Amplitude modulated data system
An amplitude-modulated data system ( AMDS ) is the term used to describe the process of transmitting additional signals that are imperceptible to the listener in addition to the radio program with the aid of amplitude-modulated transmitters in the long, medium and short wave range . To achieve this, the carrier of the transmitter is phase modulated . Since the radio program is transmitted by amplitude modulation in these frequency ranges , the AMDS signal cannot be heard when receiving with a normal radio receiver. Because of the limited bandwidth available (9 or 10 kHz), the transmission rate with AMDS is low (maximum 200 baud ).
In contrast to the RDS method in the VHF system, there is no uniform standard for the information to be transmitted, and devices that enable the decoding of AMDS information are usually not freely available in stores.
Typical information transmitted with AMDS are correction signals for GPS ( DGPS ). Such signals were transmitted, for example, until the transmitter was switched off via the Donebach long-wave transmitter operated by Deutschlandfunk on 153 kHz.
Time information is also occasionally transmitted through AMDS. For example, the long-wave transmitter of France Inter on 162 kHz and the medium-wave transmitter of Milan of the RAI on 900 kHz transmit a time signal via AMDS. The long-wave transmitter Zehlendorf , which broadcast the program of the voice of the GDR (or the German station) in GDR times and last broadcast the program of Deutschlandradio Kultur , was used in the GDR to distribute press releases for the merchant navy .
The AMDS stations will be switched off with the introduction of digital broadcasting. Internet-based processes ( Ntrip ) are offered as the successor to the AMDS / dGPS system .