A multi-channel transmission and / or reception system comprising at least N parallel processing channels and a method for decorrelating quantization noise in such a system

Abstract

Multichannel transmit and / or receive system (10), each channel comprising a DAC (14) and a sigma-delta modulator (12) whose transfer function is given as follows: OUT(z) = IN(z).FTS(z) + Q(z).FTB(z), where OUT is the output signal of the sigma-delta modulator, IN is the input signal of the sigma-delta modulator, FTS is the transfer function of the input signal, Q is the quantization noise, and FTB is the transfer function of the quantization noise. The second terms of the transfer function of the sigma-delta modulator only are distinct from each other for two channels Vi, Vj, to uncorrelated the quantization noise of distinct channels. The first term of said transfer function for channel Vi is equal to that for channel Vj. Figure for the abbreviation: Fig. 2

Description

Title of the invention: Multi-channel transmission and / or reception system comprising at least N parallel processing channels and method for decorrelating quantification noise in such a system Technical field

[0001] The invention lies in the field of multi-channel transmission or reception systems. Prior art

[0002] An example of a multi-channel radiofrequency type transmission-reception system architecture is shown in [Fig.l], comprising several parallel processing channels in transmission and reception, both for the I signals and for the Q signals in a quadrature representation. Each transmission channel actually comprises an I channel and a Q channel, each for processing the corresponding component of the input signal comprising two quadrature components. Each of the I, Q channels comprises in particular a digital-to-analog converter (DAC), a shaping low-pass filter, a frequency change stage. Each transmission channel further comprises an amplifier and a transmission antenna.Each receiving channel includes a receiving antenna, a band-pass filter, an amplifier, and also includes an I channel and a Q channel, each comprising a frequency change stage, a low-pass filter and an analog-to-digital converter (ADC).

[0003] It is often necessary, in architectures of this type used for example to perform digital beamforming, to have a large number of these channels in parallel in order to obtain a significant gain in the target direction. This large number of channels results in a large silicon surface area and can result in a large number of inputs / outputs. To limit the number of inputs / outputs, it is possible to use the same digital signal at the input of all the channels (I, respectively Q) and to perform the beamforming by operating a different delay (and / or by operating a phase shift of the signals) on each of the channels, to compensate for the difference in flight time between the signals. Even if the baseband processing is integrated into the chip, having a single input will reduce the complexity of the digital routing. The delay can also be integrated into the DAC by performing all or part of the delay on the DAC clock delay.

[0004] To have a good signal to noise ratio (SNR for English “Signal To Noise Ratio ") in the signal bandwidth and achieve a high bit rate, it is interesting to have a high number of bits in the DAC in order to reduce the quantization noise. Increasing the order of modulations in current systems tends to increase the constraints on the SNR and therefore increase the number of bits in the DAC. This high number of bits is annoying because it implies a significant complexity for the DAC (which will generate a large surface area and high consumption). The constraint on the SNR of the DAC is all the more important because when each DAC receives the same signal, the quantization noise will be the same on all channels; this noise is added coherently during beamforming and its power increases in 201og(N) where N is the number of channels.

[0005] Solutions exist to try to reduce the correlation of quantization noise between channels. Noise ("dither"), different on each of the channels, can be injected. This makes it possible to limit the increase in quantization noise to 101og(N). But this solution increases the complexity since it is necessary to generate N uncorrelated noise sources, inject the generated noise into the DAC in analog, then remove this signal at the output. This signal (dither) can increase the dynamic constraint of the signal and can create strong linearity constraints on the DAC by increasing the signal circulating in this DAC.

[0006] One of the solutions used in the state of the art is to position a sigma-delta coder upstream of the DAC: cf. WO2020105040 AL This solution makes it possible to reduce the number of bits of the DAC (and therefore to reduce consumption and surface area) in return for having a high sampling frequency, to benefit from a good oversampling factor, which is necessary for sigma-delta coding. This constraint is not necessarily a problem in the transmission architecture since it is usual to use a high sampling frequency in the transmission DAC so as not to overly constrain the analog filter following this same DAC. One of the disadvantages of this architecture is that the quantization noise, the power of which increases as a function of the frequency from the end of the signal bandwidth, must be filtered by an analog filter with a low cutoff frequency. This places strong constraints on the analog filter.

[0007] With the use of a sigma-delta encoder to perform data compression, there are simple solutions to de-correlate the quantization noise between all channels. Some of these solutions are proposed in WO2020105040 AL

[0008] A first solution consists of performing data manipulations between the output of the sigma-delta encoder and the input of the digital-to-analog converter. This solution is of limited interest since when the number of channels is high, it requires having a permutation over a long sequence to have enough different combination possibilities, which increases the complexity of the part data mixing.

