26-01-2016, 02:01 PM
(This post was last modified: 26-01-2016, 02:02 PM by ppppenguin.)
Took a closer look at the Invotron switchers this morning. There's a 1U rack with a 16x4 video router and another 1U rack with companion stereo audio router. Each rack has the router card, a PSU and a slot for a controller. Only the video unit has a controller. There's a 15 pin D connector on the back of each to slave the audio to the video.
I examined the slave interface and it's very simple. Just 4 bit data (1 of 16 inputs) and 4 enables (one for each output). It's at standard 5V TTL/CMOS levels. The data latches are 4042 which are transparent rather than edge triggered. I'm starting to wonder if it wouldn't be simpler to hook up some switches to this interface than continue with my homebrew relay switcher.
What's not so impressive is the switcher itself. Ordinary 4051 8:1 multiplexers. A pair for each output. I know from experience that the crosstalk on these isn't marvellous. Probably OK in this application but it's inherently limited by the "on" resistance and stray capacitance. When I've used this class of device in professional video switching I've always used a double break design, where each crosspoint has 2 switches in series, plus one in shunt which grounds the midpoint when the crosspoint is switched off. Even with relays I'm using this technique to get low crosstalk.
I suppose it's worthwhile running up the Invotron routers to see if they still work. At afirst glance connecting them to latching pushbuttons or rotary switches would need about 40 diodes to encode the switches to binary plus some kind of simple CMOS or TTL sequencer to run round the 4 outputs.
I've probably got some BCD encoded switches to use for test purposes.
I examined the slave interface and it's very simple. Just 4 bit data (1 of 16 inputs) and 4 enables (one for each output). It's at standard 5V TTL/CMOS levels. The data latches are 4042 which are transparent rather than edge triggered. I'm starting to wonder if it wouldn't be simpler to hook up some switches to this interface than continue with my homebrew relay switcher.
What's not so impressive is the switcher itself. Ordinary 4051 8:1 multiplexers. A pair for each output. I know from experience that the crosstalk on these isn't marvellous. Probably OK in this application but it's inherently limited by the "on" resistance and stray capacitance. When I've used this class of device in professional video switching I've always used a double break design, where each crosspoint has 2 switches in series, plus one in shunt which grounds the midpoint when the crosspoint is switched off. Even with relays I'm using this technique to get low crosstalk.
I suppose it's worthwhile running up the Invotron routers to see if they still work. At afirst glance connecting them to latching pushbuttons or rotary switches would need about 40 diodes to encode the switches to binary plus some kind of simple CMOS or TTL sequencer to run round the 4 outputs.
I've probably got some BCD encoded switches to use for test purposes.
www.borinsky.co.uk Jeffrey Borinsky www.becg.tv
