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Thursday
May022013

The mysterious Icom ID-RP2C analog/digital switch

When I received my D-Star repeater equipment from Icom Australia in 2008, the first thing on my to-do list was to pop off the covers and take a look inside!

Inside the Icom ID-RP2C D-Star controller I noticed an innocuous two position multi-pole PCB mount slide switch.  Each position was marked on the PCB; "analog" and "digital".    

I remember asking myself,  "Hmmm, D-Star is most definitely a digital radio mode...  I wonder what this switch does and why isn't it mentioned in the ID-RP2C user manual?", and then I closed the lid of the ID-RP2C, installed it in the 19" rack.  Although it did pique my curiosity the first time I saw it, I've never really given it much more thought.

There have been pictures of the interior of the ID-RP2C published on the internet for as long as the ID-RP2C has been available.  There used to be a Canadian D-Star web page that explored the interior workings of the Icom D-Star equipment.  I'm not sure who was behind it, but it was a fantastic treasure trove of technical information... but there was nothing on the digital/analog switch in the ID-RP2C.  I don't think I have ever read anything about the Analog/Digital switch in any of the D-Star forums or mailing lists I subscribe to, nor heard any discussion on-air about it.  Very intriguing!

Time now to jump forward to the present... this week I started thinking about the Analog/Digital switch again and looked through the ID-RP2C Service Manual, searching for info on the switch.  I found the section of schematic that showed S2, the strange Digital/Analog switch.  

S2 switches the data lines (RD, RC, RE, TD, TC & TE) normally going to/from IC17 to the fourth RJ45 socket (looking at the back of the ID-RP2C) to an 8 way internal socket labelled J25, mounted right next to S2. The data line designations on J25 are suffixed with the letter A... maybe for Analog??

 

At that moment the proverbial light bulb lit up above my head! A few years ago, Satoshi Yasuda 7M3TJZ (creator of the original D-Star Node Adapter/GMSK Hot Spot modem board), quietly added a single page .PDF document to the download section of his website.  Entitled shogen_4_2_8.pdf, it is an English translation of a previously unseen section of the original Japanese D-Star protocol specification document.  This section does not appear in the familiar English protocol document, shogen.pdf.

**Update** The Japanese language D-Star specification DOES show section 4.2.8.

Section 4.2.8 of the D-Star spec describes the operation of an Analog Bridge, an interface that uses an AMBE vocoder to "bridge" the digital D-Star system and an analog FM repeater. 

While it is an assumption, I'm convinced that the Analog/Digital switch, the J25 header and J24 power socket has something to do with section 4.2.8 of the D-Star specification and the operation of an Analog Bridge, linking the D-Star network to a traditional FM repeater!  There is most definitely enough space and threaded mounting posts inside the ID-RP2C for an additional PCB. There is also a blanked off cutout on the rear panel... possibly where the connection to the FM repeater would be.

 

J24 is not mentioned in the service manual, block diagram or circuit description.  But it IS in the schematic and PCB overlay diagram. J24 is connected to the 12V DC input, perfect for running an add on board.

Reading section 4.2.8, D-Star header information is transmitted on FM using 1200bps MSK. This header is identical to the traditional D-Star header transmitted by D-Star radios. 

An MSK modem and other supporting hardware would be required on the users FM radio to en/decode the D-Star header information. When the "Enhanced Analog" D-Star repeater transmits in response to an incoming DV stream on the internet connection, it would transmit the short MSK D-Star header and then the AMBE2020 vocoder would turn the DV voice packets into audio to be transmitted over FM. 

When transmitting back to the repeater, a users FM radio would send a brief MSK D-Star header for the MSK modem equipped Analog Bridge in the ID-RP2C to decode. The Analog Bridge would then encode the received FM audio with the AMBE2020 vocoder and send it via the Icom RS-RP2C gateway software as a normal D-Star DV stream. 

The MSK header data bursts would probably sound a little like MDC1200 used on commercial repeater networks.

I don't think the Analog Bridge board would need to have two AMBE chips, the FM repeater function would work through a conventional audio link between TX and RX. It would be pointless to encode AMBE audio from FM and then immediately decode it to be played back over the repeaters transmitter in FM.

Of course it is all well and good to have an Analog Bridge at the D-Star Gateway end, but you also need user radios that can generate the 1200 baud MSK D-Star header information... and these just do not exist! Icom obviously thought about D-Star specification section 4.2.8 enough to include S2, J25 and J24 on the ID-RP2C PCB, but that seems to be as far as it progressed.  Other than what can be found on the PCB overlay and schematic diagrams, there is NO mention of S2, J25 and J24 in either the ID-RP2C User or Service Manual... or indeed anywhere on the internet.

One advantage of this Analog Bridge system would be that ANYONE can listen to this enhanced FM repeater and be able to hear what is going on, even without having the ability to transmit the special MSK D-Star header data.  Of course they would not be able to communicate back through the D-Star Gateway without the ability to transmit the MSK D-Star header.

Shogen 4.2.8 does mention an "adaptor of analog FM rig" so in addition to a D-Star MSK header enhanced FM rig, there may have had been external MSK modem/display designed that could be added to any radio... maybe in the form of a "smart" microphone?  But unless someone from Icom Japan or the JARL D-Star Working Group discloses the original plans, I guess it's all speculation!

Amazing to think that analog FM cross connectivity is in the D-Star specification!  The Japanese language D-Star document I have has 50 pages!!  Shogen.pdf only has 12.  I wonder what else in the Japanese document hasn't been translated into English?

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Reader Comments (2)

The way my IC-91A acts when I turn it on (by displaying my callsign data ), it's almost as if Icom had thought about adding the MSK capabilities for analog repeaters to it, but again not following through. I do have a translated copy of the Japanese D-Star Specification document, but it's mostly diagrams like the ones you see in the radio owner's manual; nothing really helpful to figuring out how to do things.

May 10, 2013 | Unregistered CommenterMatthew N8OHU

I maybe wrong but it sounds like the IDAS system Icom and Kenwood use in the commerical world. I wonder if anyone has the IDAS controller and would look inside and compare it to the DSTAR controller

May 13, 2013 | Unregistered CommenterJames Potter n4nid

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