Bidirectional filter

• US 5,479,922 A
• Filed: 10/29/1993
• Issued: 01/02/1996
• Est. Priority Date: 10/14/1992
• Status: Expired due to Fees

• Current Assignee: Del Mar Avionics
• Original Assignee: Del Mar Avionics
• Inventors: Reichl, Philip
• Primary Examiner(s): Kamm, William E.
• Application Number:
  US08/144,696
• Time in Patent Office: 795 Days
• Field of Search: 328/167, 364/724.01, 364/724.05, 364/724.16, 364/724.17, 128/630
• US Class Current: 600/300
• CPC Class Codes: A61B 5/363 Detecting tachycardia or bradycardia
• CPC Class Codes: A61B 5/4884 inducing physiological or psychological stress, e.g. applications for stress testing
• CPC Class Codes: H03H 17/04 Recursive filters

Abstract

A system for filtering stochastic signals as embodied in stress test systems for measuring electrocardiographic (ECG) activity in response to exercise employs a bidirectional Infinite Impulse Response (IIR) filtering process over a predetermined window on a digitized version of an ECG signal to eliminate noise up to the 0.67 Hz frequency range. To eliminate phase shifting distortion in the ECG signal, the IIR filter is applied in a first direction over a predetermined window of the ECG and then applied in a second direction opposite the first direction segment over the identical window. In this manner the phase shift distortion normally associated with IIR filters is completely eliminated and superior bandwidth (as compared to FIR filters) is obtained. The response is achieved fast enough to be used in real time purpose.

Claims

1. A filter for eliminating noise in electrical signals comprising:

acquisition means capable of acquiring a digital representation of an analog electrical signal, said acquisition means including at least one analog sensor capable of generating said analog electrical signal in response to a bioelectric stimulus, and an analog to digital converter coupled to said analog sensor;

computational means including, window gating means and signal storage means for gating a series of time domain, full window (WF) data elements of said digital representation and for storing said windowed data elements for subsequent fluent and analysis of said digital representation in a series of individual windows;

filter means contained within said computational means for implementing a zero phase, bidirectional Infinite Impulse Response filtering algorithm upon each of said series of full windows in a first forward direction for the length of said data element and in a second reverse direction opposite said first forward direction for said data element length;

truncating means responsive to said filtering means for separating a first part leading data element segment, as a retained, filtered, output window (W1) of said full window (WF), and discarding a second part trailing data element segment, is a truncated window (W2) of said full window; and

combining said truncated trailing data element with new data input for another pass through said bidirectional filter.

2. The filter defined by claim 1 further comprising comparison means for determining whether said filtering means has completed filtering in each of said first and said second directions in an amount equal to said predetermined length.

3. The invention defined by claim 1 further comprising selection means for determining a first part of each of said windows to be further analyzed and a second part of each of said windows to be discarded.

4. The invention defined by claim 3, wherein said selection means further comprises:

an analysis length indicative of said first part of each of said windows to be analyzed;

storage means for retaining said analysis length number; and

window length comparison means for comparing each window against said analysis length.

5. The invention defined by claim 4 wherein said selection means further comprises retention means for retaining that part of each of said windows selected to be analyzed and elimination means for discarding that part of each said windows not selected to be analyzed.

6. The invention defined by claim 5 wherein said filter is a Butterworth filter.

7. The invention defined by claim 5 wherein said window length is about 3 seconds long.

8. The invention defined by claim 5 wherein that part of said window to be discarded is approximately 2 seconds long.

9. The invention defined by claim 4 further comprising analysis means responsive for analyzing that part of each of said windows selected to be analyzed.

10. A method of filtering analog electrical signals comprising the steps of:

digitizing said analog electrical signals into a series of digital signals;

storing said digital signals;

segmenting said digital signals into a series of windows, each of said windows being of a predetermined data element length;

filtering each of said windowed data elements by applying a bidirectional infinite impulse response (IIR) filter in a first direction for said predetermined data element length, and in a second direction that is opposite said first direction for said predetermined length;

selecting a first part of each of said filtered data elements to be output and further analyzed and a second part of each of said windows to be discarded and refiltered; and

employing means responsive to said filtering and selecting steps for reconstructing said series of digital signals and providing a filtered representation of said analog signal.

11. The method of claim 10 wherein said selecting step further comprises the following steps:

storing a predetermined number indicative of a portion of said window to be analyzed; and

comparing each of said windows with said predetermined number to identify a portion of said window which is to be analyzed.

12. The method of claim 11 further comprising the step of discarding that portion of said window not identified by said portion for being analyzed.

13. The method of claim 12 wherein further comprising the step of analyzing that portion of said window not discarded.

14. The method of claim 10 further comprising the steps of:

generating said predetermined number indicative of the desired length of said window;

storing a present length filtered variable that measures current length filtered in each direction of said bidirectional IIR filter; and

comparing said present length filtered variable with said predetermined number to determine window boundaries in each direction of said bidirectional IIR filter.

15. A method of filtering electrical signals comprising the steps of:

digitizing an analog electrical signal into a series of digital signals;

storing a value representing a predetermined time frame;

segmenting said digital signals into a series of windows, each of said windows having a length equal to said value;

filtering each of said windows by application of a bidirectional infinite impulse response (IIR) filter in a first direction for said predetermined length, and in a second direction that is opposite said first direction for said predetermined length;

storing a predetermined number indicative of a portion of each of said windows that is to be discarded;

comparing each of said filtered windows with said predetermined number and discarding that portion of each said window that exceeds said predetermined number; and

reconstructing a filtered output signal by synthesizing a series of windows consisting of that portion of each said filtered window that is not discarded in the preceding step.

16. The method of filtering defined by claim 15 further comprising the step of employing means responsive to said comparing step for discarding that portion of each of said windows indicated by said predetermined number.

17. The method of filtering defined by claim 16 further comprising the step of reconstructing those portions of said windows not discarded into a filtered representation of said series of digital signals.

18. The method of claim 15 wherein said filtering is accomplished by a bidirectional infinite impulse response Butterworth filter.

19. The method of claim 18 wherein said filtering step employs a 5 pole bidirectional infinite impulse response Butterworth filter.

20. The method of claim 18 wherein said filtering step employs a 3 pole bidirectional infinite impulse response Butterworth filter.