Setting Up Sound Levels for FM Digital Operation

FM Sound Card Digital Modes tune-up Procedure With Signalink USB Interface and Fldigi Software

Turn radio on set up on a suitable simplex frequency and set audio output to 1/3 to ½ if using ext speaker jack for your interface. If it is a fixed level from the Aux port on the radio it gets easier…only set up the radio audio to monitor the RX if needed.

Open Fldigi,set mode to bpsk-31,set cursor on the waterfall to 1500Hz, open mixer applet on computer by right-click on speaker icon.

Set up Fldigi audio….make sure it is looking at the Signalink for both TX and RX audio. (USB audio codec)

On your computer open the sound mixer and select the USB audio codec. There should be some level sliders for it on the screen.

TX audio is Play: WAV or PCM and Master. set level to 80 percent or higher…If you don’t the Signalink may not trigger the PTT on your radio.

RX audio is Record: line in or mic in.

Set audio sliders up..not muted. You should be able to adjust levels at the Signalink knobs now.

Open squelch on radio to allow white noise through.

Adjust RX audio levels by waterfall action. The noise should show a broad spectrum of random dots mostly blue, with a touch of yellow speckles. If you see vertical lines every 60 Hz then you have a ground loop hum problem. Try turning up the radio (signal source) audio and reducing the RX input gain on the Signalink sound card. If that doesn’t clean up the waterfall enough then use an isolation transformer. Once the waterfall level is set, close the radio squelch again.

Now set TX audio level.

Click the TUNE button top right. As radio transmits a single tone, listen to it on another radio. I keep a handheld radio handy for this. Set the HT volume to about 1/2 too 2/3 so it won’t distort. the tone should sound smooth, not sharp. If it pulses on and off, then the sound card sample rate is incorrect. Turn up the TX knob on the Signalink until it sounds sharp, then back off until it sounds smooth or dull. The principle is signal source high, then reduce it down the line at each step to the radio mic input so that it drives the radio about like your voice would properly modulate it.

Go to the bottom rows of buttons and click the T/R button to send an idle signal for 20 or so seconds then type your callsign for ID, then click T/R again to end TX and get a waterfall report from a buddy watching your signal. The waterfall should be fairly clean and show a double line showing up yellow, any sideband lines should be down to blue or none. On FM you don’t have to be picky about IMD readings and total lack of splatter because your signal is capturing a whole channel. As long as the other station is copying reliably it’s OK. One thing I will mention on the Signalink the Delay knob sets the TX/RX turnaround time. Most modes you want the delay short or in step with your radio. There is a reason to set the delay long and that is if you are using CW or feldhell mode in Fldigi. In that case a short delay will cause pumping of the PTT during a transmission. All the other modes transmit continually so it holds the PTT on until TX is finished sending.

Now you are ready to change to a working mode like MT63-2000, bpsk-250 or 500, or MFSK-64/pix mode.or close Fldigi and open UZ7HO Soundmodem or whatever soundcard digital program you wish to operate . The audio levels should be the same.

After the initial setup, the procedure can be shortened to the first station on the air TX a tune signal of bpsk-31 at 1500Hz, other stations click on it and take a turn TX while others watch their waterfall. If any stations TX audio settings are way out of line RX observing stations suggest correction. When you become well practiced this tune-up procedure is very quick and easy.

Under marginal FM RX conditions with dropouts MT-63 will copy due to interleaving FEC.

Dxing on FM use the Domino EX-4 mode and open the radio squelch all the way. Program DSP looks for symbol tones in the noise and prints out still when packet or voice would never get through!