Impulse response filtering of code division multiplexed signals in a capacitive sensing device
An example method of capacitive sensing includes: receiving at least one code division multiplexed (CDM) signal transmitted from different transmitters using different codes over a plurality of signal bursts, wherein a length of each of the different codes is equal to or greater than a number of the different transmitters; filtering the at least one CDM signal using an impulse response filter, wherein the filtering comprises: accumulating the at least one CDM signal over the plurality of signal bursts to obtain measurements of the at least one CDM signal; and for the measurements of the at least one CDM signal, applying different filter weights to the resulting signals over a number of the plurality of signal bursts that is greater than the length of the different codes.
1. A method for digital signal processing (DSP), comprising:
receiving at least one code division multiplexed (CDM) signal transmitted from different transmitters using different codes over a plurality of signal bursts, wherein a length of each of the different codes is equal to or greater than a number of the different transmitters; and
filtering the at least one CDM signal using an impulse response filter, wherein the filtering comprises:
accumulating the at least one CDM signal over the plurality of signal bursts to obtain measurements of the at least one CDM signal; and
for the measurements of the at least one CDM signal, applying different filter weights over a number of the plurality of signal bursts that is greater than the length of the different codes.
2. The method of claim 1 , wherein:
the impulse response filter is a triangular finite impulse response (FIR) filter, and
the different filter weights are applied according to a triangular pattern over the number of the plurality of signal bursts.
3. The method of claim 1 , wherein:
the length of each of the different codes is n signal bursts, and
the number of the plurality of signal bursts over which the filter weights are applied is equal to 2n or 2n−1 signal bursts.
4. The method of claim 1 , wherein:
the different filter weights are applied such that a total weighting applied to each of the different codes is equal.
5. The method of claim 1 , wherein:
the different codes comprise circulant codes.
6. The method of claim 1 , further comprising:
filtering the plurality of CDM signals using a boxcar filter until a threshold number of the plurality of signal bursts have been received.
7. The method of claim 1 , further comprising:
applying overlapping filters for overlapping sets of signal bursts; and
outputting reports for each of the overlapping filters.
8. The method of claim 1 , wherein the different codes used for the at least one CDM signal repeats at least once.
9. A processing system for an input device, comprising:
sensor circuitry configured to:
receive at least one code division multiplexed (CDM) signal transmitted from different transmitter electrodes using different codes over a plurality of signal bursts, wherein a length of each of the different codes is equal to or greater than a number of the different transmitters; and
a processing module, coupled to the sensor circuitry, configured to:
filter the at least one CDM signal using an impulse response filter, wherein the filtering comprises:
accumulating the at least one CDM signal over the plurality of signal bursts to obtain measurements of the at least one CDM signal; and
for the measurements of the at least one CDM signal, applying different filter weights over a number of the plurality of signal bursts that is greater than the length of the different codes.
10. The processing system of claim 9 , wherein the processing module is configured to:
filter the at least one CDM signal using a triangular finite impulse response (FIR) filter, and
apply the different filter weights according to a triangular pattern over the number of the plurality of signal bursts.
11. The processing system of claim 9 , wherein:
the length of each of the different codes is n signal bursts, and
the processing module is configured to apply the filter weights over a number of the plurality of signal bursts equal to 2n or 2n−1 signal bursts.
12. The processing system of claim 9 , wherein:
the processing module is configured to apply the different filter weights such that a total weighting applied to each of the different codes is equal.
13. The processing system of claim 9 , wherein:
the different codes comprise circulant codes.
14. The processing system of claim 9 , wherein the processing module is further configured to:
filter the at least one CDM signal using a boxcar filter until a threshold number of the plurality of signal bursts have been received.
15. An input device, comprising:
a plurality of sensor electrodes comprising a plurality of transmitter electrodes and a plurality of receiver electrodes, wherein the plurality of receiver electrodes are configured to receive at least one code division multiplexed (CDM) signal transmitted from different transmitter electrodes of the plurality of transmitter electrodes using different codes over a plurality of signal bursts, and wherein a length of each of the different codes is equal to or greater than a number of the different transmitter electrodes; and
a processing system coupled to the plurality of sensor electrodes, the processing system configured to:
filter the at least one CDM signal using an impulse response filter, wherein the filtering comprises:
accumulating the at least one CDM signal over the plurality of signal bursts to obtain measurements of the at least one CDM signal; and
for the measurements of the at least one CDM signal, applying different filter weights over a number of the plurality of signal bursts that is greater than the length of the different codes.
16. The input device of claim 15 , wherein the processing system is configured to:
filter the at least one CDM signal using a triangular finite impulse response (FIR) filter, and
apply the different filter weights according to a triangular pattern over the number of signal bursts.
17. The input device of claim 15 , wherein:
the length of each of the different codes is n signal bursts, and
the processing system is configured to apply the filter weights over a number of the plurality of signal bursts equal to 2n or 2n−1 signal bursts.
18. The input device of claim 15 , wherein:
the processing system is configured to apply the different filter weights such that a total weighting applied to each of the different codes is equal.
19. The input device of claim 15 , wherein:
the different codes comprise circulant codes.
20. The input device of claim 15 , wherein the processing system is further configured to:
filter the at least one CDM signal using a boxcar filter until a threshold number of the plurality of signal bursts have been received.