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((Short description|Sudden loss of digital signal reception))
((for|the effective financial loss resulting from increased income|benefits cliff))
[[File:Digital-cliff.svg|right|350px]]
In [[telecommunications]], the '''(digital) cliff effect''' or '''brick-wall effect''' is a sudden loss of [[Digital data|digital]] [[Television antenna|signal reception]]. Unlike [[analog signal]]s, which gradually fade when [[signal strength]] decreases or [[electromagnetic interference]] or [[Multipath interference|multipath]] increases, a digital signal provides data which is either perfect or non-existent at the [[receiver (information theory)|receiving]] end. It is named for a graph of reception quality versus signal quality, where the digital signal "falls off a cliff" instead of having a gradual rolloff.<ref>[http://www.itu.int/ITU-R/conferences/rrc/rrc-04/intersession/workshops/damaskus/docs/Presentations/ntl_1_DVB-T_systems.pdf ITU/ASBU Workshop on Frequency Planning and Digital Transmission: DVB-T Transmission Systems ''(Presentation)''] ((Webarchive|url=https://web.archive.org/web/20091228203429/http://www.itu.int/ITU-R/conferences/rrc/rrc-04/intersession/workshops/damaskus/docs/Presentations/ntl_1_DVB-T_systems.pdf |date=2009-12-28 )), Nov. 23, 2004.</ref> This is an example of an [[EXIT chart]].

The phenomenon is primarily seen in broadcasting, where signal strength is liable to vary, rather than in recorded media, which generally have a good signal. However, it may be seen in significantly damaged media that is at the edge of readability.

==Broadcasting==

===Digital television===
This effect can most easily be seen on [[digital television]], including both [[satellite TV]] and over-the-air [[terrestrial TV]]. While [[forward error correction]] is applied to the [[Broadcasting|broadcast]], when a minimum threshold of signal quality (a maximum [[bit error rate]]) is reached it is no longer enough for the [[Video decoder|decoder]] to recover.  The picture may break up ([[macroblocking]]), lock on a [[Freeze frame television|freeze frame]], or go blank. Causes include [[rain fade]] or [[solar transit]] on satellites, and [[temperature inversion]]s and other weather or atmospheric conditions causing [[anomalous propagation]] on the ground.

Three particular issues particularly manifest the cliff effect. Firstly, anomalous conditions will cause occasional signal degradation. Secondly, if one is located in a fringe area, where the antenna is just barely strong enough to receive the signal, then usual variation in signal quality will cause relatively frequent signal degradation, and a very small change in overall signal quality can have a dramatic impact on the frequency of signal degradation – one incident per hour (not significantly affecting watchability) versus problems every few seconds or continuous problems. Thirdly, in some cases, where the signal is beyond the cliff (in unwatchable territory), viewers who were once able to receive a degraded signal from analog stations will find [[DTV transition|after digital transition]] that there is no available signal in rural, fringe or mountainous regions.<ref>[http://www.wmur.com/news/18734411/detail.html Digital Transition May Leave Some Without Signal] ((Webarchive|url=https://web.archive.org/web/20110727212914/http://www.wmur.com/news/18734411/detail.html |date=2011-07-27 )), [[WMUR-TV]], February 18, 2009.</ref>

The cliff effect is a particularly serious issue for [[mobile TV]], as signal quality may vary significantly, particularly if the receiver is moving rapidly, as in a car.

[[Hierarchical modulation]] and coding can provide a compromise by supporting two or more streams with different robustness parameters and allowing receivers to scale back to a lower definition (usually from [[HDTV]] to [[SDTV]], or possibly from SDTV to [[LDTV]]) before dropping out completely. Two-level hierarchical modulation is supported in principle by the European [[DVB-T]] digital terrestrial television standard.<ref>EN 300 744, "Digital Video Broadcasting (DVB); Framing structure, channel coding and modulation for digital terrestrial television", European Telecommunications Standards Institute (ETSI), January 2009.</ref> However, layered [[source coding]], such as provided by [[Scalable Video Coding]], is not supported.

===Digital radio===
[[HD Radio]] broadcasting, officially used only in the United States, is one system designed to have an analog [[Standby (telecommunications)|fallback]]. Receivers are designed to immediately switch to the analog signal upon losing a lock on digital, but only as long as the tuned station operates in [[in-band on-channel|hybrid]] digital mode (the official meaning of "HD"). In the future all-digital mode, there is no analog to fall back to at the edge of the digital cliff. This applies only to the main channel [[simulcast]], and not to any [[subchannel]]s, because they have nothing to fall back to. It is also important for the station's [[broadcast engineer]] to make sure that the [[audio signal]] is [[synchronize]]d between analog and digital, or the cliff effect will still cause a jump slightly forward or backward in the radio program.

==Mobile phones==
The cliff effect is also heard on [[mobile phone]]s, where one or both sides of the conversation may break up, possibly resulting in a [[dropped call]].  Other forms of [[digital radio]] also suffer from this.

==Other uses==
* [[Cliff effect (economics)]] <!--The former content should be moved to that article-->

==See also==
* [[Digital television transition]]
* [[Link adaptation]]

==References==
((Refimprove|date=April 2009))
((reflist))

[[Category:Digital television]]
[[Category:Digital radio]]
[[Category:Broadcast engineering]]

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