Cliff effect

In telecommunications, the cliff effect or digital cliff 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.

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 (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. Hierarchical modulation can help with this, switching into a lower definition (usually from HDTV to SDTV, or possibly from SDTV to LDTV) before dropping out completely.

Mobile phones

The cliff effect is also heard on mobile phones, 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.

Digital radio

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 is synchronized between analog and digital, or the cliff effect will still cause a jump slightly forward or backward in the radio program.