[0009] Other solutions consist of modifying the sigma-delta encoders differently on each of the channels. One solution is to add a constant value (offset) to the input of the 1-bit quantizer. This option is easy to use, but it has the disadvantage of not breaking the rhythm of the periodic sequences of the sigma-delta encoder and therefore of maintaining identical "tones" between the channels (concentration of noise at close frequencies). Another solution of the same type consists of starting the sigma-delta encoders at different times. This is quite effective with regard to the decorrelation of the quantization noise, but as in the previous case, we will end up with noise sequences which may be just offset from each other and therefore have tones which are at the same frequencies, on each of the channels.Another solution is to provide different initial states in each of the channels, which leads to the same type of result.

[0010] A third type of proposed solution consists of modifying the coefficients of the sigma-delta encoder filter differently on each of the channels. Thus, the encoder sequences will be different from each other and the noises will not be correlated. The disadvantage is a risk on the stability of the sigma-delta encoder (by modifying the coefficients, the poles are modified) as well as a modification of the transfer function of the input signal which can disturb the quality of the transmission system.

[0011] Thus, there are either very simple solutions, which do not impact the signal, but which do not completely decorrelate the noise of one channel in relation to the others, which can present a risk if the noise is not completely white, or more complex solutions which can have an impact on the signal itself, by modifying the signal transfer function and present risks of instability. Summary of the invention

[0012] To this end, according to a first aspect, the present invention describes a multi-channel transmission and / or reception system comprising at least N parallel processing channels, N >2, each coupled to a transducer and each comprising a converter block comprising a CAN or DAC converter and a sigma-delta modulator,

[0013] the transfer function of the sigma-delta modulator of each channel Vi, i = 1 to N, being defined as a sum of two terms comprising a first term IN(z).FTS(z) relating to the signal and a second term Q(z).FTB(z) relating to the quantization noise and being stated as follows:

[0014] OUT(z) = IN(z).FTS(z) + Q(z).FTB(z),

[0015] where OUT is the output signal of the sigma-delta modulator, IN is the input signal of the sigma-delta modulator, FTS is the transfer function of the input signal, Q is the quantization noise and FTB is the transfer function of the quantization noise,

[0016] the transfer function of the sigma-delta modulator of a channel Vi being distinct from the transfer function of the sigma-delta modulator of a channel Vj, i and j distinct integers between 1 and N, to decorrelate between them the quantization noises of distinct channels,

[0017] said multi-channel system being characterized in that the second terms of the transfer function of the sigma-delta modulator only are distinct, the first term of said transfer function for channel Vi being equal to that for channel Vj.

[0018] Thus the invention makes it possible to prevent the quantification noises from the different channels from adding together in a coherent manner, without presenting the aforementioned drawbacks of the solutions of the prior art: the signal transfer function is not modified between the different channels, the quality of the signal and the stability are ensured.

[0019] In embodiments, such a system will further comprise at least one of the following features:

[0020] - the sigma-delta modulator on each of the channels Vi, Vj includes a quantizer 1 bit with hysteresis, and the hysteresis cycle is different between the channels Vi, Vj in amplitude and / or temporally;

[0021] - the sigma-delta modulator on each of the channels Vi, Vj includes a quantizer multibit; and the value of the input signal corresponding to the switching between two quantization levels on channel Vi is set distinct from the value of the input signal corresponding to the switching between these same two quantization levels on channel Vj;

[0022] - the multi-channel transmission and / or reception system comprises on each of the channels Vi, Vj, at least said sigma-delta modulator with multibit quantizer and an additional sigma-delta modulator, the output of one of said modulators being provided as input to the other of said modulators, said other sigma-delta modulator on channel Vi being identical to said other sigma-delta modulator on channel Vj;

[0023] the multi-channel transmission and / or reception system comprises on each of the channels Vi, Vj, a sigma-delta modulator with cascaded architecture comprising at least two stages, the first stage comprising a first delta-sigma modulator comprising in particular a quantizer and the second stage comprising at least a second sigma-delta modulator;

[0024] the second stage being adapted to receive as input the output of the quantizer of the first modulator and the input of the quantizer of the first modulator, to digitize, via the second comparator, a signal representative of the quantization noise of the first modulator and to shape it;

[0025] the difference between a signal proportional to the output of the quantizer of the first modulator and a signal proportional to the quantizer input of the first modulator being provided as input to the second modulator;

[0026] the output of the second stage then being subtracted from the output of the first modulator to deliver a signal devoid of at least a portion of the quantization noise of the first stage

[0027] in which the second stage of the channel Vi is different from the second stage on the channel Vj so as to differentiate between them the quantization noises of the second modulator of the channels Vi, Vj;

[0028] - the second stage comprises multipliers, each multiplier multiplying its input by a respective associated coefficient, in which, considering at least one of said multipliers, the coefficient associated with it on channel Vi is of a different value from that on channel Vj;

[0029] - a delay block is arranged in the second stage, between the output of the first modulator and the input of the second modulator, adapted to apply a delay to the signal coming from said output, the delay on channel Vi being set to a value different from that of the delay on channel Vj.

