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ATI's Glossary of Digital Signal Processing Terms

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A-law
Audio codec companding format.  A logarithmic mapping between 8 bit data space and 13 bit sample space as described in the CCITT G.711 recommendation.  A-law has 13 bits of dynamic range (78 dB).  See u-law.

AC
Alternating Current.  Power line transmission frequency is either 50 Hz or 60 Hz depending on global location.

ADC
Analog to Digital Converter.  Also known as an A/D (A to D).  The audio card's ADC connects to the Input Devices window.

ADPCM
Adaptive Differential Pulse Code Modulation.  A lossy CBR audio compression codec.  Sample coding can range from 2 to 5 bits per sample depending on the standard/mode.

AGC
Automatic Gain Control.  Adjust the input gain automatically so that clipping overload is avoided and optimal ENOB bit usage is achieved.  Also see the AGC channel mapping spaces.

aliasing
Frequency folding.  Mirror images of wrapped frequency.  Decimation without proper filtering causes aliasing which irreversibly corrupts the signal.  An anti-aliasing filter (LPF) prevents aliasing.  To experiment with the effects of aliasing see the anti-alias filter option in the Input Devices window.  See Nyquist sampling theorem.

AM
Amplitude Modulation.  AM has a carrier frequency and two sidebands (LSB and USB).  See the modulation option in the Tone Generator window.

ASK
Amplitude Shift Keying.  A data modulation scheme that involves changing the amplitude of a carrier frequency.  See AM and CW.

AWGN
Additive White Gaussian Noise.  Also known as WGN.  Constant spectral energy at all frequencies with a probability histogram that follows a Gaussian bell shaped curve.  See the Function option in the Tone Generator window.

baseband
A signal that is centered around zero Hz.  Both real and complex quadrature signals can be mixed to baseband.  The operation is performed by a DDC.  For frequency limits see Nyquist.

BER
Bit Error Rate.  A measure of data loss.

BPM
Beats Per Minute.  A measure of tempo.

bpp
Bits Per Pixel.  Color resolution.  Common color depths for video displays are 8, 16, 24, and 32 bpp.  16 bpp is (565) RGB.

BPSK
Binary Phase Shift Keying.  A data modulation scheme that involves changing the phase of a carrier frequency.  BPSK uses 2 distinct phases (0° and 180°) for 1 bit per symbol.  See PSK.

CBR
Constant Bit Rate.  Less efficient than VBR but easier to implement in fixed data stream applications.

CDMA
Code Division Multiple Access. 

CMY
Cyan Magenta Yellow.  CMY is a color space rotation of RGB.  See the RGB mapping menu in the Color Picker window.

codec
COmpressor DECompressor.  A lossy or lossless method of reducing the data bandwidth.  Here is a list of different audio codecs that baudline supports:

CW
Continuous Wave.  Morse code.

DAC
Digital to Analog Converter.  Also known as a D/A (D to A).  The Output Devices window connects to the audio card's DAC.

DAQ
Data acQuisition.  The collection and/or generation of signal.  See ADC and DAC.

dB
Decibel.  Tenth of a Bell.  A logarithmic ratio of powers or voltages.  For power dB = 10 * log10(P2 / P1);  For voltage dB = 20 * log10(V2 / V1);  It can be an absolute level when used with a 0 dB reference point or it can be a relative difference measurement.  Each sample bit contributes 6 dB of dynamic range.  16-bit samples have 96 dB of dynamic range while 24-bit samples have 144 dB of dynamic range.  True real world values are usually much less, see ENOB.

DC
Direct Current.  Constant voltage can be seen as a DC offset in the frequency domain as a peak at 0 Hz.

DDC
Digital Down Converter.  Tuner.  Mix a signal down to baseband.  A DDC operates in the complex domain and consists of a frequency mixer, a LPF, and a decimator.  See the down mixer in the Input Device window.

decimation
A decimator reduces the sample rate by discarding samples.  It is important to low pass filter in order to prevent aliasing.

DSB
Double SideBand.  See SSB

DSP
Digital Signal Processing.  The art of manipulating signals in the discrete digital domain.

DTMF
Dual Tone Multi Frequency.  Telephone signaling tones.

