23-12-2015, 08:44 PM
The audio/video switcher currently used at the Vintage Wireless Museum is an improvisation. It gives a very bare minimum functionality. Each of the 3 channels "BBC1 on Ch1", "ITV on Ch9" and "BBC2 on Ch32" can be connected to a Freeview box, DVD player or test card. Ideally each should be able to connect to the COW (Computer Originated World BBC animated logo) and an auxiliary input. The current switcher is some Avitel relay and VDA modules in a rack that originally came from the Croydon transmitter site.
There is a separate preview switcher with audio and video switched directly on the push buttons. It's an odd unit that came from who knows where.
I have started to build a new relay based switcher that will do the whole job. The photo shows 24 relay sockets mounted on the tapped strip kinldy donated by Jeremy (Pamphonica). I have plugged relays into a few of the sockets. The rest of the siwtcher will include video buffer amps based on EL2020 video op amps and a +/-12V PSU. The whole lot will be fitted on a 2U rack shelf with the BNCs and phonos mounted on an old Cox back panel that I happen to have.
The push button selectors will be on a separate 2U panel. The interconnect will likely be a 25 core ribbon crimped on to D25 connctors.
Next stage is to wire up the relays. At each crosspoint I will be using a pair of changeover contacts for video with the wipers joined together and the NC contacts earthed. This gives very low crosstalk. Ditto for audio though not so critical.
You might ask why aren't all the crosspoint positions populated. Each channel only has crosspoints for Aux, COW, DVD, its own testcard and its own Freeview tuner. The preview bank has all 9 relays fitted so it can see/hear every source. One minor wrinkle is that the BBC1 and ITV test cards can't be previewed since they are generated within the respective Auroras. To display testcard I simply send no video to the Aurora. In other words the relevant BNC input to the switcher will not be connected. However one day we may have 3x MP3 players to give different music for each channel. It was very little hassle to allow for that.
If anyone is wondering why I didn't go for an electronic solution the reason is fairly simple. A suitable 16x4 electronic switcher has been donated but a control system would need to be built that interfaces its RS232 serial port to push buttons. Since software (PIC, Arduino etc) isn't really my thing I've gone for a more agricultural solution. If a suitable control system is built one day it will be a simple job to swap the switchers.
I did actually consider mounting 48x 4053 (or perhaps HC4053) CMOS switches to do the same job. Audio and video would be separate hence the need for 48. Again I'd use double break switching and one 4053 per crosspoint. A bit lavish but it's the way to get low crosstalk. Then I decided it would probably be easier to wire up the relays. Not sure if I even have 48x 4053 in stock.
There is a separate preview switcher with audio and video switched directly on the push buttons. It's an odd unit that came from who knows where.
I have started to build a new relay based switcher that will do the whole job. The photo shows 24 relay sockets mounted on the tapped strip kinldy donated by Jeremy (Pamphonica). I have plugged relays into a few of the sockets. The rest of the siwtcher will include video buffer amps based on EL2020 video op amps and a +/-12V PSU. The whole lot will be fitted on a 2U rack shelf with the BNCs and phonos mounted on an old Cox back panel that I happen to have.
The push button selectors will be on a separate 2U panel. The interconnect will likely be a 25 core ribbon crimped on to D25 connctors.
Next stage is to wire up the relays. At each crosspoint I will be using a pair of changeover contacts for video with the wipers joined together and the NC contacts earthed. This gives very low crosstalk. Ditto for audio though not so critical.
You might ask why aren't all the crosspoint positions populated. Each channel only has crosspoints for Aux, COW, DVD, its own testcard and its own Freeview tuner. The preview bank has all 9 relays fitted so it can see/hear every source. One minor wrinkle is that the BBC1 and ITV test cards can't be previewed since they are generated within the respective Auroras. To display testcard I simply send no video to the Aurora. In other words the relevant BNC input to the switcher will not be connected. However one day we may have 3x MP3 players to give different music for each channel. It was very little hassle to allow for that.
If anyone is wondering why I didn't go for an electronic solution the reason is fairly simple. A suitable 16x4 electronic switcher has been donated but a control system would need to be built that interfaces its RS232 serial port to push buttons. Since software (PIC, Arduino etc) isn't really my thing I've gone for a more agricultural solution. If a suitable control system is built one day it will be a simple job to swap the switchers.
I did actually consider mounting 48x 4053 (or perhaps HC4053) CMOS switches to do the same job. Audio and video would be separate hence the need for 48. Again I'd use double break switching and one 4053 per crosspoint. A bit lavish but it's the way to get low crosstalk. Then I decided it would probably be easier to wire up the relays. Not sure if I even have 48x 4053 in stock.
www.borinsky.co.uk Jeffrey Borinsky www.becg.tv