[0030] According to another aspect, the invention describes a method for decorrelating quantification noise in a multi-channel transmission and / or reception system comprising at least N parallel processing channels, N >2, each coupled to a transducer and each comprising a converter block comprising a CAN or DAC converter and a sigma-delta modulator,

[0031] the transfer function of the sigma-delta modulator of each channel Vi, i = 1 to N, being defined as a sum of two terms comprising a first term IN(z).FTS(z) relating to the signal and a second term Q(z).FTB(z) relating to the quantization noise and being stated as follows:

[0032] OUT(z) = IN(z).FTS(z) + Q(z).FTB(z),

[0033] where OUT is the output signal of the sigma-delta modulator, IN is the input signal of the sigma-delta modulator, FTS is the transfer function of the input signal, Q is the quantization noise and FTB is the transfer function of the quantization noise,

[0034] according to which the operation of the sigma-delta modulator of the channel Vi is adapted with respect to the operation of the sigma-delta modulator of the channel Vj to decorrelate the quantization noises generated by the channels,

[0035] the transfer function of the sigma-delta modulator of a channel Vi being distinct from the transfer function of the sigma-delta modulator of a channel Vj, i and j distinct integers between 1 and N, to decorrelate between them the quantization noises of distinct channels,

[0036] said method being characterized in that the second terms of the transfer function of the sigma-delta modulator only are distinct, the first term of said transfer function for channel Vi being equal to that for channel Vj.

[0037] In embodiments, such a method will further comprise at least one of the following features:

[0038] - the sigma-delta modulator on each of the channels Vi, Vj includes a quantizer 1 bit with hysteresis, and the hysteresis cycle is different between the channels Vi, Vj in amplitude and / or temporally;

[0039] - the sigma-delta modulator on each of the channels Vi, Vj includes a quantizer multibit; and the value of the input signal corresponding to the switching between two quantization levels on channel Vi is set distinct from the value of the input signal corresponding to the switching between these same two quantization levels on channel Vj. Brief description of the drawings

[0040] The invention will be better understood and other characteristics, details and advantages will appear more clearly on reading the following description, given without limitation, and thanks to the appended figures, given by way of example.

[0041] [Fig.l] [Fig.l] is an illustration of a multi-channel transceiver system architecture in the Prior Art (AA);

[0042] [Fig.2] [Fig.2] is an illustration of a multi-channel transmission system in a embodiment of the invention;

[0043] [Fig.3] [Fig.3] is a schematic of a single-cell sigma-delta modulator of the first order;

[0044] [Fig.4] [Fig.4] is a graph showing the characteristic of a quantifier of a two-level output sigma-delta modulator in one embodiment of the invention;

[0045] [Fig.5] [Fig.5] illustrates an embodiment of a quantifier having the ca characteristic of [Fig.4];

[0046] [Fig.6] [Fig.6] illustrates another embodiment of a quantifier having the characteristic of [Fig.4];

[0047] [Fig.7] [Fig.7] is a graph showing part of the characteristic of a quant a sigma-delta modulator with at least three output levels in one embodiment of the invention;

[0048] [Fig.8] [Fig.8] is an illustration of a multi-channel transmission system in a embodiment of the invention;

[0049] [Fig.9] [Fig.9] is an illustration of a two-stage sigma-delta modulator with the architecture known from the prior art;

[0050] [Fig. 10] [Fig. 10] is an illustration of a method for decorrelating quantization noise in a multi-channel transceiver system in a realization mode. lization of the invention.

[0051] Identical references may be used in different figures when they designate identical or comparable elements. Description of the embodiments

[0052] A multi-channel transmission system 10, for example of the radiofrequency type implemented in the context of a millimeter wave beamforming application, comprises in an embodiment of the invention illustrated in [Fig.2], a digital-analog conversion block integrated for example in a SOC and comprising N parallel processing channels Vi, i = 0 to Nl, with N an integer greater than or equal to 2. In the case shown, N is equal to 4.

[0053] It will be noted that each processing channel Vi actually comprises two channels, one channel I and one channel Q, each for processing the corresponding component of the input signal comprising two quadrature components. When we speak of each processing channel below, we will mean each processing channel among the N processing channels of the signal I for example (or each processing channel among the N processing channels of the signal Q).

[0054] Each processing channel Vi receives the same digital signal 11 as input.