DUT
Device Under Test.  Black box.  Input a stimulus signal such as a pure sine wave, linear swept sine, or WGN to the DUT and then measure the output characterization signal.

ELF
Extremely Low Frequency.  30 - 300 Hz. 

endian
Big and Little Endian.  The ordering of 8-bit bytes within larger data structures such as shorts, ints, longs, floats, and doubles.  See the "decode format" in the Raw Parameters window and the "flip endian" operation in the Channel Mapping window.

ENOB
Effective Number of Bits.  Calculated from SINAD.  Describes the quality of an ADC in terms of bits.  See the ENOB measurement window.

FDM
Frequency Division Multiplexing.  Interleave channels in frequency.  Like a traditional AM or FM radio.

FEC
Forward Error Correction.  Improve the BER by encoding redundant data that can be used to fix bit errors.

FFT
Fast Fourier Transform.  A mathematical operation that converts between the time and the frequency domains.  O(n * log n).

FIFO
First In First Out.  An abstract streaming buffer mechanism that is useful for real-time data processing.  See "man mkfifo" and stdin.

FIR
Finite Impulse Response filter.  Performs convolution in the time domain.  A DSP algorithmic construct made up of cascaded MAC units and filter taps.  FIR filters are stable and have linear phase.  Both the decimator in Input Devices and Hilbert filters are implemented with FIR filters.

FM
Frequency Modulation.  See the modulation option in the Tone Generator window.

FPS
Frames Per Second.  The higher the frame rate the smoother the animation or scrolling.  See "video FPS" in the Stats window.

FSK
Frequency Shift Keying.  A data modulation scheme that involves switching between two frequencies called Mark and Space for 1 bit per symbol.  FSK8 would use 8 tones for 3 bits per symbol. 

full duplex
Simultaneous transmission of information in two directions.  Transmit and receive.  Baudline runs in the full duplex mode when recording a channel and using the Tone Generator at the same time.  Most Linux audio drivers can be used full duplex.  See half duplex.

GPS
Global Positioning System.  Not just an accurate method of determining position, but also an accurate source of time since each GPS satellite has an atomic clock.  GPS time is locked to UTC time plus an offset since GPS time does not account for leap seconds.  GPS timing receivers have a digital time code data port and a PPS line for high accuracy synchronization.

GSM
Groupe Speciale Mobile or Global System for Mobile Communications.  GSM 6.10 is a lossy CBR audio compression codec which has an audio sample rate of 8000 samples/second and a data rate of 13 Kbps.  GSM is also a popular global mobile phone standard.

half duplex
The transmission of information in only one direction at a time.  Transmit or receive.  Baudline is in half duplex when it is in the record or play run modes.  Many of the original PC sound cards could only operate in half duplex.  See full duplex.

heterodyne
Mix a signal with a sinusoid to move it in frequency space.  Used by radio tuners.  See DDC.

Hilbert
A transform that performs a 90° phase shift.  Typically implemented with a FIR filter that has all the odd taps being zero.  Useful for creating complex quadrature signals.  See the "Hilbert" operation in the Channel Mapping window.

HPF
High Pass Filter.  Frequencies lower than the cutoff frequency are attenuated.  A HPF coupled with a LPF is a bandpass filter.  See the "HPF" slider control in the Play Deck window.

Hz
Hertz.  Cycles per second.  A unit of frequency.  Limits are defined by the Nyquist sampling theorem.  Complex quadrature signals can have negative frequencies.

IF
Intermediate Frequency.  The output of a mixer.

IMD
InterModulation Distortion.  A two-tone signal source modulates together to create sum and difference frequencies.  Second, third, and higher order products can be present as distortion noise.

interpolation
An interpolator increases the sample rate by inserting zeros in between samples.  It is important to low pass filter in order to prevent aliasing.

LNA
Low Noise Amplifier.  The noise power is proportional to temperature (degrees Kelvin).

LPF
Low Pass Filter.  Frequencies higher than the cutoff frequency are attenuated.  LPF's are useful for anti-aliasing in decimators.  They are also useful for bandpass filtering.  See the "LPF" slider control in the Play Deck window.

LSB
Dual definitions: Least Significant Bit and Lower SideBand.  For Least Significant Bit see the "1 bit binary decode format" in the Raw Parameters window and the "bit reverse" operation in the Channel Mapping window.  Lower side band is frequency inverted around the carrier frequency, see USB.

