# Signal Processing Topics

**ACK**- In telecommunications, this is the acknowledge character in many data codes; used most commonly for an affirmative response of correct receipt.
**Alias**- A false lower frequency component that appears in sampled data acquired at too low a sampling rate.
**Amplitude Flatness**- A measure of how close to constant the gain of a circuit remains over a range of frequencies.
**Angular Rate Sensor**- A sensor that measures rotational velocity around its sensitive axis.
**Anti-Aliasing Filter**- A device that attenuates signal content outside the desired bandwidth.
**Asymmetric Waveform**- A waveform that has unequal excursions above and below the horizontal axis.
**Autocorrelation**- The autocorrelation function gives a measure of the extent to which a signal correlates with a displaced version of itself, as a function of the displacement.
**Autocovariance**- This is the degree to which a function is correlated with itself as a function of time.
**Autospectrum**- A spectrum with the coefficients of the components expressed as the square of the magnitudes
**Band Stop Filter**- An electronic network which passes signals having frequencies above and below the band-elimination frequency span.
**Bandpass Filter**- An electronic network which transfers through it a specific band of signal frequencies.
**Bandwidth**- The bandwidth of a filter is the separation between the lower and upper frequencies at which the amplitude of a sinusoidal signal is attenuated by a factor of 2.
**Bandwidth Time Product**- Used in calculating the confidence limit of a measurement.
**Base Band**- The frequency band occupied by information-bearing signals before combining with a carrier in the modulation process.
**Bins**- The individual frequencies at which the amplitudes are calculated, commonly called lines.
**Bit Error Rate**- The number of bits processed before an erroneous bit is found.
**Bit Rate**- The rate or frequency at which bits appear in a bit stream.
**Bit Stream**- A binary signal without regard to grouping.
**Blackman Window**- A weighting that is applied in the time domain to reduce leakage within a Fourier Transform analysis.
**Blackman-Harris Window**- A weighting that is applied in the time domain to reduce leakage within a Fourier Transform analysis. The Blackman-Harris window has much the same performance as the Kaiser-Bessel window, except that it suppreses the sidelobes more than 92dB at a cost of an 11% wider noise bandwidth.
**Bohman Window**- A weighting that is applied in the time domain to reduce leakage within a Fourier Transform analysis.
**Calculated Peak**- Term used to describe the spectral overall RMS level multiplied by the square root of two.
**Campbell Diagram**- A mathematically constructed diagram used to check for coincidence of vibration sources with natural resonances.
**Cauchy Window**- A weighting that is applied in the time domain to reduce leakage within a Fourier Transform analysis.
**Cepstrum**- A cepstrum is a time history defined as the power spectrum of the logarithm of the Power spectrum.
**Chirp**- A chirp is a sound that has a rapid increase in frequency.
**Circular Frequency**- The frequency of a steady recurring phenomenum in radians per second.
**Clipper Circuit**- A circuit that blocks or removes the portion of a voltage waveform above some threshold voltage.
**Coherence**- This gives a measure of the linear dependence between signal A and B.
**Colourmap**- This is a method of plotting noise or vibration versus engine speed and frequency simultaneously.
**Comb Filter**- A distortion produced by combining an electrical or acoustical signal with a delayed replica of itself.
**Common-Mode Range**- The input range over which a circuit can handle a common-mode signal.
**Composite Rating of Preference**- Combines low frequency sound and spectral balance i.e. modifiers for boom and high frequency.
**Constant Percentage Bandwidth Filter**- A band-pass filter whose bandwidth is a constant percentage of centre frequency. 1/3 octave filters are constant percentage bandwidth.
**Control System**- A group of components systematically organized to perform a specific control purpose.
**Convolution**- Convolution meaning ..intertwined, coiled…
**Correlation**- Generally speaking, a similarity between data; the extent to which data are related.
**Covariance**- A measure of the joint variability of a pair of numeric variables.
**Crest Factor**- Peak value divided by the R.M.S. value, used as a measure of the severity of a segment of data.
**Cross Correlation**- The cross-correlation function gives a measure of the extent to which two signals correlate with each other as a function of the time displacement between them.
**Cross Modulation**- The modulating of a desired signal carrier by undesired signal modulation as the result of accidental mixing of the two signals in a non-linear device.
