(spectrum of a burst from DCF39)
Overview of this chapter
See also: Spectrum Lab's main index, display settings, Controls on the left side and on the bottom of the main window (separate documents).
This graph shows the spectrum as a line plot. One axis of this graph is the frequency domain (with an optional display offset), the other is the amplitude (linear or logarithmic, depending on the current FFT output type).
(spectrum of a burst from DCF39)
With logarithmic FFT output, the amplitude scale is in decibels (about 90 decibels maximum). The 0-dB-point can be set anywhere from the display configuration dialog (or by command).
The relation between input voltage into the A/D-converter and the FFT output value in decibel (dB) is explained here (use your browser's "back" button to return..)
The displayed amplitude range can be modified in the
setup dialog.
As an overlay for the spectrum graph, a reference
curve can be displayed, a
peak-holding curve (which shows
the largest peaks from the previous XX seconds, as configured in the setup
dialog), and an average spectrum
curve (which shows the average over a selectable part of the
spectrogram).
The display colours -also the "pens" used for various curves- can also be
modified in the setup dialog.
You may right-click into the spectrum graph to open a popup-menu which allows you to:
The update-rate of this display depends on the waterfall scroll rate.
The displayed frequency range may be modified with the Time Axis panel or by pulling the (yellow or orange) frequency scale with the mouse. Hold the left button pressed and move the mouse left/right (or up/down) while the mouse cursor is over the yellow frequency axis.
There may be some programmable markers visible on the frequency scale, some of them can be moved with the mouse while others are just 'indicators' (for example: the frequency of the LO ("VFO") can be tied to one of these markers).
Small green and green circles in the graph area indicate the
data-readout-cursor in peak-detecting mode, small
red and green crosses are the readout-cursors in normal (non-peak-detecting)
mode.
See also:
This moving bitmap shows the "history" of the last recorded spectra. As time proceeds, old samples will be scrolled out of view; but they can be scrolled back with the time slider on the "Time" panel (in the upper left corner of the main window).
(stereo spectrogram with amplitude bar)
The intensity (amplitude) of a particular frequency affects the color of a pixel in this bitmap. The relation between amplitude and color can be controlled by a contrast- and brightness control panel on the left side of the main window. Additionally, you can turn on the "visual AGC" function to let the program adjust the brightness value automatically, when the noise level (in the displayed frequency range) changes.
In it's original form (with the "water" falling down), the X-coordinate of a pixel is derived from the frequency-axis, and the Y-coordinate is the time axis (depending on the "scrolling speed" which is adjustable under the display settings).
The visible frequency range can be modified by moving the frequency scale with the mouse (here: orange colored).
If the waterfall runs in RDF mode (radio direction finder), the colour of the waterfall shows the angle of arrival, while the brightness shows the signal strength. More on that in this separate document.
For long-term observations of "narrow bands", the spectrogram screen can optionally be split into multiple "strips", which will be vertically or horizontally stacked (depending on the orientation of the frequency scale: if it runs vertically, small spectrogram strips will be stacked vertically, with the newest strip on the top and the oldest on the bottom). The height of the strip is configured in the display settings. The number of strips is only limited by the screen size. Note: The multi-strip waterfall will not be redrawn completely if contrast or brightness are modified.
Like in many other components of the program, you can activate a context-specific popup menu by right-clicking into the display. Some options for the waterfall are:
If the main window shows a combination of the Spectrum Graph and the waterfall, both are separated by the scrollable frequency scale (which applies to both).
Notes:
The program can periodically save the contents of the waterfall. See periodic and scheduled actions. Experienced users can control the waterfall display via interpreter commands (even from other applications).
Normally, the colour palette of the spectrogram is only controlled through the contrast- and brightness slider on the left side of the main window. But optionally, you can turn on the "visible AGC" on the second tab of the Spectrum Display settings.
Technically, the term AGC = automatic gain control is a bit misleading. In fact, the "visual AGC" works as follows:
When painting a new line into the waterfall, the program first measures the noise level within the displayed frequency range, as explained here. Then, it subtracts this value (actual noise level in dB) from the reference value, which can be set in the display settings window (typically -100 dB). Next, this difference (deviation) is passed through a simple low pass filter, depending on the visual AGC speed (slow/normal/fast). When painting the waterfall, the low-pass filtered deviation is added to all FFT bins, before converting them into a colour values.
