EP0285413A2 — Sample rate converter

Claim 1

Independent Claim 107 words

Apparatus adapted to receive an input stream of samples representative of a time-varying signal and to generate a corresponding output stream of samples which is also representative of said time-varying signal and in which each output stream sample appears at a desired time which may differ from the time when a sample appears in said input stream, the apparatus being characterised in comprising filter circuitry adapted to receive said input stream and operatively to generate at least two intermediate samples which appear at different times, and interpolation circuitry adapted to combine selected said intermediate samples to form a sample of said output stream at said desired time.

Claim 2

Dependent Claim 107 words

Apparatus adapted to receive an input stream of samples representative of a time-varying signal and to generate a corresponding output stream of samples which is also representative of said time-varying signal and in which each output stream sample appears at a desired time which may differ from the time when a sample appears in said input stream, the apparatus being characterised in comprising filter circuitry adapted to receive said input stream and operatively to generate at least two intermediate samples which appear at different times, and interpolation circuitry adapted to combine selected said intermediate samples to form a sample of said output stream at said desired time.

Claim 3

Dependent Claim 35 words

Apparatus according to Claim 2, further characterised in that said filter circuitry comprises a plurality of transversal filters adapted for operatively generating said intermediate samples simultaneously using two or more different said nets of coefficients.

Claim 4

Dependent Claim 61 words

Apparatus according to Claims 2 or 3, further characterised in that each said set is chosen to by substantially flat in the frequency band represented by the time-varying signal and to have an envelope delay characteristic that differs from the envelope delay characteristics of another set by an amount which is nearly constant as a function of frequency within the passband.

Claim 5

Dependent Claim 37 words

Apparatus according to any preceding claim, further characterised in that said interpolation circuitry comprises a switch for selecting two said intermediate samples, and combining circuitry adapted for operatively performing a linear interpolation between said two intermediate samples.

Claim 6

Dependent Claim 39 words

Apparatus according to Claim 5, further characterised in that the coefficients for said linear interpolation are based on a measure of the difference between the sampling phase of said input stream and the sampling phase of said output stream.

Claim 7

Dependent Claim 27 words

Apparatus according to any preceding Claim, further characterised in that said output stream has a sample rate that differs from the sample rate of said input stream.

Claim 8

Dependent Claim 60 words

Apparatus according to Claim 7, further characterised in that said apparatus comprises a modem adapted to be operatively coupled to a line signal, said input stream comprising a stream of samples derived by said modem from said line signal, and said output stream comprising a stream of samples used by said modem to derive data carried by said line signal.

Claim 9

Dependent Claim 59 words

Apparatus according to Claim 8, further characterised in that said modem is a full-duplex, echo cancellation modem, said input sample stream appears at a rate related to the transmission baud rate of said modem, and said output stream appears at a rate operatively related to the transmission baud rate of a modem at the other end of said line.

Claim 10

Dependent Claim 52 words

Apparatus according to any of claims 7, 8 or 9, further characterised in that the sampling phase difference between samples of said input stream and samples of said output stream varies over time, and further comprising means for selecting said intermediate samples and controlling said interpolation circuitry based on said varying difference.

Claim 11

Dependent Claim 111 words

Apparatus according to any of Claims 1 to 6, further characterised in that said input sample stream is operatively sampled based on a clock derived from an unstable clock having a nominally fixed rate subject to jitter and frequency shifts, in that said output stream operatively appears at a nominally fixed rate, and in that said intermediate samples are adapted operatively to be selected and said interpolation circuitry is adapted operatively to be controlled in response to the difference between the sampling phase of said input stream and the sampling phase of said output stream, said apparatus further comprising means for reducing said jitter and frequency shifts in said unstable clock.

Claim 12

Dependent Claim 103 words

Apparatus according to any preceding Claim, further characterised in comprising a buffer adapted for temporarily storing said input stream samples for delivery to said filter circuitry, said samples being operatively stored in said buffer at a first rate which is derived indirectly from an external clock, said samples being operatively accepted by said filter circuitry, at a possibly different average second rate based on an external clock, said apparatus further comprising a monitor adapted for monitoring the occupancy status of said buffer and for operatively causing said average second rate to be adjusted so that said buffer is normally not full or empty.

Claim 13

Dependent Claim 69 words

Apparatus according to Claim 12, further characterised in that said apparatus comprises a modem adapted to be operatively coupled to a line signal, said input sample stream comprising samples of said line signal, in that said external clock corresponds to a baud rate of transmission on said line, and in that said second rate is a clock corresponding to said baud rate and operatively derived locally by said modem.

Claim 14

Dependent Claim 135 words

A modem in which a line signal is arranged operatively to be sampled at times governed by an unstable clock, whereby operatively to generate a first sample stream in which the samples appear with possibly changing sampling phases, and in which a converter is arranged operatively to convert the first sample stream to a second stream of samples appearing at times that are possibly different from the times of appearance of the samples in the first sample stream, characterised in comprising signal means adapted for operatively providing to said converter signals representative of the changing sampling phases in the first sample stream, whereby operatively to cause said conversion to occur in a manner that reduces the contribution of said changing sampling phases to the times of appearance of the samples in said second sample stream.