[0055] The processing channel Vi, i = 0 to Nl, successively comprises a sigma-delta modulator 12, a delayer 13 delaying the signal by a delay ir, a digital-analog converter 14, a low-pass filter 15, a frequency change stage 16 for carrying out the carrier setting, an amplifier 17 and is coupled to a radiofrequency transmission antenna 18.

[0056] In embodiments, the transmission system 10 comprises a control block (not shown), adapted to generate control signals C intended for the sigma-delta modulators 12, to control the decorrelations of the quantization noises if necessary.

[0057] The sigma-delta modulator here is a digital-to-digital sigma-delta modulator which receives a bit stream, with a precision of M bits and delivers a bit stream with a precision of K bits, with K< M and K, M integers.

[0058] A sigma-delta 12 modulator is, in a known manner, a feedback loop device performing encoding on a small number of bits (for example equal to 1, to 2, less than 5, etc.), and a high sampling frequency compared to the bandwidth of the signal to be encoded. This device shapes the spectrum of the quantization noise by pushing its power into a frequency band not occupied by the spectrum of the useful signal. This spectral separation makes it possible, thanks to filtering of the encoded signal, to maintain a signal-to-noise ratio conforming to given specifications.

[0059] A sigma-delta modulator generally comprises:

[0060] an input for receiving an input signal which, depending on the uses, is analog or, as in the example considered, digital, coded on M bits;

[0061] an output for delivering a digital output value coded on K bits; and,

[0062] one or more sigma-delta cells arranged between the input and the output of the modulator.

[0063] Each cell may be of the first order or of a higher order. In particular, some may be of the first order, and others of a higher order.

[0064] Each sigma-delta cell comprises at least the following elements:

[0065] a subtractor comprising a first input, a second input, and an output, the first input being coupled to the input of the modulator to receive the input signal or a value derived therefrom;

[0066] an integrator comprising an input coupled to the output of the subtractor, and an output; and,

[0067] a quantizer having an input coupled to the output of the integrator and an output coupled to the second input of the subtractor; the output of the quantizer is further coupled to the output of the modulator to provide the digital output value or a contribution thereto.

[0068] Each sigma-delta cell is characterized by its transfer function, connecting its output signal OUT to its input signal IN and to the quantization noise Q of the quantizer. The transfer function (z-transformed) is written as follows:

[0069] OUT(z) = IN(z).STF(z) + Q(z).NTF(z)

[0070] where STF is the input signal transfer function (Signal Transfer Function) and NTF is the noise transfer function (Noise Transfer Function), expressed here as a function of frequency; Q is the quantization noise, equal to the difference between the quantizer output and the quantizer input.

[0071] [Fig.3] schematically represents a first-order single-cell sigma-delta modulator 12, with an input signal IN, an output signal OUT, a subtractor 121, an integrator 122 and a quantizer 124.

[0072] L is the loop filter function. It is, as is known, a low-pass filter function with very high gain at low frequencies. The loop filter can be implemented as a simple integrator or as a cascade of integrators (when the input signal is analog, a DAC is additionally placed on the return path to take the digital output signal and return it to the input of the subtractor in analog).

[0073] In the case of Figure 3, STF(f) = and NTF( f ) = •

[0074] The input signal transfer function operates with a gain of 1 in the bandwidth of interest. The noise transfer function is a filter function high-pass, providing noise shaping: strong suppression of quantization noise in low frequencies while quantization noise at high frequencies outside the passband is increased.

[0075] The invention consists in decorrelating the quantification noises between the different channels, by acting only on the “Q.NTF(z)” part, without modifying the signal transfer function, to guarantee stability and quality of the signal. The “IN.STF(z)” part is therefore the same for the different processing channels, the “Q.NTF(z)” part differs at least for some of the channels.

[0076] In a first embodiment of the invention, the quantization noises are decorrelated between two processing channels by using, in the sigma-delta modulator 12 of each channel, a 1-bit hysteresis quantizer, the hysteresis applied to a channel Vi being different from that applied to the channel Vj, with i, j distinct integers between 0 and N.

[0077] In such a case, with reference to [Fig.4]:

[0078] - instead of having, on each processing channel, the output of the quantizer which switches between OUT1 and OUT2 (the two quantization levels of the 1-bit quantizer) when I crosses the Threshold value (value 0 in [Fig.4], but the Threshold value can take another predetermined value in other embodiments),

[0079] - the transfer function of the quantizer 124 is modified so that the func operation is as follows:

[0080] when the signal I is lower than the Threshold value, the switching value is equal to Threshold 1, lower than the Threshold value (in the case presented in [Fig.4]: Threshold value - h, with h real number) and when the signal I is higher than the Threshold value, the switching value is equal to Threshold2, higher than the Threshold value (in the case presented in [Fig.4]: Threshold value + h).

