Makeshift Radio Broadcast Mixer

[TomHoward]

I'm just programming up a (don't laugh, it's what I have available in the lockdown) BBC Micro:Bit to use 2 or 3 of the analogue pins to read the value of a the fader on an analogue desk and see if I can calibrate it to work at the bottom of the fader, and if it affects the audio in the fader.

I'm kind of hoping since I don't need to read the value properly, I just need to read "either absolute zero" or "not absolute zero" I'm hoping at the bottom of the fader the dead short might be readable, and the AC audio signal might also sink when the fader is at the bottom and not cause a bad reading.

 

It seems to work shorting it out with a paper clip but I'll try it on a desk tomorrow. My first preference might be to use an analogue desk if I can, as I'm not sure how long this is going to be in for or who's going to be operating it, or if there are going to be any computers at all.... It's a setup for a church to replace regular services who are partway though getting an RSL a short notice so it's all theoretical at the moment.

[DrV]

I fear you're going to be disappointed. You'll have to establish a DC voltage across the fader which will cause crackles on the audio every time you move it. That's assuming that the wiper is already AC coupled to the following circuitry. If not then it will simply swamp the signal. In the spirit of providing solutions rather than problems, however, my next suggestion is a small reed switch or Hall effect switch (you can get _very_ small ones of those) and a magnet glued to the fader knob. What you do with the output is then your choice. If you just wanted mute and no tally light you could even wire the reed switch straight across the input. The switches could be mounted on a bit of prototyping board copper side up, glued to the top panel, or even inside if there's room. All very fiddly but it would definitely work.

[TomHoward]

Aha - I have a load of miniature reed switches from when I was trying to make a lap counter for scalextric, using the magnets that keep the car to the track (it didn't work) - and I could stick a magnet in the underside of the fader knob. I'll try that too.

 

The analogue input is really sensitive and I can get it at home to detect the different between open circuit and a dead short, I was hoping I might get away with using the AC audio signal as the sense, if it is actually sunk by the fader when it is at -inf, and maybe average over a few ms to improve reliability. How it'll behave once it's connected to audio I've no idea though.

[DrV]

The problem I see with using the audio itself is that it will disappear when the user stops speaking. You're re-inventing the noise gate!

[sunray]

The faders in some of my soundcraft desks are open ended and have plenty of space for a micro switch actuator to poke into the end.

Worth checking?

 

[alistermorton]

Penny and Giles - there's a blast from the past!

 

I hadn't realised they were still going, and they do still have faders incorporating switches in their range.

 

 

 

[henny]

Ok , ive only checked this plan on the offline editor but I have a way of doing it on an x32,

 

1) Pair your mic channel with a spare unused channel on another layer eg channel 30

 

2) Set the desk's oscillator to out put white noise at odB to a spare mix buss eg bus 9

 

3) set the Source of channel 30 to be bus 9 so I gets the white noise.

 

4) Set channel 30 only to send post fader to a spare mix but eg bus 10 and nowhere else.

 

4) with a pair of cables loop your monitor out of the desk out of the back of the desk and back into a pair of spare inputs and route to another pair of spare main desk channels eg 31 & 32

 

5) Pair channels 31 & 32 and setup the compressor on theses channels as a ducker with the key source being bus 10

 

6) Send channels 31 & 32 pre fader only to bus's 11 & 12

 

7) in the desk routing send buses 11 & 12 out to whatever physical output you want to run your monitor speakers from.

 

 

 

if all goes to plan then raising the mic fader will also raise the fader of the channel with the white noise on , this white noise then triggers the ducker of the channels that have the desks monitor out patched threw them

[GaryNattrass]

Penny and Giles faders had microswitches built into them.

 

Penny and Giles - there's a blast from the past!

 

One small problem I can see with trying to measure the fader position electronically (by somehow reading the resistance/wiper voltage, perhaps) is that it will probably not have a DC wiper voltage. On an analogue mixer there's the signal it self, on a digital mixer I can imagine the faders (if they're resistive rather than linear encoders) being multiplexed so there could be digital noise on the signal too.

 

Our Calrec Apollo and Artemis desks at SKY still use P&G faders with pull for PFL and any amount of GPI functions. They have also started doing radio desks at Calrec too.

 

 

 

[dbuckley]

If you do decide to buy rather than build, A&H still make the XB-14-2, we have one in our backup studio, it seems to do everything right, and USB I/O to the computer is a useful bonus.

[bruce]

For many community broadcasters on a budget, the D&R Airmate range is a common choice. https://www.d-r.nl

[sandall]

If you do decide to buy rather than build, A&H still make the XB-14-2,

Maybe overkill for Tom's needs (& budget), but a great bit of kit for the money.

[TomHoward]

Thanks for the pointers - when this setup used to be permanent, we had two studio with a Sonifex S2 (about 14 channel frame) and a D&R Airlab 2 as well. We went down to one studio and kept the D&R but I always preferred the Sonifex as they were all soft buttons and it booted into the same state.

 

At the moment it’s gone a bit cold because they were on track to rush through an RSL but it’s all gone cold - at first the priority was disaster relief broadcasting, so they wanted to push ahead with or without the RSL granted - so Ive sourced a transmitter, mast and location - when it comes to the push they’ve now had 2nd thoughts and are waiting to hear back concrete from Ofcom about fast-tracking an RSL. (But they’ve all gone home...)

[Jivemaster]

I think I've a source of NOS P&G faders with switch at both ends.