General Hints 

Sound Card: 

MRP37 uses a direct access to the sound card hardware, thus so-called
"Soundblaster compatible" cards often will not work. So use an original
Creative Labs Soundblaster card (Pro, 16, 32 AWE, 64 AWE) whenever
possible. That does not mean that your compatible card will not work at
all; it depends on the program. Some programs will work with many cards,
some only on original SBs. Just try, but keep in mind that you may
encounter problems. 

Always install the latest driver available, since older drivers might
contain bugs. - Some programs (e. g. W95SSTV, ChromaPix) that run under
32 bit operating systems (Windows 95/98, NT) may require a 32 bit sound
card driver. Even a driver for Windows 95/98 does not automatically have
to be a 32 bit driver - sometimes it is a 16 bit driver and you will not
even know. 

Do not tune the input and output volumes to its maximum, 1/3 usually is
enough, larger volumes can cause distortion since the audio amplifiers
on sound cards are not quite linear. 

Connections: 

When using the "Microphone" input of the sound card, the signal will be
lead through the microphone preamplifier. The "Microphone" collects all
environmental sound this can cause minor performance drop. Best way is
to apply signal to the "Line In" input. Same is applicable for the
output; use "Line Out" instead of the "Speakers" output. 

When connecting to the radio, usually a direct connection will work. If
you encounter problems with hum or others (noise caused by the computer
etc.), try an audio transformer or a capacitor in the signal path to
avoid galvanic connection. Consider the impedance of the sound card and
your receiver / transmitter. 

At the receiver, use a fixed level audio output e. g. for tape
recording, if your receiver has such an output. If not, use the audio
output but tune it to about 1/3 of maximum volume - more can cause
distortion. High speed data (such as 9k6 packet radio) will need the
connection to the discriminator output of the FM chip, the normal audio
output will not work. 

Transmitting: 

If the program has transmit capability, the radio needs to be switched
to transmit mode. Sound cards do not have a PTT (push-to-talk) output,
so additional hardware might be neccessary. Many programs use a simple
device like the DSR or RTS pin of a serial port to switch the PTT, this
signal will switch a transistor which sets the PTT input of your radio
to ground level. Please read the programs documentation to see what
device will be neccessary. Sometimes (when fast switching is not
required) an audio Vox circuit or the Vox of your radio can do the
switching.