RF Hum in DMX transmitter - TIME SENSITIVE

Other Hardware General Electronics
1

Hello, Open Sourcers - I'm just over 24 hours to curtain (okay, there's no curtain because it's a black box theater) and I'm in final testing for this insane challenge I've created for myself. A newbie to Arduino, soldering, electronics. Over the last seven weeks I've logged >200 hours on this project in my "spare time" -- going from knowing nothing to knowing just enough that I think I can make this work. Here's what "this" is:

I perform a "One-Guitar Show" -- a solo, acoustic, two-act musical about a guitar that rehabilitates people while riding the Amtrak rail system called "Miss Isabella Rainsong and Her Traveling Companion." I've done the show successfully several times, but it requires a light tech and sound tech to run it. I've been wanting to take the show "on the road" while keeping costs down, so I have been building an Arduino stack to put inside my guitar so I can run the show with a button I've mounted on the top of the guitar to send out all the light and sound cues.

I tried doing this with an RF transmitter alone from the guitar (a key fob) and it wasn't proving reliable enough timingwise -- some of the triggers are in the middle of a song and timing is critical. So I stuck with my original idea of putting it all in the guitar (plus, it just seemed cooler). This means:

  • DMX RF coming off a TinkerIT DMX shield
  • Two separate R and L mono wireless transmitters off a Adafruit Music Maker MP3 player (L is for all the pre-recorded audio and R is for thunder sound effects which trigger a lightning generator)
  • A fourth wireless transmitter coming off and internally mounted ToneWood Amp for the guitar

Hardware:

  • Arduino Uno
  • Adafruit Music Maker MP3 Player shield
  • TinkerKit DMX shield
  • NightShade Electronics energyShield (basic or Pro2)

Wiring:

  • Arduino Pin 10 jumps to DMX pin 3 to avoid MP3 conflict
  • 10K resistor from 8 to GND -- sink for button
  • Button from 8 to button to 5v -- button runs to outside of the top waist of the guitar body.
  • DMX wireless tranceiver power cable goes to GND (striped neg) and 5v (+) (note, I attached the ground to the TinkerIT shield - don't know if this matters.

In testing, all of this has worked pretty well. The DMX definitely gives off noise -- a hum coming from the transmitter or the Arduino itself -- but this was positional and as long as it wasn't close to a speaker, it worked fine. Now that I have it all in the guitar, it's humming more loudly and I can't seem to position my way out of it.

When I put my hand in the guitar and near the DMX antennae, it goes away, so I'm assuming it's some sort of ground issue. I'm a real novice when it comes to electronics, so I really don't have a sense of how to deal with this and I'm running out of time to experiment. Fortunately the noise is localized to the guitar and doesn't carry into the PA, but it's enough to feel like I've got a loud mosquito honing in on me and it's really distracting. I'm also concerned that it will get louder during the performance and I won't have any way to deal with it.

Any suggestions on easily replicating my hand inside the guitar? :wink:

Attached is the code for the Arduino Mega 2650 as a text file. Here is a blog post providing more detail including a video to show you what I'm doing.

This is the first time I've posted to the community as I've been able to solve most of my many, many issues along the way by reading your posts and solutions and tutorials and videos. So very grateful. Can you help me get across the finish line in style?

If you're in the North Baltimore area, come see the show this Friday or Saturday night. Reservations are FREE at www.MissRainsong.com/events. It's also a CD release party for the companion CD to the show. Thanks in advance!

Ross

20190801RainsongMega.txt (46.7 KB)

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Like a mosquito? Then it's feedback, not a hum.

And proper positioning of power, output and input wiring will be the answer. Keep each type as far as possible from the other!!!!

If still a problem, shielded wire will be needed, with one end of the shield connected to the project common ground. You do have a common ground point, don't you?

Paul

3

How did you determine the hum was caused by the DMX RF Shield ?
To my knowledge that is a CANBUS like protocol that is used in theaters to run lighting control signals from the lighting console operator to the motorized color gel scrollers , pan and tilt spotlights and other theater lighting equipment. It is a technology that allows very long cable runs (from the ground floor of a theater to the balcony , etc).

The transmission frequency is typically 512Kb/per second which is well beyond human hearing range.
I'm not sure what could be causing the hum. What is your power source for all this equipment inside the guitar ? (hum is usually associated with AC power , not battery power so have you tried running that equipment off a lipo battery ?)

Worst case you might need to add active filters to the input of your guitar amp (line in) to filter out the noise using low pass filters. Can you list the amplifier equipment ?

DMX RF coming off a TinkerIT DMX shield

Why are you using that instead of some other wireless technology like bluetooth ?

a hum coming from the transmitter or the Arduino itself

On what do you base that assumption ?

What data can you provide that proves it is coming from the transmitter or the arduino ?

Fortunately the noise is localized to the guitar and doesn't carry into the PA

You say it is not getting into the PA so then how are you hearing it if it is not coming out of the PA speakers ?

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So I did a little more digging in the last couple of hours. The DMX RF is a Donner 2.4GHz wireless transmitter. The stack is powered by a NightShade energyShield 2 Basic. So it's DC built into the Arduino stack.

