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22 Cards in this Set
- Front
- Back
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What is an analog signal?
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continuous signal of up and down voltage fluctuations that cannot be understood by the computer
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What is a digital signal?
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Series of discrete data points assigned numerical values separated by equal intervals of time
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What is A/D conversion?
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Noise reduction and response enhancement take advantage of computer manipulations - waves must be converted to digital form in order for the computer to manipulate them
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What are the determinates of ADC reproduction quality?
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Sampling rate and dwell time- bit capacity and input voltage range
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What is Horizontal resolution?
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The resolution on the horizontal access which is related to frequency/time and is determined by the sampling rate - or the number of horizontal data points that are taken or the number of times per second the data is analyzed
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What is sampling rate?
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number of data points used to represent or attempt to replicate the analog signal - number of times per second the data is analyzed
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What is dwell time?
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Time duration between each data point - the intersample interval
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What is the nyquist frequency?
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Sampling rate must be at least twice as fast as the fastest frequency wave to be resolved - slower than required SR results in aliasing
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What is Aliasing?
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failure to sample the analog signal at a sampling rate at least twice as fast as the fastest wave to be resolved - so digital signal will have fewer waves than the analog signal
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What are the frequencies the EEG analyzes
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1 - 70 hz
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What is the ACNS stanard minimum sampling rate?
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3 times the highest frequency which is 70 hz- most machines use 200
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How are dwell time and sampling rate related?
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Reciprocal - so the faster the SR the shorter the DT
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A wave with a frequency of 50 hz should be recorded with a min sampling rate of what to prevent aliasing?
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Nyquist says 100 but ACNS says 150 |
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What are problems with horizontal resolution?
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aliasing and sampling skew
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what is sampling skew?
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ADC samples each channel independent of subsequent channles - time lapse occurs from the time to first-to-last channel is sampled and converted resulting in loss of time axis integrity or SKEW
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How do you prevent sampling skew?
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separate ADC fro each channel. Sample and hold - ADC samples channesl sequentially and then aligns data points in time.
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What is screen resolution?
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Display capabilities of the mointior and will affect the digital appearance of the data.
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What is vertical resolution?
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Verticle resolution that is related to how the voltage/amplitude is shown and is determined by the number of descrete voltage levels exist- or verticle data points that are taken - which is dependent on the number of available bits
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What is Bit capacity?
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The number of vertical levels or steps used by the ADC to analyze voltage or amp. Each bit is a power of 2. Min 11 or more are ACNS guidelines
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What is voltage range?
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range of voltage over which ADC operates - .5uV min
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What is clipping?
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Waveform tips will square off with lower bit capacity because the peak of the wave does not hit a voltage level and therefore will flatten out at the highest voltage level detected
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What determines amplitude resolution?
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The number of available voltage values which is determined by the number of bits in the A/D converter.
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