TrueRTA's audio spectrum analyzer software shows you a detailed picture of what you're hearing in real-time, that is, as it happens. The audio spectrum analyzer shows the changing spectrum of live input signals up to 48kHz (half the maximum sampling frequency). Spectrum Analyzer Real-time FFT - Eight-channel Audio Spectrum Analyzer is a set of Real-Time Multi-Channel Gauges for investigation of data accepted from any ADC you will want or 16-, 24- and 32-bit ADC of sound card. TrueRTA’s audio spectrum analyzer software designed to work on the Windows platform. The software comes packed with lots of features for the benefit of the users. It features; a high-resolution real-time analyzer, low distortion signal generator, dual trace oscilloscope, digital level meter, and a crest factor meter.

1. Real-time Audio Spectrum Analyzer Software Free
2. Real-time Audio Spectrum Analyzer Software Programs
3. Live Audio Spectrum Analyzer Software
4. Spectrum Analyzer software, free download
5. Real Time Audio Spectrum Analyzer

Note: since this article was published several of the oscilloscopes mentioned have been replaced with upgraded versions. The 16-bit ADC-216 has been replaced by the PicoScope 4262. The PicoScope 3224 and 3424 8-bit scopes have been replaced by a range of models in the PicoScope 3000 Series. The PicoScope 320x Series of budget scopes has been replaced by the PicoScope 2000 series ultra-compact oscilloscopes.

Advantages of Pico products for audio spectrum analysis

Spectrum analyzers tend to fall into two categories: so-called ‘swept’ spectrum analyzers and FFT-based spectrum analyzers. Swept spectrum analyzers work by using one or more notch filters (or mixers) to measure the signal amplitude at a given frequency, and by changing (or sweeping) the frequency of this filter a plot of amplitude against frequency can be constructed. Swept spectrum analyzers still have their place in high-frequency spectrum analysis, but for audio work they have the disadvantage that the signal must be constant for the whole period of the sweep.

FFT-based spectrum analyzers work by digitizing the signal of interest using a analog-to-digital converter (ADC). The stored values are then processed using the Fast Fourier Transform (FFT) algorithm. The advantage of this method is that the spectrum of a one-off or short-duration event can be captured. For example, using PicoScope’s trigger capabilities it is possible to capture the spectrum of a single drumbeat.

Performing spectrum analysis requires a lot of calculations, with some FFT-based spectrum analyzers taking several seconds to update a trace. PicoScope uses an optimized, high-speed routine for spectrum analysis that results in ‘real time’ results. Even on a relatively modest computer such as a 33 MHz 486 PC the spectrum analyzer can still update many times a second.

Pico PC oscilloscopes suitable for audio spectrum analysis

Although most of the PicoScope PC Oscilloscope range can be used for audio spectrum analysis, the higher-resolution devices are most suited. For high-end professional testing the ADC-216 is hard to beat. For general purpose work the PicoScope 3224 and 3424 (USB oscilloscopes) and ADC-212 (parallel port oscilloscopes) are ideal. If cost is an issue, consider the PicoScope 320x series.

The two key specifications for a FFT analyzer are sampling rate and dynamic range. A spectrum analyzer will be able to display up to one half of the maximum sampling rate. To cover the entire 20 kHz audio band this calls for a sampling rate in excess of 40 kS/s. If you are interested in testing the frequency response of amplifiers you may wish to look well beyond the 20 kHz point so a higher sampling rate is required.

The dynamic range of the spectrum analyzer is the next most important consideration. Most oscilloscopes (whether PC-based or benchtop) have an 8-bit resolution (256 steps). This limits spectrum analysis to 48 dB of dynamic range (20 log 256) The PicoScope 320x series are 8-bit devices. Unusually for oscilloscopes, the ADC-212 and, PicoScope 3224 and 3424 are 12 bit devices (4096 steps) which gives a theoretical maximum of 72 dB of dynamic range. The ADC-212 through a combination of oversampling, digital filtering and software averaging can actually improve on this theoretical 72 dB. The ADC-216 with its 16-bit resolution (65536 steps) has close to 100 dB of dynamic range.

To put these figures in context a typical tape deck would have 40 to 50 dB of dynamic range, a quality power amplifier 70 to 80 dB and a top end CD player 80 to 90 dB. As you will see below not all CD players live up to this.

The specifications of these devices are summarized in the table below.

