Editing Spectral mask

{{Short description|Limits applied to the levels of radio transmissions}}

In [[telecommunications]], a '''spectral mask''', also known as a '''channel mask''' or transmission mask, is a [[math]]ematically defined set of [[line (mathematics)|lines]] applied to the levels of [[radio]] (or [[optical]]) [[transmission (telecommunications)|transmission]]s.  The spectral mask is generally intended to reduce [[adjacent-channel interference]] by limiting excessive [[radiation]] at [[frequencies]] beyond the necessary [[bandwidth (signal processing)|bandwidth]].  [[Attenuation]] of these [[spurious emission]]s is usually done with a [[band-pass filter]], tuned to allow through the correct center frequency of the [[carrier wave]], as well as all necessary [[sideband]]s.<ref>{{cite web |title=Spectral Emission Mask |url=https://www.ni.com/docs/en-US/bundle/rfmx-nr/page/sem-overview.html |website=Emerson}}</ref>

The spectral mask is usually one of the things defined in a [[bandplan]] for each particular [[band (radio)|band]].  It is essential in assuring that a transmission stays within its [[channel (broadcasting)|channel]].  An [[FM broadcasting|FM]] [[radio station]], for example, must attenuate everything beyond ±75[[kHz]] from the center frequency by a few [[decibel]]s, and anything beyond ±100&nbsp;kHz (the channel boundary) by much more.  Emissions on further [[adjacent channel]]s must be reduced to almost zero.  

[[FM broadcasting|FM broadcast]] [[subcarrier]]s are normally required to stay under 75&nbsp;kHz (and up to 100&nbsp;kHz if [[attenuation|reduced]]) to comply with the mask.  The introduction of [[in-band on-channel]] (IBOC) [[digital radio]] in the [[United States]] has been slowed by issues concerning the subcarriers it uses &ndash; and the corresponding increase in the amount of energy in the sidebands &ndash; overstepping the bounds of the spectral mask set forth for FM by the [[National Radio Systems Committee|NRSC]] and enforced by the [[Federal Communications Commission|FCC]].  

Other types of [[modulation]] have different spectral masks for the same purpose.  Many [[digital modulation]] methods such as [[COFDM]] use the [[electromagnetic spectrum]] very efficiently, allowing for a very tight spectral mask. This allows placement of broadcast stations or other transmissions on channels right next to each other without interference, allowing for an increase in a band's total [[Channel capacity|capacity]].  Conversely, it is allowing the U.S. to eliminate [[TV]] channels 52 to 69, freeing up 108&nbsp;MHz (from approximately 700 to 800&nbsp;MHz) for [[emergency services]] and to be [[auction]]ed off to the highest bidder, while still retaining (although moving) all existing TV stations.

==References==
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[[Category:Wireless]]
[[Category:Broadcast engineering]]