HDSDR Autocorrelation Mode

This page is intended as a guide.  It is a collection of notes and links so users may learn more.

Autocorrelation is a specialised feature that analyses the content of a signal.
The autocorrelation function compares the demodulated audio signal with a delayed version of itself.

The vertical axis shows the correlation coefficient: the values are in the range -1 over 0 to +1: 
0.0 means there is no correlation between the signal and the delayed version of itself.
+1.0 means, that the signal is repeating exactly in the delay.
-1.0 means, that the signal is repeating exactly - but in opposite phase.

The horizontal scale is calibrated in milliseconds (ms), changing "RBW" alters the time resolution. The bands in the waterfall show the effect of this. The Zoom control allows fine adjustment.

Autocorrelation is applied on the demodulated audio (AF) as it is configured, just before you press spectrum. That mean changing Mode, Tune or Lo/HiCut will have an effect on autocorrelation.  http://en.wikipedia.org/wiki/Autocorrelation#Applications LC:- "I think the section Application is easiest to understand"

Digital signals/modems/systems usually have framing, that means some bits are repeated regularly. With autocorrelation one can try to measure this repetition / cycle time. Encrypted signals usually have no framing. Some signals may even have multiple frame / cycle lengths.
Example: if you have a sine with 1 Hz => that means the wave needs one full second for a full wave (phase from 0 to 2 pi). Then, correlation is +1.0 at 1000 ms. And -1.0 at 500 ms.
That is why |A.corr| is for: it just shows the absolute values of autocorrelation: Every negative correlation value is negated to positive.

By measuring out the framing, one can try to identify the digital modem/system. This enables unknown signals to be compared with known  profiles and, maybe, identified.
At a first glance some signals may just look like noise, e.g. many 2400 Bd signals, but close analysis may show a different pattern.
One such use is identifying intruders in the amateur bands, a list here http://www.iarums-r1.org/iarums/latest.pdf
Some examples of HDSDR displays, including amateur SSTV in the PDF linked at the bottom of this page: HDSDR-acf.pdf. Prepared by Alipio who requested this HDSDR feature.

Another additional method for identification of digital modem is measuring the symbol rate. The method is described for PSK signals in section https://sites.google.com/site/g4zfqradio/hdsdr-signal-measurement.

The dated Digital Signals FAQ lists many ACF (AutoCorrelationFunction) values in section 4:
The Original text can be found in the Internet Archive: http://web.archive.org/web/20060407103849/http://wunclub.com/digfaq/signals.html
A PDF version is hosted at Utility DXers Forum: http://www.udxf.nl/Digifaq53.pdf
ATTENTION: The listed ACF values are are in bits, but HDSDR just displays time in milliseconds. When knowing the bit rate of the digital signal one can convert between these different units.

This link may be interesting overview on how to identify signals: http://www.rtl-sdr.com/signal-identification-guide/
It shows many spectrum and waterfall snapshots of several signals. Autocorrelation may help on the same purpose. The autocorrelation may show differences - even when the spectrum looks identical, because the spectrum depends mostly ony the symbol rate of a signal. But autocorrelation digs deeper ..

For this purpose HDSDR does not actually need an SDR, just audio input from a radio to a soundcard. See picture at the bottom. But, of course an SDR may be used!

Below, a sample signal to show the HDSDR display. I think this is Stanag around 4206 kHz .. a zoom in time would help reading 106.6 ms for the first peak. Further peaks at multiples of 106.6 are harmonics:

And, below the zoomed spectrum showing the time more precisely.

There are three autocorrelate modes, select them by clicking "Spectrum" below the AF waterfall. Normally at the smaller display, here I have swapped the display so the signal can be critically examined. 
Modes may also be changed by keyboard shortcuts, Ctrl+S swaps the uper and lower display.

Also used often is "Cross correlation". But  this is not implemented in HDSDR. It would need a second audio signal to be selected to be compared with the received signal.

Cepstrum in a way is similar to autocorrelation. But it is not autocorrelation. It is used for voice analysis.   http://en.wikipedia.org/wiki/Cepstrum#Applications

HDSDR should be configured for your radio. The RF and AF spectrums should look good.* LC

http://www.iarums-r1.org/iarums/sound/main.html IARU Region 1 Monitoring System Soundfiles  

Keyboard Shortcuts. For full list see Help/Update (F1)

Swap position of RF/AF Display Ctrl+S
 Amplitude spectrum (Alt+N)
 AutoCorrelation (Alt+C)
 abs of AutoCorrelation (Alt+A)
 Cepstrum (Alt+V)

* I think the FM CW radars are a good example .. from the intruder list ..

* You should make the RBW much smaller, so that the time scale shows much more time/delays. Especially for radars you will need an RBW smaller than 1 Hz, cause some CW Radars have a sweep time of 1 sec.

This is a good source to sort out modes usingACF.(different lpm means different ACF)

Alan Reeves,
Jul 2, 2013, 3:11 AM