Note: If you intend to use this for PSK31 or other digital modes, you would use the 1.0µF series capacitance for TX audio. If you intend to use this for voice modes such as Echolink, you would use the 0.001µF series capacitance for TX audio.
The 0.001 µF series capacitor provides a high-pass filter to pre-emphasize the audio into the flat TNC 10Kohm audio input. Otherwise, your audio will sound like a broadcast radio with the Tone control turned fully counterclockwise (overly bassy audio).
The 1.0µF shunt capacitor provides a low-pass filter to de-emphasize the audio into the flat computer input for RX voice audio. Otherwise, your audio may sound tinny (like Tone control turned fully clockwise).
PC-Radio interface: Squelch Control
You could use VOX to control the squelch open/closing, but it is much preferable to have a hardwired connection as depicted below for smooth operation when linking repeaters etc.
This interface can be readily built with simple parts and some shielded audio cable (or use cords from old mice/keyboards).
It assumes that your radio has a TNC port, with 3..15V “shorted to ground” to transmit. You put your transmit audio from the PC Line out into the radio’s TNC port “Data In” pin, and the computer Line In to the radio’s TNC port “1200 baud Data Out” port.
If Radio Doesn’t Have TNC Port: how to interface to PC
It is also possible to use the microphone jack and speaker audio if no TNC port exists. Using the microphone jack for Line Out audio requires you to check to be sure there is no DC voltage on the mic pin! You might damage your radio otherwise. Use a 1.0 µF capacitor to block the DC.
Using the speaker jack for Line In audio is undesirable, since volume control changes upset audio levels, but sometimes that’s the only recourse….
In some cases, you may have too high an audio level. Consider then constructing a voltage divider, with a 10000 ohm series and 1000 ohm shunt resistor.