Question

The adaptive filter used by Widrow to suppress the maternal ECG during fetal heart rate monitoring has following taps and a delay respectively:

• A. 16, 1s
• B. 16, 100 ms
• C. 2, 129 ms
• D. 32, 1s

Hint:

Widrow's classic maternal ECG cancellation configuration relies on a very compact, fast-adapting structure: 2 taps coupled with an intentional propagation alignment delay of 129 ms.

Answer 1

The Correct Option is C

Concept: During non-invasive fetal heart rate monitoring using abdominal surface electrodes, the raw measured signal is highly corrupted. It contains a weak, low-amplitude fetal Electrocardiogram (fECG) heavily obscured by a much stronger maternal Electrocardiogram (mECG) signal, which can be up to an order of magnitude larger. To cleanly cancel out this overlapping maternal electrical interference without distorting the underlying fetal waveform, Bernard Widrow designed a classic adaptive noise cancelling (ANC) system. This setup requires two separate signal inputs:
• Primary Input (Abdomen): Positioned on the mother's abdomen, capturing the composite mixture of the weak target fECG and the strong, nonlinearly distorted maternal mECG interference.
• Reference Input (Chest Leads): Placed near the mother's thorax (chest) to record a clean, isolated maternal ECG signal that contains negligible fetal components.

Step 1: Reviewing Widrow's specific structural design configuration.
An adaptive filter utilizes a Least Mean Squares (LMS) optimization algorithm to dynamically adjust its internal impulse response weights. This allows it to reshape the reference chest signal to match the distorted maternal component in the abdominal lead, enabling clean subtraction. In Widrow's classic design:
• The adaptive filter architecture uses a remarkably small number of filter weights—specifically, a 2-tap Finite Impulse Response (FIR) structure.
• Because the electrical cardiac wavefront propagates through physical body tissue from the heart to the chest and abdomen at different times, a fixed time delay must be added to align the signals. To properly align these cardiac cycles before starting adaptive processing, Widrow introduced a precise pre-processing delay of 129 ms into the reference signal path.

Step 2: Matching with the provided choices.
Evaluating the unique engineering parameters of Widrow's classic experiment:
• Number of adaptive filter taps = $2$
• Propagation alignment time delay = $129\text{ ms}$ This corresponds exactly to Option (C).