DE2634114A1 - MODULATION METHOD AND ITS APPLICATION TO SINGLE-SIDED BAND MODULATORS AND DEMODULATORS - Google Patents

Description

Modulation method and its application in single sideband modulators and demodulators

The invention relates to modulation methods in which a carrier oscillation is multiplied by a modulation signal to obtain a modulated carrier. It also relates to single sideband modulators and demodulators, which comprise the modulators in which the method according to the invention is applied and which are used in particular in frequency conversion devices serving for telephone transmission.

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It is known that in order to carry out such a modulation by analog methods, the carrier oscillation and the modulation signal are superimposed on one another in a device which has a Has non-linearity, for example one in. Forward direction biased diode. Intermodulation products are obtained at the output of this device, from which one can filter takes the first order intermodulation product, which represents the desired modulated carrier wave.

This modulation can be extended using a digital process. According to the Shannon theorem any signal may be faithfully represented by a sequence of samples that is to be taken out at a clock frequency to this signal, which is greater than or equal to twice the maximum frequency contained in this signal. If one then quantizes these samples and expresses them in any digital code, for example in the pure binary code, all desired operations can be carried out on the signals by carrying out the mathematical operations corresponding to the analog operations on the numbers representing the samples. In this way, by digitizing the carrier oscillation and the modulation signal, the modulated inertia oscillation is obtained in digital form by multiplying the digital signals representing the carrier oscillation with the digital signals representing the modulation signal. It is then sufficient to convert the digital signal obtained by this multiplication into an analog signal in order to obtain the modulated oscillation

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in analog form.

Such a method, when applied to an isolated modulation operation, requires the use of two Analog / digital converters and a digital / analog converter. There is then no benefit. If, on the other hand, it is part of a more complex process, that in turn is digital is treated, it can be an advantage because the analog-to-digital and digital-to-analog converters are then all Functions common to the process and their relative weight for each function and in particular for the modulation it takes away.

Therefore, the single sideband modulators have already been proposed and demodulators, which are used in particular in telephone systems that work with frequency conversion can be used to build completely digitally. An analysis of these modulators can be found in the article [_ "I J" On Digital Single Sideband Modulators "by Sidney Darlington in IEEE Transaction on Circuit Theory, Volume CT-17, No. 3, August 1970.

The method according to the invention consists in generating a sinusoidal carrier wave and a modulation signal with a sampling frequency sample which is equal to four times the frequency of the carrier wave, and with a phase such that the sampling of a period of the carrier oscillation two positive

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Samples and then gives two negative samples, these four samples being equal in absolute terms, and then to form the product of the samples of the carrier wave and the samples of the modulation signal.

Further features and advantages of the invention emerge from the following description of exemplary embodiments Invention. In the drawings show:

FIGS. 1a and 1b sampled sinusoidal functions,

2 shows a single sideband modulator,

Fig. 3 shows a scanning modulator, and

4 shows a one-sided ribbon and the em odu I ator.

Fig. 1a shows a sinusoidal function y = sin Θ, limited to

ο
a period assumed between θ = 0 and θ = 860. It can be seen that if one samples this function at points in time corresponding to angles 45, 135, 225 and 315, four samples 1, 2, 3 and 4 are obtained which are equally distributed over time (if θ is a linear function the time is) and the-

2
have the same absolute value * (where 1 is the positive extreme value of the function) and where the first samples are positive and the last two are negative.

The respective amplitude conditions between a carrier wave

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and a modulation signal can - if a standard level of the supporting vibration is set - on one of the Let modulation signals return the impressed modulation factor. It is then justified to set a level of carrier

vibration of defining what

amount of the samples 1 results.

oscillation of what is considered to be the value of the absolute

The digital modulation operation then becomes extremely simple and consists in making a multiplication of the numbers, which represent the samples of the modulation signal by the number sequence + 1, +1, - 1 »-1, i.e. the sign of to change two consecutive samples of the same rank of the modulation signal after every four samples.

