SAMPLE RATE CONVERTER USING POLYNOMIAL INTERPOLATION

Abstract: A sample rate converter for converting the sampling frequency of an input signal from a first frequency to a second frequency. Such a sample rate converter uses interpolation means and a phase locked loop receiving the first and the second sampling frequency. The invention provides a sample rate converter which uses interpolation means implemented as polynomial interpolation means.

Description: Sample rate converter using polynomial interpolation The invention relates to a sample rate converter as described in the preamble of claim 1. The invention further relates to converting means for use in a sample rate converter. The invention further relates to a method for converting as described in the preamble of claim 7. In the United States patent US-A-4,797,845 (of the same applicant) a sample rate converter using a high over-sampling and hold, or linear interpolation at high frequencies is described. In the European patent application EP-A-0695032 a sample rate converter has been described, having a digital low pass filter with a so called piece wise continuous linear hull curve. Further is from the European patent application EP-A-0512619 (of applicant's earlier application) a sample rate converter known using a sigma delta modulator as a noise shaper. A disadvantage of the prior art a sample rate converters is that most of the prior art sample rate converters can not be implemented in software. A further disadvantage of the prior art sample rate converters is that the complexity of the converter increases exponentially with a required improved performance. An object of the invention is to provide a sample rate converter, converting means and a method that have not the disadvantages of the prior art. To this end a first aspect of the invention provides a sample rate converter as claimed in claim 1. A second aspect of the invention provides converting means as claimed in claim 6. A third aspect of the invention provides a method as claimed in claim 7. The invention can be used for asynchronous (that is input and output clock of the system are not related) as well as synchronous (fractional) sample rate conversion. Further the invention can be efficiently used in a so called embedded solution (software). No look up tables and/or hardware accelerators are necessary. The basic idea of the invention is to make use of a high order polynomial in order to calculate an approximation of the signal value at a certain time. In digital signal processing until now high order polynomials or non linear functions of time were not used as expected to be far too complex. The time at which the value is required can be determined by means that compare the incoming clock with the outgoing clock, for example a phase locked loop (PLL). The order of the polynomial depends on the number of available samples chosen to determine the actual value of the signal at the new time instance. In essence, the only correct value without approximating at the new time instance is determined by a summation of an infinite number of sin(x)/x functions, with as center value the relevant sample present in the input signal. The polynomial is used in order to calculate an approximation of the signal value at a certain time. The precision of the approximation required will increase the order of the polynomial interpolation. It is to be noticed here that in "New High Accuracy Pulse Width Modulator Based on Digital-To-Analogue Convertor / Power Amplifier", J.M. Goldberg, M.B. Sandier, IEE Proc. -Circuits Devices Syst., Vol 141, No.4, August 1994, pp 315-324 describes polynomial interpolation to improve linearity, not for a sample rate converter. An embodiment of a sample rate converter according to the invention comprises the features of claim 2. By using an up-sampler before the polynomial interpolation means, with a lower order polynomial function the same degree of resolution can be achieved. If necessary a down sampler can be used. Performance may be further improved by using the converting means to convert the signal to a sample rate which is multiple of the desired sample rate and by then converting that signal to the desired sample rate using a decimation filter. An embodiment of a sample rate converter according to the invention comprises the features of claim 4. By combining the polynomial interpolation with the look up table for higher order FIR filters the advantages of both means can be combined, whereby the look up table comprises one or more sets of coefficients and can be used for certain difference values between the input and output clock and the polynomial interpolation between the sets. An embodiment of a sample rate converter according to the invention comprises the features of claim 5. Using these formulas the polynomial interpolation can be easily implemented in a digital signal processor. An embodiment of a method according to the invention comprises the features of claim 8. By combining the polynomial interpolation with a look up table the efficiency of the method is further improved. The invention and additional features which may optionally be used to implement to advantage will be apparent from and elucidated with reference to the examples described hereinafter and shown in the figures. Herein shows : Figure 1 a block schematic example of a sample rate converter according to the invention, and Figure 2 a more detailed example of converting means of a sample rate converter according to the invention. Figure 1 shows a block schematic example of a sample rate converter SRC according to the invention, receiving an input signal Si with a sampling frequency fi and an output for supplying an output signal So with a sampling frequency fo. The sample rate converter further comprises input means IM for handling the input signal, and output means OM for handling and supplying the output signal. The input means is coupled to converting means CM for converting the input signal with the input sampling frequency to the output signal with the output sampling frequency. The converting means are controlled by a difference signal Δt depending on the difference between the input and output sampling frequencies, whereby in this example the input and output sampling frequencies are supplied to a phase locked loop PLL, which phase locked loop supplies the difference signal Δt as a control signal to the converting means. The input means and output means can comprise both a so called FIFO (first in first out) to buffer respectively the input and output signal. Further an upsampler for example in the input means can be used to improve the degree of resolution by using the same polynomial. Instead it is also possible to lower the degree of the polynomial and obtain the same degree of resolution. By using an upsampler the path of the signal between the neighboring signal values can be used to approximate the new value. Down sampling will not be necessary in most cases because it is not necessary to calculate the interpolated samples with a frequency which is (for example) four times the required frequency. Nevertheless down sampling will further improve the performance. Figure 2 shows an example of converting means CM2 for use in a sample rate converter according to the invention. The converting means comprises a polynomial interpolation part PIP. Further in this example the converting means CM2 comprises a frequency difference measurement device FDM for supplying a difference value Δt. The polynomial interpolation part PIP comprises polynomial constants summing means PS1-PS6 for obtaining polynomial constants pl-p6 depending on the difference signal Δt. These polynomial constants are supplied to constant multipliers M1-M6. In this example the interpolation polynomial is 6 Y(x) = k i A , m+n=6 ι ' =0 using the input samples y(-m), ...., y(-l), y(0),y(l),...y(n) m=n, or m=n-l with Y(-m) = y(-m) Y(0) = y(0) Y(n) = y(n) 6 m+n Y(x) = ∑A ∑a^yU-m) ; m+n=6 1=0 =0 The coefficients α ( - • are obtained by solving the set of equations Y(-m)=y(-m) Y(0)=y(0) Y(n)=y(n). The input signal Ci of the converting means CM2 is supplied to the polynomial interpolation part. This input signal is multiplied in multipliers PM1-PM5. The inputs of the multipliers PM1-PM5 are respectively supplied to the constant multipliers M1-M6. The multiplication factors pl-p6 are obtained by the polynomial constants summing means PS1- PS6 using the above formulas. The output of the multiplier PM5 is coupled to the constant multiplier M6. The outputs of the constant multipliers M1-M6 are coupled to summing means SUMP for adding all components of the polynomial to the total obtained interpolation signal Co. It is possible to combine the polynomial converting means with a look up table for higher order FIR filters. The advantages of both means can be combined in this way, whereby the look up table is used for samples nearby the incoming samples and the polynomial interpolation is used for the other (further away) samples. In the above description the idea of the invention has been described on the basis of a detail example. The man skilled in the art will be well aware of a lot of different solutions that fall within the scope of the invention concerned. As described above a combination of polynomial interpolation and a look up table can further improve the sample rate converter. Further the degree of polynomial interpolation can be amended as required. The core of the invention is to make use of a polynomial in order to calculate an approximation of the signal value at a certain time. This can be implemented in a digital signal processor or in software. Further the invention can be used by both asynchronous and synchronous sample rate converters.

