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How to modify your scanner or receiver with a discriminator output

Why would you need a discriminator output?

If you want to monitor digital modes that use FSK or PSK (e.g. POCSAG, FLEX, ERMES and AIS), you get the best results if you use the unfiltered audio from your receiver. You can find that signal at the discriminator of your receiver. A discriminator is the 'heart' of an FM receiver. After the discriminator, audio filters and amplifiers follow that may heavily distort a digital signal. If you want to decode and monitor digital (FSK and PSK) systems seriously, a discriminator tap is an absolute prerequisite.

Only a few scanners have a discriminator output as a standard feature. In most cases, you need to 'operate' on your scanner to make the discriminator signal available to the outside world. On this site, you can find pictures and descriptions of over 200 scanners, receivers, pagers and (marine) VHF transceivers that I modified with a discriminator tap.


Several paging standards exist. The most common ones are:
telecom tower
  • POCSAG. Developed in the late 1970s in the UK, but adopted as a worldwide paging standard: CCIR Radio Paging Code No. 1
  • FLEX. Developed by Motorola, used in many networks worldwide
  • ERMES. European paging standard, developed in the 1990s. Never became a commercial success; implemented in only a few European countries
On this page, you can find more information how you can decode and monitor paging signals.


AIS (Automatic Identification System) is a shipboard broadcast system using GMSK. It operates in the VHF maritime band. With the right software, you can plot the positions of ships received on a map.

On this page, you'll find more information how to decode and monitor AIS signals.
nautical map

Other Systems

Other systems that benefit from the undistorted demodulated audio from the discriminator output are e.g.


Good luck modifying your scanner with a discriminator tap. Be careful. A scanner contains sensitive components that are easily damaged. Let someone else make the modification if you don't have sufficient soldering experience. If you can't find someone qualified in your area and you live in the European Union, I can make the modification for you. See the Order page for more information.

Should you have suggestions or comments, feel free to contact me.

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