开云体育

Is there any news of AGC being added to the QMX transceiver some time soon?

The reason I ask is that I just had a VERY painful experience using my freshly built QMX. The headphone output can suddenly jump from reasonably loud to EXTREMELY LOUD AND CLIPPED when listening to strong stations. The clipped audio is at such a high level that it can cause pain and is potentially damaging to the ear. I'm reluctant to use headphones with the radio now I know this is a possibility, which is a shame as that's how I intended to use it when out /p.....?

This is in no way a complaint. I know the radio is a work in progress and it's pretty incredible as it stands, but some sort of audio limiting or AGC really should be added ASAP to prevent further hearing damage for us poor listeners.?

Cheers,
Ronan
MM0IVR

Hi Ronan,

I had exactly the same problem, but luckily I was listening via a powered speaker at the time, so it limited the ear damage.

I have stopped using my working QMX until AGC is implemented as I don't want to experience that again!

73,
Jon G4IVV

I'm primarily a sound engineer by trade and have some algorithms/code from some audio dsp I've written which I'm willing to share if desired

I posted a couple days ago about a super simple albeit kludged option involving some diodes to limit the voltage on the headphone cable.? Might be worth trying.

On Thu, Aug 24, 2023 at 02:19 PM, N1EDC wrote:

I posted a couple days ago....

I saw that. Similar things were tried with the QCX and do work up to a point but they're not a proper solution. I'd rather not have to bodge the hardware when the problem can be sorted out gracefully in firmware.

I'm sure Hans will get round to it during his daily spare 12 seconds.... ?

Joking aside, this really should be a priority as there is very real potential to cause hearing damage with these sudden violent jumps in volume. Similar to KE0WVA, my day job is in music and sound production so I kinda need what's left of my hearing to stay undamaged.

Cheers,
Ronan?

Ronan,
I was the guy that posted the limiter circuit back when. Here is a link to it:
No, it does not distort until levels get quite loud. This is essentially the same as what the telephones utilized for nearly a century so nothing new.
I did publish a simple RF AGC circuit in January 2021 page 33 (since improved) but is not applicable here. Hans will eventually implement AGC in the QMX as a superior firmware feature. This is a true Software Defined Radio and the audio processing is entirely digital, making Automatic Gain Control just a software code entry. Values beyond a selected level are simply re-mapped to provide low distortion compression. Just like Flex and the other "Big Boys" do.
In the meantime, you might try an external jack (if you don't want to use a speaker) with the resistor and limiting diodes. It's cheap and effective. Try it yourself and see if it works. There will always be naysayers and detractors but I always advocate trying something just to be sure. Especially something so simple and cheap. Pennies really.
It's a hobby. Have fun!
72, Don

Hi Don,
Yes, I remember your original posts and did try the diode method on a couple of QCX's until the little AGC module was developed. As I said, I'm sure Hans will include AGC limiting in a future firmware update so I'll not bodge this radio for the time being.

An external adaptor is an option, as you say, but it's yet another thing to pack/forget/lose.... besides, it's just limiting the level of an already badly distorted signal. Much better to have the levels controlled gracefully at an earlier stage in firmware and avoid those nasty square waves in the first place.

Cheers,
Ronan

I don't think anyone is arguing that the diodes are the best solution to the problem long term, but they can be done now with minimal cost and effort, and will help protect your hearing.? They will also cause no problems when the eventual polished solution is released as they only have an effect at very loud audio levels.

I'm not super concerned about "bodging" a radio that is home built to begin with - I always considered that part of the fun so long as the modifications were made well, but an external adapter would solve that problem for you, and you only have to worry about forgetting it until the polished firmware solution is released.

This is a very serious problem and in my opinion the QMX should NOT be used with headphones as there is a very real risk of hearing damage.? The first time it happened to me, I was using some JVC headphones (32 Ohms).? I was probably on 40m and I must have had the "volume" up quite high when trying to hunt a weak POTA station.? The next station that transmitted was a powerful station which was EXTREMELY loud!? It was so loud and intense that I reared back in my chair and swatted the headphone clean off of my head and they ended up on the floor.? I felt pain in my ears for over an hour after that so it is likely that I suffered some degree of hearing damage.? It's hard to quantify exactly, but I would say the volume level felt about the same as someone screaming in your ear from and inch or two away!

I shelved the QMX until version?1_00_014 of the firmware was released.

I decided to give the radio another try.? This time I was using a more expensive set of headphones which I normally use with my FTDX10.? Unfortunately, the same thing happened to me again.? This time I managed to rapidly yank the headphones off of my head in a somewhat controlled manner but I very nearly ended up damaging an expensive pair of prescription glasses.

I do CW POTA activations and I prefer using a headset as the Morse code can be quite annoying to other park users.? My QMX is going back on the shelf until this issue gets sorted out.? In the mean time, I am investigating various AGC / limiter circuits, both passive and active.? In addition to the simple diode circuit described earlier, I have found a couple of other circuits which I will attach to this post.? I would very much appreciate any other recommendations or information.

Kind Regards, Lyndon