PSK Connections and Duplex Mode
Connecting a radio to a computer for effective DSP work requires a bit of extra attention. I strongly recommend using some sort of interface such as those manufactured by West Mountain Radio, MFJ, Buck Rogers K4ABT or other similar units. These units are quite handy providing the required impedance matching between different radios and the low level input and output of the computer sound card.
Receive Audio Path
This is the path the audio your radio will take to the computer. As a general rule, DO NOT connect this audio to the microphone input on any sound card. This input is designed for the frequency response of the normal cheap microphone and filtered for the normal human voice. They also have a normal impedance of 600 - 1000 ohms. Connecting this input directly to you radios speaker can cause an impedance mismatch and load the radio. Too much audio output from the radio could damage the sound cards input channel. (Note: I have one of these as evidence).
As a general rule, tapping audio from the radio speaker is not a good idea. Most radios today provide between 2 to 5 watts of power to drive a speaker. In my case, I get 2.5 watts to drive my speaker. This is about 2 volts peak-to-peak of audio. The sound card is much happier with something on the order of less than 200 mV. If you have no other option for receive audio, then you must use some sort of device to decrease the output level. A potentiometer to decrease the level and a unity gain opamp would do the trick. One of the above mentioned interface devices will also do the trick.
Most of the current production radios provide a low-level audio output from the radio. Consult to owners manual and try to locate a data port or a phone patch-output jack. These outputs are low-level ( 200mV or less) and high impedance to keep from loading the devices they are connected to.
Connect your receive audio to the LINE INPUT on the sound card either directly or via your interface device. Although an interface device is not required, most are designed to provide a high degree of isolation between the radio and computer. A trip to the local radio store can yield a few of those snap-on type RFI chokes. These will help if you are experience any stray RF problems.
Transmit Audio Path
This is the path the PSK audio from the sound card takes to the radio. Connecting this directly to the microphone input is acceptable as long as you can get the output from the sound card low enough not to overdrive the microphone input. This will vary between different DSP software. This method is not very practical because you now loose the functionality of the microphone. Do not connect this audio directly to the auxiliary input found on some desktop microphones. When the transmitter is keyed, not only will you transmit the PSK audio, but any background noise the microphone picks up. Connection via an interface box is preferable. Most boxes I've seen provide a mic connector on the front panel and lock out the station microphone during PSK operation.
If you chose to use the microphone jack for PSK audio, make certain you do not have any compression or audio processor activated during PSK operations. These features will do more harm than good to your transmitted signal. Adjust the microphone level such that you just barely have ALC action then back off until the ALC is not visible on the meter. This is a good starting point.
If you have a data port or a phone patch port, then there will be a data-in or phone patch-in jack. These inputs to the radio are low-level, high impedance, and isolated from the front panel microphone circuits and not affected by compression or processing. These jacks are the preferred method of feeding sound card audio to the radio.
This mode is an internal function of the DSP program. In full duplex mode, the receive audio path, in the software, is left open. This will allow you to receive audio while transmitting. In half duplex, the receive audio path in software is shutdown and the transmitted audio is wrapped around through the audio channel. Using the half duplex mode with any PSK program will allow you to see the transmitted signal in the waterfall and spectrum display. In the full duplex mode you will see the the receive audio in the spectrum display and water fall display. The full duplex mode was designed for satellite use where there are 2 radios involved, one for transmitting and one for receiving.
We can use the full duplex mode to our advantage when using a single radio to help the recovery from transmit to receive. With the single radio concept, when the radio is activated to transmit, the receive audio is muted and no audio is sent to the sound card. The DSP software computes no signal and therefor the buffers are not filled with data. When returning to receive, it will only take 1 - 2 cycles to start the process of looking for a signal. One cycle is 256 mS. In half duplex mode, the software buffers are filled with full signal data from the transmitted signal that is wrapped around to the software receiver. Returning to receive will require 10-12 cycles for the software to figure out there is no longer this full strength signal.
Many of today's radios have a monitor function that allows you to 'monitor' your transmitted signal via the station speaker. If you find it necessary to use this monitoring function and you are using a speaker output path for receive audio to the sound card, do not use the full duplex mode of operation. You will feed a distorted audio stream of the monitored signal that will conflict with the software providing possible garbage on the screen and send the DSP into areas of the unknown. If you plan on using the monitor function, select an audio output form the radio that does not contain the monitored audio.