Delayed mute circuit for Digital ATV use... still work in progress!

For a while now i have been trying to find a work-around for a problem when using Digital ATV for duplex qsos through our repeater GB3KM, the main problem being delayed digital audio being returned to my headset about half a second after i have spoken and in a long over this can get extremely confusing, try it and you will see what i mean!!

The ideal solution would be to use two different sound sub-carriers for different users but at present this is not practical so i have gone for a delayed muting system of the returned audio. There may be other applicatioons for this circuit such as internet linking but the delays on there are too variable at present...


The circuit:

SCHEMATIC - TBD




Most components are not critical, the pic code will easily compile on most PICs without much trouble as it is so simple, in my case the PIC16F688. The type of muting circuit is not critical, the use of a FET with a couple of resistors to effectively ground the audio path rather than a switch will work too. The input audio amplifier is not too critital as long as it has enough gain and the ouput smoothing cap is not too large.


Connections needed:
Local audio (audio sent to digital transmitter - Line Level)
Receiver audio output (in my case the receive audio from the repeater receiver - Line Level)
Monitor audio input


How it works:
The microphone audio feeding my digital transmitter is tapped off into an audio amplifier and then rectified to a DC voltage level, this voltage level is limited by a zener and then goes to an input pin on the pic - just use ADC instead?!
Inside the pic there is a buffer that has an adjustable size (delay), the incoming audio logic level is passed through the buffer and then to an output pin on the pic.
This output pin drives a simple mute circuit which is installed in the return audio path between the receiver and the speaker/headphones.


This is an example of the dc level for "hello Rob"

Voltage from audio detector: _____________------___-----__________


The PIC sample time is set to 100mS which makes the steps of adjustment equal to this.
Here is what the levels on the PIC would look like, first is the audio detector output and second is the mute control 500mS delayed from the PIC.

Input logic level : _____________------___-----__________

Output logic level: ___________________------___-----____

This system gives some gaps in between words which helps a lot with a full duplex qso but does depend on the input level pot to the detector circuit.
If the input level is too high it will keep the mute on more often between words due to the time constant of the detector circuit. If the input level is too low it will not mute full words, only the peaks which can be useful but not so pleasant to listen to.

An ADC input on the PIC is used along with an analogue pot to give you control of the delay time from 0-2500mS in 100mS steps. There is no means of displaying the actual delay setting but it can be tweaked during operation for best word sync !!