- Cliff effect
In telecommunications, the (digital) cliff effect or brickwall effect describes the sudden loss of digital signal reception. Unlike analog signals, which gradually fade when signal strength decreases or electromagnetic interference or multipath increases, a digital signal provides data which is either perfect or non-existent at the receiving end. It is named for a graph (shown at right) of reception quality versus signal quality, where the digital signal "falls off a cliff" instead of having a gradual rolloff.
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, which is at the edge of readability.
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 broadcast, when a minimum threshold of signal quality (a maximum bit error rate) is reached it is no longer enough for the decoder to recover. The picture may break up (pixellation), lock on a freeze frame, or go blank. Causes include rain fade or solar transit on satellites, and temperature inversions 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 signal 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 after digital transition that there is no available signal in rural, fringe or mountainous regions.
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. However, layered source coding, such as provided by Scalable Video Coding, is not supported.
HD Radio broadcasting, officially used only in the United States, is one system designed to have an analog 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 hybrid digital mode (the official meaning of "HD"). In the future all-digital mode, there is no analog to fallback to at the edge of the digital cliff. This applies only to the main channel simulcast, and not to any subchannels, 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 synchronized between analog and digital, or the cliff effect will still cause a jump slightly forward or backward in the radio program.
In economics, the "cliff effect" refers to a positive feedback loop, where downgrading a single security can have a disproportionate cascading effect. This has become pronounced with respect to the assessment of credit risk in a bank's portfolio. If a Credit Rating Agency has the expectation that the credit risk of a position rises, it will downgrade its rating. As a consequence, a bank faces additional capital charges in order to comply with national capital requirements. Especially during the subprime mortgage crisis banks had to increase their capital substantially, because many ratings were considerably downgraded. In a time banks needed their capital most to cope with high losses, this term emerged in the consultative process on reforms regarding existing capital requirements, namely criticizing the procyclical "cliff effect".
- ^ ITU/ASBU Workshop on Frequency Planning and Digital Transmission: DVB-T Transmission Systems (Presentation), Nov. 23, 2004.
- ^ Digital Transition May Leave Some Without Signal, WMUR-TV, February 18, 2009.
- ^ EN 300 744, "Digital Video Broadcasting (DVB); Framing structure, channel coding and modulation for digital terrestrial television", European Telecommunications Standards Institute (ETSI), January 2009.
- ^ American Association Banker's comment on a consultative paper of a part of Basel III, April 15, 2010.
Wikimedia Foundation. 2010.
Look at other dictionaries:
Cliff Compton — Ring name(s) Cliff Compton Dice Domino Domino … Wikipedia
Cliff Hagan — No. 6, 17, 16 Small forward / Guard Personal information Date of birth December 9, 1931 (1931 12 09) (age 79) Place of birth Owensboro, Kentucky … Wikipedia
Cliff Compton — en 2007. Nom(s) de ring Cliff Compton Dice Domino Domino Taille 1,91 m Poids 111 kg Naissance … Wikipédia en Français
Cliff Norton — Born Clifford Charles Norton March 21, 1918(1918 03 21) Chicago, Illinois, U.S. Died January 25, 2003(2003 01 25) (aged 84) Studio City, California, U.S … Wikipedia
Cliff Compton — Nacimiento … Wikipedia Español
Cliff Richard — Sir Cliff Richard OBE Cliff Richard in November 2009 Background information Birth name Harry Rodger Webb Born … Wikipedia
Cliff Watson — For the motorcycle speedway rider, see Cliff Watson (speedway rider). Cliff Watson Personal information Full name Clifford H. Watson Born 26 April 1940 (1940 04 26) (age 71) Stepney, London, England … Wikipedia
Effect size — In statistics, an effect size is a measure of the strength of the relationship between two variables in a statistical population, or a sample based estimate of that quantity. An effect size calculated from data is a descriptive statistic that… … Wikipedia
Domino (Cliff Compton) — Cliff Compton Cliff Compton Pseudo(s) de lutte Cliff Compton Dice Domino Domino Taille 1,91 m Poids … Wikipédia en Français
Lake-effect snow — Lake Effect redirects here. For the American literary journal, see Lake Effect (journal). Lake effect precipitation coming off Lake Erie and into Western New York, as seen by NEXRAD radar, October 12–13, 2006 … Wikipedia