NMT (Nordisk MobilTelefon) was an automatic, analog cellular telephone system, primarily designed by Östen Mäkitalo. In Sweden, the system existed on two bands;

NMT 450 (Forward: 463-467.5 MHz, Reverse: 453-457.5 MHz), in operation between late 1981 and January 1, 2008.

NMT 900 (Forward: 935-960 MHz, Reverse: 890-915 MHz), in operation between 1986 and 2000.

This system used FFSK signaling at 1200 bps which had a very characteristic sound. Anyone who has ever used an NMT phone or had a conversation with an NMT subscriber will have heard one part of the signalling: the FFSK chirp which occured at regular intervals during a conversation, and also during handover.

Those of us with scanners have probably heard more components of the signaling. Specifically: control channel, call setup and clear down, and ringing.

The system also had certain peculiarities. For example, sometimes when calling a NMT unit, the system would leak static and/or FFSK signaling back to the caller before ringing began. And also, when an NMT unit hangs up, the base station would sometimes fail to mute outgoing audio from the NMT subscriber which at that point no longer was transmitting, letting the caller, again, hear static interspersed with bursts of FFSK.

In Sweden, NMT 450 was operated by the incumbent telco Telia. The service was scheduled to be decommisioned on December 31, 2007 (In reality the shutdown occured about 24 hours later). Knowing the system was going to be shut down, and because I wanted to preserve as much of these sounds as possible, I began recording as much as I could, at about 1-2 months prior to the shutdown date.

The work consisted of recording the forward frequency range using my scanner (an Icom IC-R10, with the recordings being made through a discriminator tap output modification for maximum clarity), and also making numerous calls to NMT units (010-2, 010-6 prefix) from my home phone line. (Served from an Ericsson AXE10 switch; recording were made using an Edirol R-09 recorder connected to a homemade telephone audio interface).

The purpose of these recordings is historical preservation. Much of my inspiration to do this came from Evan Doorbell, who worked tirelessly to record and preserve the sounds of the North American telephone network of the 70's. If you have the slightest interest in telecommunications history and/or the subject of "phreaking", I highly recommend his presentations. You can find them on the Phone Trips page. I recommend getting the mp3's from ftp://ftp.wideweb.com/GroupBell

Update, May 2018: The Phone Trips website isn't really being updated anymore. Evan Doorbells presentations are now primarily located at http://www.evan-doorbell.com/, he has also posted most of his presentations on Sound Cloud. Also, check out his Twitter account at https://twitter.com/evandoorbell

My recordings are being presented in an uncompressed form. I decided against using lossy compression, such as mp3, because it would mangle the high frequency components of some of these recordings.

I would like to thank olle at toolcrypt.org for extending the Tronquito application with the ability to decode NMT signalling. I have included the output from his Tronquito branch (snabel) with each of the recordings where it's applicable.

Scanner recordings

Telephone recordings

Additional recordings

NMT specifications

Programs

Pictures

Image	Description
	Östen Mäkitalo plaque located outside a lecture hall at the Kista campus of the Royal Institute of Technology (KTH), Sweden.
	Östen Mäkitalo plaque located near the entrance of the Kista campus of the Royal Institute of Technology (KTH), Sweden.

Last updated: September 15, 2021.