FI103745B - Signal processing method and device - Google Patents

Abstract

The invention relates to digital signal processing and specificly to level control of a pulse density modulated (PDM) signal generated by a sigma-delta modulator. A single-bit pulse density modulated PDM signal is generated by a first sigma-delta modulator (2) being an analog modulator, for instance. Level control is performed by multiplying the single-bit pulse density modulated PDM signal by a multibit multiplier (300) to obtain a multibit number stream, which is reconverted into a single-bit PDM signal by a second digital sigma-delta modulator (4). In accordance with the invention, the performance of the second sigma-delta modulator (4) is better than that of the first sigma-delta modulator (2), as to the signal-to-noise ratio. Thus, the most significant factor in the total signal-to-noise ratio (SNR) is the noise level of the first sigma-delta modulator (2), by which the PDM signal was originally generated. In the subsequent second sigma-delta modulator (4), the PDM signal can then be attenuated as much as is the difference between the SNR performances of the modulators without any decrease in the total signal-to-noise ratio. A relative amplification of the PDM signal is provided in this manner.

Description

«103745

Signal processing method and apparatus

The invention relates to digital signal processing, and in particular to level-5 control of a pulse density modulated (PDM) signal produced by a sigma-delta modulator.

The basic signal processing operations, multiplication and addition may be performed in known signal processing blocks, or the analog signal may be converted to digital using an A / D converter and digitally performed the desired signal processing operations. The results can be converted again to analog signals using a D / A converter. The A / D and D / A conversion is performed at a specific sample rate and at a specific resolution.

Sigma-Delta converters have become more common as A / D and D / A converters lately. In a Sigma-Delta A / D converter, the conversion of an analog signal to a baseband digital signal is biphasic. In the first step, the input signal is converted by a Sigma delta modulator to a one or a few bit over-sampled signal. In a second step, this oversampled one or few bit signal is decimated using digital filtering to baseband. Sigma-delta techniques and converters are described, for example, in:

20 [1] An Overview of Sigma-Delta Converters, P.M. Aziz et al., IEEE

Signal Processing Magazine, January 1996, pp. 61-84.

[2] '' Oversampling Delta-sigma Data Converters: Theory, Design and Simulation ', J.C. Candy et al., IEEE Press NJ 1992, pp. 1-25.

[3] Design Design Methodology for Sigma-Delta Modulation, B.P. Agrawal et al., 25 ali, IEEE Transactions on Communications, Vol. COM-31, March 1983, pp. 360- 370.

The oversampled sigma-delta modulator output signal is a pulse-rate-modulated (PDM) representation of the input input signal. In Sigma-delta A / D converters, a modulator converts an analog signal into a pulse density mode-30 (PDM) format. A PDM sianal is a one or a few bit (e.g. 2-4 bits) oversampled signal. The relative density of the pulses in the PDM signal determines the amplitude of the input signal. In the frequency domain, the baseband part of the spectrum of the PDM signal forms the signal band and the higher frequencies of the spectrum have quantization noise produced by the noise-modifying function of the sigma-delta modulator 35. The resolution at the signal frequencies has thus been changed to the oversampling rate. The noise-modifying ability of the Sigma-delta modulator 103745 2 is known to depend on its degree of order, and higher order modulators more efficiently transfer quantization noise out of the signal band. Increasing the oversampling rate also makes the signal band relatively narrower and the proportion of noise falling on it to be smaller. The amount of noise in the signal band 5 in the Sigma delta modulator can also be controlled by the modulator transfer function, e.g. by setting zeros to the transfer function at suitable frequencies.

