PTT cable - testers wanted

K0TX · April 5, 2024, 7:48pm

I’m trying to make portable operations more portable and stimulate the development of more digital modes apps.

Computers are great, but cellphones and tables are also capable computers - cheaper and with much better battery life. Digirig Lite in a USB-C cable form factor will work for newer devices that are capable to act as a USB host. For older and cheaper devices I have designed a passive PTT cable.

I have a batch of these made and they are available in store to early adopters:

Here are the terms:

Zero risk - play with it and send it back for refund within 30 days if it doesn’t work to your satisfaction
Please report your results in this thread. I would love to hear what worked for you: mobile device, radio, app/mode, settings

Schematic of the internals for those interested:
cable-ptt.pdf (90.4 KB)

Constrainted · April 6, 2024, 12:31pm

Thanks for this opportunity.

To be clear, this can generate a tone on audio-R for PTT and payload on audio-L, right? There is no signal on GPIO3 for PTT capability, right?

73 Constrainted

K0TX · April 6, 2024, 5:21pm

Yes, let me clarify. At this time we’re looking at three options in the order of descending complexity and price:

Interface	Devices	Power	PTT	Computing Device	Use Cases
Digirig Mobile	Sound Card, Serial Port	From USB	RTS, CAT	Computers, USB capable phones and tablets	basic operation, CAT control, serial programming
Digirig Lite	Sound Card	From USB	GPIO3, R-VOX	Computers, USB capable phones and tablets	basic operation
PTT cable	None	Passive	R-VOX	phones and tablets	basic operation

The benefit of VOX-based PTT (either by tone or by payload audio) is that it doesn’t require any specific hardware support on the computing device. This is especially significant on the locked down mobile platforms such as iOS where any custom hardware support requires expensive and involving certification effectively closing door to hobby projects. Even with less locked down platforms like Android, there is an additional benefit of simplified app development.

Mike2 · April 14, 2024, 6:46pm

Hi.

With several hours trying the experimental PTT cable with a Baofeng UV-5R and a Quansheng UV-K5 there is not much success to report. Could get nothing from the Baofeng. Was able to receive APRS with the Quansheng, but not transmit.

Am using APRSdroid 1.6.3d 2024-04-03. Full volume on phone or less than full volume, it doesn’t seem to matter.

Have a Btech APRS-K2 cable which has never worked. Tried it again. Still didn’t work.

Reset the radios to factory, SQL to 1, VOX to 1,5,10 and all in-between, no difference, doesn’t work, although on the Quansheng, sometimes when I set VOX to 1 it will briefly trigger the red LED transmit light, but only once. Tried various volume settings on the radios mostly between 1/4 and 3/4 volume. Think the Quansheng liked a little more than 50%. The orange on the volume control knob has a longer finger on it that helps show where the volume is set.

Used an outdoor antenna 25 ft up slim jim and the factory HT antennas. On the Quansheng both antenna’s received ok with the farthest point being 96km away with the slim jim and about 40km with the factory antenna, but there was no transmit. Note that for the Quansheng HT factory antenna to work, the radio had to be outdoors. Would not work indoors with the factory antenna.

The Baofeng would not work at all regardless of antenna. Even the plain radio set to 144.390 is inconsistent. Probably just me but I’ve never had good luck with the 2 Boafengs I have. On again, off again, always something. Never solid.

Am semi-rural and using any HT is a major challenge. Regardless of mode, can usually receive something but transmission is always tough with a factory HT antenna, if it works at all. Usually it doesn’t. In this case however, I would think the red led light for TX would at least go on, but it doesn’t. Apparently it doesn’t even try to transmit.

Am at a loss.

No plug and play as yet. Not that I expected that.

Want to add for the type of customer aspect that I am regular guy. Not an engineer or high tech wiz. Willing to fiddle around a bit but with that in mind and doing the best I can, we are in need of some instructions/directions that ought to work–which I’m sure we are trying to figure out. Am hoping others will give this a try too and compare notes.

UPDATE: I knew no sooner than I would post that something would pop up. Some progress with the Quansheng. Red TX led now going on!

Here are the settings for the Quansheng UV-K5 radio: SQL 1, VOX 7, volume knob–the longer orange finger–at the back edge of the knob protector built on the right side of the radio, or about 2 o’clock.

APRSdroid settings: Volume on my phone about 60%. When the recent log is shown the word AFSK shows near the top line of my screen, have the volume to the K in AFSK. In the connection settings using AFSK, have connect log check marked, high quality demodulation shut off–not check marked, audio output is voice call, frame sync at 600ms.

Still not getting picked up on APRS.fi but the TX is happening and my other radios I think hear it. Just the sound of the transmit that is.

