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Digital-to-digital conversion using non-uniform sample rates
| Details |
Inventors: Wilson, James; Cellini, Ronald A.; Sobol, James M.;
Assignee: Analog Devices, Inc. (Norwood, MA)
Primary Examiner: Pellinen; A. D.
Assistant Examiner: Kost; Jason
Attorney, Agent or Firm: Wolf, Greenfield & Sacks
A method and apparatus for digital-to-digital conversion using sigma-delta modulation of the temporal spacing between digital samples. The method and apparatus of the present invention provides for sigma-delta modulation of the time base such that noise produced by non-uniform sampling are frequency-shaped to a region (i.e., shifted to higher frequencies) where it can be removed by conventional filtering techniques. In one embodiment, the digital data is interpolated by fixed ratio and then decimated under control of a sigma-delta modulated frequency selection signal that represents, on average, the data rate of the incoming digital data stream. Thereafter, the digital data is interpolated under control of a sigma-delta modulated frequency selection signal that represents, on average, the data rate of the digital data to be output by the converter and then decimated by a fixed ratio. The first and second frequency signal selection numbers are modulated using n-th order m-bit sigma-delta modulators. The method and apparatus converts the data rate of the incoming digital data stream to the data rate of the first n-th m-bit sigma-delta modulator and then converts the digital data stream from the first sigma-delta modulator to an output data rate determined by the second n-th order m-bit sigma-delta modulator. |
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DETAILED DESCRIPTION The present invention overcomes the limitations of the prior art by providing a method and apparatus for digital-to-digital conversion using non-uniform sampling.
In one embodiment of the invention, the apparatus includes a first interpolator or other comparable circuitry such as a sample and hold circuit for receiving digital signals at a first data rate and for supplying the digital signals at a first increased data rate and a first decimator, coupled to the first interpolator, for decimating the digital signals at the first increased data rate to provide digital signals at a second data rate.
In one embodiment, a first sigma-delta modulator is coupled to and controls the first decimator and provides a first sigma-delta modulated output signal representative of the first data rate and controls the first decimator to provide the digital signals at the second data rate.
This part of the invention interpolates digital data by a fixed ratio and then decimates the interpolated digital data by a variable ratio depending on the second data rate desired.
Depending on the performance requirements of the particular application, the decimated digital data at the second data rate may then be filtered to remove, for example, sigma-delta noise introduced by the variable decimation of the digital data at the first increased data rate.
The digital data at the second data rate is supplied to a second interpolator or other comparable circuitry such as a sample and hold circuit that receives the digital data at the second data rate and supplies the digital signals at a second increased data rate.
A second decimator is coupled to the interpolator and decimates the digital signals at the second increased data rate to provide digital signals at a third data rate.
In one embodiment, a second sigma-delta modulator is coupled to and controls the second interpolator and provides a second sigma-delta modulated output signal representative of the third data rate to control the second interpolator to provide the digital signals at the second increased data rate so that, upon decimation by the second decimator, the digital signals are at the third data rate
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