The "visual AGC" avoids that the spectrogram image will not get too dark or too bright if the input signal level changes. This happens, for example, if the spectrogram shows a shortwave radio signal, and the path loss changes dramatically, or (for some reason) the local noise level rises.
One of the downsides of the visual AGC is, you cannot tell the signal strength
(voltage, power, or whatever) from the colour in the spectrogram display.
You may be fooled by the AGC, when a narrow-band signal "disappears", which
in fact is just overwhelmed by broadband noise. Without the AGC, you would
have seen that the noise went up. So be sure to turn on this AGC only
if you really need to.
An optional amplitude bar can be displayed on the side of the waterfall. It can show the "peak" value of the input signal in the time domain. Unlike the waterfall display, the amplitude bar is not limited to a certain frequency range. It often looks like a seismogram (and in fact, has been used as such already):
The background of the amplitude bar is blue. The amplitude from the first input channel adds green colour, the amplitude from the second channel adds red colour (if the analyser is configured for dual channel mode, of course). So if the bars from both channels overlap, the result is white.
Other data can be plotted in the amplitude bar, too. For example, the red curve in the screenshot above shows the current noise level. Their pen colour, content, and scaling range are all defined in the "watch window" (where they can also be plotted in a separate window). Do define which of the watch-window's data channnels shall be plotted into the amplitude bar, open the display configuration screen.
The 3D spectrum adds another dimension (the time) to the display, but is
often less easy to read than the waterfall display.
To switch the main display window to "3D Spectrum", open the spectrum display dialog (from the main menu: "Options".."Display Settings"), then select "3D Spectrum" in the combo list labelled "Show".
It is important to select a "strong" colour palette, with as many colour transitions as possible, and to adjust the displayed amplitude range carefully. Without a suitable colour palette, you will hardly see anything on the 3D spectrum. The colour palette is controlled the same way as for the spectrogram.
Furthermore, it helps to turn on the option "Amplitude Grid" in the display settings for a better readability of the amplitudes. But still, in the authors opinion, a 3D spectrum display is not particularly suited to read the amplitudes accurately. But it can help to see the amplitudes (y axis) versus frequency (x axis) and time (z axis, here: pointing from the foreground into the background).
Special options (which only apply to the 3D spectrum) are on an extra tab in the configuration window. Besides that, the following options also affect the 3D spectrum display:
Under certain conditions, reassigned spectrograms provide more resolution along the frequcency- and the time-axis than the classic spectrogram shown above. For comparison, a zoomed conventional spectrogram (1st) and a time/frequency reassigned spectrogram (2nd) with the same FFT parameters :
More about reassigned spectrograms in a separate document; a few configurations with reassigned spectrograms can be recalled from the Quick Settings menu.
The correlogram is a rarely used function. It is a special graphic representation of the 'similarity' of two signals, or the 'randomness' in a signal.
More specific, it can be used as a cross-correlation plot (if the two inputs of the analyser are connected to different signal sources), or an auto-correlation plot (if both inputs are connected to the same source. The correlator itself doesn't care about that).
The graphic shows a plot of the correlations r(h) versus h (the time lags). In Spectrum Lab, the correlogram uses the same fourier-transformed sample blocks used for the "normal" spectrogram. In fact, the correlogram runs side-by-side with the main spectrum display (spectrum graph and/or spectrogram, as explained in an earlier chapter).
For that reason, the range of time lags delivered by the correlator depends on the FFT size of the main spectrum display.
Example: A soundcard delivering 11025 samples/second, feeding a 65536 point FFT, will fill a buffer with 65536 points (in the time domain) in 5.94 seconds. The maximum displayable time lag range will be +/- 2.97 seconds then, with lag zero in the middle. The following screenshot shows the spectra and correlogram of a strong noise signal with weak sine wave added. A 0.5 second delay line (using SL's test circuit) was added between the signal generator and channel #1 of the analyser. Channel #2 was directly fed with the test signal (no delay).
The displayed lag range can be modified by pulling the scale with the mouse (the same way as with the frequency scale in the other display modes).
The output of the correlator is *not* normalized to the average input amplitude. Instead, the following scaling and sign convention was chosen (quite arbitrarily):
Due to the FFT windowing, the coefficients at the edges of the window (extreme lags) may be attenuated. This may be compensated in a future version of SL, if required for some application (it can be avoided by using a rectangular FFT window ). At the moment (January 2009), the correlator / correlogram is so rarely used that putting more effort in it doesn't seem justified.
