How Sysid Works
Accurate in Five Ways
Sysid Measurements
How SYSid Works
1.  SYSID excites the system being measured with a broadband (log chirp, chirp, MLS or impulse), narrowband (sinusoidal) or user-defined stimulus. SYSid synchronously records and then analyzes the system's response to the stimulus. SYSid can sample and analyze two input channels simultaneously, preserving their relative phase relationship. 2.  SYSid offers fast and easy measurement of frequency response, impulse response, phase response, group delay, noise floor, harmonic and intermodulation distortion, spectral contamination and linearity/monotonicity. Other analysis available include 1/3 octave through 1/12 octave band response, energy-time curve, reverse energy time curve and 3D "waterfall" plots. 3.  SYSid employs advanced signal processing techniques to make robust measurements. Using synchronous averaging techniques, the system can reduce the effect of high ambient noise levels to elicit more accurate results under extreme test conditions.
Accurate in Five Ways
  • High Quality Hardware
    Low noise, sixteen bit analog-to-digital conversion on two line-level input channels at sample rates up to 50 kHz is provided by Ariel's DSP-16+ PC board. The DSP-16's S/N is typically 88 dB, prior to any time domain averaging.
  • Time Domain Averaging
    Time domain averaging (also referred to as synchronous, or vector averaging) is a powerful technique for increasing a measurement system's signal-to-noise ratio. SYSid repetitively excites the system and synchronously averages the result; SYSid assumes a linear, time-invariant system that has an impulse response time less than the length of the stimulus. Consequently, the component of the system response due to the excitation will be reinforced with each averaging period, while all other system noise is reduced. This includes any noise generated by the electronics of the test instrument that is independent of the signal. The result is an honest-to-goodness increase in dynamic range: by 3 dB for every doubling of the number of averages. Averages are calculated with 32-bits of precision and the user can specify the number of averages, from one to 32,768 (for a total dynamic range of 130dB S/N). Time domain averaging is used on all measurements, including distortion and spectral contamination.
    • Frequency and Time Response
  • Proper Measurement Techniques
    SYSid permits the system to "settle" to steady state before accumulating its response, eliminating transient artifacts which would otherwise degrade the results. This is done by ramping up the stimulus level from zero during the first average, then discarding the results, as well as those from the next (full-power) stimulus.
  • Low Crest Factor Stimuli
    SYSid offers a choice of stimuli: chirp (a rapidly-swept sine wave), log chirp (more energy into low frequencies), and MLS (Maximum Length Sequence). All have low crest factors to greatly increase the total energy delivered to the system during the measurement interval, and to enhance the signal-to-noise ratio of the measurement.
  • FFT Computation
    The FFT length may range from 64 to 32,768 points, in one or two channels simultaneously. The FFT is computed on the PC's math coprocessor. For a 32k point FFT measurement at an 8 kHz bandwidth, SYSid can measure impulse responses of up to 2.048 seconds, with a frequency resolution of 0.49 Hz.
    • SYSid Measurements
    Transfer Function Analysis
    The user can set the broadband stimulus type (log chirp, chirp, MLS, impulse or data file), stimulus output level, number of time-domain averages, maximum analysis frequency (1 kHz to 25 kHz), and length of FFT. SYSid's two independent channels allow measurement of either channel individually, both channels simultaneously, or the ratio of the two channels. Ratiometric transforms permit deconvolution of the near field response from the far field response, and impedance measurements which are used to compute Theile-Small parameters. SYSid can display phase or group delay along with the amplitude response. A delay correction factor is available to compensate for a constant system time delay, such as the propagation time for sound to travel from a speaker to a microphone.

    Distortion

    Normalized Transfer Function
    Transfer function plots can be normalized by a previously stored response file, to subtract the response of the system, leaving only the response of the the device under test, or to convert a mic's output to a precise sound pressure level. The name of the file used for normalization is displayed on the screen.

    Distortion Measurements
    SYSid provides automated measurement and display of intermodulation and harmonic distortion without additional equipment. Three distortion curves (second harmonic, third harmonic and THD plus noise) are shown for both distortion vs. frequency and distortion vs. level at a user selected frequency. An output vs. input amplitude linearity/monotonicity curve is also plotted, which is used to test the compression (non-linearity) of a system at a variety of levels.

    Sub-Octave Analysis
    SYSid can plot the spectral energy in bands from one octave down to 1/12 octave wide. The reverse time energy curve (waterfall) may also be displayed in octave or third-octave bands.

    Narrowband Analysis
    SYSid permits pure tone (sinusoidal) stimulus at a user-selectable frequency and amplitude for analyzing a system's response to pure tones. Narrowband analysis is useful in many measurement situations ranging from evaluating analog-to-digital and digital-to-analog systems to nonlinear system characterization.

    Waterfall Plots
    SYSID can display the amplitude vs. frequency vs. time characteristics of the system under measurement as a three-dimensional 'waterfall' display. The user can specify the width of the time window used. The reverse energy time curve can also be displayed as a waterfall in continuous frequency, octave or third-octave bands.

    Flexible Graphic Display
    SYSid presents the results of all measurement functions as a graphic display. All pertinent measurement parameters (e.g., stimulus level, maximum analysis frequency, input mode, FFT length, and more) as well as the time and date, are automatically logged on the top two lines of the screen. User comments can also be added to the display. The display options include:

    • Log or linear frequency axis
    • Linear or dB amplitude axis
    • Log or linear time axis
    • Horizontal (frequency or time) limits
    • Vertical (amplitude or phase) limits

    Hard Copy
    A resident utility, SNAP, is included with SYSid to dump the system's results to an HP inkjet printer.

    How Sysid Works
    Accurate in Five Ways
    Sysid Measurements