**Cross Spectrum**- The cross spectrum F
_{xy}(f) of f_{x}(t) and f_{y}(t) is the forward Fourier Transform of the cross correlation function R_{xy}(T). The cross spectrum is in general complex. **Cyclic Redundancy Check**- An integrity checking process for block data.
**DAC**- Abbreviation of Digital-to-Analogue Converter.
**DAQ**- Abbreviation of Data Acquisition.
**Data Acquisition**- Collecting and measuring electrical signals from sensors, transducers, and test probes or fixtures and inputting them to a computer for processing.
**Data Processing**- The execution of a systematic sequence of operations performed upon data. Synonymous with information processing.
**Date Stamp**- Information added to data to indicate the date at which it was collected.
**DC Component**- The dc value of an waveform that has a value other than zero.
**Definition of a Sound**- This may be described by the time at which the sound arrives at the listening position.
**Degenerative Feedback**- Feedback in which the feedback signal is out of phase with the input signal.
**Delta-Sigma Modulating ADC**- A high-accuracy circuit that samples at a higher rate and lower resolution than is needed and pushes the quantization noise above the frequency range of interest.
**Differentiation**- A mathematical operation that differentiates a signal or function.
**Digital Audio Compression**- Any of several algorithms designed to reduce the number of bits required for accurate digital audio storage and transmission.
**Digital Filter**- Any filter accomplished in the digital domain.
**Digital Signal Processing**- The term used for calculations performed on digital signals.
**Digital-to-Analogue Converter**- Converts a digital number into a corresponding analogue voltage or current.
**Digitization**- Any conversion of analog information into a digital form.
**Direct Sequence Modulation**- A spread spectrum modulation scheme that directly modulates the information-bearing data with a spreading code sequence.
**Discrete Fourier Transform**- The digital version of the fourier transform.
**Discrete Gabor Transform**- An algorithm for transforming data from the discrete time domain to the joint time-frequency domain.
**Dropout**- Any discrete variation in signal level during the reproduction of recorded data which results in a data-reduction error.
**Effective Number of Bits**- Figure of merit for an A/D converter describing how many bits of effective resolution, this is below the actual A/D word size.
**Electronic Filter**- Any of various electric, electronic, acoustic, or optical devices used to reject signals, vibrations, or radiation of certain frequencies while passing others.
**Equivalent Rectangular Bandwidth**- This is the effective bandwidth of an auditory filter.
**External Trigger**- A voltage pulse from an external source that triggers an event such as A/D conversion.
**Far-End Crosstalk**- A common telephony impairment created by physical twisted wire pairs sharing a bundle.
**Fast Fourier Transform**- An algorithm, or digital calculation routine, that efficiently calculates the discrete Fourier transform from the sampled time waveform.
**Filter**- Any of various electric, electronic, acoustic, or optical devices used to reject signals, vibrations, or radiation of certain frequencies while passing others.
**Filtering**- Attenuates components of a signal that are undesired: reduces noise errors in a signal.
**Finite Impulse Response Filter**- A commonly used type of digital filter. Digitized samples of the audio signal serve as inputs, and each filtered output is computed from a weighted sum of a finite number of previous inputs.
**FIR Filter**- A commonly used type of digital filter. Digitized samples of the audio signal serve as inputs, and each filtered output is computed from a weighted sum of a finite number of previous inputs.
**Flattop Window**- An amplitude weighting of the time signal used with gated continuous signals to give them a slow onset and cut-off in order to reduce the generation of side lobes in their frequency spectrum.
**Flicker Noise**- A type of low frequency noise where the power spectral density is inversely proportional to the frequency.
**Fluctuation Strength**- This is similar in principle to roughness except it quantifies subjective perception of slower, up to 20Hz, amplitude modulation of a sound.
**Fourier Analysis**- A mathematical analysis of waves, discovered by the French mathematician Fourier (1768-1830).
**Fourier Series**- Decomposes periodic functions or periodic signals into the sum of a set of simple oscillating functions.
**Fourier Transform**- The mathematically rigorous operation which transforms from the time domain to the frequency domain and vice versa.