The ToneWood Amp is what was picking up the frequency of the transmitter. When I removed it, the feedback went away. Essentially, the TWA has a speaker in it that uses the soundboard of the guitar to amplify the guitar and add effects (like reverb, tremolo, and plate). I am able to replicate what the TWA does using an effects box at the mixer, so I'm good for now. It's too bad, because my next step (some months from now) was to hack the TWA (with the company's help -- their developer was going to update the firmware for this application) and be able to change the guitar effects at different times during the show.

So for tomorrow's show, I have a solution that will get me through Friday and Saturday. But I'd like to understand how to limit or eliminate this issue for future uses and get back to using the TWA if I can.

You do have a common ground point, don't you?

I connect the DMX to ground through the DMX shield. But keep in mind, this is all in a guitar -- a graphite guitar, so I'm not sure it's all grounded in the end. Again, my understanding of electronics is limited -- not sure how you get that to ground. One note is that I didn't have the right male-to-female in adapters for the DMX shield, so I soldered wires to the transmitter (it is essentially a three-prong XLR cable connector) and connected those wires to the shield posts with the provided clamp screws (GND, D- and D+). I also attached the power for the transmitter to 5V and GND on the DMX shield. Those were not shielded wires that I used. Could that be an issue?

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And thank you!

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Audio and 2.4 Ghz is not my area of expertise but 2.4 Ghz is so far above human hearing that I think that the carrier signal can be ruled out which leaves the input stage.

When it comes to audio signals in the microphone/pre-amp stage, they are so sensitive to picking up hum
that as a general rule, any wire that isn't the ground wire should be shielded. If you could see electromagnetic waves (like Superman), you would see that all those wires are totally immersed in a bath
of electromagnetic soup, so yeah they should be shielded. I assume you are not aware that the D+ and D-
are differential pair 5V signals used in DMX equipment. They are the equivilent of CAN H and CAN L in CANBUS. Without seeing the schematic it's hard to say if those are the culprit. You just have to hope for the best and plan for the worst. Electronics is not too forgiving to people who break it's rules. It's pretty much a strict religious discipline. It would probably not be untrue to say that Electronics Engineering techs and Engineers are more religious than the Pope in Rome with regards Electronics. Sins are NOT forgiven.
You might want to remember that. I like to say there is a special Hell reserved for people who don't read
datasheets, it's a place where nothing works and everything burns up and bursts into flames when you turn on the power. (a bit of an exaggeration) but for the record I have personally witnessed a small 8-pin IC explode and burst into flame and glow red, melting the plastic solder-less breadboard due to one little (or maybe big) violation of the rules. I had to replace the breadboard.

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Silly idea for a quick fix. Can you use a foot switch for DMX so you can put the transmitter on the floor away from the preamp?

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I guess there's only one way to find out.

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Well, I must have upset the EE gods royally last night as they caused all sorts of mischief in the hours before the show and, just when I thought things were stable again, the whole thing stopped working about 10 or 15 minutes into the show. But because this is more about the music and story, I just soldiered on with no fancy lighting and explained how the ending was supposed to work (which is lighting/blacklight dependendent) after the show. It was still pretty magical despite the epic tech fail.

So I think the central issue I'm having is that I've isolated the Arduino and components all inside the carbon fiber body of the guitar with no place for ground to go. Tonight, I'm going to give using the Arduino at the control table plugged into my computer, with my wife pushing the button at the beginning of every scene and song. That should pretty much take care of it except for a couple of moments when timing is everything, but I'll give her some cues for that. If I can't get that working in the next two hours, we will just do audio from my laptop like the old days and be content with flipping the power on and off on the lights.

Then after tonight is in the books, I'll take a little break from the mayhem that has been my deep dive trial by fire into all things Arduino and electronics. After the rest of my life has been attended to, I'll look at going back to my original plan B, which is keeping the electronics out of the guitar and just making a remote control key fob type trigger. When I tried that before, I wasn't getting enough signal and it wasn't reliable enough (using the Adafruit one button momentary RF trigger). That was several weeks ago and I have learned much since then. I'll be able to take my time developing a solution that will be reliable. If you have suggestions about the best way to remote trigger the Arduino Mega (now that I have plenty of pins to play with), I'm all ears (or, I guess, eyes).

10

Final update on this topic. Saturday night's show went without a hitch on the technical front. Once I moved the Arduino stack out of the guitar, resoldered some of the connections, removed the battery, and ran power from the USB, it all worked flawlessly. The EE gods were mollified and all was well.

I wrote a bunch of new Serial.println messages so that my wife would know when to push the button for one of the 37 lighting/audio cues during the show.

I now need to build a remote trigger solution that will work consistently. The button can't be:

  • infrared or anything that requires line of site (I can't point the device at the receiver)
  • on the floor as a stomp button (too many places I need to be on stage)
  • bigger than a key fob

The button needs to be:

  • highly reliable at a distance of up to 50 feet
  • instantaneous to trigger music cues in time with a live performance
  • small enough to be very discrete / hidden
  • silent or nearly silent - no loud click!

Other future enhancements:

  • I might be trying to add some lights inside the guitar for a glow effect in one scene.
  • I want to have a metronome feature where I can set beats per minute flashes -- either embedded in the guitar where only I can see it or on my lighting rig behind the audience.
  • Then there's the synchronized drone dance over the heads of the audience... :wink:

Any suggestions about adding a nano to the guitar that won't be bothered by ground issues would be appreciated!

Thanks for all your help!

Ross

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Have you looked at any of these ?