Unit	Resolution	Sampling rate	Spectrum range	Dynamic range
ADC-216	16 bits	333 kS/s	166 kHz	> 95 dB
PicoScope 3224	12 bits	20 MS/s	10 MHz	> 70 dB
PicoScope 3424	12 bits	20 MS/s	10 MHz	> 70 dB
ADC-212/100	12 bits	100 MS/s	50 MHz	> 80 dB
ADC-212/3	12 bits	3 MS/s	1500 kHz	> 80 dB
PicoScope 3206	8 bits	200 MS/s	100 MHz	> 50 dB
PicoScope 3205	8 bits	100 MS/s	50 MHz	> 50 dB
PicoScope 3204	8 bits	50 MS/s	25 MHz	> 50 dB

Using the ADC-216 to test CD players

To show the sort of performance you can expect with the ADC-216 spectrum analyzer we decided to test two CD players. We chose a ‘budget’ portable model and a high-quality unit from Quad. One channel of the ADC-216 was connected directly to the portable CD player. The PicoScope trace below shows a pure 1 kHz tone from a test CD. As expected the result is a sharp peak at 1 kHz. The second, third and fifth harmonics are clearly visible showing distortion caused by the CD player. The peaks around 18 kHz are caused by the switching power supply inside the CD player's mains adaptor. If the CD player is run on batteries this noise disappears.

Real-time Audio Spectrum Analyzer Software Free

Next we repeated the experiment with the Quad CD player. As expected the results were much improved, the 5th harmonic is the most significant, 96 dB down on the main signal. The window showing measurements and harmonics is a separate program that took data from PicoScope (using DDE) to automate audio measurements. This program is no longer required since these measurements along with many others are now built in to the main PicoScope program. You can access the measurements from the settings menu—see help file for further details.

Crosstalk is an important performance indicator that can easily be measured with a spectrum analyzer. We played a 10 kHz sinewave (–10 dB) on the right channel of each CD player in turn (measured with the ADC-216). Ideally no signal would be present in the right channel, on the portable CD player the crosstalk is visible 60 dB down on the signal on the left channel.

On the Quad CD player, the crosstalk is at least 90 dB down.

An ideal CD player should have a flat frequency response over the whole audio spectrum. The specifications of our portable CD player stated a 20 Hz to 20 kHz response within 3 dB. We tested this using a sinewave that sweeps from 0 to 20 kHz. Plotting such a frequency response is not possible with many FFT spectrum analyzers as they take a quick snapshot of the signal then take several seconds processing and displaying the results. The result tends to be that only one frequency peak gets captured during the sweep. PicoScope’s data collection and processing are optimized for speed—even on a relatively slow PC (33 MHz 386) the spectrum analyzer has a near-instantaneous ‘real time’ update rate. The sinewave used for our test takes about 30 seconds to sweep from 20 Hz to 20 kHz. In this time PicoScope performs 100s of FFTs rather than the 2 or 3 that most FFT spectrum analyzers can manage. To display the frequency response as a single line rather than a moving peak, we used PicoScope’s peak detect function as shown below. As you can see the –3 dB point is not the 20 kHz claimed by the data sheet, but is nearer 16 kHz.

When the test was repeated on the Quad CD player, the frequency response was almost flat to 20 kHz. It also exhibits a sharper drop off after 20 kHz.

Several customers have asked for our advice on what type of signal source is best for testing amplifiers. The problem is finding a signal generator or sinewave source with a low enough distortion figure. We have been particularly impressed with the Black Star LDO100 low-distortion oscillator. The trace below shows its output at 1 kHz when plugged directly into an ADC-216.

DL4YHF's Amateur Radio Software: Audio Spectrum Analyzer ('Spectrum Lab') Last updated: October 2020. Current stable version : Spectrum Lab V2.95 b3 (2020-10-04) Most recent 'beta' : Spectrum Lab V2.95 b? (2020-..-..)