**Frequency Shift**- A sudden change in the frequency of a signal.
**Gaussian Noise**- Another name for Random Noise.
**G Weighted Overall Level**- This purportedly reflects human response to infrasound.
**Hamming Window**- An amplitude weighting of the time signal used with gated continuous signals to give them a slow onset and cut-off in order to reduce the generation of side lobes in their frequency spectrum.
**Hanning Window**- An amplitude weighting of the time signal used with gated continuous signals to give them a slow onset and cut-off in order to reduce the generation of side lobes in their frequency spectrum.
**Hanning-Poisson Window****Harmonic Analysis**- Statistical method for determining the amplitude and period of certain harmonic or wave components in a set of data with the aid of Fourier series.
**High Pass Filter**- A filter that passes signal frequencies above a specific, or cut off, frequency is called a high pass filter.
**Image Frequency**- An undesired frequency capable of producing the desired frequency through heterodyning.
**Infinite Impulse Response Filter**- A commonly used type of digital filter. This recursive structure accepts as inputs digitized samples of the audio signal, and then each output point is computed on the basis of a weighted sum of past output (feedback) terms, as well as past input values.
**Instantaneous Amplitude**- The amplitude at any given point along a wave at a specific instant in time.
**Instantaneous Value**- The magnitude at any particular instant when a value is continually varying with respect to time.
**Integrating ADC**- An ADC whose output code represents the average value of the input voltage over a given time interval.
**Interaural Cross-Correlation Coefficient**- The measure of the difference in the sounds arriving at the two ears of a listener.
**Intermodulation**- The mixing of two or more signals in a nonlinear element to produce signals at new frequencies which are sums and differences of the input signals or their harmonics.
**Intermodulation Distortion**- Distortion produced by the interaction of two or more signals. The distortion components are not harmonically related to the original signals.
**Jitter**- The short-term instability of a signal.
**Joint Time Frequency Analysis**- A method of analysis that simultaneously provides both time and frequency information.
**JTFA**- Abbreviation of Joint Time Frequency Analysis.
**Kaiser Bessel Window****Kalman Filter**- A technique for estimating an unknown state of a linear dynamic system given observations of the system which have additive (Gaussian) noise.
**Lead Term**- A control system component that anticipates future inputs based on the current signal trend.
**Leakage**- In a Fourier Transform the signal is assumed to be periodic. If a pure sine wave does not repeat exactly within the time window, it is truncated. this truncation will lead to the frequency domain resultant being smeared (leakage) and not a single frequency.
**Linear Distortion**- Magnitude and phase distortion.
**Low Pass Filter**- A filter with a characteristic that allows all frequencies below a specified rolloff frequency to pass and attenuate all frequencies above.
**Lower Frequency Cutoff**- The lowest frequency a circuit will pass.
**Lower Sideband**- All difference frequencies below that of the carrier.
**Mains Frequency**- Electricity ac supply frequency; 50 Hz in UK, 60 Hz in US.
**Mark**- An interval during which a signal is present.
**Maximum Length Sequence**- A type of pseudorandom binary sequence.
**Minimum Discernible Signal**- The weakest input signal that produces a usable signal at the output of a receiver.
**Modulated Wave**- A complex wave consisting of a carrier and a modulating wave.
**Modulation**- The process, or the result of the process, by which a characteristic of one wave is varied according to some characteristic of another wave.
**Moving Picture Experts Group**- A working group within SMPTE who set, among other things, specifications for compression schemes for audio and video transmission.
**Multiplexing**- A method for simultaneous transmission of two or more signals over a common carrier wave.
**Near-End Crosstalk**- Impairment typically associated with twisted-pair transmission, where a local transmitter interferes with a local receiver.
**Noise Climate**- This is sometimes calculated as L
_{A10}- L_{A90}and represents the fluctuations in noise. **Nonlinear Distortion**- New spectral components are produced.
**Notch Filter**- An electronic network which passes signals having frequencies above and below the band-elimination frequency span.
**Order Analysis**- Order analysis is simply frequency analysis where the frequency axis of the spectrum is expressed in orders of rpm rather than in Hz or rpm.
**Overall RMS Level**- A measure of the total RMS magnitude within a specified frequency range.
**Overdriven**- When the input signal amplitude is increased to the point that the transistor goes into saturation and cutoff.