As shown in Fig. 1b, such a sequence of samples represents a sinusoidal function as well as a cosinusoidal one Function since the difference between the two corresponds only to a difference in the time origin. Thus, if the samples 1, 2, 3 and 4 represent a sine, samples 5, 6, 7 and 8 will represent a cosine corresponding to the Number sequence will correspond to +1, -1, -1, +1. Thus, if, as is sometimes required, two carriers out of phase by 90 are modulated with one and the same modulation signal are to be carried out, on the one hand, the multiplication given above and, on the other hand, the multiplication with the sequence of Numbers taken at the same points in time + 1, - 1, - 1, +1

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off, these multiplications being based on simple changes in sign let back.

This method is particularly advantageous for use in single sideband modulators in which the phase shift method is applied. This procedure is also known as the Hartley procedure and is detailed in the article [2] "The Phase Shift Method of Single Sideband Signal Generation" by Donald E. Norgaard in the journal Proceedings of the IRE, Volume 44, December 1956> described. In particular, it requires two modulations through 90 phase-shifted carrier oscillations.

The Charge Coupled Device (CCD, or Bucket Brigade Device, BBD) make it possible to produce all kinds of devices by means of which the signals can be handled by sampling, but without the samples being digitized. The operation of these arrangements is described in the article [3] "Charge Coupled Devices" by Gilbert F. Amelio in the journal "Scientific American", Vol. 250, No. 2, February 1974. The use of such arrangements for the production of filters is described in the article [4] "Transversal Filtering Using Charge Transfer Devices" by Denis D. Buss and others in the journal "IEEE Journal of Solid-state Circuits", Volume SC 8, No. 2, April 1973. The calculation of the filters produced according to this technique corresponds to the known calculation of the digital filters, but in practice

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Execution works directly with the samples, without digitize them.

The method according to the invention is particularly suitable in devices in which one does not use digitized samples works because it consists in changing the sign of these samples periodically, which is done with a simple inverter circuit can be done.

Fig. 2 shows a single sideband modulator which can be used in a remote prechfrequenzumsetzaniage. There are only those devices are shown, by means of which the levels are set both at the input and at the output of this modulator and it is believed that the various devices shown signals yeast which level compatible with the facilities it is intended to receive. The numbers given as examples relate to European telephony technology and do not deprive the invention of its general applicability .

This modulator contains filters 210 to 215, summers 216 _s 217 and 223, attenuators 219 and 220, modulators 221 and 222 and a demodulator 218.

The speech signal Tw to be modulated is passed through the filter 210 filtered so that it is based on the standardized voice band of

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300 to 3 400 Hz is limited. This voice band filter is a filter using charge transfer devices according to the technique described in Article Q 4 J. The sampling frequency used in this filter is chosen to match that used in filters 211 and 212, so that the scanning processes are not multiplied.

The filtered signal to be modulated then passes through the two processing circuit branches which are the same Structure, but have different parameters.

The first processing circuit branch contains the summer 216, the filters 211 and 213, the attenuator 219 and the Modulator 221.

The reference signal R, its function, is added to the filtered signal to be modulated in the summer 216 is described below. A composite signal is obtained in this way.

This composite signal is filtered in the filter 211, which is an all-pass filter that causes a constant phase shift of all frequencies contained in the signal, from which it is crossed. This filter is realized with charge shifting devices and the sampling frequency is chosen to be sufficiently high to obtain error-free operation, while at the same time those lying outside the band Lobes characteristic of scanning filters

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are suppressed sufficiently. It has been shown that the frequencies above 200 kHz are suitable and that 256 kHz was a good value. ^

The phase-shifted composite signal that the filter 211 leaves, thus represents a spectrum that consists of the voice band from 300 to 3400 Hz, which at every multiple the sampling frequency is repeated up and down. It is required those beyond 3400 Hz To eliminate components of this spectrum so as not to generate signals that are later disruptive. That is due to the large distance between the lobes, the first interference frequency if the sampling is carried out at 256 kHz, starts at (256 - 3.4) kHz = 252.6 kHz. You then came to use a summary filter, for example from a simple RC element. This filter 213 thus delivers a smoothed signal.