[0081] The Threshold value around which hysteresis takes place is the same for channels Vi, Vj.

[0082] [Fig.5] illustrates a possible functional implementation of such a 1-bit quantizer (or comparator) with hysteresis, implementing a zero comparator and a multiplier.

[0083] Another solution for the functional implementation is to use, as shown in [Fig.6], a threshold comparator and a multiplexer which will choose h or - h depending on the output level of the comparator, which avoids the implementation of a multiplier.

[0084] In this embodiment, different hysteresis values ​​are used on separate channels Vi, Vj. Thus, even with the same constant input 11, the noises will have different cycles and will be decorrelated from each other.

[0085] The respective hysteresis values ​​are for example controlled via a C command generated by the control block.

[0086] The hysteresis will have a low value (h for example of a value less than or equal to 1 / 10, or even less than or equal to 1 / 1000 for example) to limit the impact on the quantization noise. Indeed, the higher the hysteresis, the greater the power of the quantization noise can be. Another possibility is that the hysteresis value, h, in each channel (or at least one of the channels) is controlled to vary over time, to limit the impact of this hysteresis.

[0087] In one embodiment, the hysteresis values ​​on two distinct channels Vi, Vj are set so that the following minimum differences are respected between two channels.

[0088] In one embodiment, the same hysteresis value h is used on different channels, or even on all the channels, but the hysteresis is controlled (for example by a command C) so as to intervene at different respective times, the transfer functions of the quantizers therefore not overlapping for two distinct channels.

[0089] It will be noted that the hysteresis may or may not be inverting, that is to say that there may be an addition or a subtraction in the feedback loop and that the hysteresis cycle may not be symmetrical with respect to the Threshold value.

[0090] In another embodiment of the invention, the sigma-delta modulator 12 this time comprises a quantizer 124 which takes the form of a multibit quantizer, which therefore comprises 3 or more distinct quantization thresholds. In a multichannel system, it is advantageous to reduce the number of bits of the DAC without necessarily aiming for an Ibit DAC, the 1-bit converter posing a certain number of stability problems for high-order modulators.

[0091] In this embodiment, one channel is distinguished from another by a modification specific to the channel on one or more of the quantization thresholds of the multibit quantizer 124 present in this channel, as represented in [Fig.7].

[0092] The dotted transfer function is the “ideal” transfer function of the quantizer 124 with quantization thresholds Ib I2, ... Ip, distributed regularly (for n = 1 to p, number of quantization thresholds, the output value OUT is equal to Sn for an input value I between I„4 and In). The real transfer function of the quantizer 124 as implemented is that in solid lines on which, for each or some of the thresholds, an offset of the quantization threshold is introduced with respect to the ideal transfer function: the output value OUT is equal to Sn for an input value I between (In-i- en.i) and (In- en). Thus the values ​​of Si, S2, ... , Sp, lo, L, L,..., IP,... are the same from one channel to another, but each offset value e0, .. .ep (or at least one, or even some of them) is fixed so that it varies from one channel to another, allowing the decorrelation of quantization noises fication.

[0093] In one embodiment, these values, specific to the channels, are controlled by a control signal C generated by the control block.

[0094] The impact of this shift will be low since the shift value will be low compared to the quantization step q, typically the shift values ​​are set to be between O.OOlq and O.lq.

[0095] In one embodiment of a multi-channel transmission system 110 implementing this solution and shown in [Fig.8], in each processing channel Vi there are two successive sigma-delta modulators 12, 121, the output of one being connected to the input of the other. The first sigma-delta modulator 12 has a multi-bit quantizer whose thresholds are modified by shift injection as explained above and which will compress the signal on K bits (K <M) à une fréquence d’échantillonnage Fsl. L’optimisation du nombre de bits et de la fréquence d’échantillonnage peut être choisie pour minimiser le débit de données. La sortie de ce premier codeur sigma delta 12 est ensuite injectée dans le second modulateur sigma-delta 121 fonctionnant à une fréquence Fs2 qui va encore comprimer le signal, sur L bits (L<K), L= 1 bit par exemple.The advantage of this structure is that the signal of the second 1-bit modulator has an input signal with quantization noise from the first stage already decorrelated from one channel to another. A filtering and / or oversampling block can be placed between the two modulator stages to optimize the two modulators separately and to filter part of the quantization noise from the first stage. The advantage of this noise introduced at the first quantizer compared to noise introduced at the input is that this noise is shaped by the sigma-delta modulator.

[0096] The sigma-delta modulators 121 used on the different channels may be identical in one embodiment.