Claims: CLAIMS: 1. Sample rate converter for converting the sampling frequency of an input signal from a first frequency to a second frequency, comprising input means for receiving the input signal with the first sampling frequency, output means for supplying an output signal with the second sampling frequency, converting means for converting the input signal into the output signal using interpolation means, and a phase locked loop receiving the first and second frequency for controlling the converting means, characterized in that the interpolation means are polynomial interpolation means. 2. Sample rate converter according to claim 1, characterized in that the input means comprise an up-sampler. 3. Sample rate converter according to claim 2, characterized in that the output means comprises a down sampler. 4. Sample rate converter according to claim 1, characterized in that the converting means comprise a look up table with pre-calculated FIR coefficients. 5. Sample rate converter according to claim 1, characterized in that the polynomial interpolation means use wherein Y(x) = the interpolated signal y(j-m) = the input sample 6. Converting means for use in a sample rate converter according to claim 1. 7. Method for converting the sampling frequency of an input signal from a first frequency to a second frequency having the steps of : receiving the input signal with the first sampling frequency, converting the input signal into an output signal with the second sampling frequency using interpolation, whereby the converting is controlled by a signal depending on the difference between the first and second sampling frequencies, characterized in that the converting of the input signal into the output signal is obtained by polynomial interpolation. 8. Method according to claim 6, characterized in that the method uses a combination of a look up table and the polynomial interpolation for converting the input signal.

Inventors: PUTZEYS BRUNO J G, PUTZEYS, BRUNO, J., G.
Applicants: KONINKL PHILIPS ELECTRONICS NV, PHILIPS SVENSKA AB, KONINKLIJKE PHILIPS ELECTRONICS N.V., PHILIPS AB
Application year: 1999
Main classification: H04L
IPC codes: H03H17