Claim 15

Dependent Claim 135 words

A modem in which a line signal is arranged operatively to be sampled at times governed by an unstable clock, whereby operatively to generate a first sample stream in which the samples appear with possibly changing sampling phases, and in which a converter is arranged operatively to convert the first sample stream to a second stream of samples appearing at times that are possibly different from the times of appearance of the samples in the first sample stream, characterised in comprising signal means adapted for operatively providing to said converter signals representative of the changing sampling phases in the first sample stream, whereby operatively to cause said conversion to occur in a manner that reduces the contribution of said changing sampling phases to the times of appearance of the samples in said second sample stream.

Claim 16

Dependent Claim 25 words

A modem according to Claim 14, further characterised in that said unstable clock is operatively provided from a data terminal equipment associated with said modem.

Claim 17

Dependent Claim 23 words

A modem according to Claim 14, further characterised in that said unstable clock is a clock operatively derived from said second sample stream.

Claim 18

Dependent Claim 65 words

A modem according to Claim 14, further characterised in comprising a stable clock adapted operatively to deliver pulses at a nominal rate related to the rate of pulses of said unstable clock, and in that said signal means comprises means adapted operatively to generate said signals based on detecting changes in phase of said unstable clock pulses relative to the pulses of said stable clock.

Claim 19

Dependent Claim 82 words

A modem according to Claim 17, further characterised in that said signal means includes an add or delete pulse circuit adapted for operatively generating a derived clock from said stable clock by adding or deleting pulses to or from said stable clock, and a comparator adapted from operatively generating comparisons of the phases of said unstable clock pulses with the phases of pulses of said derived clock, said add or delete pulse circuit being adapted to operate in response to said comparisons.

Claim 20

Dependent Claim 120 words

A modem according to any of Claims 14 to 18, further characterised in that said converter comprises conversion circuitry and a buffer adapted for temporarily storing samples of said first sample stream for delivery to said conversion circuitry, said samples being arranged operatively to be stored in a said buffer at a first rate derived from said unstable clock, in that said conversion circuitry is adapted to accept said samples at a possibly different second rate based on said signals provided to said converter, and in that a monitor is provided for operatively monitoring the occupancy status of said buffer and for operatively causing said first rate to be adjusted so that said buffer is normally not empty or full.

Claim 21

Dependent Claim 90 words

A modem according to any of Claims 14 to 18, wherein said first sample stream represents a time-varying signal, said second sample stream is also representative of said time-varying signal, and each second stream sample appears at a desired time, further characterised in that said converter comprises filter circuitry adapted operatively to receive said first stream and to generate at least two intermediate samples which appear at different times, and interpolation circuitry adapted to combine selected intermediate samples to form a sample of said second stream at said desired time.

Claim 22

Independent Claim 101 words

A method for generating an output stream of sample from an input stream of samples representative of a time-varying signal, output stream of samples also being representative of said time-varying signal, the and each output stream sample appearing at a desired time which may differ from the time when a sample appears in said input stream, the method being characterised in comprising the steps of receiving said input stream and generating, by filtering, at least two intermediate samples which appear at different times, and interpolating selected said intermediate samples to form a sample of said output stream at said desired time.

Claim 23

Independent Claim 101 words

A method for generating an output stream of sample from an input stream of samples representative of a time-varying signal, output stream of samples also being representative of said time-varying signal, the and each output stream sample appearing at a desired time which may differ from the time when a sample appears in said input stream, the method being characterised in comprising the steps of receiving said input stream and generating, by filtering, at least two intermediate samples which appear at different times, and interpolating selected said intermediate samples to form a sample of said output stream at said desired time.

Claim 24

Independent Claim 126 words

A method of operating a modem in which a line signal is sampled at times governed by an unstable clock to generate a first sample stream in which the samples appear with possibly changing phases and a converter converts the first sample stream to a second stream of samples appearing at times that are possibly different from the times of appearance to the samples in the first sample stream, the method being characterised in comprising providing to said converter signals representative of the changing sampling phases of the samples in the first sample stream, whereby to cause said conversion to occur in a manner that reduces the contribution of said changing sampling phases to the times of appearance of the samples in said second sample stream.

Claim 25

Independent Claim 126 words

A method of operating a modem in which a line signal is sampled at times governed by an unstable clock to generate a first sample stream in which the samples appear with possibly changing phases and a converter converts the first sample stream to a second stream of samples appearing at times that are possibly different from the times of appearance to the samples in the first sample stream, the method being characterised in comprising providing to said converter signals representative of the changing sampling phases of the samples in the first sample stream, whereby to cause said conversion to occur in a manner that reduces the contribution of said changing sampling phases to the times of appearance of the samples in said second sample stream.

Sample rate converter

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