This smoothed signal reaches the attenuator 219, which is used to automatically adjust the gain, as described below. This attenuator can, for example, be a potentiometer circuit that has a fixed resistor and a field effect transistor connected as a variable resistor.

The attenuated signal coming from the attenuator 219 modulates a carrier P in the modulator 221. This carrier

is one of the twelve standardized carriers of a basic telephone group, their frequencies each by 4 kHz from 64 to 108 kHz

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are graded. According to the invention, this carrier is sampled at a frequency four times greater than its own Natural frequency, and the modulation is done by changing the sign of the damped signal during two out of every four Samples of the carrier is changed.

The second processing circuit branch contains a summer 217, the filters 212 and 214, the attenuator 220 and the modulator 222.

The differences in the processing performed on the filtered signal to be modulated are in this second circuit branch the following:

the reference signal R has the same frequency and the same

öO

Amplitude like. the signal R, but it is opposite this

90 out of phase;

- the all-pass filter 212 has a phase shift characteristic,

ο which is 90 out of phase with that of filter 211. The main thing is that the one consisting of these two filters

ο The arrangement shifts the phase of the mixed signals by 90 against each other. For example, you can only give one of the filters a

ο
Give a phase shift of 90, while the other does not cause a phase shift, but serves to compensate for the inevitable delay caused by the first filter. However, it is generally easier to distribute the phase shift properties between the two filters in order to

ο to get a difference of 90 between the two. Of the

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Carrier P _0n has the same frequency and the same amplitude as carrier P, but is around 90 relative to it

phase shifted, which corresponds to the shifting of the points in time at which the sign of the attenuated signal is changed, corresponds to one sample.

The modulated carriers originating from the modulators 221 and 222 are added in the summer 223. The expression "Addition" is taken in an algebraic sense and the direction chosen (addition or subtraction) is accordingly the direction of the phase deviation caused by the filters 211 and 212 and corresponding to the direction of the phase deviation between the beams P and P in such a way

o 90

assume that the lower sideband is obtained, which corresponds to the modulation of the selected carrier, since one in telephony transmits the lower sidebands. These options are explained in the article [2J.

Due to the sampling of the carriers, the spectrum of the addition signal coming from the summer 223 becomes parasitic Contain signals which, in the case of the first 64 kHz carrier, which is sampled at 256 kHz, start at 192.3 kHz and then at higher frequencies are repeated. The filter 215 allows these parasitic signals to be eliminated. It can also do this be very summary, due to the large distance between the. parasitic signals and the useful signals, and

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can for example consist of a simple RC element. However, one chooses the cut-off frequency of this filter above 108 kHz (12th carrier), so that one generalized Has facilities that can be used by all carriers. In this case, the assignment of such Modulator to a carrier is determined solely by the carriers P and P supplied by external devices will.

It is necessary to comply with the very strict standards (blocking attenuation> 80 dB) which exist in telephone equipment, an automatic gain control is required to be provided for the two processing circuit branches and in particular for that part of these branches that contains the CCD or contains BBD filters, which are the most unstable. Members of these branches.

For this purpose, the signals R and R _{Qn are} signals with the same frequency and the same amplitude, for example with a frequency of 800 Hz, which is a common test frequency. she

ο
are shifted 90 in phase so that they are in antiphase after passing through filters 211 and 212. After modulating the carriers P and P in this way, they become at the

O 90

summation carried out in the summer 223 disappear.

