Principals of Digital Signal
Recording
How do we represent a continuously
variable signal digitally?
• Sampling
– Sampling rate – number of measurements per unit
time
– Sampling depth or quantization – number of
gradations by which the measurement can be
recorded
How do we represent a continuously
variable signal digitally?
• Sampling
– What would be the advantage to higher sampling
rates?
How do we represent a continuously
variable signal digitally?
• Sampling
– What would be the advantage to higher sampling
rates?
• Nyquist limit
How do we represent a continuously
variable signal digitally?
• Sampling
– What would be the advantage to higher sampling
rates?
• Nyquist limit
• Aliasing
– What would be the disadvantage?
• Data size
• Compute time
How do we represent a continuously
variable signal digitally?
• Sampling
– What would be the advantage to greater sampling
depth?
• Finer resolution
– What would be the disadvantage?
• Data size
• Possibly compute time
How do we represent a continuously
variable signal digitally?
• Sampling
– A note about data size and compute time:
• New data size = increase in quantization x number of samples x number of electrodes!
Filters used in EEG
What is a filter?
What is a filter?
• Filters let some “stuff” through and keep
other “stuff” from getting through
– What do we want to let through?
– What do we want to filter out?
What is a filter?
• The goal of filtering is to improve the signal to
noise ratio
– Can the filter add signal?
Different Kinds of Filters
•
•
•
•
Low-Pass (High-Cut-Off)
High-Pass (Low-Cut-Off)
Band-Pass
Notch
• Each of these will have a certain “slope”
How do Filters Work?
• Notionally:
– Transform to frequency domain
– Mask some parts of the spectrum
– Transform back to time domain
Are There Any Drawbacks?
• Yes
• Filters necessarily distort data
– Amplitude distortion
– Latency distortion
• Forward/backward/zero-phase
Recommendations
• Should you filter?
– Yes, when necessary to reveal a real signal
• Problem: how do you know it’s “real”
– No, always look at the unfiltered data first
• What filters should you use?
– Depends on your situation (e.g. what EEG band
are you interested in? Do you have 60Hz line
noise?)
– General rule: less aggressive filters are less
distorting