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Thread: audio input/output noise

  1. #1

    audio input/output noise


    I have made a few projects and I can't get the audio output completely free of noise. I use teensy 3.5 or 3.6 plus audioShield.
    I have read many threads and try hard to avoid ground loops, but my output (or input) is never completely quiet, even with battery operated projects.

    For exemple, I have connected the audio output from a pro audio mixer to the line input of the Teensy audioBoard. I monitor the line input with the peak object to trigger events when audio is detected. Dealing with low threshold levels is hard due to constant noise. My mixer is a good one, and it is grounded. I have also tried with a battery powered mixer (a professional one) but I still have some noise. I have checked mixers output and it is free of noise.

    I certainly miss something somewhere, so I need guidance to solve this problem. The noise sounds like "digital noise" : high pitch noise, fast small cracks, no hum.
    I have in my "junk box" a pair of Jensen audio transformers. How can I wire them to isolate completely the audio input of the teensy ?


  2. #2
    Junior Member
    Join Date
    Sep 2020
    Could be completely unrelated but I had added some code to my audio project to read two axis of a joystick (to use as pitch/mod - MPK style) so in loop() it was doing regular analogRead()s. I was getting "noise" on the audio so I basically went through the code "#if 0"ing bits of it out of action to see if I could locate the source of the noise and it was when I removed the stuff doing the analogRead()s for the joystick that the noise stopped. So you could try an approach like that perhaps? And if you specifically have code doing analogRead()s (as you might if you are reading Pots and things) then perhaps it's that?

  3. #3
    Thanks for your answer.

    Unfortunately, at the moment there is no pots. I must search elsewhere.

  4. #4
    Senior Member
    Join Date
    Apr 2019
    Hello, if it sounds like a digital noise, is it from the code? Does the noise appear on USB digital audio output if you add a USBAudio object and compile with Audio?

  5. #5
    Senior Member
    Join Date
    May 2015
    As much as possible, switch to digital audio transmission.

  6. #6
    Senior Member
    Join Date
    Jun 2018
    Hello emmanuel63,

    This is a common issue that I've faced over the years while working with pro-audio. It is my professional opinion that the reason for the noise is coming from the quality of the DAC, and wires that are being used. I've played with the SGTL5000 and I found that I was getting similar noise from it. This isn't much different than the noise I'm used to combating when connecting a PC audio output to a pro audio mixer. The chips used in PCs just aren't professional quality.

    I'm not knocking it like it's no good, it's a very well spec'd chip. In my opinion though, it's just not great for pro audio. If you think about it - it makes sense. For consumer audio, we are talking very low wattage. A bluetooth speaker, or consumer grade sound system, only pushes 5 watts/25 watts/100 watts maybe? When you are hooking up to your pro audio mixer, I assume you also have pro audio speakers pushing 1000+watts! It's 10+ times louder than consumer grade stuff. So the noise is amplified quickly and therefore more audible. These low-end DACS (or codecs) are designed for the low volume or headphone solutions, not for pushing out to hundreds of people over a pro audio system. :-)

    To directly answer your question on how to eliminate it - I've had success with Transformers (aka Hum Eliminators, Ground Loop Isolator, Noise Eliminator, etc). It's very possible that this would eliminate the noise in *most* environments, but they aren't perfect. They will shape your audio somewhat, and usually result in lower bass response. Here is one that would probably work but there are many of them out there. You can also buy the actual components found inside them and put them on a PCB... But this is adding another part to your system, and complexity in terms of wiring and so forth.

    To truly eliminate the noise you should consider a higher quality DAC, and consider using balanced audio cables. You can use the hum eliminator mentioned above, and then direct the audio into a balance box, and then out to your mixer... But again, more complexity and cost. Plus it's not truly balanced, because it's coming out the DAC as single ended. The ultimate solution is to choose a high quality dac with balanced outputs...

    This is why I'm designing my "Teensy Pro Audio" modules. My kickstarter campaign should launch shortly and if successful I will sell them to the community. You should look at my PCM5242 DAC module. They are in development but my final "pre-production order" will arrive next week. If they check out - they are ready to build in quantities. I've been listening to my development modules over 1000 watt speakers for months and it sounds perfectly clear, no buzz, no hum, no clicks, no pops. When the music stops, you hear no hiss... Silence - it's great.

    I hope you can get to the bottom of it soon, and if you have any questions about your noise, or my project please let me know!


  7. #7
    Senior Member
    Join Date
    Jun 2018
    PS - after re-reading your post I realize you are doing INPUTS into the teensy. I wrote my post regarding outputs but the same theories apply. I also have an ADC card in pre-production that I will reveal in short time too. This ADC also has high quality balanced inputs.... The link will be on the PCM5242 forum post when details are posted.

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