Hello everybody! I'm trying to remove power line hum from a signal and have two questions concerning this. 1. How high do the harmonics go? (In general, round numbers) 2. How much frequency variations (+/-) should I expect in the fundamental 50 Hz component? The reason for asking these questions is that I am having a hard time choosing between a fixed comb-notch filter and something that tracks the frequency variations by adapting... Thanks in advance JT
Power line hum question
Started by ●November 10, 2008
Reply by ●November 10, 20082008-11-10
DJT wrote:> Hello everybody! > > I'm trying to remove power line hum from a signal and have two questions > concerning this. > > 1. How high do the harmonics go? (In general, round numbers) > > 2. How much frequency variations (+/-) should I expect in the fundamental > > 50 Hz component? > > The reason for asking these questions is that I am having a hard time > choosing between a fixed comb-notch filter and something that tracks the > frequency variations by adapting... > > Thanks in advanceThe harmonic structure depends on the source. A "dirty" SCR controller or an arking power-line insulator can be heard on an AM radio. The frequency stability depende on the grid. 60 years ago, electric clocks in New York City would lose as much as three seconds during the day and evening and make it up overnight. It has been much better than that for many years now. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●November 10, 20082008-11-10
On 11 Nov, 02:26, "DJT" <JanTorgrims...@hotmail.com> wrote:> Hello everybody! > > I'm trying to remove power line hum from a signal and have two questions > concerning this. > > 1. How high do the harmonics go? (In general, round numbers)It depends. If you have lots of heavy-duty electrical motors nearby, you might expect more harmonics than if you are in suburbia.> 2. How much frequency variations (+/-) should I expect in the fundamental > > � �50 Hz component?It depends. Different power sources may have different variations.> The reason for asking these questions is that I am having a hard time > choosing between a fixed comb-notch filter and something that tracks the > frequency variations by adapting...Try the simpler solution first and see if it is good enough. If it isn't, try a more elaborate solution. Rune
Reply by ●November 10, 20082008-11-10
DJT wrote:> Hello everybody! > > I'm trying to remove power line hum from a signal and have two questions > concerning this. > > 1. How high do the harmonics go? (In general, round numbers)Infinitely high. The rolloff factor ballpark is ~10dB per octave.> 2. How much frequency variations (+/-) should I expect in the fundamental > 50 Hz component?Very accurate in the long term; short term accuracy ~ 1e-4.> The reason for asking these questions is that I am having a hard time > choosing between a fixed comb-notch filter and something that tracks the > frequency variations by adapting...No point to do an adaptive filter. The random fluctuations of the waveform smear the spectrum. Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
Reply by ●November 10, 20082008-11-10
On Nov 10, 8:58�pm, Vladimir Vassilevsky <antispam_bo...@hotmail.com> wrote:> DJT wrote: > > Hello everybody! > > > I'm trying to remove power line hum from a signal and have two questions > > concerning this. > > > 1. How high do the harmonics go? (In general, round numbers) > > Infinitely high. �The rolloff factor ballpark is ~10dB per octave. > > > 2. How much frequency variations (+/-) should I expect in the fundamental > > 50 Hz component? > > Very accurate in the long term; short term accuracy ~ 1e-4. > > > The reason for asking these questions is that I am having a hard time > > choosing between a fixed comb-notch filter and something that tracks the > > frequency variations by adapting... > > No point to do an adaptive filter. The random fluctuations of the > waveform smear the spectrum. > > Vladimir Vassilevsky > DSP and Mixed Signal Design Consultanthttp://www.abvolt.comOn a slightly related note; did you know that if you are analyzing a recording and wish to know when and where it was made, you can extract the hum on the recording, track the tiny fluctuations in frequency over the duration of the recording, and correlate these with the frequency variations that occur in the power grids of various countries? These fluctuations are recorded (for just this purpose), so you can tell both the time it was recorded as well as the country (just search the database for a correlation peak). Since all generators are syncronously linked, the frequency variations are valid for the entire country (or continent, even). This doesn't work if you are running off a battery, and are living in a cave ..... Bob Adams
Reply by ●November 10, 20082008-11-10