Main Site: www.qsl.net/dl4yhf/spectra1.html Download/Backups: dl4yhf.ssl7.com/spectra1.html (gone for good?), dl4yhf.darc.de/spectra1.html ('backup'). ('freenet' site deleted since their hosting service isn't free anymore)

Spectrum Analyzer with Waterfall Display and real-time audio processing

This program started as a simple FFT program running under DOS a long timeago, but it is now a specialized audio analyzer, filter, frequency converter,hum filter, data logger etc (see history). You can download it from this site. Or look into the manual (in HTML format), even though the manual included in the archive will be more up-to-date. Furthermore, the same manual has occasionallybeen converted into a single PDF (SpecLab_Manual.pdf), but any attemptto create a common index and table of contents for this PDF, using OpenOffice (with proper page numbers instead of the hyperlinks)has failed miserably - see note in the preface of the PDF document.
If you are looking for a short description in German language, look here. The revision history is here. How to use SpecLab with SDR-IQ is described here;how to use SpecLab with PERSEUS (without an annoying virtual audio cable)is here,and how to use SpecLab with software-defined radios supporting ExtIO (instead of a soundcard),like FiFi-SDR, RTL-SDR, FunCube, SDRplay RSP, etc etc, is described here.
If you need help getting the program running, or have questions about aparticular function, look into theSpectrum Lab User'sforum at groups.io (not at Yahoo anymore)..

(SpecLab screenshot in 'Colour Direction Finder' mode, VLF spectrum,colour~bearing)

Spectrum Lab runs under Windows 98, 2000, ME, XP (home and professional),Linux/WINE, but obviously not under Windows Vista. The reason why it doesn'twork under 'Vista' is unknown. Since I don't use Vista myself, there's littleI can do about this. Use Linux/WINE, or a virtual PC running XP (inside yourshiny power-hogging Vista machine).

Features

• Input can be taken from the soundcard, but the 'audio utility' interface makes it possible to create an interface to any A/D converter you like (for example a PIC with serial interface). A plugin for Winamp allows to analyse audio streams from the internet (see download section).
  Besides the soundcard and the interfaces supported by the 'audio utilities', SpecLab directly supports SDR-IQ and SDR-14 (by RFSpace, Inc), and PERSEUS (by Microtelecom s.r.l.).
• Demodulated output is usually sent to the soundcard, but can be sent anywhere else (including an MP3- or OggVorbis encoded stream using Winamp/Oddcast and a special Audio-I/O DLL, which acts as an input plugin for Winamp).
• Selectable and adjustable waterfall color palette allows you to change the contrast of the waterfall display during and after receiving spectrum lines ('contrast' & 'brightness' - sliders)
• Frequency range can be adjusted while running, the old part of the waterfall display will automatically be re-drawn without stopping the audio processing.
• Special waterfall mode for 'Radio Direction Finder' with colour-coded azimuth display, based on DF6NM's Wideband Direction Finder with Colour Encoded Spectrogram Display. Now with up to three 'notches' (adjustable nulls in the synthesized antenna pattern) to suppress noise from certain directions.
• Special 'multi-strip' waterfall mode for long term observation of relatively narrow bands
• Reassigned spectrogram for higher time- and frequency resolution, if certain conditions are fulfilled (since 2009-06)
• Support for image-cancelling direct conversion receivers ( quadrature input via stereo soundcard )
• Very sharp, FFT-based audio filters which can also be used to shift and invert frequencies, with auto notch, denoiser, etc. New (2007-02): Write your own plugin for the FFT filter !
• Waterfall display may run from top to bottom, or from right to left (good for HELL modes)
• Calculation of peak levels, peak frequencies, noise levels, effective values, SINAD, etc
• File logging and file analysis with WAVE files ( now with 'Triggered Audio Recorder' + pre-trigger option )
• Periodic or scheduled actions, for example to capture the screen as BMP or JPEG file
• Audio sample rates from 8000 to 192000 samples per second.
  192 kS/second only tested with a few cards, using ASIO drivers and / or standard multimedia driver. With extra hardware (like software-defined receivers with high-speed USB interfaces), even higher sampling rates (and thus bandwidths) are possible.
• 16-bit ADC resolution giving about 90dB of input range (plus additional FFT gain!)
• 24-bit resolution possible with certain cards (tested with Audigy 2 ZS; 2004-05)
• Frequency resolution in the sub-milliHertz range (exceeding the stability of the soundcard's clock generator).
• FFT output is buffered in an array with adjustable size for long-term observations (with 'scroll-back-in-time' while analysis continues). Furthermore, the FFT output can be recorded (exported) in various formats.
• Signal generators with selectable waveform, frequency, and optional modulation, plus noise generator
• Hum filter to remove 50 Hz (or 60 Hz) plus harmonics, based on Paul Nicholson's algorithm for a multi-stage comb filter with automatic tracking. If you are interested in improving your Natural Radio reception, read Paul's Notes on Domestic VLF Reception.
• Frequency converter to shift low 'IF'-frequencies (e.g. 17.2 kHz) to audible tones (650 Hz, etc) in real-time. Can be used in a 'soft-VLF-RX'. Other demodulators in DSP-blackbox.
• Decoder for some time-code transmitters: MSF(60kHz), HBG(75kHz), DCF77 (77.5kHz) can now be used to set your PC clock to a high accuracy. All you need is your longwave receiver and the soundcard.
• Modulator and decoder for some 'experimental' digital communication modes like PSK31, BPSK, QPSK, FSK, multi-tone HELL, MSK (minimum shift keying since 2004-12), transmission and reception of letters with a small 'terminal' window.
  A DLL interface is available to use Spectrum Lab for new digital transmission modes (but not in use yet).
• Fast waterfall with up to 200 waterfall lines per second, making it possible to decode even fast Morse code by eye (though it wasn't intended for that, it's fun to watch)
• versatile export function (text-file based, for post-processing with Excel etc.). Brian, CT1DRP has used the export functions to create spectrograms and signal graphs of the 136kHz band.
• Built-in HTTP server so you can write a (simple) user interface for SpecLab in HTML (intended to be used in a LAN, not as a fully-fledged web server).
• Communication with other applications through a simple message-based system, which is also used by SpecLab to communicate with the RDF calculator (range-and-bearing calculator + map plotter, link may work on the main site only).

