Biosignal sampling & quantizaiton Flashcards
Bioelectrical singals
generated by nerves & muscles, elecrochemical changes within & between cells > generation of action potetnail = flow of ions across membrane
ECG, EGG, EEG, EMG
Biomagnetic signals
- magnetic fields generated by brain/organs, typically accompanying electric field
- Biomagnatism: measurement of magnetic signals associated with specific physiological activity
- SQUID & MEG
SQUID
superconduction quantum interface device
- very precise magnetic sensors or magnetometers
- monitoring of magentic activity
MEG
Magnetoencepalography
- Principle: recording magnet fields in different positions produced by electrical currents occuring naturally in the brain
- technique for mapping brain activity
- allows to backcalculate intracellular current
- assessment of perceputal and cognitive brain processes
Biochemical signals
- changes in concentration of various chemical agents in the body
- measure: changes in partial pressure of oxygen and carbon dioxide
- determine: normal levels
Biomechanical signals
motion, displacement, force, pressure and flow
Bioacoustic signals
special subset of biomechanical signals that involve vibrations
- bioogical events produce acoustic noise: flow of blood, respiratroy system, joints
- measured at the skin surface via microphones and accelerometers
- photoacoustic tomography of arthritic joints
Biooptical signals
optical light ot induced attributes of biological systems
- fluorescnet characteristic of amniotic fluid
- dye dilution method (estimate cardiac output)
- red and infrared light (measurement of blood oxygen)
characterisitcs of biosignals
- continuous
- discrete
- deterministic
- random /stochastic
continuous biosignals
- almost all biosignals are continuous
- vary with time → converted into discrete signals → analyzed by computer
discrete biosignals
- a series of specific signal values acquired at specific time points
deterministic biosignals
described as periodic or transient mathematical funcitons
- periodic: usually composed of the sum of differnet sine waves ot sinusoid components
- transient: nonzero or vary over a finite time interval → decay to a constant value
random / stochastic biosignals
- contain uncertainity in describing parameters
- mathematical funcitons cannot be used to precisely describe random signals
- are most often analysed using statistical techniques
- biosignals most often have some unpredictable noise or change in parameters
components of signal acquisation
- A/D converters
- signal sampling
- quantization errors
A/D converter
- transforms biological signals form continuous (analog) wavefrom to discrete (digital) sequences
- compunter controlled voltmeter: measures input signal and gives a numeric representation as output
signal sampling
- Definition: process by whcih a continous singal is converted into a discrete sequence in time
- x(t) = analog singal → recording of the amplitude value of x(t) every T second
sampling rate/ frequency fs
= equal to inverse of the sampling period → 1/T in Hz (s^-1)
- sampling rate used to dicretize a continous or analog singal → critical for generation of accurate approximation of waveform
- too low sampling rate leads to distortions
Nyquist sampling theorem
f (nyquist) = 2 *f(max)
- the minimum sampling rate used should be at least twice the macimum frequency of the original signal
quantization
definition: process of mapping a large set of input values to a smaller set by rounding values to some unit of precision → transformed into binary values
Resolution of the A/D converter
- quantizer with N bits is capable of representing 2^N possible amplitude values
input voltage range / 2^N = voltage resolution
quantization error
- analog signal approximated by quantization function (digital) → set to best approx. value of the digital output line
