Decodium Mobile over Decolink

How to bring the radio's audio — and rig control — from the shack computer to your phone: at home on the same network, or anywhere at all through the server.

Decodium 4.0 Mobile Decolink · v3 protocol 17 August 2026

Before you start

On the computer

Decolink running next to the radio, with the rig's USB CODEC connected. It sends the audio and, if you want, serves the CAT.

On the phone

Decodium 4 Mobile. Everything is set in one place: the ⚙ gear at the top, sections Station, Audio and CAT / Rig.

Only for the relay

An approved account on decolink.ft2.it. On a local network you don't need one: no registration, nothing leaves the house.

The three connections

Decolink and Decodium Mobile must be on the same mode: it is the commonest mistake, and it gives no clear message.

Mode	When	Who calls whom	Port	To open
LAN	Phone and computer on the same WiFi	The computer fires the audio at the phone's IP	5555	Nothing
Relay	Away from home, mobile data included	Both reach out to the server	5555	Nothing
Home	Away from home, without the server	The phone calls your home address	5555	UDP 5555 on the router

The relay is the one that always works: neither side has to be reachable from outside, because both are the ones making contact with the server. It is also the only one that needs an account.

Route A · local network

On the computer · Decolink

Send the audio to the phone's address

Radio audio → pick the transceiver's USB CODEC.It is the input the received signal arrives on, not the computer's microphone.
Mode → Direct LAN
Host → the phone's IP address on your home WiFi.On the phone you find it in Settings → WiFi → the network you are connected to. It looks like 192.168.1.42.
Port → 5555
Press Start. The level should move with the band noise.

On the phone · Decodium Mobile

Start listening

⚙ → Station: callsign, locator, report.Without callsign and locator the messages cannot be composed.
Audio → Connection → LAN
Port → 5555, the same one set in Decolink
Press Receive. “Listening on port 5555” appears: from then on the waterfall fills up as soon as the computer starts sending.

On LAN the phone waits, it does not call

It is the computer that has to know the phone's IP. If the router hands out addresses in rotation, tomorrow that IP will belong to someone else and the audio will not arrive: reserve a fixed address for the phone in the router, or check the Host field every session.

Route B · relay on the server

Once only · from the browser

Request access to the station

Open https://decolink.ft2.it/registrati
Fill in email, password, callsign and the station you want to connect to.
Wait for approval.The request stays pending until the station owner accepts it: they decide, it is not automatic. They also set your role.

Owner: listens and transmits — it is their station
Operator: listens and transmits — authorised by the owner
Listener: receive only — the transmit button stays off

On the computer · Decolink

Bring the station into the relay

Server → decolink.ft2.it, then email and password, and press Sign in.If you leave the station running on its own, tick “remember the password”: after a restart it comes back with nobody at the keyboard.
Mode → Relay + station
Host → decolink.ft2.it · Port → 5555
Profile → PCM 48 kHzCompressed profiles get down to twenty-five times less, but save them for later: first make sure the link holds up like this. The table below says what each one costs.
Press Start.

On the phone · Decodium Mobile

Sign in with your credentials

⚙ → Audio → Connection → Relay
Fill in the sign-in fields and press Enter.
Check the status line: it should give your callsign, the station and the role.If it says “You are in as a listener”, you receive but do not transmit: that is the role, not a fault.
Port → 5555, then Receive.

Sign-in: decolink.ft2.it
Email: the one you registered with
Password: the one for your Decolink account
Relay: leave empty: it takes the sign-in address
Port: 5555

The permit lasts an hour

The server issues a pass that expires after sixty minutes. With “Remember the password” on, the app renews it by itself a minute before; without it, after an hour the audio stops with no explanation and you have to sign in again by hand. On the phone the password lives in the system's encrypted keychain, out of the backups.

Route C · the phone calls home

For those who would rather not go through the server and have an address reachable from outside. In exchange you have to get your hands into the router.

On the router: forward port UDP 5555 to the shack computer's address.
In Decolink: mode The phone calls home, port 5555. The Host field stays empty: it is the phone that makes contact.
On the phone: connection Home, in the address field your DynDNS name (for example iu8lmc.ddns.net), port 5555, then Receive.

With a home connection behind CG-NAT — many fibre lines and nearly every mobile one — this mode cannot work: nobody outside can reach you. That is exactly the case the relay exists for.

Commanding the radio

Audio and CAT are two separate things: you can receive without CAT, but without CAT the phone does not know what frequency you are on and cannot change band or key the transmitter.

Source	How it connects	Where it works
TCP to the PC	In Decolink tick “Serve the CAT to the phone”; on the phone put the computer's IP and port 4532	Local network
USB-OTG	A cable from phone to radio, no computer in between	Anywhere, with the radio within cable reach
Audio channel	The commands travel inside the audio link that is already open	Anywhere — the right choice with Relay or Home

With the audio channel there is nothing extra to open: for it to work two things must be true — the network audio already started, and CAT switched on in Decolink.

How much bandwidth it takes

The profile is chosen in Decolink and applies to the whole link. Between the first and the last there are three orders of magnitude, and that is no detail: it decides which links are possible at all.

Profile	Bandwidth	When
PCM 48 kHz	808 kbit/s	Works with everything, no compression. It is the starting point: 364 MB per hour.
PCM 12 kHz	232 kbit/s	Plenty for SSB, which only occupies 2.7 kHz. If the phone plays it sped up, go back to 48.
Voice (Opus)	32 kbit/s	Phone: twenty-five times less than PCM. Needs an updated phone at the other end.
CW (Opus)	20 kbit/s	Narrow-band telegraphy: forty times less than PCM.
Digital	146 kbit/s	Compressed without losing a bit: FT8 and the like decode just as they do on PCM, because it is the same signal.
CW keying	2.4 kbit/s	Only the keying instants. You lose all the context — QSB, QRM, anyone calling off frequency — but it goes through anything.

Below 20 kbit/s the wrapper weighs more than the contents: every packet carries 28 bytes of IP and UDP headers, and lowering the codec further stops helping. That is why several frames are grouped into one packet: at 40 ms per packet you save 18% of the bandwidth, and the extra delay cannot be felt.

When nothing arrives

“Listening on port 5555 — the PC must send the audio here”You are in LAN mode but you wanted the relay. On LAN the phone just waits: no registration goes out, and you sit waiting for audio nobody will send. Change the connection to Relay.

“Name not resolved”The address in the field does not exist, or the phone has no network. Check you have not included https:// in the relay field.

“Port 5555 busy”Another app is holding that port. Close it, or use a different one — changing it in both programs.

“server unreachable”This is a connection problem, not a password one: the app tells the two apart on purpose. Check you have network and try again.

The audio starts and stops after a few secondsThe relay drops anyone silent for fifteen seconds. If it keeps happening, the network is losing packets: switch to the Voice profile, which takes twenty-five times less than PCM.

The waterfall moves but nothing decodesNearly always it is the clock: digital modes stand or fall on the exact second. Let the phone take the time from the network.

Trying it without a radio

The Demo button, next to Receive, lights up a made-up band with five stations and decodes scrolling by. It is there to show how the app behaves before connecting anything — useful for learning where to look, before going hunting for a fault that isn't there.