custom vox switch

[kbs]

Hi,
I want to make a custom VOX project to switch between two mono audio signals.
It will switch Sig1, Sig2 or ground through to SigOut.
Sig1 is the received audio from a transceiver.
Sig2 is a MP3 player.
SigOut is the signal passed to the audio amplifier driving a speaker.
SigOut could be Sig1, Sig2 or ground(ed).

I've built basic Vox circuits that will do the sensing. As a microphone is not involved, no need to inhibit vox whilst radio receiving loud signals. This project will be switching audio sources regularly, could an audio mixer amp IC be the best approach? Is there one that rapidly fades out the signal before changing source and rapidly fades in the new signal? Perhaps also offering mute?

Is the whole thing crying out for a simpler approach?

Thanks in advance of any advice.

What the project needs to do;
When Sig1 is below threshold, SigOut = Sig2.
When Sig1 is above threshold, SigOut = Sig1.
Optionally (ideal): when Sig2 is not present, SigOut = Sig1.

When Sig2 is selected and Sig1 rises above threshold, Sig1 needs to be selected within 100us.

When Sig1 is selected and Sig1 falls below threshold, Sig1 needs to remain selected for a period of 2 seconds. Sig1 should (now) remain selected until a continuous period >2seconds where Sig1 is below threshold has elapsed.

Lastly, the project needs to disconnect (mute) SigOut when the transceiver is transmitting (normally 1w, occasionally 25w) @150Mhz. I cannot interface with the radio directly, perhaps I could put a few turns round the transceivers coax?
Thanks in advance of any advice.

 

[alec_t]

If you're not too fussed about hi-fi, perhaps a cheap analogue gate IC would be the basis?

 

[kbs]

sounds good

Hi Alec,

Thanks for the quick response.

Have you had personal experience of this kind of switching?
I'm not worried about audio quality (only MP3 anyway).

Should I expect 'noisy' switching between the two sources?

I was looking at 74HC4066N from RS.

Thanks again

 

[alec_t]

I did this years ago with a 4066 but can't recall whether the switching was noisy. More recent ICs may give better quality, but I'm no expert. Might be worth a PM to Audioguru ?

 

[audioguru]

A CD4066 does not produce noise. It is a switch that is about 120 ohms in series with the signal or it is open.

 

[kbs]

Hi,
4066 it is then.
Thank you both for the advice.
Keith

 

[alec_t]

Be aware that (IIRC) the audio will get distorted if the input voltage swings close to the 4066's Vdd or ground. So biasing at ~ half the supply voltage may be advisable to use the most linear part of the gate transfer characteristic.

Edit: I don't know if ramping the gate control voltage would result in fade-in/out of the signal. Might be worth a try.

2nd edit: Just ran a simulation. Not sure of the accuracy of the 4066 model I used, but the sim showed that ramping the gate control voltage (over a ~20ms period) doesn't give much of a fade effect but it helps to reduce clicks.