Tagged biosignal
8 pieces
- · 7 min read
You Can Be 90 Percent in the Green Zone and Measuring Nothing
A model that predicts close to the population mean can place most of its points in a clinical error grid's acceptable zones and post a respectable error percentage, while explaining less variance than the mean itself. Check the variance explained against a mean baseline, under a subject-disjoint split, before believing any accuracy figure from a physiological model.
- · 7 min read
Cadence Lock: When Your Heart-Rate Monitor Starts Counting Your Steps
Optical heart-rate algorithms separate the cardiac signal from motion by picking the dominant periodic component. When step frequency converges on heart rate, which happens routinely in field sports, the algorithm can lock onto cadence and report steps per minute as beats per minute. It is confident, stable and wrong, and nothing on the display indicates it. The validation protocol has to contain the condition where the two frequencies meet.
- · 7 min read
Tracker or Model: The Wearable Fork
Two wearables can look identical and be different products. One computes a quantity the sensing can actually resolve; the other infers a quantity it cannot, and covers the gap with a model. The fork is decided when the sensing is chosen, months before there is data, and it determines what you are allowed to claim.
- · 7 min read
What a Regulator Asks About Your Biosignal Front End That Your Bench Never Did
Your bench asks whether the signal looks right. A reviewer asks what the system does when it is wrong, across every unit you will ship, in conditions you did not choose. Those are different questions, and the second one is answered in the analog front end months before there is software to test.
- · 7 min read
Common Mode Rejection: How a Biosignal Front End Hears a Whisper at a Concert
When you measure off a body you are not measuring your signal. You are measuring your signal plus the person plus the room, all summed at the amplifier, and the thing you want is microvolts while the mains hum coupling into the person can be a thousand times larger. The entire job of the front end is pulling the small thing out from under the large thing, by throwing away everything that appears equally on both inputs.
- · 8 min read
The Driven Reference Electrode: Your Front End's Hero and Its Hidden Risk
The third electrode is a feedback loop wrapped around a person. It senses the body's common mode and drives against it, which is what turns a hope about electrode symmetry into an actively regulated quantity and gives you rejection figures worth quoting. It is also a deliberately low-impedance connection between your circuit and the body, which makes it a stability problem, a safety-limited path, and the exact node any injected current will come home through.
- · 7 min read
The Physical Ceiling: What No Model Can Recover From Your Sensor
Whatever your sensor and that first little circuit throw away is gone. You cannot get it back later with software. A photo that comes out blurry does not become sharp with editing, and the fancy tools that appear to sharpen it are inventing plausible detail rather than recovering real detail, which is exactly the distinction that matters when the output drives a decision about a person.
- · 16 min read
The Return-Path Bug That Kills Your Biosignal Front End
On a closed-loop bio device, the instant you turn the actuator on, the signal can vanish to a flat line, and it is not coupling. The current you inject is a loop that has to come home, and it takes the lowest-impedance path, often the exact electrode protecting your front end. It forces a voltage the amplifier cannot reach, the amplifier rails, and your sensor goes blind. The fix is a decision about where the current returns, not a better part.