modulation-like distortion of music at low volumes

Discussion in 'The Projects Forum' started by fez, Oct 15, 2012.

  1. fez

    Thread Starter Member

    Dec 6, 2009
    I'm having this weird problem where music distorts at LOW volumes, rather than high in this project of mine.

    I have my electric guitar going into a hacked (as in, torn apart and modified slightly) usb headset - the sss 1623a, through's it electret mic input, and I have audio coming out of the headset's audio-out lines. Now, this in itself was working excellently - very little noise, and virtually no lag in recording thanks to ASIO drivers. Now I'm trying to extend this further by wirelessly transmitting the audio from the headset via a PLL-based FM-radio transmitter - the WLS03281M from Seeed Studio. This is where the weird thing happens that the audio I receive on my mp3player-cum-radio is noisy, and gets even noisier when I lower the volume on my computer (i.e. volume out from the headset)! And yet, when I exchange the headset's audio-in wires for the computer's own internal audio out, the audio is much much better, hence tolerable. What is up with the weird modulation-like noise when the volume is low? Surely some of you have experienced it when working with audio circuits?

    I experienced it myself when earlier I had I fed the headset's output directly to my guitar amp - the exact same kinda noise, at low (headset-output) volumes. Back then I resolved it simply by replacing the headset's output capacitance (~uF, see datasheet schematic) with a much smaller value (~pF) - I thought that perhaps the time constant had gotten too large because of the large input impedance the amp presents. And I turned out to be right in the sense that the weird noise did indeed go away, I could now reduce the volume on my computer and the amp would play it nice and well, only, just not like a speaker, cause you know... it's an amp.

    But now, with the FM transmitter module, I seem unable to make the noise go away. I simply use a coupling capacitor to connect the headset's headphone-out to the transmitter's L/R-in. I've tried different values of capacitances but the symptom doesn't go away - the only noticeable change is the transmitted audio gets very thin with small coupling capacitances (as it should because there's a high-pass filter at work and with small coupling caps the HPF cutoff is kinda high up in the audio spectrum). So it would seem that there's something other than a large RC constant at play? But the noise is so similar and has the same characteristics! What do I do?
    I have two samples attached - soundcardaudio.mp3 is music recorded from my MP3 player radio when the FM transmitter was taking audio-in from the computer. usbaudio.mp3 is the case when the FM transmitter was taking audio-in from the usb headset. In both files, I turned down the volume on the computer around the end - notice the noise differences, both during constant, loud volume, and during the volume decrease.

    Another problem here is that the headset's audio-out has a DC bias of 1.7V for some reason, whereas my computer's headphone out had no DC bias. Even more so, the transmitter's L/R-inputs also have a DC bias of 0.7V (the transmitter as a whole is operating at 5V)! So capacitive coupling cannot be avoided.

    To make sure my usb headset audio-out isn't malfunctioning, I connected my headphones to it through 100uF coupling caps (as per datasheet schematic) and played the same music - it was completely clean, no problems. This is the very reason I haven't discussed my guitar-input connection to the usb headset, because the problem is at its OUTPUT and has no dependency on the input to function correctly. Also, no, I do not believe there is something wrong with my mp3 player since nothing like this particular kind of noise has happened with me on any other FM station/channel.

    Also note that yes the FM transmitter is fine too, I have been able to program it successfully. Though there are several aspects of it (e.g the charge pumping, LSI test etc) that I don't understand.

    Please help!
    Last edited: Oct 15, 2012
  2. Audioguru

    New Member

    Dec 20, 2007
    I have no idea what causes the "weird modulation-like noise". I heard it on both of your MP3 recordings but the USB audio had much louder noise than the sound card.

    You say you are feeding the (high level) guitar signal into the (very sensitive) electret mic input on the USB headphone. Doesn't it produce severe overload distortion?

    I hope you connected the headset outputs to the line-level inputs on the transmitter and not into the very sensitive microphone inputs.