[0097] This cascade of two sigma-delta is easier to integrate on the same integrated circuit, and allows better optimization of the interface represented in dotted lines in [Fig.8], compared to the case where a single sigma-delta modulator would be used to go from M bits to L bits (with in such a case the following alternative to be respected: either very high resolution with low bit rate, or very low resolution with a high bit rate), knowing that the signal at the input of the modulator 12, which comes from the digital communication system, has a high number of bits (resolution typically 10 or 12 bits with a bit rate depending on the modulation of the data for example 50 Mb / s), while it is desirable to reduce the constraints at the level of the DAC 14 (for example L=1 bit and very high sampling frequency).

[0098] In another embodiment of the invention, the 12 sigma-delta modulator of [Fig.2] is then a 12” cascaded architecture sigma-delta modulator, of which the architecture - known - is represented in [Fig.9] in an example.

[0099] This sigma-delta modulator with 12” cascaded architecture comprises in the case shown two sigma-delta modulators. The first sigma-delta modulator is here of order 2. It receives as input a signal vinet comprises respective blocks 30, 32, 35, 38 multiplying the signal which is respectively supplied to them as input by the respective coefficient ab kb k2, k3, two integrators 33, 36, a quantizer 37, two subtractors 31, 34.

[0100] The input of the quantizer 37 is provided as input to a multiplier 40 by a coefficient a, the output of the quantizer 37 is provided as input to a multiplier 41 by a coefficient b, the outputs of the multipliers 40, 41 are provided as input to a subtractor 44 delivering the signal vin2 as input to the second sigma-delta modulator.

[0101] The second sigma-delta modulator is here of order 1. It comprises a subtracter 45, a block 47 multiplying the signal which is respectively supplied to it at input by the respective coefficient k4, an integrator 48 and a quantizer 49.

[0102] The output Y2 of the second sigma-delta modulator is added to the output Yi of the first modulator multiplied by the transfer function z 1 and by the coefficient c; the result of the sum is multiplied twice by the transfer function (1- z1); the resulting signal is summed to Yb z ' in the adder 55, which outputs the signal Y.

[0103] The operating principle is as follows: the first modulator processes the input signal vin and delivers the digital signal Yi:

[0104] Y1(z)=a1.vin(z).FTSl(z)+Q1(z).FTBl(z)

[0105] where vin(z) is the input signal, Qi(z) the quantization noise of the quantizer of the first modulator, FTSl(z) the transfer function of the input signal and FTBl(z) the transfer function of the noise. Here FTSl(z) is equal, in an approximation, to z2 and FTBl(z) is equal, in an approximation, to (1-z ')2, the approximation errors being a function of kB k2, k3.

[0106] The quantization noise of the first quantizer 37 is extracted by taking the difference between the output signal and the input signal in the subtractor 44. The two inputs of the subtractor 44 can be weighted differently (by the coefficients a, b) to compensate for the coefficients inside the first modulator so as to obtain at the output of the subtractor 44 a signal vin2 depending only on the quantization noise of the first modulator:

[0107] vin2(z)=a. Qi(z)

[0108] This signal vin2 is then digitized by the second sigma delta encoder, of the first order, which delivers Y2 as output, then shaped to reproduce the transfer function of the first stage.

[0109] Y2(Z)=Q^Z) FTS2(z)+Q2(Z) FTB2(z)

[0110] where Q2(z) the quantization noise of the quantizer of the second modulator, FTS2(z) the transfer function of the input signal of the second modulator (here FTS2(z) ~ z1) and FTB2(z) the transfer function of the noise (FTB2(z) ~ (1-z ').

[0111] Finally this signal is subtracted from the output of the first stage to cancel this quantization noise. The residual quantization noise in the signal Y is then that of the second stage which is generally, as in the case of [Fig.9], at least a first-order modulator whose output noise is ultimately shaped at least to the third order (the sum of the orders of the 2 modulators). These cascaded architectures are known for their unconditional stability.

[0112]

[0113] Y(z)=Y1(z).z1+Y2(z)*(l-z1)2=a1.Vin(z).FTSl(z).FTS2(z)+Q2(z).FTB2(z).(l-z1)2

[0114] assuming that a=l, k5=l and c=0.

[0115] In one embodiment of the invention using such a modulator with cascaded architecture, a decorrelation of the noise of the second sigma-delta modulator is implemented between the different processing channels Vi, i = 0 to NL Very simple solutions can be implemented. The output signal of the first modulator can be injected into the second modulator with a different weight in each of the channels.

[0116] For example, by replacing the coefficient a by (a+a) on one or more processing channels with a value specific to the channel concerned (a = a; on channel Vi and a = Oj on channel Vj, i and j distinct between 0 and Nl and a,, cq), we differentiate the input of the second modulator on channel Vi compared to channel Vj.