**Overlap Processing**- Overlap processing is desirable when using a Hanning Window because it ensures against loss of data for parts of the signal that occur near the beginning and end of the window.
**Overmodulation**- A condition that exists when the peaks of the modulating signal are limited.
**Oversampling**- Sampling at a rate higher than the sampling Nyquist theorem.
**PAM**- Abbreviation of Pulse Amplitude Modulation
**PDM**- Abbreviation of Pulse Duration Modulation.
**Peak to Average Power ratio**- Usually expressed in decibels and used to determine the robustness of a modulated signal against nonlinear distortion.
**Percent of Modulation**- The degree of modulation defined in terms of the maximum permissible amount of modulation.
**Periodic Wave**- A waveform that undergoes a pattern of changes, returns to its original pattern, and then repeats the same pattern of changes.
**PFM**- Abbreviation of Pulse Frequency Modulation.
**Phase Angle**- The number of electrical degrees of lead or lag between the voltage and current waveforms in an ac circuit.
**Phase Delay**- The time or angular difference between two signals.
**Phase Modulation**- Angle modulation in which the phase of the carrier is controlled by the modulating waveform.
**Phase Shift**- The time or angular difference between two signals.
**Picket Fence Effect**- The true spectrum of the signal being analyzed may have peaks at frequencies between the lines of the FFT spectrum, and the peaks in the FFT spectrum will not be at exactly the correct frequencies.
**Positive Alternation**- The part of a sine wave that is above the reference line.
**Positive Feedback**- Feedback in which the feedback signal is in phase with the input signal.
**Post Triggering**- The technique used on a DAQ board to acquire a programmed number of samples after trigger conditions are met.
**Power Standing Wave Ratio**- The ratio of the square of the maximum and minimum values of a wave.
**PPM**- Abbreviation of Pulse Position Modulation.
**Pre Triggering**- The technique used on a DAQ board to keep a continuous buffer filled with data, so that when the trigger conditions are met, the sample includes the data leading up to the trigger condition.
**PSWR**- Abbreviation of Power Standing Wave Ratio.
**PTM**- Abbreviation of Pulse Time Modulation.
**Pulse**- Signal characterized by a steep rise from and decay toward an initial level.
**Pulse Amplitude Modulation**- Pulse modulation in which the amplitude of the pulses is varied by the modulating signal.
**Pulse Compression**- Any technique for modifying or processing a pulse so that the autocorrelation function is greater than one.
**Pulse Duration**- The period of time during which a pulse is present.
**Pulse Duration Modulation**- Pulse modulation in which the time duration of the pulses is changed by the modulating signal.
**Pulse Frequency Modulation**- Pulse modulation in which the modulating voltage varies the repetition rate of a pulse train.
**Pulse Modulated Field**- An electromagnetic field produced by the amplitude modulation of a continuous wave carrier by one or more pulses.
**Pulse Modulation**- A form of modulation in which one of the characteristics of a pulse train is varied.
**Pulse Oscillator**- A sine-wave oscillator that is turned on and off at specific times.
**Pulse Output Power**- The ratio of the average output power to the pulse duty factor.
**Pulse Position Modulation**- Pulse modulation in which the position of the pulses is varied by the modulating voltage.
**Pulse Repitition Rate**- The average number of pulses in unit time during a specified period.
**Pulse Time Modulation**- Pulse modulation that varies one of the time characteristics of a pulse train.
**Pulse Width**- Duration of time between the leading and trailing edges of a pulse.
**Pulse Width Modulation**- Pulse modulation in which the duration of the pulses is varied by the modulating voltage.
**PWM**- Abbreviation of Pulse Width Modulation.
**Quadrature Amplitude Modulation**- A combined phase- and amplitude-modulation scheme that is used to modulate the carrier signal in phase quadrature.
**Quadrature Phase-Shift Keying**- A digital phase-modulation scheme that is used to modulate the carrier signal in phase quadrature.
**Quantization Error**- When a continuous time signal is digitized, because there isn′t an infinite number of discrete digital levels, the difference between the actual analog value and the digital representation of that value is defined as the quantization error.
**Quasi Periodic**- A deterministic signal whose spectrum is not a harmonic series, but nevertheless exists at discrete frequencies.
