Circuit Notebook 104 - Automatic Video/Audio Switch

In February 2010 Repeater GB3TM on the coast of Anglesey was up-graded to include a digital DVB-S output on 23cms as an alternative to the existing analogue one. The change-over between analogue and digital outputs is controlled by a DTMF tone on the incoming audio (Key 1 for digital, Key 2 for analogue).

The repeater transmits analogue TV in beacon mode, which is the fall-back condition. Once switched to digital output it continues in this mode for 15 minutes before returning to analogue. It can be switched between either at any time.

Streaming the output of the repeater is done by receiving the signal at my home (a 37 mile path), converting the video and sound in my PC and then sending it on line to the BATC server. I receive the signal from the repeater on two receivers simultaneously, a Comac SL25/12 for DVB-S and a Maspro for analogue. While I am in the shack I can manually switch between the outputs of the two receivers depending on which signal mode is being transmitted. I needed an automatic switch which would automatically select the valid signal.

It is not possible to detect whether or not a digital signal is present by monitoring the video output of the digital receiver as it continues to output a 'no signal' caption. So there is a video output at all times. On the other hand, the Maspro analogue satellite receiver outputs noise in the absence of a valid signal and normal video when a signal is present. The detection of this condition is used to select the correct video signal to be output. The detection is performed by a video detector circuit, first described in Circuit Notebook No. 83, CQ-TV 204 November 2003.

The block diagram given in Fig.1 shows the basic arrangement. The outputs of the two separate receivers are taken to a change over relay. In practice, two relays are used, one for video and one for audio and LED indicators. If the repeater transmission is analogue then the analogue receiver produces a valid video output and this is detected and the relay is switched to the analogue receiver. If the analogue video output is not valid then, in essence, the digital output will be, so relay is switched to this receiver. This means that no matter which mode of transmission is used there will be a valid signal available for streaming.

The complete circuit is shown in Fig.3.