Recently, solutions have been proposed in the literature whereby signal processing operations using PDM signals may be performed to a limited extent. 10 This achieves the known benefits of digital signal processing, such as accuracy, repeatability, insensitivity to interference, etc. When the signal is processed directly in oversampled PDM format, it does not need to be converted to Nyquist pulse code modulated (PCM) signal for signal processing. Hereby, the decimation and interpolation filters that produce 15 baseband PCM signals from the PDM signal can be omitted at the signal processing points. This is a significant advantage because the sigma-delta modulator that generates the PDM signal is generally small and simple in circuit implementation, while the decimation and interpolation filter is often a large and complex circuit structure which requires a large amount of circuit area in an integrated circuit implementation . For example, [4], "Design and Analysis of Delta-Sigma Based HR Filters," by DA John et al., IEEE Transactions on Circuits and Systems-II: Analog and Digital Signal Processing, Vol. 40, No. 4, April 1993, 233-240, discloses a multi-input A / D converter in which each input is filtered separately and summed together before a common desmixing filter, such as an audio mixing table.

An important form of signal processing is signal level control: gain and / or attenuation. This feature is very useful especially in audio applications such as the aforementioned audio mixing table-30. Thus, it would be advantageous to be able to perform level control directly on the PDM signal as well. In the above article [4], there is shown in Fig. 1 a sigma-delta suppressor wherein the oversampled 1-bit signal (PDM) is multiplied by the multi-bit coefficient a1 and the resulting multi-bit signal is supplied to a digital sigma-delta modulator which outputs 1-b The multiplier of the PDM-35 signal, the 1-bit PDM signal, is implemented as a 2-input multiplexer (selector) which, depending on the state of the incoming PDM signal, converts the output to a1 or -a1. The article also provides a digital sigma-delta filter suitable for this purpose. The attenuator can be implemented when said multi-bit coefficient is less than one. With a Sigma-delta modulator feedback value of b and a coefficient a of said, a training ratio of a / b is obtained.

The problem with this known solution is that the PDM signal could only be attenuated, so all multiplications had to be made to one with lower coefficients. Amplification of the PDM signal is not considered possible because, due to the structure of the sigma-delta modulator, the input value of the moduloator cannot exceed or even come close to the value of the modulator feedback. The Sigma-delta modulator is a conditionally stable structure and the output signals of the integrators escape when the input exceeds a certain value. In an analog sigma-delta modulator, the input value should normally be in the range of 0.3 to 15 0.7 times the feedback value, depending on the modulus degree and structure. article [3]. Amplifying the PDM signal in such a circuit would require that the incoming signal be multiplied by a higher number than the feedback. Even if the input of the AD modulator is very small and could in principle be much amplified by setting the input signal values (a) above the feedback value (b), the resulting PDM signal has only +1 and -1 values (one-bit case). Multiplying would result in momentarily too large values for the modulator. On average, the PDM signal density and energy would still be low, but instantaneous values would quickly drive the modulator unstable.

It is an object of the present invention to provide a signal processing method and apparatus which also enables the relative amplification of a PDM signal without significantly increasing the noise level.

The objects of the invention are achieved by a signal processing method comprising the steps of generating an N-bit pulse density modulated signal with a first sigma-delta modulator, where N = 1,2, ...; adjusting the level of the pulse-30 density-modulated signal by a) multiplying the N-bit pulse-density-modulated signal by a multi-bit multiplier whose output is an M-bit signal, where M> N, b) converting the M-bit signal to N-bit The method according to the invention is characterized by converting an M-bit signal to an N-35 bit pulse density modulated signal with a digital sigma-delta modulator 103745 4 having a better signal-to-noise ratio than said first sigma-delta modulator.

The invention also relates to a signal processing system comprising a first sigma-delta modulator for generating an N-bit 5 pulse density modulated signal, where N = 1,2, ...; pulse rate modulated signal level control means, comprising: a) a multi-bit multiplier having an input of an N-bit pulse rate modulated signal and an output of an M-bit signal, where M> N, b) a digital sigma-delta modulator to M-N bit signal . The system 10 according to the invention is characterized in that said digital sigma-delta modulator has a better signal-to-noise ratio than said first sigma-delta modulator.

The single bit pulse density modulated PDM signal is produced by a first sigma-delta modulator which is e.g. an analog modulator.