Also am in Michigan and on my front porch outdoors it kind of faces northwest. With the factory Quansheng HT antenna, I was receiving from Wisconsin, farthest was 211km, which is about 130 miles…with the HT sitting on a patio table. Cool.

Now, the next step is to get my signal out and heard. Will report what happens.

UPDATE 2: APRS.fi has me now. Someone about 15 miles away heard me. However this was with the 25ft outdoor slim jim antenna, not the factory HT antenna. Will add updates as I learn more. Main thing. the PTT cable works!!!

UPDATE 3: What was learned

If the settings that worked for me do not work for you given that you might have a different radio, phone, or app, please consider these generalizations.

There are settings for the radio and settings for the app software. Both the radio and the software must each be set correctly and must also balance with or against each other as they work together.

For the radio it seems there are 3 areas to focus on.

The volume knob on the radio
The VOX setting
The squelch setting (probably set at 1)

For the app there are also a couple of things that seem to matter. Regarding APRSdroid that is.

The volume setting on your phone or tablet
The connection AFSK settings, especially the frame sync setting, which involves the timing of the app with the radio’s VOX setting. Getting these settings to be ok with each other got transmit to work for me. Trial and error.

If one or more of the settings is beyond some certain magic and unknown range, something will not work.

Currently getting this combination to work is a seat of the pants effort. I hope the settings I found will work again, but I don’t know. They work now. With my radio, phone, and app software version.

Also, wouldn’t you know it, my old Btech APRS cable now works perfectly with the settings I found!

WHAT?? Yes, hours and hours in the past and more hours presently, used up because of a few simple settings I could not comprehend or see. There must be a better way. An idiot proof special meter of some sort?

If these settings keep working for me, I will have a quick “plug and play” setup. If they don’t, back to square one.

Here’s to hoping you will figure this out faster than I did.

K0TX · April 15, 2024, 4:53am

Mike,
Thank you for your detailed report. Helluva effort. This cable is very new and I’m only starting to collect the info for the documentation. Your findings will also help other operators.

I understand that you achieved the transmission by your radio’s VOX. This is certainly an option that is always available. The cable also implements VOX of its own so it can be used with transceivers that do not support this feature internally or it is not good enough.

The audio payload is on left channel while the VOX is on the right channel. Some apps like APRSdroid don’t differentiate and send payload into both channels, others like WoAD support PTT tone on one channel while payload on another. If you want to trigger cable’s VOX-PTT then you want maximum volume output your phone/tablet can deliver.

Another setting to look out for is “No-Op Preamble” or similar. In APRS Droid it can be found in Preferences->Connection Preferences->Frame Sync Prefix. This generates a tone of the configurable duration before sending payload giving VOX and radio time to get going. This is usable regardless if you use VOX in radio or in cable.

Please keep sharing any of your new findings.

Mike2 · April 15, 2024, 4:41pm

Thank you. Yes the “No-Op Preamble” timing was one of things that made the difference in getting the cable to work for me. That and getting the radio’s VOX setting to be happy with that timing. Or perhaps vice versa. I don’t know.

A few thoughts.

From the perspective of someone who has purchased a BTECH APRS cable that appeared to not work (be junk), I think you would agree that that is not a good feeling to associate with a product.

From a business perspective, it is much easier selling and supporting a product that people love and buy more of versus a product that leaves them with a failed unfulfilled experience.

Regarding this, I wonder if a checklist or an instruction sheet are enough to achieve a good experience out of the box with this cable. There are so many variables at play, most of which are adjustable, causing success or failure depending on the choices made.

The cable is in the middle. A bridge between hardware and software. A “smart cable” even though I hate the term SMART on anything, but a cable that has an led light or something, a standard point of focus, that could provide feedback as to what was happening on either end of the connection would be super helpful.

As it is, we adjust both ends of the cable in blindness. Something beyond persistence or high IQ to help to sort the proper settings out would be greatly appreciated.

Thank you for your designs and products. Love the digirig. Would like to love this PTT cable too, but you know, it beat me up pretty good!

I have WoAD but have never used it. Haven’t gotten that far yet. But it’s good to know there is a PTT option for that with your cable. Will update if/when I get to it. Sounds like a great feature.

N5XHI · April 27, 2024, 12:08am

Here is my setup for now. I like the Digirig Lite option with a USB-C cable.
I am looking for an option for my wife’s iPhone running radio mail. It has a 8 pin lighting plug. The adapters are 8 pin to TRS. I don’t know if this cable would be the answer for me and the iPhone?

N5XHI · April 27, 2024, 12:49am

I am looking at this adapter for the iPhone. If you make the Digirig Lite with USB-C the PTT by tune should work. Option #2 use the above cable with a lighting to TRS cable and a TRS to TRRS adapterand the radio cable. With four cables there might be a lost of gain/volume? Thank you for looking at other options for us smartphones users.