(screenshot of 'Components' window from an older version)

Features which this analyzer does not have (but others do...):

• Due to its 'Laboratory' nature, this analyzer is not as easy to use as ARGO (by I2PHD and IK2CZL, available on Alberto's homepage: www.weaksignals.com ).
  

Some Applications

AMSAT-DL's Earth-Venus-Earth experiment on 2.4 GHz at the Bochum radio telescope

In March 2009, a group of radio amateurs successfully bounced a radio signaloff Venus, over a distance of (2*) 45 million kilometers. A short descriptionof the reception technique (software) can be foundhere; more details are on theAMSAT-DL website, and in the AMSATjournal soon. Thanks to DD5ER, DJ1CR, DJ4ZC, DH2VA, DK8CI, DL1YDD, G3RUH,ON6UG, and everyone else who contributed, for being part of the team.
The configuration used for the (2.4 GHz) EVE test is part of the installationpackage now (EVE-SDR-IQ-5kHz_2G4.usr); we used the SDR-IQ for this. If youwant to try something similar and have a PERSEUS receiver, useEVE-Perseus_2G4.usr instead.

Beacon Logger for DI2AG (July 2005)

How to use Spectrum Lab to produce field strength plots of DI2AG, an experimentalmedium wave beacon on 440 kHz in southern Germany. It realizes some ideaswe spoke about at the Ham Radio fair in Friedrichshafen (June 2005). Moreinfo in this document (up to now, in german language only) :Beschreibung des Einsatzes von SpectrumLab als Bakenlogger für DI2AG. Update 2007: DI2AG has moved to 505kHz now (which is a new amateur radio band in many countries - with the exceptionof Germany..).

Hellschreiber Modes in Spectrum Lab (Nov. 2004)

There is a special narrow-band transmission mode in Spectrum Lab's 'digimodeterminal' called Chirped Hell, based on an idea by Markus, DF6NM. We usedit on the amateur longwave band (136 kHz) to make some narrow band transmission.A special property of Markus' Chirped Hell principle is the relatively lowcrest factor, so it can be used to transmit characters (and even small images)quite effectively, but (unlike sequential multi-tone hell) you need a lineartransmitter. An image received on a waterfall in 'QRSS 3' mode may look likethis:

LowFER Receiver Using a 'Software' IF

This article by LyleKoehler, KØLR, describes a simple 'software defined' receiverand some other (easier-to-use) alternatives to SpecLab too. Don't miss Lyle'sdownconversion circuits which he sucessfully uses to the receive US-AmericanLowFER beacons. The last part describes how to log fieldstrengths of LowFERbeacons with Spectrum Lab's plot window.

G7IZU Radio Reflection Detection Page

This nice website by Andy G7IZUmonitors Meteor Shower, Aurora Sporadic-E and Solar Flares in real time,mostly using reflections of radio signals in the 50 MHz region.

