Video sender

A video sender (also known as a DigiSender, wireless video sender, AV sender or audio-video sender) is a device for transmitting domestic audio and video signals wirelessly from one location to another. It is most commonly used for sending the output of a source device, such as a satellite TV decoder, to a television in another part of a property and provides an alternative to cable installations.

A wide range of video sender technologies exist including analogue wireless (radio), digital wireless (spread-spectrum, WiFi, Ultra-wideband) and digital wired (power-line communication). Other, less common, technologies also exist, such as those that utilise existing ethernet networks.

Overview

Typical video sender applications include the transmission of television audio and video signals from a lounge to a bedroom or from a CCTV camera to a display, as well as interconnecting appliances with audio, video and IP (Internet Protocol) requirements.

Most video sender systems will consist of three separate components, a transmitter, a receiver and a remote control relay (also known as a magic eye or IR blaster). The transmitter is responsible for transmitting or broadcasting a connected audio-video device, while the receiver outputs that transmitted audio-video signal to a connected television. The remote control relay permits infrared remote controls to operate the equipment whose output is being transmitted.

As connectivity standards have changed in the television and audio-video markets, so it has in the video sender market, with older models usually featuring SCART and/or composite video and newer models featuring HDMI as their key means of connection to host equipment. More recently RJ45 connectors for LAN have been added to some models.

Analogue Wireless

Analogue video senders have the advantage of low manufacturing costs as the audio and video signals are simply modulated onto a carrier at 2.4 GHz or 5.8 GHz. They do, however, have the adverse effect of causing reduced bandwidth to local WiFi networks and, in some cases, WiFi networks can cause picture interference on the video sender signal. More information can be found in the article on electromagnetic interference at 2.4 GHz. To avoid this, some video senders now use a spread-spectrum technology similar to WiFi and can co-exist with wireless networks and share available bandwidth.

Usually there are four FM transmit channels, A, B, C & D, with stereo audio on 6.0 MHz and 6.5 MHz FM subcarriers added to the composite video baseband. These different channels can often be used to overcome the adverse effects of nearby WiFi networks.

The reverse remote control channel is usually fixed at 433.92 MHz, using whatever modulation is on the 34 kHz to 45 kHz IR remote "carrier". ASK/OOK schemes such as RC5 and RC6 work best over the RF link as the receiver uses a data slicer and AGC designed for ASK/OOK with Manchester encoding.

Analogue wireless video senders can achieve typical operating distances of up to 60 metres (clear line of sight) with DVD quality (720x576) video resolution and stereo audio.

Digital Wireless

Digital video senders are quickly becoming the most popular solution and combine the use of a System on Chip (used for picture and sound encoding/decoding) with a means of transmitting the signal, such as spread-spectrum, WiFi and ultra-wideband.

Spread-spectrum

Spread-spectrum techniques are methods by which a signal is deliberately spread in the frequency domain, resulting in a signal with a wider bandwidth. These techniques are used for a variety of reasons, including the establishment of secure communications, increasing resistance to natural interference, noise and jamming, and to limit power flux density.

Some manufacturers use proprietary versions of WiFi technology, enabling typical operational ranges of up to 80 metres in-building. By also employing externally mounted antennas, operational ranges in excess 2000 metres (clear line-of-sight) have been achieved and several such models are sold under the DigiSender brand.