The visible frequency scale is located between spectrum graph and waterfall (if both are visible). It is usually horizontal ("X-axis") and shows only a part of the processed audio spectrum (which is defined by the FFT settings). You can add or subtract a user-defined offset if you want. The frequency scale may look like this:
The colored rhombic symbols on the frequency scale are markers (they may be simple indicators but also versatile control elements). The markers can be controlled via interpreter commands in the frequency marker table, which you can activate by double-clicking on a marker. You can move a marker by holding the left mouse button pressed. For example, a frequency marker can be connected to a signal generator's frequency, to the local oscillator of the audio frequency converter, to the AFC center frequency of the digimode decoder ,etc.
To show certain 'frequencies of interest' without using one of the programmable markers, a 'Radio Station' Frequency List can be loaded into Spectrum Lab. Frequencies of radio stations appear as thin coloured lines in the main frequency scale and in the spectrum graph.
(VLF spectrum/spectrogram with 'Radio Station' display)
Clicking into the frequency scale (on a particular 'frequency of interest', not on one of the frequency markers) with the right mouse button, opens the frequency scale's popup menu:
The menu contains some frequently used functions. Most of the entries should speak for themselves, some are explained in the next chapters.
The displayed frequency range can be modified by pulling the visible frequency scale with the mouse (left button pressed). Or right-click into the interesting part of the frequency scale and select "Zoom In" or "Zoom Out" in the popup menu.
Alternatively you can enter the "edge frequencies" (Min + Max) in the frequency control panel on the left side of the main window:
(frequency scale
control panel)
More information about the frequency control panel is here.
Splitting the frequency scale into two ranges
Can be activated from the display settings dialog or from a popup menu which opens when you click the frequency scale with the right mouse button. Use it, for example, if you want to have an "audio band overwiew" on the left side of the screen and a zoomed display of a certain frequency range on the right side. The separator between both scale sections can be moved with the mouse (not necessarily in the center of the screen !).
To modify the frequency range of a frequency scale section, first click into the section on the visible frequency scale. The frequency scale control panel will then show "Freq1" or "Freq2" instead of "Freq", and the edit fields will apply to one section only.
On the frequency control panel, you can enter a frequency offset which will be added to the displayed frequency. This does not affect the internal processing, it is just for the "optics".
If you want to SUBTRACT the displayed frequency from a certain value (for example because you have an LSB receiver tuned to 138kHz), enter the value "-138k" in this field. After three seconds, the entered value will become effective (the entered value will be normalized), and the frequency scale will be updated with the new settings. (Note: for LSB receivers, you should additionally activate the "LSB mirror" in the settings menu).
No matter if your soundcard has one or two input channels, the main frequency analyser can be switched to "two-channel" mode. Both channels can be tapped to different points in the test circuit, for example channel #1 can be connected to the left audio input, and channel #2 to the right audio input. Or, channel #1 may be connected to the "input signal" (before the DSP chain) and #2 to the "output signal" (which goes to the D/A converter).
The channel selection can be modified in the
circuit/component window, which can be opened
from the "View/Windows" menu.
If two input channels are active for the spectrum analyser (showing two different signals on one screen), the frequency scale will be split into two sections automatically.
For some special "weak signal" applications, Spectrum Lab offers different ways and stages of averaging. Averaging can be superior to simply increasing the FFT size, if the observed signal is "broad" (incoherent; phase-, frequency- or amplitude modulated, etc).
There are various stages of (spectrum-) averaging, which will be explained in some depth later:
water.avrg_max returns the number of FFTs added in each line
of the waterfall (you can use this function to calculate the "gain" from
this incoherent averaging, as explained somewhere below).
This is a simple "second" spectrum analyser, not as versatile as the display in the main window. You can use it to display another portion of the spectrum, or analyze another audio source if you have a stereo soundcard (or 2-channel ADC).
To open a window the second spectrogram, enter the "View/Windows" menu of the main window, then select "Second Spectrogram". To activate the spectrum analyser for the second window, and select the source, use the "Mode" menu of the second spectrogram window.
Notes:
Last modified: 2008-11-30 (updated a few screenshots)