The appeal is simple: keep one radio, reach every network. An openSPOT4 Pro carries a real vocoder chip, so it can convert between modes that no software-only hotspot can bridge. The reality has one significant gap, and it is worth knowing before you choose which radio to keep.
An openSPOT has two sides. On one side is the radio in your hand, talking to the device over the air on a single frequency. On the other side is the network the device is logged in to. Cross mode is simply letting those two sides be different modes, with the device converting between them in the middle.
That is why one radio can be enough. The radio never changes mode. The network does.
| Your radio | D-STAR | DMR | C4FM | NXDN | P25 |
|---|---|---|---|---|---|
| D-STAR | Yes | Yes | Yes | No | |
| DMR | Yes | Yes | Yes | No | |
| C4FM | Yes | Yes | Yes | Yes | |
| NXDN | Yes | Yes | Yes | No | |
| P25 | No | No | Yes | No |
Read a row as "my radio is this mode, and these are the networks I can reach". Every D-STAR entry above requires the Pro.
The standard openSPOT4 converts in software and has no D-STAR cross mode at all. Its list is DMR to C4FM and NXDN, C4FM to DMR, NXDN and P25, NXDN to DMR and C4FM, and P25 to C4FM.
There is a second difference that shows up on the air rather than on a spec sheet: the standard model cannot adjust the audio level of a converted stream, which its own manual says may leave you hearing others loudly while they hear you quietly, or the reverse.
Three of these modes share a speech frame. DMR, C4FM in DN mode and NXDN all carry AMBE+2 at 2450 bits per second of speech plus 1150 of error correction, so converting between them is repacking rather than converting, and software can do it.
D-STAR is different. It carries the older AMBE at 2400 plus 1200. The channel total is the same 3600 bits per second, which is why people assume it should interchange, and the split is different, so it does not. Reaching D-STAR means decoding the speech all the way back to audio and encoding it again, and that is what the Pro's vocoder chip is for.
P25 is different again, using IMBE, which is why it sits apart in the matrix.
It reaches C4FM only. There is no P25 to DMR, no P25 to D-STAR and no P25 to NXDN in the device, in either direction.
And the one path that exists carries conditions:
Even on the Pro, this particular conversion is described as software based rather than as work done by the vocoder chip.
C4FM has two voice sub modes, and which one you need depends on where you are going:
| Going to | Your C4FM radio must be in | If it is not |
|---|---|---|
| DMR, NXDN or D-STAR | DN, Digital Narrow | Silence at the far end, with no error shown anywhere |
| P25 | VW, Voice Wide |
The same applies in reverse. A C4FM station transmitting in the wrong sub mode reaches your D-STAR, DMR, NXDN or P25 radio as silence, and nothing on either radio explains why.
This one behavior accounts for most reports that a cross mode setup has stopped working.
Each mode identifies people differently, so something has to translate. The device uses an association between callsigns and IDs, and what happens when there is no association is worth knowing.
| Path | What is sent |
|---|---|
| DMR radio out to D-STAR | The callsign associated with your DMR ID. With no association, a configured default callsign, and if that is unset, your DMR ID as plain text in the callsign field |
| D-STAR radio out to DMR | The DMR ID associated with your callsign |
| D-STAR or NXDN calls coming back | The callsign associated with the far station's ID |
| D-STAR network in to a DMR radio | All of it arrives as a group call on TG9. TG9 must be your transmit contact or in a receive group list, or you will not hear it |
| Out to NXDN | A configured default destination ID, rather than a talkgroup you pick per call |
| Private DMR calls in to an NXDN radio | Converted to NXDN group calls |
The practical consequence: keep the ID association current, and expect a bare number rather than a callsign at the far end when it is missing.
Everything above is one small box doing the work that otherwise takes a chain of programs on a Pi. The trade is the usual one.
| openSPOT4 Pro | The software bridges | |
|---|---|---|
| Reaching D-STAR | Yes, with the onboard chip | No converter exists for DMR or C4FM to D-STAR. It takes a transcoding reflector |
| Reaching P25 | From a C4FM radio only | From DMR or C4FM, with software transcoding |
| Reaching M17 | Not supported | Yes, from DMR, with software transcoding |
| Setup | A web page and a few settings | Config files, ports and services to keep running |
| Cost | The device | Free, on hardware you likely already own |
| Claim | Checked against | Strength |
|---|---|---|
| The full matrix, including every P25 restriction | SharkRF's own openSPOT4 manual, the cross modes pages, read per source mode | The manufacturer's own documentation |
| D-STAR pairs need the Pro, and the Pro has an AMBE vocoder chip | The same manual, quoting "onboard transcoding hardware with a genuine AMBE vocoder chip" and marking the D-STAR pairings Pro only | Primary |
| The standard model cannot adjust converted audio level | The same manual | Primary |
| Which C4FM sub mode is needed for which destination, and that the wrong one gives silence | The manual, stating it separately on each mode's page | Primary, stated several times |
| TG9 for incoming calls and the selective call requirement | The manual | Primary |
| Identity mapping and the fallbacks when no association exists | The manual | Primary |
| Why D-STAR needs real conversion while the other three do not | The vocoder rates: AMBE 2400 plus 1200 against AMBE+2 2450 plus 1150, from the vocoder maker's own published split and the shared error correction routine in the MMDVM source | Two origins, plus the device's own capability split as corroboration |
| M17 is absent | Its absence from every cross mode page in the manual | An absence, stated as one |
Prepared for n6jet.com. Firmware changes: the capabilities above come from the manufacturer's current manual, and a firmware update may add or alter them. The software alternative to all of this is covered on the DMR and Fusion cross modes pages.