EMRFD Message Archive 6928
Message Date From Subject 6928 2011-11-28 02:25:59 N1EU Smooth audio T/R switching This is slightly off-topic, involving AF and not RF design. I'm in the process of replacing all AF circuitry (after the product detector) in an Omni VI+. Can anyone suggest a book or Web site with a good presentation on audio T/R switching that avoids pops/clicks/thumps/etc? My audio sources are ssb rx, cw rx, sidetone, and ssb monitor. SSB and CW rx are discreet since I'm using separate low pass filters. I've got all the control lines. I just need a way to multiplex the 4 sources and avoid audio transients during T/R switching.
Thanks & 73,
6930 2011-11-28 21:28:00 Harold Smith Re: Smooth audio T/R switching Barry,
Audio switching can be more difficult than it seems. I remember spending
weeks on popless transitions (primarily power on/off, though) when I was
working for a TV manufacturer. The biggest thing you want to remember with
T/R switching is not to switch DC in the signal path. Charging and
discharging coupling caps to power supply voltages causes very large
transitions, which translate at the speaker or phones to loud pops.
Ideally the DC on both sides of every coupling cap in the receiver stays
the same whether or not the receiver is muted.
My preferred method is to find or make a fairly high impedance point - a
series resistor, typically - and ground the output end of it through a
2N7000 and a good sized electrolytic cap. Note that the 2N7000 should have
a large resistor shunting it, so the low side of the cap is always at DC
ground. The T/R switch line (high during transmit) goes to the gate of the
2N7000. Its drain is connected to the low side of the 'lytic, and its
source to ground.
The other part of the issue is timing. You don't want the transmitter
delivering power until the receiver is safe. Depending on RF front end
circuitry and power levels, that can mean more than merely muting the audio.
I hope this has been some help.
de KE6TI, Harold
6931 2011-11-28 21:28:06 ehydra Re: Smooth audio T/R switching Basically you have several practicable choices druing switching state:
1. Electronically controlled reduced volume control
2. strong low-pass to remove intermodulation because of DC difference
3. DC removable (i.e. offset-compensation)
4. zero-point transition switching
Look for anti-pop etc. audio switches and amplifiers. The preferred
method depends heavily on your control infra-structure.
> This is slightly off-topic, involving AF and not RF design. I'm in--
> the process of replacing all AF circuitry (after the product
> detector) in an Omni VI+. Can anyone suggest a book or Web site with
> a good presentation on audio T/R switching that avoids
> pops/clicks/thumps/etc? My audio sources are ssb rx, cw rx,
> sidetone, and ssb monitor. SSB and CW rx are discreet since I'm
> using separate low pass filters. I've got all the control lines. I
> just need a way to multiplex the 4 sources and avoid audio transients
> during T/R switching.
> Thanks & 73, Barry N1EU