DigiRig V1.9 sound card and Icom 2820 dual-band FM radio
Note: the DigiRig has a COM port thus the PTT function can be over the COM port using the RTS or DTR pins for SEND.
This makes it an ideal sound card for any of the “linked modes” (VARA FM) via Winlink Express or VAR AC applications.
I just hooked up my 3 yr old DigiRig V1.9 to a 10 yr old Icom 2820 (dual band FM rig with DATA jack 6 pin MINI DIN)
The DigiRig RADIO cable (4 pin, TRRS) hooks up to the Icom 2820’s 6 pin MINI DIN plug, but this cable has the RX audio on Pin 4 (9600 baud).
This is RECEIVE audio. From pin 4 the receive audio is wide-bandwidth and FLAT, often called the 9600 baud cable (grey/green color)
The other 3 wires in this cable are for TX audio and a PTT and ground.
This is the RADIO cable, not the SERIAL cable.
There is nothing plugged into the SERIAL cable socket at this time.
This is NOT the optional Digirig cable that picks up RX audio on PIN 5 (1200 baud, filtered, and narrow bandwidth).
Either RX cable should work fine, but PIN 4 is Wide-Bandwidth (6 kHz) and PIN 5 is narrow-bandwidth (3 kHz - filtered like speaker audio)
Unless I were trying to run VARA FM “wide” this cable to pick up RX audio on PIN 4 would be required.
Tx audio is hooked up to PIN 1 (the only TX audio input on the MINI DIN port.
But the ICOM 2820’s MENU is set to transmit via 1200 baud (pre-emphasized and filtered) similar to the rig’s mic jack.
I did have the option to set the Radio’s TX audio also to wide-band 9600 baud (with no audio filter)
but for now, the TX audio is set to narrow bandwidth option (1200 baud).
On most FM mobile radios the MENU SETUP determines the TX audio path.
NOTE: Tx is by PTT on the COM port (set to 9600 baud, 8 bit, 1 stop bit, no parity checking)
Thus, this setup does NOT rely on VOX for PTT (send) like a SignaLink.
SignaLinks are MONO only (left channel) and the PTT is a bit finicky with VOX and DELAY (hang-time).
However - the DigiRig is very small size and has no knobs to adjust TX audio, RX audio.
You do the levels adjusting in the software settings, and in the Windows settings.
MY Windows 10 levels
Tx audio = Speaker out audio level is set to -22 dB (10%)
This would have been less attenuation if I had set the radio to TRANSMIT at 9600 baud.
Fldigi Tx attenuator is set to -8 dB for the RF spectrum set to amplitude carrier = amplitude 1st sideband for MOD INDEX of 1.5
Thus, the DEVIATION is equal to: MOD INDEX x AUDIO freq = 1.5 x 1.5 kHz = 2.25 kHz deviation (about ideal)
Here is the RF spectrum centered on VFO = 145.690 MHz, showing that the amplitude carrier = amplitude 1st sidebands
Recorded by a RSP duo (hardware) and RSP Spectrum Analyzer (software) shown below.
NOTE: no PL tone sent on simplex, so each multiple “sideband” is 1.5 kHz from the RF carrier on 145.690 MHz
This is very different spectrum compared to AM or SSB.
If you try this on a repeater frequency with a PL tone, the spectrum looks much more complex, so adjust TX level with NO PL tone.
note: since I am transmitting at 1200 baud, the higher pitch sounds will have a greater deviation (just like human audio with a microphone).
It does not seem to cause any problems (my experience) with receiving stations set up for 1200 or 9600 receive audio.
Ideally - you might want all stations on a 2m FM net to be set up for 1200 baud (filtered) or all stations to be set up for 9600 baud (unfiltered) but
with the narrow bandwidth FLDIGI modes on FM radios like MFSK, OLIVIA, THOR, Cont, AFSK RTTY, or even VARA narrow ==> it does not seem to matter.
Almost all of these modes require a bandwidth of less than 2.5 kHz, centered at 1500 Hz.
Here is the resulting RX AUDIO (received on Pin 4 with flat audio or 9600 baud, wide 6 kHz bandwidth)
but only recorded up to 2500 Hz.
The amplitude is fairly constant over the audio range 500 - 2500 Hz (where Fldigi operates on most modes)
de k3eui barry
