I'm not knowledgeable in DSP, but maybe someone here can help. I have a cheapo electret microphone that is giving a bad hum, at 15754hz and 60hz Any idea for a problem here or a quick fix. It's a cheapo so hopefully you can point me to something simple like installing a new ground or something easy. -- Phil
microphone hum
Started by ●January 31, 2006
Reply by ●January 31, 20062006-01-31
Phil wrote:> I'm not knowledgeable in DSP, but maybe someone here can help. > > I have a cheapo electret microphone that is giving a bad hum, at 15754hz > and 60hz60hz is probably from not having a common ground. That's common for desktop computers hooked up to stereos or tvs and getting lines or that hum. As for 15754hz that's probably aliasing from the sample rate you chose. A simple low pass FIR could filter PCM samples for you and eliminate the hum. You could make it a bandpass around 100hz-4000hz if you're doing voice. Tom
Reply by ●January 31, 20062006-01-31
On 30 Jan 2006 20:58:25 -0800, "Tom St Denis" <tomstdenis@gmail.com> wrote:> >Phil wrote: >> I'm not knowledgeable in DSP, but maybe someone here can help. >> >> I have a cheapo electret microphone that is giving a bad hum, at 15754hz >> and 60hz > >60hz is probably from not having a common ground. That's common for >desktop computers hooked up to stereos or tvs and getting lines or that >hum. > >As for 15754hz that's probably aliasing from the sample rate you chose.The combination of 60Hz and 1575xHz points to interference from a (probably north american) television set or video signal. 60Hz is the vertical frequency, 15750Hz is the horizontal frequency. Regards, Allan
Reply by ●January 31, 20062006-01-31
Phil- Sorry this is probably a really basic question, but I'm new to electronics/dsp. Read your post with interest. How did you determine the frequencies of the hum? Spectrum analyzer? Thanks, Mike
Reply by ●January 31, 20062006-01-31
Phil wrote:> I'm not knowledgeable in DSP, but maybe someone here can help. > > I have a cheapo electret microphone that is giving a bad hum, at 15754hz > and 60hz > > Any idea for a problem here or a quick fix. It's a cheapo so hopefully > you can point me to something simple like installing a new ground or > something easy.You have an analog problem, and you're tight that it has to do with grounding. There are better groups than this for your question, but you're here. so here goes: The 60 Hz is power-line pickup. That commonly happens either through electrostatic pickup because of inadequate shielding -- the shield must be grounded, of course -- or because of hum currents circulating in a ground loop. In your case, I suspect bad shielding (broken ground wire?) because you are also picking up the horizontal frequency of a monitor. Poke around and see what changes. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●January 31, 20062006-01-31
Mike wrote:> Phil- > > Sorry this is probably a really basic question, but I'm new to > electronics/dsp. Read your post with interest. How did you determine > the frequencies of the hum? Spectrum analyzer?Oscilloscope? Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●January 31, 20062006-01-31
Yup, 15734 Hz is NTSC. On a clear day I can pick this signal up from a distance of 100+ meters. This is surprising since high frequencies have much more attenuation in air than lower bass frequencies. Just shows how loud the retrace can be and how adaptable the human brain is at notching noise signals out. For an audio file, a spectrogram, some analysis, and a bit of TV flyback fun see: http://www.baudline.com/mystery_signal/12_answer.html Enjoy,
Reply by ●January 31, 20062006-01-31
Great replies, thanks, a couple of clarifications in response. The mic is plugged into plug-in-power of my minidisc recorder which is probably 1 to 9vdc, and the recorder itself operates on 1.2 v battery, but I am near the computer monitor or tv when I noticed it, but it doesn't always indicate hum and I couldn't figure out why it was not consistent even when I play with the wires to try and locate a break. I opened up the housing and notice some pinching in the wires but also thought the casing was sometimes shorting to the ground but insulating it was not a positive fix. Being a cheapo mic, I suspected the ground wire is not as good as it should be. I just checked my monitor and it is 68.76khz and vert 85.22 hz. But I believe I was near the tv when it showed humming. Also I thought it was curious that I could cup my hand around the wire or housing and cause it to go away, but it wasn't consistent. Again maybe it was still relative to the monitor. Anyway I don't think it is in the mics but the ground wire. I can touch the condenser case and get severe hum but I think I am directly interferring with the caps' operation when I do that. In a ground wire fix do you all recommend I just run another wire from the condenser to the plug or does it have to be some type of shielding wire that wraps around the signal cable too. My analyzer is my goldwave audio editor I use I can catch the general area of hum freq. Then I can zoom in on the hum and count 10 waves to get a pretty accurate indication of the rate based on the 44100 sample freq. But I don't know how to tell if it is an aliased freq. I'll do some more experimenting with the tv and monitor but a shielding suggestion would be helpful at this time if it is not too complicated. I need the full audio spectrum for the best live recording. 20-20k. Jerry Avins permanently inked into the cyberlog:> Phil wrote: >> I'm not knowledgeable in DSP, but maybe someone here can help. >> >> I have a cheapo electret microphone that is giving a bad hum, at >> 15754hz and 60hz >> >> Any idea for a problem here or a quick fix. It's a cheapo so >> hopefully you can point me to something simple like installing a new >> ground or something easy. > > You have an analog problem, and you're tight that it has to do with > grounding. There are better groups than this for your question, but > you're here. so here goes: > > The 60 Hz is power-line pickup. That commonly happens either through > electrostatic pickup because of inadequate shielding -- the shield > must be grounded, of course -- or because of hum currents circulating > in a ground loop. In your case, I suspect bad shielding (broken > ground wire?) because you are also picking up the horizontal > frequency of a monitor. Poke around and see what changes. > > Jerry-- Phil
