The Telekom code transmitter 1 still works after 29 years
Found while tidying up the desk of a colleague who retired after more than 25 years at the company: a “Telekom code transmitter 1”
ALT
This device generates the corresponding DTMF tones when the buttons are pressed and outputs them from a speaker on the back. It was placed on the microphone shell of the telephone receiver so that these signals could be transmitted. This was used until the early 2000s to remotely access an answering machine.
The code transmitter has “4.95” on the back as the production date. At that time, pulse dialing using mechanical pulses (from a rotary dial telephone) was still very popular with analogue connections. It was only at the end of the 1990s that all exchanges were converted to support multi-frequency dialing using DTMF tones. DTMF is just the English abbreviation for Dual-Tone Multi-Frequency.
With DTMF, when you press a key, two sine tones from the audible range are played superimposed. This can be transmitted over a telephone line with a limited frequency range.
Until the end of the 1990s, the mobile phone network was not yet widespread, but there was a telephone booth on virtually every corner. Many people had an answering machine at home. Now there was a need to listen to this answering machine from a trip or from the office.
For this purpose, the answering machines could decode DTMF tones and be operated with them. After skipping your own announcement by pressing a button, a four-digit* code was usually requested. Only then did you get to the main menu of the answering machine. Using the number keys, messages could be listened to, skipped back and forth or deleted. Finally, storage space was very limited.
* Under the original post on Mastodon, Stefan Baur comments: “Four digits? Bwahaha. Double digits, only the newer ones had three digits. Four digits at most for software AB (telecom convenience feature or mobile phone voicemail).”
Telephone booths were often still connected via pulse dialing. Therefore, their keypad could not be used to generate the DTMF signals. So such small devices were sold as an additional product. Some were so small that they fit on a key ring.
Today, DTMF is still used for dialing and in hotline customer menus. The code transmitter can still be used 29 years later. It still works perfectly. We no longer have any use for it in the office.
However, it would breathe new life into my FeTAp 611 rotary dial telephone (in orange) at home, because the FritzBox has no longer supported pulse dialing for a few years. So I can only accept calls, but I can’t use them to make calls.
(Lars, first published here: