Lessons Learned — A difficult but successful MultiMode project

How the W6CX DV Repeater moved to MultiMode Digital Voice

W6CX’s Dashboard & Equipment List

In January 2021, the Mount Diablo Amateur Radio Club’s Technical Committee decided to experiment with upgrading our highly successful ICOM D-STAR repeater to digital voice MultiMode. I was asked to assemble and configure the new repeater. The original objective was to enable both D-STAR and DMR (later we also enabled Fusion). Our repeater sits atop Mount Diablo in the San Francisco East Bay and enjoys a wide footprint. We wanted to make this resource available to more club members and other RF users.

It was a difficult journey, but most of the problems were avoidable, and I’d like to pass along what I learned in hopes others can avoid my mistakes.

Here’s a summary of my key learnings

      • Don’t try to use a Motorola XPR 8400 repeater (or similar such as Quantar) with MultiMode if you want DMR to work
      • Check that RX audio levels don’t exceed the modem’s max voltage and if necessary, insert resistance in the audio line 
      • If DMR is planned, then use a Service monitor, otherwise use a SDR-RTL dongle with open source software to tune/calibrate
      • If problems, ask for help from more knowledgeable repeater experts

Background
Testing began with a Motorola XPR 8400 repeater in analog mode.  This repeater is extremely well built, rated 100% duty cycle at full power.  Initially I used a Repeater Builder STM32-DVM MMDVM modem board.  This board and associated software on the Pi take over as the repeater controller.  After several failures (that it was later discovered were not the fault of that modem), I moved to a ZUM Radio MMDVM-Pi board.  Both modems are products of long design and test cycles and are first rate.  We achieved success with the ZUM modem and it is now in production.

Process

        • I configured the 8400 for analog, flat unprocessed audio both in and out, then using the trial and error tuning of the RX and TX pots on the modem, I quickly got D-STAR to work and C4FM (Fusion) as well. I figured I was almost done….
        • Moving onto DMR, after very methodically tuning, I got DMR TX to pretty much work, and RX mostly worked in simplex/DMO mode, but worked only 10% of the time in full duplex.  I tried everything, including placing ferrite beads on all cables into the Pi and modem, but no success.
        • I posted questions on both the OpenDV MMDVM forum and the Repeater Builder forum. On OpenDV, I got considerable help, in particular from Graziano – IZ5IGB.  Without those posts from him, I would likely have given up. Thank you, Graziano.
        • Graziano’s posts and articles emphasized that if you don’t possess the technical knowledge to do this right, you should seek expert help. My first mistake was to wait too long to ask.
        • Long-time Digital Voice veteran and friend Don – WB5EKU advised me on the project via QSOs.  He is owner and trustee of K6PUW, a large-footprint MultiMode repeater covering much of the greater Los Angeles area.  He also highly recommended that I get help and use a service monitor.
        • It is now well-known that DSP receivers cause a phase shift in the RX signal sent to the modem. IZ5IGB was one of the first to document this phenomonem. Check his 2017 (Italian) article.  Given DMR’s TDMA, timing between radio and repeater is critical, and the phase shift prevents DMR full duplex from working. The solution is changing the MMDVMHost DMRDelay setting from 0 to some value that compensates for that RX delay. This works well on DSP repeaters like the MTR 2000, where DMRDelay of 165 solves the phase-shift problem.
        • But Scott Zimmerman – N4XCC, designer of the Repeater Builder modem, posted that he had tried  to configure MultiMode for DMR on a Quantar repeater, whose radios are also of DSP design. He concluded that the amount of the Quantar’s phase shift varied by audio frequency. Given that most MMDVM users deploy radios without this “feature,” the MMDVM software does not address this kind of phase shift. The DMRDelay setting assumes the phase shift is constant across the audio spectrum, which for most DSP radios, it is.  So it appears the MMDVM software cannot deal with variable phase shifts that vary by audio frequency.
        • Scott suggested I should give up on the 8400. I thought since it worked 10% of the time, I could fine-tune to success. Second big mistake.
        • Scott’s post prompted me to acquire an external analog radio (started out with a GM300). This allowed me to reset the DMRDelay back to zero. At that time, I also began testing with the ZUM Radio MMDVM board.  Its designer, Jim McGlaughlin – KI6ZUM, had participated with G4KLX in parts of the original MMDVM modem design.
        •  And I finally sought help from Trevor, WA6JAU, who oversees club repeaters at our repeater site.  He found that the audio voltage from the GM300 exceeded the modem’s maximum, so he added resistance to the RX audio line.  That change and switching to an analog receiver solved our TDMA timing issues.  Then, using a service monitor in conjunction with the MMDVMCal program, we were also able to achieve the correct deviation for DMR transmissions.
        • It no longer made sense to continue using the 8400 just for its transmitter, so I bought a pair of Motorola CDM1250s on eBay. I tuned audio settings using an SDR-RTL USB stick with SDRSharp and MMDVMCal to achieve Bessel null of the carrier.
        • Our MultiMode repeater went on the air Nov 20, 2021. Due to user interest, we decided to also enable C4FM mode, and that was done remotely without any problems.  Bit error rates are low on D-STAR, DMR and C4FM, and I have not needed to do any extra tuning from home.  From our early tests, the repeater seems to have at least as good range as our previous D-STAR repeater.

Conclusion
The process was long and I made some key mistakes. But I learned a lot and it feels great to achieve success. I hope this article will save time for others who might be starting a similar project.

Jim – K6JM
December 12, 2021

Useful Links

While I can read some German, I made liberal use of Google Translate whle researching this project.

IZ5IGB – Il ponte Motorola MTR 2000 e la MMDVM, storia di un fidanzamento difficile
(The Motorola MTR 2000 and the MMDVM, the Story of a Difficult Project)
https://www.grupporadiofirenze.net/2017/03/20/ponte-motorola-mtr-2000-la-mmdvm-storia-un-fidanzamento-difficile/ 

DL4OCH – Abgleich von Repeatern mit MMDVM-Modem
(Configuration of Repeaters with MMDVM Modem)
https://web.archive.org/web/20180508152849/http://dl4och.de/MMDVM_Abgleich_V0.02.pdf

KP4MD – Frequency Deviation Measurement with an RTL-SDR Dongle
Although this is not about MMDVM, it describes how to install and configure SDRSharp to function as a Spectrum Analyzer
https://www.qsl.net/kp4md/freqdev.htm

N6IB – Interfacing the MMDVM to a pair of CDM 1250 Uhf Radios
https://www.bi7jta.org/files/CDM1250_GM338_notes-for-interfacing-the-cdm-1250-uhf-radio-with-the-mmdvm-board.pdf

IW8ELN – MMDVMCal procedura di calibrazione per MMDVM NTH Made in Digiland
(MMDVMCal Calibration Procedure for MMDVM)
https://digilanditalia.wordpress.com/2018/05/05/mmdvmcal-procedura-di-calibrazione-per-mmdvm-nth-made-in-digiland/

N5AMD – How to tune your MMDVM repeater with an SDR for low BER
https://n5amd.com/digital-radio-how-tos/tune-mmdvm-repeater-sdr-low-ber/