Circuit and method for sampling frequency conversion

Claim 2

A sampling frequency converter for converting sampled input data having an input sampling frequency into sampled output data having a selected output sampling frequency, the converter comprising: circuit means (5, 6) for determining a relative ratio between periods of said input data samples and output data samples and generating an output signal representing the relative ratio; averaging means (7, 8, 10) for averaging said output signal of said circuit means (5, 6) for a predetermined time duration and generating an averaged output; filter coefficient generating means responsive to said averaged output for generating data representative of a set of predetermined filter coefficients derived from a low pass filter impulse response; and sampling filter circuit means (2) for converting said sampled input data into said sampled output data in response to said filter coefficients.

Claim 3

A converter according to claim 1, wherein said circuit means (5, 6) includes a phase locked loop circuit (5) connected to be supplied with a clock signal representing the frequency of one of said sampled input and sampled output data for frequency multiplying the supplied clock signal to generate a counting signal, and counter means (6) for counting said counting signal during every period of a clock signal representing the frequency of the other of said sampled input and sampled output data, thereby generating said output signal representing the relative ratio.

Claim 4

A converter according to claim 1 or claim 2, wherein said averaging means (7, 8, 10) includes a circuit having a transfer function H(Z) = (1-z -n )/(1-z⁻¹).

Claim 5

A converter according to claim 1, claim 2 or claim 3, wherein said filter coefficient generating means includes an addressable memory for storing said set of predetermined filter coefficients and address generating means for generating memory addresses for reading out the filter coefficients.

Claim 6

A converter according to claim 4, wherein said filter coefficient generating means includes adder means (18) for adding the averaged output, a fixed value, and an output of said adder means (18) for generating a memory address.

Claim 7

A converter according to claim 5, wherein said filter coefficient generating means includes an overflow check circuit (12) connected to be supplied with the output of said adder means (18) to generate a control signal for selecting said fixed value.

Claim 8

A sampling frequency converter for converting sampled input data having an input sampling frequency into sampled output data having a selected output sampling frequency, the converter comprising: over-sampling means (205) connected to be supplied with said sampled input data for increasing the sampling frequency of said sampled input data by a factor 2 k , to provide oversampled data; buffer memory means (206) connected to be supplied with said oversampled data for temporarily storing same and for reading out the stored oversampled data at a read-out rate determined by a ratio between periods of said input sampling frequency and said output sampling frequency; averaging means (209, 212, 213, 215, 216) for averaging the periods of said input sampling frequency and for averaging the periods of said output sampling frequency for a predetermined time duration; filter coefficient generating means (217) responsive to said averaged periods of said input sampling frequency and to said averaged periods of said output sampling frequency for generating data representing a set of predetermined filter coefficients derived from a low pass filter impulse response; variable filter means (207) for receiving the oversampled data read out from said buffer memory means (206) and the filter coefficient data generated by said filter coefficient generating means for producing converted data samples; and output means (208) for supplying the converted data samples at said output sampling frequency.

Claim 9

A sampling frequency converter for converting sampled input data having an input sampling frequency into sampled output data having a selected output sampling frequency, the converter comprising: over-sampling means (205) connected to be supplied with said sampled input data for increasing the sampling frequency of said sampled input data by a factor 2 k , to provide oversampled data; buffer memory means (206) connected to be supplied with said oversampled data for temporarily storing same and for reading out the stored oversampled data at a read-out rate determined by a ratio between periods of said input sampling frequency and said output sampling frequency; averaging means (209, 212, 213, 215, 216) for averaging the periods of said input sampling frequency and for averaging the periods of said output sampling frequency for a predetermined time duration; filter coefficient generating means (217) responsive to said averaged periods of said input sampling frequency and to said averaged periods of said output sampling frequency for generating data representing a set of predetermined filter coefficients derived from a low pass filter impulse response; variable filter means (207) for receiving the oversampled data read out from said buffer memory means (206) and the filter coefficient data generated by said filter coefficient generating means for producing converted data samples; and output means (208) for supplying the converted data samples at said output sampling frequency.

Claim 10

A converter according to claim 7, comprising buffer control means (210) for generating buffer control signals to be supplied to said buffer memory means (206) to control the read out of oversampled data from said buffer memory means.

Claim 11

A converter according to claim 7, wherein said averaging means comprises local clock generating means (209) for generating a local clock signal, first means (212) for determining a relative ratio between the period of said input sampling frequency and the period of said local clock signal, second means (213) for determining a relative ratio between the period of said output sampling frequency and the period of said local clock signal, and first and second averaging means (215, 216) for averaging each of said ratios.

Claim 12

A sampling frequency converter according to claim 9, wherein said first averaging means (215) includes a first adder (251) connected to be supplied with a measured relative time difference between an input sampling period and said local clock period and with an estimated relative time difference therebetween such that said adder (251) is operative to determine an error between the measured and estimated relative time differences, error monitor means (253) connected to be supplied with the error determined by said first adder (251) for producing a correction output, a second adder (255) connected to be supplied with the correction output of said error monitor means (253) and a previous estimate of the input sampling period produced by said second adder (255) for generating a corrected estimate of the input sampling period, and a third adder (252) connected to be supplied with the corrected estimate of the input sampling period generated by said second adder (255) and a previous corrected estimate for generating said estimated relative time difference between said input sampling period and local clock period and supplying same to said first adder (251).

Claim 15

A method according to claim 11, wherein the step of determining a relative ratio comprises providing an input clock signal having the input sampling frequency and providing an output clock signal having the output sampling frequency, frequency multiplying (5) a selected one of the input or output clock signals, and counting (6) the frequency-multiplied clock signals during every period of the other non-multiplied clock signal, thereby generating an output signal representing the relative ratio.

Claim 16

A method according to claim 11 or claim 12, wherein the step of generating predetermined filter coefficients comprises storing a set of predetermined filter coefficients and generating memory addresses for reading out selected filter coefficients.

Claim 17

A method according to claim 13, wherein the step of generating memory addresses comprises generating (18) a summation output by adding the averaged relative ratio and a fixed value, and using the summation output to generate a memory address.

Claim 18

A method according to claim 14, wherein the fixed value is generated only when said summation output is less than said averaged relative ratio and said output sampling frequency exceeds said input sampling frequency.

Claim 19

A method according to claim 14, wherein a first fixed value is generated when said summation output falls within a range between first and second amounts, said second amount being a predetermined fraction of said averaged relative ratio, and a second fixed value is generated when said summation output is less than said first amount or greater than said second amount, provided that said output sampling frequency is less than said input sampling frequency.

Claim 22

A method according to claim 17, wherein the step of averaging comprising generating (209) a local clock signal, determining (212) a relative ratio between the period of said input sampling frequency and the period of said local clock signal, determining (213) a relative ratio between the period of said output sampling frequency and the period of said local clock signal, and averaging (215, 216) each of said ratios.

Claim 23

A method according to claim 18, wherein the step of averaging the relative ratio between the input sampling period and the local clock period comprises adding (251) a measured relative time difference between the input sampling period and the local clock period and an estimated relative time difference therebetween to detect an error between the measured and estimated relative time differences, producing (253) a correction output in response to the detected error, adding (255) the correction output and a previous estimate of the input sampling period for generating a corrected estimate of the input sampling period, and adding (252) the corrected estimate of the input sampling period and a previous corrected estimate for producing said estimated relative time difference between said input sampling period and said local clock period; and wherein the corrected estimate of the input sampling period is the averaged relative ratio between the input sampling period and the local clock period.