MAC
Multiply Accumulate operation.  A += B[x] * C[x];  An important building block used internally by many DSP algorithms.

MFSK
Multi Frequency Shift Keying.  A data modulation scheme that involves coding frequency combinations with bits.  A variation of FSK but with more than two frequencies and there can be more than one frequency active at a time in a parallel fashion. 

modem
MOdulator DEModulator.  Communications device.  A method of transmitting and receiving digital binary data over an analog channel.  See modulation.

modulation
The process of encoding binary data or waveform information for analog transmission.  Opposite of demodulation.  Here is a list of different modulation types:

mixer
Dual definitions: /dev/mixer for adjusting the gain of a soundcard and a frequency mixer (multiplication) for a DDC type operation.

MSB
Most Significant Bit.  See the "1 bit binary decode format" in the Raw Parameters window, and the "bit reverse" operation and "bit Raster" transform in the Channel Mapping window.

NTP
Network Time Protocol.  Used for accurate clock synchronization over the Internet.  See "man ntpq" and the -calibratesr command line option.

Nyquist
Nyquist sampling theorem.  For real signals; half the sampling rate is known as the Nyquist frequency limit.  This is the highest frequency that can be represented by a particular sampling rate.  Complex quadrature signals have twice the bandwidth for a given sample rate.  For baseband complex signals the frequency range is { -Hz, 0, +Hz } where Hz is half the sample rate.  For non-baseband signals the frequency range is twice that of a real signal.

OFDM
Orthogonal Frequency Division Multiplexing.  A multicarrier data modulation scheme that spreads multiple QAM-like channels over FDM.  Each channel has its own dynamically adjustable modulation parameters that maximizes the total signal bandwidth and reduces the effect of interference and multipath.

OSD
On Screen Display.  Text overlays like a heads up display.  Uses minimal screen real estate in the display of important information.  Most baudline display windows use OSD for cursor and data specific metrics.

overlap
The amount to move the FFT sliding window for smooth and continuous spectral slice processing.  The overlap controls spectrogram scrolling speed, time axis resolution, and FPS.  See the overlap slider in the Scroll Control window.

PCM
Pulse Code Modulation.  It defines the analog and digital signal mapping (see ADC and DAC).  An analog signal is quantized into discrete digital samples at a uniform sample rate.  See Nyquist sampling theorem.

PDF
Probability Density Function.  The probability distribution of sample amplitudes.  See the Histogram window.

PM
Phase Modulation. 

PPM
Parts per million.  Used as error units. 

PPS
Pulse per second.  Used for timing synchronization.  See GPS.

PSD
Power Spectral Density.  A method of scaling the spectra amplitude axis such that it becomes a continuous function of frequency. 

PLL
Phase locked loop.  A feedback loop that consists of a phase detector, LPF, and VCO.  The PLL is a phase tracking circuit whose output is a clock signal that is phase locked to the input.  Some useful applications: clock synchronization, frequency multiplier, and FM discriminator.

PSK
Phase Shift Keying.  A data modulation scheme that involves changing the phase of a carrier frequency.  See BPSK and QPSK.  8-PSK uses 8 distinct phases for 3 bits per symbol.  See also FSK and QAM.

QAM
Quadrature Amplitude Modulation.  A data modulation scheme that is a combination of PSK with ASK.  256-QAM has 8 bits per symbol.  QAM has better data throughput and better spectral efficiency than PSK but it requires a higher SNR to achieve an equivalent BER.

QoS
Quality of Service.  A general measure of signal quality and service availability.  Frequency bandwidth, phase jitter, latency (delay), BER, SINAD, SNR, and THD are common signal quality performance metrics.  Service availability is typically measured in units of the NINES where each 9 digit represents another order of magnitude of reliability.  7 nines = 99.99999% = 3 seconds of downtime per year.

QPSK
Quadrature Phase Shift Keying.  A data modulation scheme that involves changing the phase of a carrier frequency.  QPSK use 4 distinct phases (0°, 90°, 180°, 270°) for 2 bits per symbol.  See PSK.

quadrature
Complex I/Q sampling.  Two channels; in-phase (I) and quadrature (Q) create a complex signal with twice the standard Nyquist bandwidth. 

