BASEBAND

Baseband is an adjective that describes signals and systems whose range of frequencies is measured from 0 to a maximum bandwidth or highest signal frequency; it is sometimes used as a noun for a band of frequencies starting at 0.

• A baseband bandwidth is equal to a highest frequency of a signal or system, or an upper bound on such frequencies. By contrast, a non-baseband bandwidth is the difference between a highest frequency and a nonzero lowest frequency.
• A baseband signal is a signal that can includes frequencies that are very near zero, by comparison with its highest frequency (for example, a sound waveform is a baseband signal, whereas a radio signal is not).
• A baseband channel (or system, or network) is a channel (e.g. a telecommunications system) that handles baseband signals directly.
• A signal "at baseband" is usually considered to include frequencies from near 0 Hz up to the highest frequency in the signal with significant power.

In general, signals can be described as including a whole range of different frequencies added 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.

The concept of baseband signals is most often applied to real-valued signals, and systems that handle real-value signals. Fourier analysis of such signals includes a negative-frequency band, but the negative-frequency information is just a mirror of the positive-frequency information, not new information. For complex-valued signals, on the other hand, the negative frequencies carry new information. In that case, the full two-sided bandwidth is generally quoted, rather than just the half measured from zero; the concept of baseband can be applied by treating the real and imaginary parts of the complex-valued signal as two different real signals.

A signal at baseband is often 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 the usual double-sideband amplitude modulation (AM) is that, usually, the range of frequencies the signal spans (its spectral bandwidth) is doubled. Thus, the RF bandwidth of a signal (measured from the lowest frequency as opposed to 0 Hz) is usually twice its baseband bandwidth. Steps may be taken to reduce this effect, such as single-sideband modulation; the highest frequency of such signals greatly exceeds the baseband bandwidth.

The figure shows what happens with AM modulation:

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

• Bandwidth