VE2AZX precise frequency measurements

Jacques, VE2AZX, describeshere how SpectrumLab can be used for precise frequency measurements in the millihertz region.Don't miss his detailed description - it's actually better than SpecLab'sbuilt-in help system ;-)

Bat Converter

If you have a suitable soundcard, you can use SpecLab to make ultrasonicbat calls visible and audible in real-time. In the menu, select 'QuickSettings'..'Natural Radio / Animal Voices' .. 'Bat Converter'. This configurationrequires a soundcard with true 96 kHz sampling rate, and a PC with at least1.7 GHz. The software shows the call in a fast spectrogram (with high timeresolution, but little frequency resolution), and converts the ultrasounddown to audio, notches out constant-frequency 'carriers' (like your CRT monitor'sline sync frequency, which is annoying if it's in the 'bat band'), and finallypasses the downconverted and filtered signal through an automatic gain controlstage.

As a test, I used a cheap miniature electret microphone to makethis recording of bat sounds(96 kHz, 16 bit, mono .. right-click to save it). If you play it with a normalaudio player, you will hardly hear anything. But if you play it into SpecLab(menu 'File'..'Audio Files'..'Analyze and Play', with the 'Bat Converter'setting loaded, you will hear the bat calls. I don't know which species itis yet, but they seem to be quite frequent in this part of Germany.

FFT Filter Plugins

The FFT-based filter in Spectrum Lab is already very versatile, but it canstill be extended with a 'filter plugin'. These plugins come in the formof a special windows DLL which can be loaded from the filter control panel.Such plugins can be written with any C / C++ Compiler (recommended: BorlandC++Builder V4 or DevCpp V4.9.9.2; the latter is a free development systembased on the GNU / MinGW compiler). To develop your own plugin, downloadthis FFTFilter Plugin package - it contains all required information requiredto write a filter plugin, and a sample plugin written in the 'C' programminglanguage. NOTE: THE DLL INTERFACE IS STILL 'SUBJECT TO CHANGE' !

Download Spectrum Lab..

To check if you have the latest version of Spectrum Lab, compare the textin the main window's title bar with the revision number and compilation dateat the top of this document. For example, if you have 'Spectrum LaboratoryV2.2 b1', it is way too old. If you have SL already installed, you can checkif it's up-to-date by selecting Help .. Check for Updatein the main menu. The primary download site for Spectrum Lab is here:

• Primary: www.qsl.net/dl4yhf/speclab/install_speclab.zip,
  (V2.95 b3, compiled 2020-10-03 or later, with new output switches, a bugfix in the noiseblanker, Ogg/Vorbis audio file reader and -writer, wave files with 24-bit integer and 32-bit floating point samples, GPS (NMEA) decoder also through the soundcard, input resampler optionally using the GPS PPS output for synchronisation, support for Winrad-compatible ExtIO-DLLs (also for FiFi-SDR), new controls for the filter passbands on the main frequency scale... see revision history.
  The latest additions were:
  • OpenWebRX-alike server for remote live audio and waterfall in a web browser (details here)
  • Extended CAT/CI-V protocol to display broadband spectra from an IC-7300, IC-7610, IC-7851, etc(?)
  • Forwarding of CI-V messages from and to multiple clients (e.g. WSJT-X, RS-BA1) on extra serial ports
  • Timestamped event queue for the Conditional Actions
  • Non-differential encoding for experimental (B)PSK transmissions
  • New sample formats received over the serial port (e.g. from the GPSDO currently under development)
  • Modified the reader for non-compressed, timestamped web streams (and their logfiles; *.dat)
  • Modified triggered audio recorder
  • Modified COM port enumeration because a stupid Bluetooth driver blocked the application for many seconds
  • Spectrum buffer can be retrieved as JSON object via SL's integrated web server
  • Different buffer sizes for audio processing and soundcard I/O
  • Modified Audio-I/O DLL interface (bugfix in the configuration dialog)
  • Support for RTL-SDR (via ExtIO-DLL)
  • Improved speed, at the expense of compatibility with ancient 80486
  • EbNaut-recorder configuration files
  • Compressed Ogg/Vorbis file selectable for recording audio (besides non-compressed Wave audio)
  
  
• Backup: http://dl4yhf.darc.de/speclab/install_speclab.zip.
  
• Installer-less ZIP: www.qsl.net/dl4yhf/speclab/SpectrumLab_without_Installer.zip.
  