Level control is accomplished by multiplying the single-bit pulse density modulated (PDM) signal by a multi-bit coefficient so as to obtain a multi-bit reading stream which is converted back to a single-bit PDM signal by another sigma-delta modulator, preferably a digital modulator.

According to the basic idea of the invention, the performance of said second sigma-delta modulator for converting the multi-bit reading stream back into a PDM signal is better in terms of signal-to-noise ratio than said first sigma-delta modulator. Thus, in the total signal to noise ratio (SNR), the noise level of the first sigma-delta modulator on which the PDM signal was originally generated is the most significant factor. Thereby, in said later second sigma-delta modulator, the PDM signal can be attenuated to the same extent as the difference in the SNR performance of the modulators without degrading the overall signal-to-noise ratio. For example, if the SNR of the first sigma-delta modulator has a maximum excitation of 90 dB and the SNR of the second sigma-delta modulator is 110 dB, the second modulator can suppress 30 PDM signals by almost 20 dB without reducing the signal-to-noise ratio. This is possible because in the latter modulator, in addition to the signal, of course the noise of the first modulator in the signal band is also attenuated, approaching the noise floor set by the structure of the second modulator.

The PDM signal is thus scaled to a slightly lower level without loss of performance. Although the signal is also attenuated by the second sigma-delta modulator, the attenuation may be less than the aforementioned difference in performance (103745 above in Example 20 20 dB) to achieve relative gain. When the PDM signal is attenuated less than said performance difference, the total signal-to-noise ratio is the same as that of the preceding analog modulator. In the case of the example, the nominal signal level can be fixed to a point where the first modulator gives a full signal and the second modulator attenuates the signal by 20 dB. The second-order attenuation is set to C. Thus, in the example case, the total signal-to-noise ratio is obtained at 90 dB with a signal between +20 and 0 dB and 90 + 20 - (c) when c is between 20 and 110 dB. and thus system attenuation between 0 and 90 dB.

The invention will now be further described in connection with preferred embodiments, with reference to the accompanying drawings, in which:

Fig. 1 is a block diagram showing a PDM level controller of the invention connected after an analog sigma-delta A / D converter;

Fig. 2 is a graph showing the noise and signal levels of an analog sigma-delta converter 15 and a digital sigma-delta converter as well as the available adjustment range as a function of frequency;

Figure 3 is a block diagram showing a multichannel PDM level controller.

Referring to FIG. 1, an analog sigma-delta modulator 2 converts an analog input signal 20 at input 1 into an A / D conversion to a 1-bit pulse density modulated (PDM) format. The modulator 2 may be, for example, any of the sigma-delta A / D converter structures described in Article [1]. Assume modulator 2 is a third-order sigma-delta modulator with a signal-to-noise ratio of about 100dB. A one-bit PDM signal, which can obtain values: 25 +1 and -1, is supplied to PDM level controller 3.

The PDM level controller 3 according to the preferred embodiment of the invention comprises a digital modulator 4 and a preceding multiplier 300. The level control is performed by multiplying the single bit pulse density modulated (PDM) signal by the multi bit coefficient a in 300 to obtain a multi bit 30 bit as a signal with a digital sigma-delta modulator 4.

In the case of a single-bit PDM signal, the multiplier 300 may be implemented by a simple multiplexer or selector which outputs + a or -a depending on whether the input value is +1 or -1. The output 35 of the multiplier 300 is thus a multi-bit read stream consisting of + a and -a. The multiplier 300 may have a structure similar to that described in [4]. The multiplier may have a single fixed factor or the value of the multiplier may be adjustable. In the preferred embodiment of the invention shown in FIG. The coefficients may be, for example, according to Table 1. The table shows the values of 32 coefficients a, which give a control range of +12 dB ....- 34.5 dB in 1.5 dB increments.