If the gain of the two circuit branches is not the same, the sum of these is at the output of the attenuators

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219 and 220 removed signals are not zero and the error signal resulting from this sum allows the attenuators 219 and 220 to be controlled so that the equilibrium is restored. Synchronous demodulation of these two can be used to achieve this error signal execute anti-phase signals in the synchronous demodulator 218, one of the two signals P or P being used as the demodulation reference signal. If you only get a single error signal the attenuators 219 and 220 must be reversed work. They can be the same if the demodulator 218 has two Supplies error signals with opposite signs.

A particularly advantageous embodiment of the modulators 221 and 222 is an amplifier with switchable To create reinforcement, as shown in FIG. This amplifier contains a differential amplifier 31, resistors R, R, R and a switch 32.

The switch 32 is built up from static devices, for example from field effect transistors , because of the rapidity of the phenomena. It is controlled by the P signal.

The assumptions that are common in these circuit diagrams are made, i.e. the value of the resistors R, R and R. is large compared to the output impedance of the signal generators and compared to the input impedance of the operational amplifier small ·, the gain of this amplifier being very large and

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its output impedance is very small.

When the switch 32 is closed, the positive input of the amplifier 31 as well as the resistor R are connected to ground, which does not affect the level of the signal E. The gain of the amplifier is then conventional

R3
Way is given by - · --— and is equal to - 1 when R ₁ = R ₃ .

When the switch 32 is open, the signal E is applied to the positive input without attenuation due to the values of the impedances. In a manner known per se, the negative feedback exerted on the minus input endeavors to eliminate the voltage difference between the plus and minus input. The voltage at the minus input is thus essentially equal to E. Therefore the current flowing through the resistor R is zero and consequently the current in the resistor R is also zero. Thus the voltage at the output S is equal to E and the gain of the amplifier is equal to + 1 _β

Therefore, for R = R, one has an amplifier whose

3 I.

Strengthening is equal to +1 or - 1, depending on whether the switch 32. is open or closed.

This amplifier can thus take on the task of a modulator according to the invention, since by applying the

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Modulation signal to the terminal E and the samples of the carrier wave to the terminal P, the direction of the modulation signal is reversed according to the direction of the samples of the carrier. For this purpose, however, no pulses such as pulses 1 to 8 (FIGS. 1a and 1b) are used, but in the usual manner rectangular pulses of maximum width, which are shown by the hatched zones in FIGS. 1a and 1b. In this way the switch 32 will be closed for the positive hatched zones and opened for the negative hatched zones, or vice versa, depending on the desired direction of modulation.

FIG. 4 shows a single sideband dem odu I ator corresponding to the modulator shown in FIG. Conditions is described. The phase shift demodulation method used in this demodulator is detailed in essay 5 "The Phase-Shift Method of Single-Sideband Reception "by Donald E. Norgaard in the journal" Proceedings of the.IRE ", Volume 44, December 1956.

This demodulator includes filters 424 and 425 and 410 bis 414, summer 416, 417, 423, attenuators 419 and 420, modulators 421 and 422 and a demodulator 418.

The signal to be demodulated, for example a basic telephone group signal is and consists of twelve carriers,

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those staggered from 64 to 108 kHz and each in the lower Are sideband modulated, is applied to the input BLU, from where it arrives at two processing circuit branches, which have the same structure but different parameters.

The first processing circuit branch includes the modulator 421, the filters 424, 411 and 413, the summer 416 and the attenuator 419.

The signal to be demodulated is modulated in the modulator 421 by the first carrier P, which is the same Carrier is like the carrier P used in the modulator of Figure 2 and the modulation method is exactly the same.

In fact, in a product modulator, the modulation operation is the same as the demodulation operation, except that that it is an exchange between the modulated and signals to be modulated. So you get different Intermodulation products, for example the voice band from 300 to 3,400 kHz for the first 64 kHz carrier and on the other hand higher products, the lowest frequency of which is 64 χ 2 - 3.4 = 124.6 kHz.

It is thus very easy to isolate a smoothed demodulated signal in the demodulated signal obtained in this way, which only contains the voice band by using the filter 424, which is due to the large distance between the Signals can be a simple RC element.

