Signals Flashcards
In biomechanics, we are dealing with values from sensors that have limitations based on…
- Sampling
- Sensor error
- Environmental error
A signal has ____ that needs to be dealt with
noise
How can we deal with noise
By sampling properly
Smoothing data
Filtering data
Knowing some basics of signals and the transformations that we intend to perform on them
What is the Nyquist-Shannon Sampling theorem
In sampling an analog signal, the goal is to represent the original signal as accurately as possible with as few discrete points as possible. Sampling frequency must be a certain level in order to be accurately represented by discrete data
How do we ensure that sharp direction changes in the signal are not missed
Use sampling frequencies 5-10 times the Nyquist Frequency are often used
Do we deal with the noise before or after doing calculations
before
What are 3 smoothing techniques
- Curve fitting
- Polynomials
- Splines
- Moving averagea
What is curve fitting
Fitting a best-fitting function to the data
What is moving averages
Moving average over a window of samples gets rid of noisy spikes
What are 4 components to a signal
Frequency
Amplitude
Offset
Phase shift
What is frequency
The number of repeating cycles per unit time
What is amplitude
The magnitude of change
What is the offset
The distance of the mean amplitude from zero
What is the phase shift
A horizontal offset
Signals can be decomposed and reconstructed from…
a number of basic functions (sine waves)
Once can occur once a signal is decomposed
It can be represented in a frequency graph of the basic sine waves (frequency domain)
What is frequency domain
A time-domain signal is converted to a frequency-domain signal with a mathematical technique called a Fourier Transform
What is digital filtering
A way of reducing the frequency content of the signal so that it just obtains the signal you are interested in without the noise
What is a common use of fourier transforms
To find the frequency components of a time domain signal that is mixed with noise
Why can a filter be used to remove noise?
Noise is often of a different frequency than the intended signal
What are 5 types of digital filtering
low pass high pass band-pass band stop notch
What is a problem with digital filtering
Phase distortion (usually phase-lags occur, shifts signal)
What is the most common source of noise in human motion capture data
small errors in the position of the biomechanical markers during the digitization process
Noise in motion capture data is usually…
Low amplitude and high frequency
What is a low pass filter
A filter which multiplies everything above a certain point by 0 - Just interested in low frequencies
What is a high pass filter
A filter which multiplies everything before a certain point by 0 - Just interested in high frequencies
What type of filter most commonly used to smooth kinematic data
A low-pass filter
What is the aim of filtering
Arrive at the highest signal : noise ratio
When is noise in motion capture data amplified
each time the data are differentiated of integrated
acceleration data arrived at by double-differentiating displacement data are typically smoothed using…
a low pass filter to increase the signal:noise ratio
What is the highest voluntary frequency in human locomotion
Less than 30 Hz