[0117]

[0118] Vin2'(z)=a.Q1(z)+ a. a1.Vin(z).FTSl(z)+ aQ^zj.FTBUz)

[0119] This therefore amounts to injecting into the second modulator a part of the signal and a part of the noise of the shaped modulator, this additional signal then influencing the quantization noise of the second modulator. By taking different values ​​of a for different processing channels Vi, the noise of each of the cascaded modulators is differentiated between the channels.

[0120] This additional signal injected into the second stage shaped by the global transfer function will have no impact on the SNR at the output of the modulator. The signal undergoing the noise shaping stages after the second modulator (by (1-z ')2 in our example), its influence will be zero on the output. We can even cancel its influence at the output of the second modulator via the coefficient c, by adapting it to the value of a.

[0121] Any modification of the second modulator differentiating the processing between the channels can be implemented: for example by modifying the quantizer as seen previously, or if the second modulator is a second-order modulator (with the coefficients k'i, k'3 instead of the coefficients kb k3 in blocks of the second modulator similar to blocks 32, 35 of the first modulator), or differentiating the channels by distinct values ​​of k\ and / or k'3. Another solution consists of injecting the output of the first modulator into the second modulator with a variable delay (the same delay being also injected on the input of the first quantizer injected into the second modulator) and different for different channels. All sorts of modifications can be made to this second modulator differently on each of the channels (or between at least some of the channels) to guarantee the decorrelation of the quantization noise without modifying the signal transfer function.

[0122] Another possibility with these cascaded structures is to vary the value of the coefficients a over time with variations that are different from one channel to another.

[0123] The example shown in [Fig.9] includes two sigma-delta modulators but there can be a greater number of sigma-delta modulators in cascade, the rth modulator having as input the noise of the (rl)th modulator of rank, r>l.

[0124] In embodiments, some of the different implementations of the invention presented above are combined with each other within a single multi-channel transmission system.

[0125] Ideally, the invention is implemented such that there are no two processing paths where the resulting quantization noise is identical. In embodiments, at least two of the processing paths have different quantization noises from each other.

[0126] The invention further proposes, with reference to [Fig. 10], a method 100 for decorrelating quantization noise in a multi-channel transmission system, according to which: - in a step 101, an input signal is processed on a plurality of parallel processing channels Vi, i = 0 to Nl, each of said channels being coupled to a transducer and comprising a converter block comprising a CAN or DAC converter and a sigma-delta modulator, - in a step 102, the operation of the sigma-delta modulator of channel Vi is adapted with respect to the operation of the sigma-delta modulator of channel Vj to decorrelate the quantization noises generated by the channels, so that the second term of the transfer function of the sigma-delta modulator for channel Vi is distinct from the second term of the transfer function of the sigma-delta modulator for channel Vj, the first term of said transfer function for channel Vi being equal to that for channel Vj.

[0127] To do this, the method comprises one and / or the other of the different adaptations described above with reference to the multi-channel transmission system 10.

[0128] The invention has been further described above in the context of radiofrequency signals, but it is of course equally applicable to other signals, for example audiofrequency, ultrasound, etc., the radiofrequency antenna then being replaced by a transducer device adapted to the type of signals handled.

[0129] Furthermore, the invention has been described more precisely above with reference to a multi-channel transmission system, but it applies similarly to multi-channel reception or multi-channel transmission / reception systems, where, in relation to the above, the DAC converters are replaced by the ADC converters in the processing channels also comprising the sigma-delta modulators. The same applies to the noise decorrelation method according to the invention.

[0130] The invention is of course applicable to other types of application than beamforming.

Claims

Claims

1. Multi-channel transmission and / or reception system (10) comprising at least N parallel processing channels (V0, VI), N >2, each coupled to a respective transducer (18) and each comprising a converter block comprising a CAN or DAC converter (14) and a sigma-delta modulator (12), the transfer function of the sigma-delta modulator (12) of each channel Vi, i = 1 to N, being defined as a sum of two terms comprising a first term IN(z).FTS(z) relating to the signal and a second term Q(z).FTB(z) relating to the quantization noise introduced by the sigma-delta modulator (12) and being stated as follows: OUT(z) = IN(z).FTS(z) + Q(z).FTB(z), where OUT is the output signal of the sigma-delta modulator, IN is the input signal of the sigma-delta modulator, FTS is the transfer function of the input signal, Q is the quantization noise and FTB is the transfer function of the quantization noise, the transfer function of the sigma-delta modulator of a channel Vi being distinct from the transfer function of the sigma-delta modulator of a channel Vj, i and j distinct integers between 1 and N, to decorrelate the quantization noises of distinct channels from each other, said multi-channel system (10) being characterized in that, for distinct channels Vi, Vj, the second terms of the transfer function of the sigma-delta modulator only are distinct, the first term of said transfer function for channel Vi being equal to that for channel Vi.