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The first reference signal R is added to the smoothed demodulated signal in the summer 416, its

Task is described below. A composite signal is obtained in this way.

This composite signal is filtered in the filter 411, which has the same structure as the filter 211 and the same Has the task of delivering a phase-shifted signal.

This phase shifted signal is filtered by the low pass filter 413 which does the same job as the Has filter 213 and provides a smoothed phase-shifted signal.

This smoothed phase shifted signal goes into variable attenuator 419 which does the same job as the attenuator 219 has and a first attenuated signal supplies.

The second processing circuit branch includes modulator 422, filters 425, 412 and 414, summer 417 and 414 the attenuator 420.

The differences in processing involved in the demodulated Signal executed in this second circuit branch are as follows:

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the second carrier P has the same frequency and the same

so

Amplitude like the carrier P, but it is opposite this

ο
ο
90 out of phase;

- The second reference signal R _n ^ has the same frequency and

9O

same amplitude as signal R, but it is opposite

ο
this phase shifted by 90; and

- The filter 412 has a phase shift characteristic, the opposite that of the filter 411, in the same way as that of the filter 212 is 90 out of phase with that of filter 411 ·.

This second processing circuit branch supplies a second attenuated signal, which is added to the first attenuated signal in the summer 423. The term "addition" is assumed in the algebraic sense, as for the modulator, and corresponds to the totality of the phase shifts used, the choice of the available possibilities, which are described in the article [5J, being made so that the lower sideband results.

If namely the signal applied to the input BLU only contained a single single sideband modulated carrier, one would have by modulation in one of the modulators 421 or directly the usable voice band that would be sufficient to extract the higher modulation products through filtering '. There In fact, eleven more single sideband-modHed carriers are applied to the BLU input, one also receives at the

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Output of the modulators 421 and 422 the modulations that are transmitted by the carriers that are smaller than the demodulated carrier and are located outside the voice band every 4 kHz, as well as the modulations that are transmitted by the carriers that are greater than the demodulated Carriers are staggered by 4 kHz each and the first of which is in the voice band. This is because the lower sideband of the next higher carrier can just as well be viewed as the upper sideband of the demodulated carrier. However, the signal obtained in this way is not understandable, because its spectrum is shifted and in an inverted position - (300 to 3 400 Hz - 3 700 to 600 Hz) _; but it is

mixed with the useful signal and makes it incomprehensible. For this reason it is necessary to receive a To apply a process opposite to that used in the broadcast.

In the same way as with the transmission, it is necessary to provide an automatic control of the gain of the two circuit branches. For this reason, the two reference _{signals R and R 0n are introduced} , which are in the detector 418

O 9O

be synchronous demodulated, which outputs an error signal that the attenuators 419 and 420 in opposite Sense controls.

The addition signal from summer 423 contains

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thus the modulation in the voice band from 300 to 3400 Hz, which comes from the carrier that you want to demodulate, and also the modulations of the lower carriers, which are stepped beyond this voice band by 4 kHz. For example, if you demodulate the third carrier, which is 72 kHz, you have low frequency signals in the 300-3400 Hz (desired signals), 4,300-7,400 Hz (68 kHz carrier), and 8,300-11,400 Hz bands (64 kHz carrier). In order to keep the desired telephone signal at the output T , these signals are filtered with the low-pass filter 410, which corresponds to the filter 210 and eliminates the frequencies above 3400 Hz. This filter is a CCD or BBD filter that operates at the same sampling clock frequency as filters 411 and 412.

The application of the modulation method according to the invention in such single sideband modulators and demodulators in which CCD or BBD filters are used thus allows the production of simple and economical devices.