**Real Time Monitoring**- Observing measured results as they are happening.
**Real Time Operation**- What is perceived to be instantaneous to a user. This is particularly applicable to Hardware in Loop systems where any simulation of a system must operate in real time to allow inputs and outputs to operate as per real time.
**Rectangular Window**- An amplitude weighting used to truncate continuous time signals to fit within the length of the DFT window.
**Recursive Filter**- A filter in which there is no need to store past measurements for the purpose of calculating current estimates.
**Rest Frequency**- The carrier frequency during the constant-amplitude portions of a phase modulation signal.
**Ringing Oscillator**- A sine-wave oscillator that is turned on and off at specific times.
**Ripple Frequency**- The frequency of the ripple current.
**Root Mean Square**- This is the square root of sum of squares of individual observations divided by the total number of observations made.
**Sample and Hold**- A circuit that captures and holds an analog signal for a finite period.
**Sample Rate Conversion**- The process of converting one sample rate to another.
**Sampling**- The process of representing the amplitude of a signal at a particular point in time.
**Sampling Theorem**- States that a band limited continuous waveform may be represented by a series of discrete samples if the sampling frequency is at least twice the highest frequency contained in the waveform.
**Selectivity**- The ability of a receiver to select the desired signal and reject unwanted signals.
**Shaping Circuit**- A circuit that alters the shapes of input waveforms.
**Signal Ground**- The common return or reference point for analog signals.
**Sine Wave**- A waveform corresponding to a single-frequency periodic oscillation that can be mathematically represented as a function of amplitude versus angle in which the value of the curve at any point is equal to the sine of that angle.
**Spatial Filtering**- Enhancing an image by increasing or decreasing its spatial frequencies.
**Spectral Centroid**- This is the centre of gravity of a frequency spectrum.
**Spectrum**- A spectrum is a definition of the magnitude of the frequency components that constitute a quantity.
**Spectrum Analyser**- An instrument for measuring, and usually recording, the spectrum of a signal.
**Spectrum Density**- Limit, as the bandwidth approaches zero, of the mean-square amplitude divided by bandwidth.
**Speech Intelligibility**- A measure of sound clarity that indicates the ease of understanding speech.
**Splatter**- Unwanted sideband frequencies that are generated from over modulation.
**Spread Spectrum Modulation**- Digital modulation scheme that spreads the system bandwidth far greater than the minimum required bandwidth.
**Square Wave**- A periodic wave that alternately assumes one of two fixed values (high and low) with the transition time between these two levels being negligible.
**Stage Support**- Early energy (20 to 100ms), sound source on the stage 1m away from the microphone.
**Standing Wave Ratio**- The ratio of the maximum to the minimum points of a wave.
**Step by Step Counter**- A counter that provides an output for each cycle of the input in one-step increments.
**SWR**- Abbreviation of Standing Wave Ratio.
**Time Delay Spectrometry**- A method for obtaining anechoic results in echoic spaces.
**Time History**- A continuous record of the variation of a physical quantity (e.g. displacement, acceleration, force, etc.) with time.
**Time Series**- A sample of data values collected at equally spaced points in time.
**Tracking Filter**- A low-pass or band-pass filter which automatically tracks the input signal.
**Trellis-Code Modulation**- Combined coding and modulation technique.
**Trigger**- A short pulse, either positive or negative, that can be used to cause an electronic function to take place.
**Turnover Frequency**- Also known as Nyquist Frequency.
**Uniform Window**- The uniform, or rectangular, window does not modify the signal amplitude at all.
**Unmodulated**- A carrier signal without modulation.
**Velocity Servosystem**- A servosystem which controls the speed of the load it is driving.
**Waterfall Plot**- A series of spectral maps taken at regular intervals of time or at regularly spaced shaft speeds,
*similar to the flow of a waterfall*. **Waveform Analysis**- Observation displays of voltage and current variations with respect to time or by harmonic analysis of complex signals.
**Wavelet**- The wavelet allows a rapidly changing time history (e.g. door slam noise) to be investigated in the frequency domain.
**Windowing****Z Transform**- A mathematical method used to relate coefficients of a digital filter to its frequency response, and to evaluate stability of the filter.