Systems and methods for variable rate conversion
Poly-phase filters are used to offer an efficient and low complexity solution to rate conversion. However, they suffer from inflexibility and are not easily reconfigured. A novel design for rate converters employ poly-phase filters but utilize interpolation between filter coefficients to add flexibility to rate conversion. This interpolation can be implemented as an interpolation of the poly-phase filter results. Additional approximations can be made to further reduce the amount of calculations required to implement a flexible rate converter.
1. A variable rate converter comprising:
a poly-phase filter having a nominal input sampling rate, an output sampling rate and a plurality of phases, each phase corresponding to a phase index and comprising a plurality of filter coefficients based on the nominal input sampling rate and the output sampling rate; and
an approximator configured to approximate the poly-phase filter at a phase offset derived from an input sample having a variable rate.
2. The variable rate converter of claim 1 , wherein the approximator includes an interpolator.
3. The variable rate converter of claim 2 , wherein the interpolator is a linear interpolator.
4. The variable rate converter of claim 2 , wherein the interpolator is a cubic interpolator.
5. A variable rate converter comprising:
a poly-phase filter having a nominal input sampling rate, and an output sampling rate and a plurality of subfilters, each subfilter comprising a plurality of phases, each phase corresponding to a phase index and comprising a plurality of filter coefficients based on the nominal input sampling rate and the output sampling rate; and
means for approximating each poly-phase subfilter at phase offsets derived from an input sample having a variable rate.
6. The variable rate converter of claim 5 , wherein the means for approximating comprises a means for interpolating between results produced by each poly-phase subfilter with phase indices near the phase offset.
7. The variable rate converter of claim 6 , wherein the means for interpolating uses a linear interpolation.
8. The variable rate converter of claim 6 , wherein the means for interpolating uses a cubic interpolation.
9. A variable rate conversion method comprising:
applying a poly-phase filter to an input sample having a variable rate, the poly-phase filter having a nominal input sampling rate, and an output sampling rate and a plurality of subfilters, each subfilter comprising a plurality of phases, each phase corresponding to a phase index and comprising a plurality of filter coefficients based on the nominal input sampling rate and the output sampling rate; and
approximating each poly-phase subfilter at phase offsets derived from an input sample having the variable rate.
10. The variable rate conversion method of claim 9 , wherein the approximating comprises interpolating between results produced by each poly-phase subfilter with phase indices near the phase offset.
11. The variable rate conversion method of claim 10 , wherein the interpolating uses a linear interpolation.
12. The variable rate conversion method of claim 10 , wherein the interpolating uses a cubic interpolation.