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Claims (8)

Patent claims: 1. Modulation method, which consists in that with a and the same frequency a sinusoidal carrier wave and a modulation signal is sampled and that in each Sampling time the product of the sample obtained on the carrier wave and that obtained on the modulation signal Sampling is formed, characterized in that the sampling frequency is four times the frequency of the carrier oscillation is and that the phase of the sampling is chosen so that in a period of the carrier oscillation two positive samples and then two negative samples where these four samples are equal in absolute magnitude. 2. Modulation method according to claim 1, characterized in that that the product is obtained by taking the sign of two consecutive samples of the same Range of the modulation signal is changed every four samples. 3. Single sideband phase modulator for performing the Method according to claim 1 or 2, characterized by: a CCD or BBD baseband filter that receives a signal to be modulated and a filtered signal to be modulated supplies; 709807/1179 - A first processing circuit branch with a Summer which allows a first reference signal to be superimposed on the filtered signal to be modulated, and a Composite signal delivers, with a CCD or BBD phase shift filter, that with the same sampling period as the baseband filter works, allows the phase shift of the signal mixture and delivers a phase-shifted signal, with summary Filter devices that filter the phase-shifted signal and deliver a smoothed signal, with an attenuator, that attenuates the smoothed signal and provides a first attenuated signal, and with a modulation circuit that the modulating the attenuated signal onto a first carrier and providing a first modulated carrier; - A second processing circuit branch, which has the same structure as the first circuit branch, to the filtered receives a modulating signal, a second reference signal, ο that is the same as the first, but is 90 out of phase, and uses a second carrier that is the same as the first, ο
but is phase shifted by 90, with a phase shift filter whose phase shift characteristic is shifted by 90 with respect to that of the phase shift filter of the first circuit branch, and delivers a second attenuated signal and a second modulated carrier; - A circuit for synchronous demodulation of the first and the second attenuated signal, which is one of the reference signals used and provides error signals that the attenuators of the 703807/1179 26341U control both processing circuit branches in opposite directions; a summing circuit which adds the first and the second modulated carrier and supplies an addition signal; and A circuit that filters the addition signal summarily and outputs a desired single sideband signal. 4. Modulator according to claim 3, characterized in that the sampling period of the CCD or BBD filter is one Frequency above 200 kHz. 5. Single sideband phase demodulator for performing the Method according to claim 1 or 2, characterized by: ~ a first processing circuit branch with a Modulation circuit which allows a signal to be demodulated and a demodulated signal to be modulated on a first carrier Emits signal, with a circuit that filters this demodulated signal summarily and a smoothed one demodulated signal provides, with a summing circuit that allows the smoothed demodulated signal a first To superimpose reference signal, and a signal mixture delivers, with a CCD or BBD phase shift filter, the ■ allows the composite signal to be shifted in phase, and provides a phase-shifted signal, and with a circuit for the summary filtering of the phase-shifted signal, 709807/1179 26341U which provides a smoothed phase-shifted signal, and with an attenuator that the smoothed phase-shifted Attenuates signal and provides a first attenuated signal; a second branch of processing circuitry that has the same The structure of the first circuit branch that receives the signal to be demodulated has a second carrier that corresponds to the first ο
Carrier is the same but is 90 out of phase and uses a second reference signal that is the same as the first but is phase shifted by 90, with a phase shift filter whose phase shift characteristic compared to that of the phase shift filter of the first circuit branch ο
is shifted by 90 and which provides a second attenuated signal; - A circuit for synchronous demodulation of the first and the second attenuated signal, which is one of the reference signals used and provides error signals which the attenuators of the two processing circuit branches in opposite Steer directions; a summing circuit which adds the first and second attenuated signals and delivers an addition signal; - a CCD or BBD filter with the same sampling period how the phase shift filter of the processing circuit branches works and a desired one in the addition signal Voice signal isolated. 6 ..- demodulator according to claim 5, characterized in that 709807/1 179 that the sampling period of the CCD or BBD filter is one Frequency above 200 kHz. 7. Telephone transmission device operating with frequency conversion, characterized in that it has at least one modulator according to claim 3 or 4 contains. 8. Telephone transmission device operating with frequency conversion, characterized in that it contains at least one demodulator according to claim 5 or 6. 709807/1179