Baseband

From Academic Kids

All signals are comprised of a whole range of different frequencies added up together. In telecommunications in particular, it is often the case that those parts of the signal which are at low frequencies are 'copied' up to higher frequencies for transmission purposes, since there are few communications media that will pass low frequencies without distortion. Then, the original, low frequency components, are referred to as the baseband signal. Typically, the new, high frequency copy is referred to as the 'RF' (radio frequency) signal, although it need not be at the particular frequencies set aside for radio.

More precisely, a signal "at baseband" is usually considered to comprise all relevant frequencies from 0Hz up to the highest frequency component in the signal with significant power. The signal is then usually modulated in order that it may be transmitted. Modulation results in shifting the signal up to much higher (RF) frequencies than it originally spanned. A key consequence of this process is that, usually, the range of frequencies the signal spans (its spectral bandwidth) is doubled. Thus, the RF bandwidth of a signal is usually twice its baseband bandwidth. Steps may be taken to reduce this effect, such as filtering the RF signal prior to transmission. The figure shows what happens:

Missing image
Baseband_to_RF.png
Comparison of the baseband version of a signal and its RF version, showing the typical doubling of the occupied bandwidth.

The simplest definition is that a signal's baseband bandwidth is its bandwidth before modulation and multiplexing, or after demultiplexing and demodulation.

The composite video signal created by devices such as most newer VCRs, game consoles and DVD players is a commonly used baseband signal.

Not to be confused with

  • Broadband - generally refers to transmission of data over numerous frequencies
  • Wideband - a communications medium or signal that spans a large (continuous) range of frequencies, or is wide compared to something else
  • Narrowband - the opposite of wideband.

See also

fr:Bande de base ja:ベースバンド

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