such a pilot signal can he produced in various ways. An attractive method for the production of the pilot signal is the following.
The numerical values of the signal samples which `are supplied by sampler l~ to digital filter 3, are increased by a fixed amount. This causes spectral lines in the fre-quency spectrum to be produced according to Fig. 2a at the frequ~nc~es f1 t 2I1, .,. The spectral l:lne at 2fl (= 1l8oo Hz) shLf`ts by modulation wlth the spectral l:Lne nt 6900 Hz of`
carrier wave source 6 (Flg. 2c), to the line carrier frequency of 2100 Hz (~ig. 2d) and forms here the desired pilot signal.
In the case of ~the 4800 bit/sec data transmitter a conversion of the numerical values +3, +1, ~ 3, to ~4, ~2, 0, -2 results in a pilot signal which is 6 dB below the maximum signal level.
.
Claims
THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS
CLAIM:
Modulation- and filtering device for digital signals comprising means for filtering the digital signals and means for modulating the filtered digital signals, on a modulator carrier wave characterized in that signal samples of the digital input signal, which occur at a first sampling frequency are converted in a digital filter to signal samples of a second sampling frequency, which is twice as high as the modulator carrier wave frequency and the signal samples, at the second sampling frequency are modulated in the modulator with signal samples of the modulator carrier wave which occur at the same second sampling frequency the digital filter being adapted to have a band-pass filter characteristic.
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS
CLAIM:
Modulation- and filtering device for digital signals comprising means for filtering the digital signals and means for modulating the filtered digital signals, on a modulator carrier wave characterized in that signal samples of the digital input signal, which occur at a first sampling frequency are converted in a digital filter to signal samples of a second sampling frequency, which is twice as high as the modulator carrier wave frequency and the signal samples, at the second sampling frequency are modulated in the modulator with signal samples of the modulator carrier wave which occur at the same second sampling frequency the digital filter being adapted to have a band-pass filter characteristic.
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| NL7412095A NL168099C (en) | 1974-09-12 | 1974-09-12 | MODULATION AND FILTER DEVICE FOR DIGITAL SIGNALS. |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1063178A true CA1063178A (en) | 1979-09-25 |
Family
ID=19822081
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA234,814A Expired CA1063178A (en) | 1974-09-12 | 1975-09-04 | Modulation and filtering device |
Country Status (10)
| Country | Link |
|---|---|
| US (1) | US4003002A (en) |
| JP (1) | JPS555942B2 (en) |
| AU (1) | AU496789B2 (en) |
| BE (1) | BE833281A (en) |
| CA (1) | CA1063178A (en) |
| DE (1) | DE2540473C3 (en) |
| FR (1) | FR2285032A1 (en) |
| IT (1) | IT1042394B (en) |
| NL (1) | NL168099C (en) |
| SE (1) | SE404118B (en) |
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| DE3237619C2 (en) * | 1982-10-11 | 1985-04-18 | Karl Dipl.-Phys. Dr. 3550 Marburg Meinzer | Method and device for digital data transmission |
| DE3314603A1 (en) * | 1983-04-22 | 1984-10-25 | Siemens AG, 1000 Berlin und 8000 München | METHOD FOR DIGITAL SQUARE AMPLITUDE MODULATION |
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Family Cites Families (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| NL6713667A (en) * | 1967-05-13 | 1969-04-09 | Philips Nv | |
| IT958720B (en) * | 1972-03-27 | 1973-10-30 | Cselt Centro Studi Lab Telecom | TRANSCODER FOR TRANSMISSION OF NUMERICAL SIGNALS |
-
1974
- 1974-09-12 NL NL7412095A patent/NL168099C/en not_active IP Right Cessation
-
1975
- 1975-08-27 US US05/608,392 patent/US4003002A/en not_active Expired - Lifetime
- 1975-09-04 CA CA234,814A patent/CA1063178A/en not_active Expired
- 1975-09-08 AU AU84607/75A patent/AU496789B2/en not_active Expired
- 1975-09-09 SE SE7509996A patent/SE404118B/en not_active IP Right Cessation
- 1975-09-09 JP JP10859875A patent/JPS555942B2/ja not_active Expired
- 1975-09-09 IT IT27052/75A patent/IT1042394B/en active
- 1975-09-10 BE BE159924A patent/BE833281A/en not_active IP Right Cessation
- 1975-09-11 DE DE2540473A patent/DE2540473C3/en not_active Expired
- 1975-09-11 FR FR7527892A patent/FR2285032A1/en active Granted
Also Published As
| Publication number | Publication date |
|---|---|
| FR2285032B1 (en) | 1978-06-23 |
| NL168099B (en) | 1981-09-16 |
| JPS5153444A (en) | 1976-05-11 |
| NL168099C (en) | 1982-02-16 |
| JPS555942B2 (en) | 1980-02-12 |
| IT1042394B (en) | 1980-01-30 |
| AU496789B2 (en) | 1978-10-26 |
| DE2540473A1 (en) | 1976-04-01 |
| US4003002A (en) | 1977-01-11 |
| AU8460775A (en) | 1977-03-17 |
| DE2540473C3 (en) | 1978-10-26 |
| FR2285032A1 (en) | 1976-04-09 |
| SE404118B (en) | 1978-09-18 |
| SE7509996L (en) | 1976-03-13 |
| NL7412095A (en) | 1976-03-16 |
| DE2540473B2 (en) | 1978-03-09 |
| BE833281A (en) | 1976-03-10 |
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