table 1

Multiplier a Gain (dB) 872 +12.0 734 105 617 9.0 519 7.5 437 6.0 368 4.5 309 3.0 260 1.5 219 0 184 -1.5 155 -3.0 130 -4.5 110 -6.0 92 -7.5 78 -9.0 65 -10.5 55 -12.0 46 -13.5: 39 -15.0 33 -16.5 28 -18.0 23 -19.5 20: 2TOP 16 -22.5 14 -24.0 12 -25.5 10 -27.0 8 -28.5 7 -30.0 6 -31.5 5 -33.0 4 -34.5 7 103745

Digital modulator 4 is a fourth-order modulator comprising adder 400-403, integrators 404-407 and quantizer 408, and feedback loops 409-412 having feedback ratios r1-r4, respectively. It should be noted that the detailed implementation and structure of the modulator 4 is not in itself relevant to the invention. It is of interest to the invention only that the performance of the modulator 4 is better than that of the modulator 2, as will be explained below. The input of modulator 4 is said reading current consisting of + a and -a. The output 5 of the modulator 4 is a 1-bit oversampled PDM signal. The level 10 of the PDM signal in the level controller 3 is adjusted by the ratio a / r1. Due to the unstable nature of the Sigma delta modulator, the input value of modulator 4 cannot approach the internal reference voltage value of the modulator, i.e., the coefficient a must be less than the feedback factor Π. Therefore, the PDM signal can only be attenuated by multiplier 300.

However, at the system level, i.e., between input 1 and output 5, gain can be achieved when the digital sigma-delta modulator has a higher noise reduction performance than the modulator 2. The noise reduction performance may be higher with modulator 4, e.g. 20 cells or a higher oversampling ratio, or some combination of these. In the embodiment of Figure 1, modulator 4 is a fourth-order modulator, while modulator 2 is a third-order modulator. When a higher modulator (or otherwise better noise reduction performance) follows the smaller modulator in the PDM signal processing path, the noise level of the lower modulator is determined by the overall system noise ratio (SNR) of the system. In the case of Figure 1, the signal-to-noise ratio at output 5 is thus primarily determined by the signal-to-noise ratio of modulator 2. The performance of the modulator 4 must be at least the desired amplification need, and preferably further suitable. 30 stability margin, better than the signal-to-noise ratio of modulator 2 to incoming PDM signal. When the signal-to-noise ratio of the level-regulator 3 modulator 4 is significantly better than that of the incoming PDM, the level-control means can lower the entire level of the PDM-signal without substantially reducing the signal-to-noise ratio. This is possible because not only the payload signal but also the noise contained in the PDM signal is attenuated. The signal is thus scaled to a slightly lower level for performance degradation. Although the PDM signal is also attenuated in modulator 4, it is possible in attenuator 4 to attenuate the signal less than said difference in performance of modulators 2 and 4 and achieve relative gain.

Let us consider, by way of example, the operation of the level control according to the invention with reference to the graph of Figure 3. Assume that analog modulator 2 is a third order modulator with a signal to noise ratio of about 100 dB. Modulator 4 is a fourth-order digital modulator with a signal-to-noise ratio of about 120 dB, or about 20 dB better than modulo 2. The desired adjustment range is +12 dB to -34.5 dB in 1.5 dB increments. To ensure the stability of modulator 4, the ratio a / r1 is 0.5, i.e. -6 dB. The value of reference r1 is calculated as a function of maximum attenuation (-34.5 dB) and required accuracy (<0.3 dB). Assume a reference number of 1744. Now, a gain of +12 dB corresponds to a multiplication of the incoming PDM by 15 by 872 and a maximum attenuation by a multiplication of the PDM of 4. The above table 1 lists all different values of a and constant 1744. With a performance difference of 20 dB between modulators 2 and 4 and a stability margin of 6 dB, the available gain range is about 14 dB.

In the present example, the signal-to-noise ratio remains at approximately the same range of +12 ... -1.5 dB as it is after modulator 2. With greater attenuations, the noise of the input signal itself is attenuated below the noise levels 22 of the modulator 4, whereby the attenuated payload signal 25 and the noise floor 22 determine the signal-to-noise ratio at the output 5.

