Richard G7LEE

Richard G7LEE

OHIS Audio Interface Standardisation for Multiple Operator working

5 minute read

I came across the Open Headset Interconnect Standard recently, hat-tip G1LRO with his Universal Radio Controller. OHIS’s elevator pitch summarises the problem well

The Amateur Radio community has standards for DC power (13.8v +/- 15%, Anderson Powerpole connectors, etc) and RF (50 ohm coax, PL-259/BNC/SMA connectors, etc). But we have no such standards for the interface between the user and the radio. Is the microphone a dynamic, or electret? Is it balanced, “pseudo” balanced, or unbalanced? […] There are so many different standards for microphone, headphone, and PTT that it is improbable that one could take their preferred headset and connect it to any radio without an adapter. […] With the Open Headset Interconnect Standard, or OHIS, the club/EOC can build/buy one adapter for every radio which stays with that radio, and the user only needs to build/buy one adapter for their specific headset which stays with their headset, and now they can achieve full interoperability with only O(N) adapters.

As the old saw says1, “The wonderful thing about standards is that there are so many of them to choose from.”

XKCD

https://xkcd.com/927/

We have seen this movie before. Raynet users of a certain vintage will recall the CAIRO standard that we used for Exercise Blue Ham.

CAIRO (Communications Audio Interface for Remote Operations)

Not sure if Raynet still use it, there’s not much from Google about Raynet and CAIRO, and the original CAIRO website from the 1990s now goes to a dis, the Internet Archive has a copy

I’ve used CAIRO personally, it is similar enough to OHIS, indeed a CAIRO to OHIS adapter would be easy and purely passive, as CAIRO mandated the same electret input standard. OHIS have done well to standardise the DC powering at 5V limited to 200mA, I was always a little uncomfortable with CAIRO’s presentation of 12V on a DIN plug pin, particularly if there’s a car battery behind it even via a 20A fuse.

HT radio two pin connector - Kenwood, Baofeng

Most but not all HT radios use the two-pin connector Icom came up with for the IC2E from 1980. This is now often called the Kenwood connector Whatever devilry was it that tempted them to put the PTT through the mic line2, making it a pain to wire commonly grounded mic, speaker, PTT accessories to a HT? Because of this weirdness you can connect a properly wired CAIRO accessory (with independent Mic L earth) to a HT with a CAIRO adaptor, but you can’t wire a HT accessory to a regular transceiver unless you do something else with PTT.

6-pin Mini-DIN packet/data connector

There is also a Japanese 6-pin mini-DIN Packet/data connector which is fading as modern rigs present an all-digital USB audio/CAT interface for computer data modes. Confusingly this carries analogue audio, not data. It could probably be adapted to OHIS because while there’s no electret bias on the mic connector, there is a +5V supply that could be pressed int oservice for that role, with a suitabel blocking capacitor to the rig. Mini-DIN pins are fragile, and this is even more fraught when stuck on a back-panel next to a mini-DIN socket with a different number of pins for the CAT connector, as on the FT897.

too many RJ-45s on our rigs with different signals and different power pins

There’s a lot to be said for OHIS. For now I will stick with CAIRO because OHIS has one dire hazard for me. I have a FT897 and an Anytone both of which present a RJ453 connector for the mic socket, neither are OHIS, and both carry power, on different pins. I heard of one unfortunate chap who let the magic smoke out of a Yaesu FTM-500 because Yaesu don’t share the mic pinout with you like every other radio manual I have seen, and he shorted the mic power trying to igure out what was what. CAIRO saves me from that, there is no way a CAIRO DIN connector can get plugged into the RJ45 of my radios4. However, I am all set for a CAIRO to OHIS adapter if it catches on.

I can’t help feeling that the RJ-45 is the Achilles heel of OHIS. It’s not a particularly robust connector, Wi-Fi came along just in time in the late 1990s to save us all from the snapped off hook connector from our LAN cables for office laptops, and that hook makes it fiddly to get the mic out of my FT897, you can’t just snap it off else the plug leaps out at the end of the curly cable to hit you in the eye. OHIS isn’t going to make friends with the poor fellows who plug OHIS into their legacy rigs - four of the eight pins are ground so you have a 50% chance of shorting the mic power contacts. As they say in the doc

Since the RJ-45 is a very common connector, all OHIS sockets Should be labeled clearly as OHIS.

manufacturers and their fancy microphones

Hopefully that won’t happen to too many people and OHIS will gain ground. It resolves one issue many manufacturer RJ45 connectors miss - they are so busy putting features onto the mic that they haven’t got enough contacts left over to carry receive audio. That is what’s probably going to stiff OHIS for manufacturer support - there are no spare pins to support data lines for fancypants features or even a basic up/down facility. Marketing isn’t going to be happy with a dumb microphone for your new shiny rig. I quite like the dumbness of CAIRO/OHIS. I’m not that keen on fancy mics with lots of rig features, I can see the point of up/down keys for a mobile op, but frequency entry keypads and the like, not so much5. I do wish OHIS hadn’t gone quite so bonkers with the extra ground wires. Sure, you need a separate ground wire for the mic6, due to the low level, but audio L and R can share grounds with the PTT. CAIRO retains a separate ground for the mic but that’s it.

  1. Probably Andrew Tanenbaum ↩

  2. Of course some manufacturers ring the changes on which pins are which, or make the shunt resistance finicky. ↩

  3. Strictly speaking an 8P8C connector but everyone knows it as an RJ45. ↩

  4. I can, of course, plug the wrong CAIRO adapter into the wrong rig, these adapters need obvious labelling ↩

  5. the cynic in me wonders if the fancypants mic buttons are there to compensate us for the miserable rubbish touch screens manufacturers foist upon radio amateurs when they moved away from real front panel controls, along with dire UI design. ↩

  6. In practice most of the time I short mic L to ground at the CAIRO DIN plug (which goes into the very short transceiver adapter) and I haven’t observed a problem, though I can understand that if you are at the end of 50m of audio cable you don’t want to do that at the operator end. ↩