What is baudline?
Mystery Signal
A list of baudline related digital signal processing acronyms, definitions, and terms.

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.

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

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

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.

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.

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.

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

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

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.

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.

Bit Error Rate.  A measure of data loss.

Beats Per Minute.  A measure of tempo.

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

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.

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

Code Division Multiple Access. 

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

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

Continuous Wave.  Morse code.

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

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

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.

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

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.

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

Double SideBand.  See SSB

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

Dual Tone Multi Frequency.  Telephone signaling tones.

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.

Extremely Low Frequency.  30 - 300 Hz. 

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.

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

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

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

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

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

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.

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

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

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.

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.

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.

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

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.

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.

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

Intermediate Frequency.  The output of a mixer.

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.

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

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

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.

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.

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

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. 

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

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

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

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.

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

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.

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.

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.

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.

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.

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

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.

Phase Modulation. 

Parts per million.  Used as error units.  See the -calibratesr command line option.

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

Power Spectral Density.  A method of scaling the spectra amplitude axis such that it becomes a continuous function of frequency.  See the -psd command line option.

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.

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.

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.

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.

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

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)

Radio Frequency Interference.  A source of signal noise.

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.

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

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

Software Defined Radio. 

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.

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

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

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.

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.

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.

Standard Error.  A streaming UNIX pipe mechanism.  The output from the -debugrate and the -debugmeasure command line options can be redirected with the shell ">&" and the "|&" operators.

Standard Input.  A streaming UNIX pipe mechanism.  See the -stdin command line option to use baudline as a data sink.

Standard Output.  A streaming UNIX pipe mechanism.  See the -stdout command line option to use baudline as a data source.

Test and Measurement.  See DUT.

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

Time Division Multiple Access. 

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

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.

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

Ultra High Frequency.  300 - 3000 MHz. 

Ultra Low Frequency.  DC - 30 Hz. 

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.  See the -utc command line option.

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

Voltage Controlled Oscillator. 

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. 

Very High Frequency.  30 - 300 MHz. 

Very Low Frequency.  3 - 30 kHz. 

White Gaussian Noise.  See AWGN.

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

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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