The invention has been described above in connection with a 1-bit PDM signal.

However, the invention can also be directly applied to a multi-bit PDM signal, for example 2-4 bits.

Referring to Figure 1, in the preferred embodiment of the invention illustrated, the analog modulator 2, the multiplier 300 and the digital modulator 30 4 are shown in series. In practice, these units may be spaced apart in the signal processing system with other signal processing steps between them. An example of such a signal processing system is shown in Figure 3.

Fig. 3 shows three analog input signals 31, 32 and 33, 35 which are supplied to respective analog sigma-delta modulators 34, 35 and 36. Modulators 34, 35 and 36 produce PDM signals 37, 38 and 39, 103745 9 respectively, which are supplied to the multipliers. 40, 41, and 42, respectively. Multipliers 40, 41, and 42 produce multi-bit read streams 43, 44, and 45, respectively, which are summed in 46 to multi-bit read streams 47. The signal 47 is converted by a digital sigma-delta modulator 48 into a PDM signal 49. The multipliers 40-42 may have the same structure as the multiplier 300 in Figure 1. The modulator 48 may have the same structure as the modulator 4 in Figure 1. An application of the signal processing apparatus of the type 3 of Figure 3 is an audio mixing table.

The invention can be applied to the level control of the PDM signal in all 10 sigma-delta structures. In addition to audio applications, typical applications include IIR and FIR filter structures.

It will be obvious to a person skilled in the art that as technology advances, the basic idea of the invention can be implemented in many different ways. The invention and its embodiments are thus not limited to the examples described above but may vary within the scope of the claims.

Claims (10)

10 103745 The patent claimed a claim A signal processing method comprising the steps of generating an N-bit pulse rate modulated signal by a first sigma-delta modulator, wherein N = 1,2, ..., 5 adjusts the level of the pulse rate modulated signal a) by multiplying the N-bit pulse rate modulated signal by a bit signal, wherein M> N, b) converting the M bit signal to an N bit pulse rate modulated signal by a digital sigma delta modulator, characterized in that converting the M bit signal into an N bit bit pulse rate modulated sigma performance with respect to signal-to-noise ratio is better than with said first sigma-delta modulator. Method according to claim 1, characterized in that the level control step further comprises the step of providing a relative gain of the pulse density modulated signal by multiplying said N-bit pulse density modulated signal by a coefficient corresponding to said attenuation less than said difference in performance. Method according to claim 1 or 2, characterized in that a digital sigma delta modulator is used which has a better noise modulation performance than the first modulator due to one or more of the following factors: higher order number, multi-bit quantization, multi-bit feedback, higher oversampling. A signal processing system comprising a first sigma-delta modulator (2) for generating an N-bit pulse rate modulated signal, where N = 1,2, ..., pulse density modulated signal level control means (3) comprising a) a multi-bit multiplier (3). 300) having an input as an N-bit pulse density modulated signal and an output as an M bit signal, where M> N, b) a digital sigma-delta modulator (4) for converting an M-bit signal to an N-bit pulse rate modulated signal. 103745 said digital sigma-delta modulator (4) has a better signal-to-noise ratio than said first sigma-delta modulator (2). A system according to claim 4, characterized in that the level control means (3) has a relative gain when the coefficient of the multi-bit multiplier (300) corresponds to a damping smaller than said performance difference. System according to Claim 4 or 5, characterized in that said digital sigma delta modulator (4) has a better noise reduction performance than the first modulator (2) due to one or more of the following factors: higher order number, multi-bit quantization, multi-bit feedback, higher oversampling. System according to one of Claims 4 to 6, characterized in that the first modulator (2) is an analog sigma-delta modulator. A system according to any one of claims 4 to 7, characterized in that the system is a digital filter of pulse density modulated signals, such as an HR or FIR filter. A system according to any one of claims 4-8, characterized in that the system is an audio system. System according to one of Claims 4 to 9, characterized in that the value of the coefficient of the multiplier (300) is adjustable in stages. 103745 12