Baseband

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Spectrum of an exemplary baseband signal with bandwidth  B.

In communications engineering , the baseband is the natural frequency range of the useful signal (lower limit frequency equal to or close to 0 Hz).

The occupied frequency range with the value of bandwidth  B expressed. Real baseband signals have both positive and negative spectral components; Therefore, the bandwidth extending symmetrically from -B to  + B . Digital modulation types, on the other hand, are usually processed in the complex baseband, which extends asymmetrically from −B / 2 to  + B / 2 .

In radio technology , the useful signal to be transmitted lies “in the baseband” before the modulation and after the demodulation , so-called baseband position ; other words for it are baseband range, low-pass range or low-pass position. For example, a sound event with 1000 Hz in the baseband corresponds to an electrical signal with a frequency of 1000 Hz.

In the other stages of the transmission (i.e. after the modulation and before the demodulation) the useful signal is not in the baseband position but is modulated onto a carrier ; it is then in the carrier frequency range or in the intermediate frequency range , more generally in a bandpass position. The sound event with 1 kHz just mentioned as an example would consist of the three frequencies 999, 1000 and 1001 kHz in the bandpass position (AM modulation without carrier suppression on a medium wave carrier of 1000 kHz).

Applications

As a concrete example of baseband transmission , analog telephony with a landline connection should be mentioned: In this case, frequencies on the telephone connection in the baseband of 300 Hz to 3400 Hz are transmitted. Analogously to the above example, a certain tone with a frequency of 1000 Hz, for example, is represented in the telephone connection cable as an electrical signal with a frequency of 1000 Hz.

Other applications of baseband transmissions can be found, for example, in the LAN area with Ethernet and Token Ring . Even television signals , which SCART - run or video terminals on the TV are located in the baseband - unlike signals via antenna sockets .

If, for example, multiplexing is used on lines for the simultaneous transmission of several signals, baseband transmission is no longer possible. In the case of frequency division multiplexing, the baseband signal is shifted to a higher frequency range by modulation.

In radio transmissions , it is always necessary to modulate the baseband signal onto a carrier frequency, since the low frequencies of a baseband signal are usually difficult or impossible to transmit by radio. By choosing different carrier frequencies, different transmitters can transmit at the same time without interfering with one another. In the case of pure baseband transmission in a shared transmission channel, there would be massive mutual influences and interference.

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