View Parameters for Amplitude Spectrum Display
These view parameters determine how the collected data are analyzed and displayed.
Frequency Range (F)
17 options are available for Frequency Range (F). They are: 1 Hz, 2 Hz, 5 Hz, 10 Hz, 20 Hz, 50 Hz, 100 Hz, 200 Hz, 500 Hz, 1 kHz, 2 kHz, 5 kHz, 10 kHz, 20 kHz, 25 kHz, 50 kHz,100 kHz.
This parameter is initially selected by the software such that the selected Frequency Range is able to accommodate from 0 to ½ of the sampling frequency. You can subsequently change the Frequency Range if necessary.
An important principle in digital signal processing is the "Nyquist Sampling Theorem" which states that any signal can be represented if sampled at least twice the rate of the highest frequency of interest. This means that if you wish to measure a 3,000 Hz signal, the sampling rate must be greater than 6,000 Hz, otherwise aliasing will occur. The possible range of sampling rates are limited by the capability of the sound card.
In Amplitude Spectrum Display, the horizontal axis can be displayed in linear, logarithmic, octave, 1/3 octave, 1/6 octave, 1/12 octave, or 1/24 octave scale.
Frequency Multiplier
The Frequency Multiplier is the zooming factor for the horizontal axis. There are 10 options available: ×1, ×2, ×5, ×10, ×20, ×50, ×100, ×200, ×500, ×1000.
When "×1" is selected, the full Frequency Range is displayed over the width of the view.
If you change the Frequency Multiplier to "×N" which is greater than 1, then only 1/N of the full Frequency Range is displayed over the width of the view, with a horizontal scrollbar at the bottom which allows you to scroll over the full range of the Frequency.
Channel A Display Range
In Amplitude Spectrum Display, there are two modes for the vertical axis:
Absolute Mode
The vertical axis is scaled in engineering unit. All data points are plotted based on their absolute values, in V(rms), dBV(rms), dBu(rms), dBSPL, or dBFS. Note that by definition, the reference voltages for dBV(rms) and dBu(rms) are 1 V(rms) and 0.775 V(rms) respectively. 1 dBu(rms) in amplitude spectrum is equivalent to 1 dBm in power spectrum when the load is 600 ohms.
The reference voltages for dBSPL for both channels need to be calibrated. Please refer to the section for Sound Pressure Level Calibration Factor in the previous Chapter.
The reference voltage for dBFS is the full-scale voltage (1/2 Vpp).
You can specify the Display Range for Channel A. Available options are: Off, 100 µV, 200 µV, 500 µV, 1 mV, 2 mV, 5 mV, 10 mV, 20 mV, 50 mV, 100 mV, 200 mV, 500 mV, 1 V, 2 V, 5 V, 10 V, 20 V, 50 V.
When "Off" is selected, the data in Channel A will not be displayed.
Relative Mode
The vertical axis is scaled in relative value, in either linear or dBr (logarithmic) scale. All data points are plotted based on the relative value with regard to the maximum value in the current measurement. In this mode, the vertical axis ranges from 0 to 1 where 1 corresponds to the highest absolute vertical value in the measurement.
When "Off" is selected, the data in Channel A will not be displayed.
Channel A Multiplier
The Multiplier for Channel A is the zooming factor for A axis. There are 5 options available: Off, ×1, ×2, ×5, ×10.
When "Off" is selected, the full Display Range for Channel A is displayed over the height of the View.
When "×1" is selected, initially the full range is displayed over the height of the view with a vertical scroll bar on the left of the view. You can use the scroll bar to move the data curve for Channel A up and down.
If you change the multiplier to "×N" which is greater than 1, then only 1/N of the full range is displayed over the height of the view, with a vertical scrollbar on the left of the view. You can use the scroll bar to scroll over the full Display Range.
Channel B Display Range
Absolute Mode
You can specify the Display range for Channel B. Available options are: Off, 100 µV, 200 µV, 500 µV, 1 mV, 2 mV, 5 mV, 10 mV, 20 mV, 50 mV, 100 mV, 200 mV, 500 mV, 1 V, 2 V, 5 V, 10 V, 20 V, 50 V.
When "Off" is selected, the data in Channel B will not be displayed.
Relative Mode
When "Off" is selected, the data in Channel B will not be displayed.
For single channel measurement, this control will be disabled.
Channel B Multiplier
The Multiplier for Channel B is the zooming factor for B axis. There are 5 options available: Off, ×1, ×2, ×5, ×10.
When "Off" is selected, the full Display Range for Channel B is displayed over the height of the View.
When "×1" is selected, initially the full range is displayed over the height of the view with a vertical scroll bar on the right of the view. You can use the scroll bar to move the data curve for Channel B up and down.
If you change the multiplier to "×N" which is greater than 1, then only 1/N of the full range is displayed over the height of the view, with a vertical scrollbar on the right of the view. You can use the scroll bar to scroll over the full Display Range.
For single channel measurement, this control will be disabled.
FFT Size
This parameter is applicable to all types of views in the Spectrum Analyzer. 16 options are available: 128, 256, 512, 1024, 2048, 4096, 8192, 16384, 32768, 65536, 131072, 262144, 524288, 1048576, 2097152, 4194304.
The selected FFT size directly affects the resolution of the resulting spectra. The number of spectral points is always 1/2 of the selected FFT size plus one. Thus a 1024-point FFT produces 513 spectral points.
The frequency resolution of each spectral point is equal to [Sampling Frequency]/[FFT Size]. For instance, if the FFT size is 1024 and the Sampling Frequency is 44100 Hz, the resolution of each spectral point would be:
44100 / 1024 = 43.07 Hz
Larger FFT size provides higher spectral resolution but take longer time to compute.
If the FFT size is greater than the number of data points per scan (record length), then zeros will be padded at the end of the actual measurement data during FFT computation. It should however be noted that the real frequency resolution is equal to [Sampling Frequency]/[Number of data points] in this case, although the apparent FFT frequency resolution is [Sampling Frequency]/[FFT Size]. In other words, zero padding does not improve the real frequency resolution although it does provide more spectral points via interpolation.
If the FFT size is less than the number of data points per scan (record length), then the measurement data will be split into different segments with the size of each segment equal to the FFT size. The last segment of data will be dropped if its size is not equal to the FFT size. The final result will be obtained by averaging the FFT results from all segments. It should be noted that this approach is used for Amplitude Spectrum, Auto Correlation Function, Cross Correlation Function, Coherence Function, Gain and Phase, and Impulse Response, except Phase Spectrum where only the first segment of data is used. The averaging method here is referred to as Intra-Frame Averaging as compared to the Inter-Frame Averaging set through [Setting]>[Spectrum Analyzer Processing]>[Inter-Frame Processing]>[Averaging].
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