Adaptive sampling
A method and apparatus is disclosed for adaptively sampling an analogue signal to increase the sampling rate in the presence of high frequency content within the signal, for example, QRS complex of an ECG signal. In one aspect, a change in a derivative of the analogue signal is used to control a voltage-controlled oscillator to provide a clock signal for an analogue-to-digital converter. The change in the derivative is compared to an automatically controlled threshold value. The clock signal controls the sampling rate of the analogue-to-digital converter so that the sampling rate is increased from one level, where only P and T waves are present to another higher level when the QRS complex has been detected.
1. A method for adaptively sampling an analogue biopotential signal, the method comprising the steps of:
a) determining a derivative of the analogue biopotential signal;
b) detecting a change in the derivative of the analogue biopotential signal;
c) sampling the analogue biopotential signal using a sampling frequency; and
d) using the change in the derivative of the analogue biopotential signal to control the sampling frequency of the analogue biopotential signal, wherein the process of controlling the sampling frequency of the analogue biopotential signal comprises comparing the change in the derivative of the analogue biopotential signal with an automatically controlled threshold value, and wherein the automatically controlled threshold value is generated from an expected average activity value and an extracted average activity value, the extracted average activity value being derived from the analogue biopotential signal, and the expected average activity value being derived from a beat frequency of the analogue biopotential signal.
2. The method according to claim 1 , wherein the analogue biopotential signal comprises an electrocardiogram signal and the process d) further comprises deriving the expected average activity value from a beat frequency of the electrocardiogram signal.
3. The method according to claim 1 , wherein the process c) comprises adjusting the sampling frequency to sample the analogue biopotential signal in accordance with the comparison between the derivative of the analogue biopotential signal and the threshold value.
4. The method according to claim 3 , wherein the sampling frequency is increased to correspond with a high frequency component of the analogue biopotential signal.
5. The method according to claim 3 , wherein the sampling frequency is decreased to correspond with other lower frequency components of the analogue biopotential signal.
6. The method according to claim 1 , wherein the process c) is carried out using an analogue-to digital converter.
7. An apparatus for adaptively sampling a high frequency component of an analogue biopotential signal, the apparatus comprising:
a sampling device configured to sample the analogue biopotential signal at a sampling frequency; and
a control device configured to control the sampling device, the control device comprising an activity detection device configured to determine a derivative of the analogue biopotential signal and to use a change in the derivative of the analogue biopotential signal to alter the sampling frequency of the sampling device, the activity detection device comprising a comparator configured to compare the change in the derivative of the analogue biopotential signal with an automatically controlled threshold value which is generated from an expected average activity value and an extracted average activity value, the extracted average activity value being derived from the analogue biopotential signal, and the expected average activity value being derived from a beat frequency of the analogue biopotential signal.
8. The apparatus according to claim 7 , wherein the comparator generates a difference between the extracted average activity value and the expected average activity value that is used to automatically control the threshold value.
9. The apparatus according to claim 8 , wherein the threshold value is increased when the extracted average activity value is greater than the expected average activity value.
10. The apparatus according to claim 8 , wherein the threshold value is decreased when the extracted average activity value is less than the expected average activity value.
11. The apparatus according to claim 7 , wherein the activity detection device comprises a differentiator configured to generate the derivative of the analogue biopotential signal.
12. The apparatus according to claim 7 , wherein the threshold value is derived from the derivative of the analogue biopotential signal.
13. The apparatus according to claim 7 , wherein the sampling device comprises an analogue-to-digital converter configured to convert the analogue biopotential signal into a digital representation thereof in accordance with the sampling frequency.
14. The apparatus according to claim 7 , wherein the control device comprises a digital signal processor (DSP).
15. The apparatus according to claim 14 , wherein the DSP is implemented in an application specific integrated circuit.
16. A device for adaptively sampling a high frequency component of an analogue biopotential signal, the device comprising:
means for determining a derivative of the analogue biopotential signal;
means for detecting a change in the derivative of the analogue biopotential signal;
means for sampling the analogue biopotential signal using a sampling frequency; and
means for controlling the sampling frequency of the analogue biopotential signal, the means for determining further configured to use a change in the derivative of the analogue biopotential signal to alter the sampling frequency of the means for sampling, the means for determining a derivative comprising a means for comparing the change in the derivative of the analogue biopotential signal with an automatically controlled threshold value, wherein the automatically controlled threshold value is generated from an expected average activity value and an extracted average activity value, the extracted average activity value being derived from the analogue biopotential signal, and the expected average activity value being derived from a beat frequency of the analogue biopotential signal.