• 'BETA': www.qsl.net/dl4yhf/speclab/install_speclab_beta_2020_06_25.zip
  (This is seriously 'under construction', and is really 'only for beta testing' yet..):
  Contains all the above, plus some new (unstable) features which are not required for 'normal' use:
  Automatic closing of audio devices shortly before system suspend ('PC sleeping'), and automatic re-opening after resume ('wakeup from sleep'), and a tiny subset of the OpenWebRX server functionality which can be used to put your IC-7300 or IC-9700 'online'.
  If the beta-installer doesn't exist, there is no 'beta' at the moment, so use the release version above.
  Details about the new installer (with separate 'Program' and 'Data' folders) are here. If the new installer doesn't work on your system (especially Vista / Windows 7), please report your observations on the Spectrum Lab User's group at groups.io - thanks in advance .
  

As an alternative, check the download section onKo Versteeg's website .
Don't trust any other download site - neither 'softpedia', nor 'freedownloadmanager',nor anyone else because you never know what those sites did to/with the files.Some of those sites claim they performed an extra 'virus check', but noneof those site's owners has ever asked me (the author of Spectrum Lab) for permissionto re-distribute or even re-package the installer, so 'by default' I would not trust any of these sites.
The archive includes EXE-file, sample setting files, help system (in HTMLformat) and a few other goodies, but not utilities below.

Some additional utilities which may be useful:

• Winamp output plugin to send audio data directly from Winamp to Spectrum Lab (not via soundcard)
• FFT filter plugins allow you to insert your own subroutines (compiled into a DLL) into the signal path, where signals are processed in the frequency domain .
• Soundcard utility sourcecodes contains an example to send an audio stream to Spectrum Lab, using WM_COPYDATA messages.
  The zipped archive also contains the 'C' headers with structure definitions for interaction between SL and other applications (audiomsg.h) .

If you want to write your own program to control Spectrum Lab, readthis document which explains howto communicate with it using simple WM_COPYDATA messages (the same datastructures are used to exchange audio via UDP or TCP, too).

Download modified 'htdocs' filesto use Spectrum Lab like an OpenWebRX server

You don't need this if you don't intend to use SL as 'Web Receiver' (server) !

Since 2020-06, the HTTP server integrated in Spectrum Lab can optionally'host' a modified variant of HA7ILM's Open Web Receiver,as described in the Spectrum Lab manual.Because the OpenWebRX 'htdocs' folder contains a large bunch of Javascriptmodules which are not required for normal use, these files are notcontained in the Spectrum Lab installer (downloadable from a link in theprevious chapter). Instead, if you want to use Spectrum Lab like anOpenWebRX server, get those slightly modified files from here:
www.qsl.net/dl4yhf/speclab/OpenWebRX_for_SpectrumLab.zip
In addition to Spectrum Lab itself, the above files (based on the originalOpenWebRX 'htdocs' folder) are all you need to get this running. Just unpack these files into a folder of your choice (e.g. 'C:OpenWebRx'),and let Spectrum Lab's HTTP server know where those files are asdescibed here. There are no dependencies to install, you neither need Python nor CSDR, since the entire HTTP server is integrated as a C/C++ module in Spectrum Lab.The demodulated audio and waterfall is simply taken from SL itself (or from an IC-7300 / IC-9700's 'Spectrum Scope' display). Thus unlike a multi-channel Kiwi SDR, all clients connected to our server will actually see and hear the same, and only one of the userscan control the receiver - if you allow him to. But it's good fun if, during a QSO, you can give the fellow ham the URL (or IP address) of a website where he can listento his signal in your receiver, or see how crowdedthe band is on

Real-time Audio Spectrum Analyzer Software Programs

your end (instead of using one of thehundreds of WebSDRs and KiwiSDRs around the globe).
The original OpenWebRX itself was available (*) on Github, see

Live Audio Spectrum Analyzer Software

github.com/simonyiszk/openwebrx

Spectrum Analyzer software, free download

Also don't miss Andras' BSc thesis about OpenWebRX,which (in 2019) could be downloaded from
sdr.hu/openwebrx

Real Time Audio Spectrum Analyzer

Note:

The Github repository has been archived by its owner (Andras, HA7ILM), since the original OpenWebRX development has been discontinued in 2019-12-29. There may be forks from the original project still actively developed (the Kiwi Web SDR is also based on it), but I didn't check if those forked HTML- and Javascript modules can also be hosted by Spectrum Lab.
Since the original OpenWebRX was published by HA7ILM under Affero GPL v3 license, the modified files are distributed here under the same license. Unlike the original OpenWebRX license, there is no commercial license available for it (neither for the modified OpenWebRX files nor for Spectrum Lab).