Reply by ●January 31, 20062006-01-31
Phil wrote:> Great replies, thanks, a couple of clarifications in response. > > The mic is plugged into plug-in-power of my minidisc recorder which is > probably 1 to 9vdc, and the recorder itself operates on 1.2 v battery, > but I am near the computer monitor or tv when I noticed it, but it > doesn't always indicate hum and I couldn't figure out why it was not > consistent even when I play with the wires to try and locate a break. I > opened up the housing and notice some pinching in the wires but also > thought the casing was sometimes shorting to the ground but insulating > it was not a positive fix. Being a cheapo mic, I suspected the ground > wire is not as good as it should be.Either the wire is continuous or it isn't. A piece of #30 is enough. What does "as good as it should be" mean?> I just checked my monitor and it is 68.76khz and vert 85.22 hz. But I > believe I was near the tv when it showed humming.Clearly. That's the only source of 15,750.> Also I thought it was curious that I could cup my hand around the wire > or housing and cause it to go away, but it wasn't consistent. Again > maybe it was still relative to the monitor. Anyway I don't think it is > in the mics but the ground wire. > > I can touch the condenser case and get severe hum but I think I am > directly interferring with the caps' operation when I do that.Bingo! The microphone pellet isn't grounded? Then it's an antenna of sorts. Tie it to the cable shield or put in a shielded container.> In a ground wire fix do you all recommend I just run another wire from > the condenser to the plug or does it have to be some type of shielding > wire that wraps around the signal cable too.Don't make a ground loop. Tie the pellet to the nearby shield. most of the time, that's the best way.> My analyzer is my goldwave audio editor I use I can catch the general > area of hum freq. Then I can zoom in on the hum and count 10 waves to > get a pretty accurate indication of the rate based on the 44100 sample > freq. But I don't know how to tell if it is an aliased freq. > > I'll do some more experimenting with the tv and monitor but a shielding > suggestion would be helpful at this time if it is not too complicated. > I need the full audio spectrum for the best live recording. 20-20k.... Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●February 1, 20062006-02-01
Jerry Avins permanently inked into the cyberlog:> Phil wrote: >> Great replies, thanks, a couple of clarifications in response. >> >> The mic is plugged into plug-in-power of my minidisc recorder which >> is probably 1 to 9vdc, and the recorder itself operates on 1.2 v >> battery, but I am near the computer monitor or tv when I noticed it, >> but it doesn't always indicate hum and I couldn't figure out why it >> was not consistent even when I play with the wires to try and locate >> a break. I opened up the housing and notice some pinching in the >> wires but also thought the casing was sometimes shorting to the >> ground but insulating it was not a positive fix. Being a cheapo mic, >> I suspected the ground wire is not as good as it should be. > > Either the wire is continuous or it isn't. A piece of #30 is enough. > What does "as good as it should be" mean?There were some pinches but I don't think I am correct about that.> >> I just checked my monitor and it is 68.76khz and vert 85.22 hz. But >> I believe I was near the tv when it showed humming. > > Clearly. That's the only source of 15,750.I'm not clear on how high freq alias but do you mean the 68.76 is aliased as 15.75> >> Also I thought it was curious that I could cup my hand around the >> wire or housing and cause it to go away, but it wasn't consistent. >> Again maybe it was still relative to the monitor. Anyway I don't >> think it is in the mics but the ground wire. >> >> I can touch the condenser case and get severe hum but I think I am >> directly interferring with the caps' operation when I do that. > > Bingo! The microphone pellet isn't grounded? Then it's an antenna of > sorts. Tie it to the cable shield or put in a shielded container.Ok, I suspect something was amis there, I had played with various things along with touching the case, and I did notice shorting the ground to the case relieved it, also there is a small tab to the case near the ground solder joint and It appears it should have been part of the ground joint or not, I wasn't sure and now you've helped me there. Can I solder it or will I destroy the condenser. Its close enough to the case, about 1 mm, that I should be able to do it.>> In a ground wire fix do you all recommend I just run another wire >> from the condenser to the plug or does it have to be some type of >> shielding wire that wraps around the signal cable too. > > Don't make a ground loop. Tie the pellet to the nearby shield. most of > the time, that's the best way. > >> My analyzer is my goldwave audio editor I use I can catch the general >> area of hum freq. Then I can zoom in on the hum and count 10 waves to >> get a pretty accurate indication of the rate based on the 44100 >> sample freq. But I don't know how to tell if it is an aliased freq. >> >> I'll do some more experimenting with the tv and monitor but a >> shielding suggestion would be helpful at this time if it is not too >> complicated. I need the full audio spectrum for the best live >> recording. 20-20k. > > ... > > Jerry-- Phil
