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Old 15th October 2012, 05:30 PM   #1
fez79 is offline fez79  Pakistan
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Unhappy modulation-like distortion of music at low volumes

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!

(I wasn't able to attach the FM transmitter's datasheet - too big. But just google "Seeed Studio WLS03281M")
Attached Files
File Type: zip SSS 1623 headset datasheet.zip (501.3 KB, 4 views)
File Type: zip usbaudiomp3_soundcardaudio.mp3.zip (947.7 KB, 3 views)
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Old 16th October 2012, 08:00 AM   #2
qusp is offline qusp  Australia
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its most likely noise on your USB ground
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