Source of Noise at 1khz + each harmonic?

Source of Noise at 1khz + each harmonic?

Postby safiire » Tue Nov 15, 2011 8:14 pm

Hello, I have created a wavetable polysynth with Maple and the Audio Codec Shield.

I have been sampling the output of the ACS through my external soundcard and running it through a the spectral analyser from Ableton Live, and I notice that regardless of which program I have flashed onto my Maple, I see a -88 dBFS spike at 1000hz and all harmonics of 1000 after that until it aliases. This noise is present immediately upon starting the Maple, before any of my programs run.

I've eliminated a few sources, it is not coming from my MIDI cable on Serial3, or my soundcard, and not specific to any programs flashed on there.

I kinda assume it is from the USBSerial interface (I have called USBSerial.end() though), but it is not specific to the computer it is plugged into either, it does it on both computers I've powered it by so far. I don't have a battery set up to power the Maple, but I might try that.

Has anyone else with ADS and a Maple seen this, and perhaps know how to get rid of it? Preferably still allowing me to power it by the USB cable.

Thanks :)

-Saf
safiire
 
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Re: Source of Noise at 1khz + each harmonic?

Postby safiire » Tue Nov 15, 2011 8:17 pm

Also, btw, the Audio Code Shield is really awesome! Thanks :)
safiire
 
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Re: Source of Noise at 1khz + each harmonic?

Postby guest » Wed Nov 16, 2011 4:49 am

i seem to recall a similar problem
and it had to do with the usb
i think SerialUSB.end() doesnt actually shut everything off
i will look around for the fix i made

if you run the stock sinewave generator program
does it give the same problem?
what is the volume when it is full up?
im curious about the relative noise level of -88dB on your device
the codecshield is only 16b
so thats -96dB for the lowest noisefloor

the other thing it might be
is a slight difference between the codecshield and your soundcard
in terms of sampling rate
so they beat against each other
there isnt much in terms of an output filter on the codecshield
so there will be a fair bit of 44khz in the output

can you hear the noise on headphones
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Re: Source of Noise at 1khz + each harmonic?

Postby guest » Wed Nov 16, 2011 6:13 am

i found it
from the good folks at leaflabs

as a workaround, try including nvic.h and calling
nvic_irq_disable(NVIC_USB_LP_CAN_RX0); from within setup().

and if that doesnt do it
and if you happen to have an arduino
see if the arduino gives the same response
if it does
then id assume its an aliasing problem
between the codecshield and soundcard
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