Robert Adams wrote: ...> On a slightly related note; did you know that if you are analyzing a > recording and wish to know when and where it was made, you can extract > the hum on the recording, track the tiny fluctuations in frequency > over the duration of the recording, and correlate these with the > frequency variations that occur in the power grids of various > countries? These fluctuations are recorded (for just this purpose), so > you can tell both the time it was recorded as well as the country > (just search the database for a correlation peak). Since all > generators are syncronously linked, the frequency variations are valid > for the entire country (or continent, even). > > This doesn't work if you are running off a battery, and are living in > a cave .....Wow! just like tree-ring dating! Jerry P.S. Wise guys will please refrain from asking why one would want to date a tree ring. -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●November 10, 20082008-11-10
Jerry Avins <jya@ieee.org> wrote:>Robert Adams wrote:>> On a slightly related note; did you know that if you are analyzing a >> recording and wish to know when and where it was made, you can extract >> the hum on the recording, track the tiny fluctuations in frequency >> over the duration of the recording, and correlate these with the >> frequency variations that occur in the power grids of various >> countries? These fluctuations are recorded (for just this purpose), so >> you can tell both the time it was recorded as well as the country >> (just search the database for a correlation peak). Since all >> generators are syncronously linked, the frequency variations are valid >> for the entire country (or continent, even).>Wow! just like tree-ring dating!Sounds like something some expert witness would come up with. Not to say it isn't valid. :-) Steve
Reply by ●November 10, 20082008-11-10
On Nov 10, 9:40�pm, spop...@speedymail.org (Steve Pope) wrote:> Jerry Avins �<j...@ieee.org> wrote: > > >Robert Adams wrote: > >> On a slightly related note; did you know that if you are analyzing a > >> recording and wish to know when and where it was made, you can extract > >> the hum on the recording, track the tiny fluctuations in frequency > >> over the duration of the recording, and correlate these with the > >> frequency variations that occur in the power grids of various > >> countries? These fluctuations are recorded (for just this purpose), so > >> you can tell both the time it was recorded as well as the country > >> (just search the database for a correlation peak). Since all > >> generators are syncronously linked, the frequency variations are valid > >> for the entire country (or continent, even). > >Wow! just like tree-ring dating! > > Sounds like something some expert witness would come up with. > > Not to say it isn't valid. :-) > > SteveI learned about this from an AES paper, so it must be true! ("forecnsic audio") Bob
Reply by ●November 11, 20082008-11-11
"Robert Adams" <robert.adams@analog.com> wrote in message news:8e9934f0-451b-4b2f-a673-9259b24e37d3@c36g2000prc.googlegroups.com...> On a slightly related note; did you know that if you are analyzing a > recording and wish to know when and where it was made, you can extract > the hum on the recording, track the tiny fluctuations in frequency > over the duration of the recording, and correlate these with the > frequency variations that occur in the power grids of various > countries? These fluctuations are recorded (for just this purpose), so > you can tell both the time it was recorded as well as the country > (just search the database for a correlation peak). Since all > generators are syncronously linked, the frequency variations are valid > for the entire country (or continent, even).I have doubts about the feasibility. The frequency measurement will produce something like one useful bit per minute; so the recording has to be too long to be practical. Also, the fluctuations of the speed of the recording are likely to be of the same order as the power frequency variations. Plus phase shifts, propagation delays, waveform distortions, etc... Vladimir Vassilevsky DSP and Mixed Signal Design Consultan http://www.abvolt.com
Reply by ●November 11, 20082008-11-11
On Mon, 10 Nov 2008 20:35:12 -0500, Jerry Avins wrote:> DJT wrote: >> Hello everybody! >> >> I'm trying to remove power line hum from a signal and have two >> questions concerning this. >> >> 1. How high do the harmonics go? (In general, round numbers) >> >> 2. How much frequency variations (+/-) should I expect in the >> fundamental >> >> 50 Hz component? >> >> The reason for asking these questions is that I am having a hard time >> choosing between a fixed comb-notch filter and something that tracks >> the frequency variations by adapting... >> >> Thanks in advance > > The harmonic structure depends on the source. A "dirty" SCR controller > or an arking power-line insulator can be heard on an AM radio. The > frequency stability depende on the grid. 60 years ago, electric clocks > in New York City would lose as much as three seconds during the day and > evening and make it up overnight. It has been much better than that for > many years now. > > JerryJerry, you forgot to tell him to eliminate the hum at the source! (I _think_ this is a different guy than the last one). It's still good advise. OP: There's been a thread on this recently, if you google for it you'll see lots of discussion. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" gives you just what it says. See details at http://www.wescottdesign.com/actfes/actfes.html
