FM radio

Under FM stereo means the analog transmission method for stereophonic signals for transmission over frequency-modulated transmitter as they come on the FM band ( radio) used. The method is described in ITU Recommendation BS.450 (English Transmission standards for FM sound broadcasting at VHF ) set. The Stereotonübertragung for analog terrestrial television for bilingual and AM stereo is different from the one explained here FM Strereo for FM radio.

General

One of the main requirements for the FM stereo system was a full compatibility with mono devices, which is why the most obvious way to use a transmitter for each audio channel was eliminated.

Instead, the pilot tone - division multiplexing ( FM MPX) is used. Thus, the baseband signal is composed of three parts:

  • The sum signal from the left and right channels (L R) in the baseband.
  • The difference signal from the left and right channels (LR) according to the method of amplitude modulation with suppressed carrier at a frequency of 38 kHz, which is modulated onto the stations that broadcast.
  • Additionally a 19 kHz signal, called pilot tone is modulated. It is used to detect the recipient of the stereo signal and demodulating the difference signal and is later removed by a notch filter. Some devices offer these filters in addition explicitly as so-called MPX filter Can be selected on.

This baseband signal as the sum is transferred directly by means of frequency modulation in the FM band.

The frequency of the pilot tone is just the half of the frequency of the suppressed carrier for the differential signal:

The pilot tone at 19 kHz is sent as a substitute for the suppressed 38 kHz carrier. It requires only max. Would be 10 % of the total modulation swing of the FM transmitter and so much less than a direct transfer of the carrier with 38 kHz is required. This increases the cost of encoding and decoding, but the only way to get a good signal - to-noise ratio of the LF signal ( on a " mono " switched receiver achieves almost the same signal quality as an equally powerful mono channels).

On the receiver side the support obtained in the stereo decoder to recover the difference signals of left and right channels by frequency doubling. For a good stereo separation lowest possible phase error between the pilot tone and 38 - kHz carrier in the decoder must be adhered to.

With the help of summing the stereo decoder, the signals for the left and right channels are formed after the extraction of the sum and difference signal:

A mono receiver will output all signals again; The difference signal is inaudible because of its high frequency and also the pilot tone is not perceived by most people because of the frequency near the threshold of hearing and its low noise level.

With the ARI system, or its successor system, the Radio Data System (RDS ), another subcarrier at 57 kHz was introduced, which is used for the transmission of non-acoustic, digital information data with low bandwidth as the station name. Depending on the radio station and broadcast region located above the RDS carrier or additional other services such as in North America Direct band subcarrier of 67.65 kHz with a bandwidth of 9 kHz. Direct band is used for the digital broadcasting of mobile data by software companies such as Microsoft.

History

In Germany, already the Reichs- Rundfunk -Gesellschaft ( RRG ) had tentatively involved in the production of stereo broadcasts. Continuing this tradition of Sender Freies Berlin this topic took place in the mid -1950s re-embark and radiated from the December 26, 1958 tentatively from stereo broadcasts. In the first experiments, the specific for the two speakers of the radio listener modulations were aired yet two separate FM transmitter and received with two normal receivers. The tests were very successful at the Berlin listeners. But it was clear from the outset about the fact that finally only one method out of the question, in which a single VHF channel is used, the carrier frequency is modulated twice.

For this double modulation, there were a number of proposals of American and European professionals. Stronger towards the end of the 1950s put an effort to create uniform standards for the transmission of stereophonic broadcasts in Europe. The Technical Commission of the European Broadcasting Union ( EBU) established a working group, which theoretically and experimentally studied the participation of experts of broadcasting, postal administrations and the industry companies from a number of European countries, the technical aspects of broadcasting stereophonic and the various proposed methods for their suitability examined. It was primarily two problems: first, to avoid the crosstalk, that is, as low as possible for holding the two modulation interference channels and, secondly, to the mono- compatibility. In the Federal Republic the steps necessary to solve these problems, studies from the Institute of Radio Technology (IRT ) have been carried out in cooperation with the broadcasters, the Federal Postal and industry.

As the best method proved to this day by the U.S. companies General Electric and Zenith Electronics Corporation developed, described in this article above " pilot tone - division multiplexing ", which was finally in 1962 recommended by the EBU as standard and introduced in most countries of Western Europe.

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