From: Giovanni C <uc...@gm...> - 2011-11-12 16:47:21
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Hi, while trying to recording the "stereo" silence, from line input left/right, (i.e. line input floating), I found some noise, as depicted in the following: http://old.nabble.com/file/p32831818/Schermata%2B2011-11-12%2Ba%2B17.36.15.png This noise bursts have a period of about 250msec (4Hz), and inside are modulated by a 50Hz (20 msec) quasi sinusoidal wave: http://old.nabble.com/file/p32831818/Schermata%2B2011-11-12%2Ba%2B17.39.33.png The overall spectrum is reported below: http://old.nabble.com/file/p32831818/Schermata%2B2011-11-12%2Ba%2B17.36.15.png This file is obtained by using the following command line: rec -L -c 2 -s -2 -r 16000 -q -t ogg -f ./file.ogg trim 5 0:10 All capture devices are shutdown, except AUX line input. All lights, Monitors, Scopes, probes, whatever, in the range of 5 meters is switched off. Overo has wifi On. This noise is always present: I does not matter if I supply the board from PSU or from battery. Noise is ALWAYS present. Which could be the noise source? how do I remove it? Thanks in advance. -g -- View this message in context: http://old.nabble.com/overo-fire%3A-noise-in-audio-line-input-tp32831818p32831818.html Sent from the Gumstix mailing list archive at Nabble.com. |
From: Clifford H. <cli...@gm...> - 2011-11-13 00:08:36
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On 13/11/2011, at 3:48 AM, Giovanni C wrote: > while trying to recording the "stereo" silence, from line input left/right, > (i.e. line input floating), Why is the line input floating? If you want to know what the quiescent noie floor is, you should tie it to ground with a resister having the same impedance as your source. > This noise bursts have a period of about 250msec (4Hz), and inside are > modulated by a 50Hz (20 msec) quasi sinusoidal wave: Do you have 50Hz main power? This could be a switch mode power supply whose control loop is operating in discontinuous (squegging or blocking-oscillator) mode. These things radiate like crazy, which makes amateur radio operators sad... Overlaid on the 50Hz cycles, is there a periodic signal in the 10's of KHz present (possibly aliased by the Overo's sample rate)? That would be the frequency of the SMPS oscillator... Since you get the problem also when running from batteries, it's either generated by the Overo itself, or is radiated noise (not conducted) from an external source - so take it further away. If you get the same thing, it's on-board. Clifford Heath. |
From: Giovanni C <uc...@gm...> - 2011-11-13 08:42:49
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Hi Clifford, thank you for your reply. Actually by comparing the schematic of my custom board with that of tobi, I am confident problem comes from separation of analog from digital ground: in the tobi board the analog ground is ties to the ground of aux line connector; in mine it tied to a big common ground plane. Sampling rate is at 16KHz and, as I said before, there is no light, video, radio or whatever switched on in the range of 5 meters. >From a TI forum I learned that the internal impedence of auxin/auxout line is about 5k Ohm: if so, I could make another test putting a resistor of 5k between each input and ground. Please confirm. Thanks. -g Clifford Heath wrote: > > On 13/11/2011, at 3:48 AM, Giovanni C wrote: >> while trying to recording the "stereo" silence, from line input >> left/right, >> (i.e. line input floating), > > Why is the line input floating? If you want to know what the quiescent > noie floor is, you should tie it to ground with a resister having the > same impedance as your source. > >> This noise bursts have a period of about 250msec (4Hz), and inside are >> modulated by a 50Hz (20 msec) quasi sinusoidal wave: > > Do you have 50Hz main power? This could be a switch mode power > supply whose control loop is operating in discontinuous (squegging or > blocking-oscillator) mode. These things radiate like crazy, which makes > amateur radio operators sad... > > Overlaid on the 50Hz cycles, is there a periodic signal in the 10's of KHz > present (possibly aliased by the Overo's sample rate)? That would be > the frequency of the SMPS oscillator... > > Since you get the problem also when running from batteries, it's either > generated by the Overo itself, or is radiated noise (not conducted) from > an external source - so take it further away. If you get the same thing, > it's on-board. > > Clifford Heath. > > -- View this message in context: http://old.nabble.com/overo-fire%3A-noise-in-audio-line-input-tp32831818p32834163.html Sent from the Gumstix mailing list archive at Nabble.com. |