RF
Radio Frequency spectrum.  See these different RF bands:
  • ULF  (DC - 30 Hz)
  • ELF  (30 - 300 Hz)
  • VGC  (300 - 3400 Hz)
  • VLF  (3 - 30 kHz)
  • VHF  (30 - 300 MHz)
  • UHF  (300 - 3000 MHz)


RFI
Radio Frequency Interference.  A source of signal noise.

RGB
Red Green Blue.  Color space.  888 RGB is 24 bpp which represents 8-bits for each of the red, green, and blue components.  565 RGB is 16 bpp.  See CMY.

RMS
Root Mean Square.  The square root of the mean of the squares.  sqrt(sum(x[i]^2)/n);

RPM
Revolutions per minute.  A measure of rotation.  See the RPM measurement window.

SDR
Software Defined Radio. 

SFDR
Spurious Free Dynamic Range.  A measure of dynamic range which is the ratio of the input signal to the strongest non signal element (spurious peak).  See the SFDR measurement window.

SINAD
Signal to Noise and Distortion Ratio.  Also known as THD+N.  A measure of signal purity.  See the SINAD measurement window.

SNR
Signal to Noise Ratio.  Useful for measuring the characterization of the noise floor.  See the SNR measurement window.

spectrogram
The spectrogram display is a flat three dimensional plot that is composed of spectrum slices.  The axis are time vs. frequency vs. color (spectral energy).  See the Spectrogram window.

SPL
Sound Pressure Level.  Intensity of sound measured in dB.  The 0 dB SPL reference point is 0.0002 ubar (20 uPa) which is sometimes called the Threshold of Hearing.  SPL meters typically measure in dBA and dBC where (A) and (C) are frequency weightings.  See the different weighting curves in the Equalization window.

SSB
Single SideBand.  FDM with one of the two sidebands suppressed making either upper (USB) or lower (LSB) sideband.  Sometimes the carrier is also suppressed.  The benefit of SSB over DSB is better spectral efficiency and reduced power.


stdin
Standard Input.  A streaming UNIX pipe mechanism. 

stdout
Standard Output.  A streaming UNIX pipe mechanism. 

T&M
Test and Measurement.  See DUT.

TDM
Time Division Multiplexing.  Interleave channels in time.  Examples: sound card PCM data, Compact Discs (CD's), and voice carrying T1's.

TDMA
Time Division Multiple Access. 

THD
Total Harmonic Distortion.  A measure of nonlinearity.  Units can be dB or percent (%).  See the THD measurement window.

u-law
An audio codec companding format more correctly known as mu-law.  A logarithmic mapping between 8 bit data space and 14 bit sample space as described in the CCITT G.711 recommendation.  u-law has 14 bits of dynamic range (84 dB).  See A-law.

USB
Dual definitions: Universal Serial Bus and Upper SideBand (see SSB). 

UHF
Ultra High Frequency.  300 - 3000 MHz. 

ULF
Ultra Low Frequency.  DC - 30 Hz. 

UTC
Coordinated Universal Time.  Also known as Greenwich Mean Time (GMT).  Global scientific observations use UTC for time synchronization.  From a UNIX shell try the command "date -u" for your computer's time displayed relative to UTC. 

VBR
Variable Bit Rate.  More efficient than CBR but more difficult to implement in real-time streaming applications since the data bandwidth is dynamic.

VCO
Voltage Controlled Oscillator. 

VGC
Voice Grade Channel.  It can refer to an analog channel with a frequency range of 300 to 3400 Hz.  Or it can refer to a digital 64 Kbps PCM DS0 communications channel. 

VHF
Very High Frequency.  30 - 300 MHz. 

VLF
Very Low Frequency.  3 - 30 kHz. 

WGN
White Gaussian Noise.  See AWGN.

XOR
bitwise eXclusive OR.  A = B ^ C;  A reversible operation.  See the "XOR" operation in the Channel Mapping window.

xruns
Overruns.  Underruns.  An error case that happens when either the input sample buffers are full or the output sample buffers are empty.  An xrun results in a loss of data or an audible click.  This error is usually caused when the data bandwidth requirements exceed the CPU power.  See "potential drops" in the Stats window.

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