2. VJ- Multi-channel transmission and / or reception system (10) according to claim 1, in which the sigma-delta modulator (12) on each of the channels Vi, Vj (V0, VI) comprises a 1-bit hysteresis quantizer, and the hysteresis cycle is different between the channels Vi, Vj in amplitude and / or temporally.

3. Multi-channel transmission and / or reception system (10) according to claim 1 or 2, in which the sigma-delta modulator (12) on each of the channels Vi, Vj comprises a multibit quantizer; and the value of the input signal corresponding to the switching between two quantization levels on the channel Vi is fixed distinct from the value of the input signal corresponding to the switching between these same two quantization levels on the channel Vj.

4. Multi-channel transmission and / or reception system (110) according to claim 3, comprising on each of the channels Vi, Vj, at least said sigma-delta modulator (12) with multibit quantizer and an additional sigma-delta modulator (121), the output of one of said modulators being provided as input to the other of said modulators, said other sigma-delta modulator on the channel Vi being identical to said other sigma-delta modulator on the channel Vj.

5. Multi-channel transmission and / or reception system (10) according to one of the preceding claims, in which on each of the channels Vi, Vj, the sigma-delta modulator is a sigma-delta modulator (12”) with cascaded architecture comprising at least two stages, the first stage comprising a first delta-sigma modulator comprising in particular a quantizer and the second stage comprising at least a second sigma-delta modulator; the second stage being adapted to receive as input the output of the quantizer of the first modulator and the input of the quantizer of the first modulator, to digitize, via the second comparator, a signal representative of the quantization noise of the first modulator and to shape it; the difference between a signal proportional to the output of the quantizer of the first modulator and a signal proportional to the input of the quantizer of the first modulator being provided as input of the second modulator;the output of the second stage then being subtracted from the output of the first modulator to deliver a signal devoid of at least part of the quantization noise of the first stage in which the second stage of the channel Vi is different from the second stage on the channel Vj so as to differentiate between them the quantization noises of the second modulator of the channels Vi, Vj.;

6. Multi-channel transmission and / or reception system (10) according to claim 5, in which the second stage comprises multipliers, each multiplier multiplying its input by a respective associated coefficient, in which, considering at least one of said multipliers, the coefficient associated with it on channel Vi is of a different value from that on channel Vj.

7. Multi-channel transmission and / or reception system (10) according to claim 5 or 6, in which a delay block is arranged in the second stage, between the output of the first modulator and the input of the second modulator, adapted to apply a delay to the signal coming from said output, the delay on channel Vi being set to a value different from that of the delay on channel Vj.

8. Method for decorrelating quantization noise in a multi-channel transmission and / or reception system (10) comprising at least N parallel processing channels, N > 2, each coupled to a respective transducer (18) and each comprising a converter block comprising a CAN or DAC converter (14) and a sigma-delta modulator (12), the transfer function of the sigma-delta modulator (12) of each channel Vi, i = 1 to N, being defined as a sum of two terms comprising a first term IN(z).FTS(z) relating to the signal and a second term Q(z).FTB(z) relating to the quantization noise introduced by the sigma-delta modulator (12) and being stated as follows: OUT(z) = IN(z).FTS(z) + Q(z).FTB(z), where OUT is the output signal of the sigma-delta modulator, IN is the input signal of the sigma-delta modulator, FTS is the transfer function of the input signal, Q is the quantization noise and FTB is the transfer function of the quantization noise,according to which the operation of the sigma-delta modulator of channel Vi is adapted with respect to the operation of the sigma-delta modulator of channel Vj to decorrelate the quantization noises generated by the channels, the transfer function of the sigma-delta modulator of a channel Vi being distinct from the transfer function of the sigma-delta modulator of a channel Vj, i and j distinct integers between 1 and N, to decorrelate between them the quantization noises of distinct channels, said method being characterized in that, for distinct channels Vi, Vj the second terms of the transfer function of the sigma-delta modulator only are distinct, the first term of said transfer function for channel Vi being equal to that for channel Vj.,

9. Method for decorrelating quantization noises according to claim 8, according to which the sigma-delta modulator (12) on each of the channels Vi, Vj comprises a 1-bit hysteresis quantizer, and the hysteresis cycle is different between the channels Vi, Vj in amplitude and / or temporally.

10. Method according to one of claims 8 or 9, according to which the sigma-delta modulator (12) on each of the channels Vi, Vj comprises a multibit quantizer; and the value of the input signal corresponding to the switching between two quantization levels on channel Vi is set distinct from the value of the input signal corresponding to the switching between these same two quantization levels on channel Vj.

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