Intelecommunications, the(digital) cliff effect orbrick-wall effect is a sudden loss ofdigitalsignal reception. Unlikeanalog signals, which gradually fade whensignal strength decreases orelectromagnetic interference ormultipath increases, a digital signal provides data which is either perfect or non-existent at thereceiving 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.[1] This is an example of anEXIT 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.
This effect can most easily be seen ondigital television, including bothsatellite TV and over-the-airterrestrial TV. Whileforward error correction is applied to thebroadcast, when a minimum threshold of signal quality (a maximumbit error rate) is reached it is no longer enough for thedecoder to recover. The picture may break up (macroblocking), lock on afreeze frame, or go blank. Causes includerain fade orsolar transit on satellites, andtemperature inversions and other weather or atmospheric conditions causinganomalous 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 findafter digital transition that there is no available signal in rural, fringe or mountainous regions.[2]
The cliff effect is a particularly serious issue formobile 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 fromHDTV toSDTV, or possibly from SDTV toLDTV) before dropping out completely. Two-level hierarchical modulation is supported in principle by the EuropeanDVB-T digital terrestrial television standard.[3] However, layeredsource coding, such as provided byScalable Video Coding, is not supported.
HD Radio broadcasting, officially used only in the United States, is one system designed to have an analogfallback. Digital radio 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 inhybrid digital mode (the official meaning of "HD"). In the proposed all-digital mode, there is no analog to fall back to at the edge of the digital cliff. This applies only to the main channelsimulcast, and not to anysubchannels, because they have nothing to fall back to. It is also important for the station'sbroadcast engineer to make sure that theaudio signal issynchronized between analog and digital, or the cliff effect will still cause a jump slightly forward or backward in the radio program.
The cliff effect is also heard onmobile phones, where one or both sides of the conversation may break up, possibly resulting in adropped call. Other forms ofdigital radio also suffer from this.
 | This articleneeds additional citations forverification. Please helpimprove this article byadding citations to reliable sources. Unsourced material may be challenged and removed. Find sources: "Cliff effect" – news ·newspapers ·books ·scholar ·JSTOR(April 2009) (Learn how and when to remove this message) |