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
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 (
macroblocking), 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 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
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.
Digital radio HD Radio broadcasting, officially used only in the United States, is one system designed to have an analog
fallback. 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 in
hybrid 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 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. ==Mobile phones==