how to filter out AC line noise from recording?

[Jeff DLB]

I have some recordings that have AC line noise.

Is there a way to filter out the noise? I tried using Audacity, but got very poor results with the noise removal and equalization filters.

In the future, I suppose I will use batteries (what a pain) or perhaps a battery box for the mics (would that help?).

-Jeff DLB

Edited March 13, 2006 by Jeff DLB

[danielbb90]

I used the noise removal in Audicity and it worked perfectaly fine for me!

Try messing about with the strenth of it until you can get the best results for whatever you were recording!

I can't really help past that!

[greenmachine]

Is it a 50/60 Hz hum or broadband noise? Hum would be pretty easy to filter, whereas broadband noise wouldn't. Do you have a short sample?

[Jeff DLB]

The noise seems to be much more broad-band than a pure 60Hz tone.

Attached is a noise sample (17sec, 2.8MB, WAV) with some talking and instrument sounds. I could not find a period of pure noise when nobody was making other sound, which is part of the problem when trying to filter the noise with Audacity. (I should try to get a pure-noise sample at home, though I'm not sure whether my AC has the same noise profile.)

-Jeff DLB

line_noise.wav

[xatax]

Try the Noise Reduction Package for Sound Forge.

[greenmachine]

If you manage to get a pure noise profile (same equipment, same settings, same location etc.), you can try to substract that from the original recording (open both files, invert the pure noise, match it to the original recording and mixdown). I'm not familiar enough with Audacity to tell you how to do it in detail, but it should be possible.

Using a battery box most propably won't help to avoid noise. Recording without ac power seems to be your best bet.

Edited March 14, 2006 by greenmachine

[surripere]

I had a problem and raised a thread on the forum about noise when recording from vinyl with AC power connected, I was advised to try a ground loop isolator. I didn't go down this route, I just record with the battery but it might be worth looking at as it is quite annoying. I just didn't fancy spending the money on something I wasn't sure would work and I get by okay with battery only for vinyl recording.

Here is the thread with a link to the ground loop isolator

http://forums.minidisc.org/index.php?showt...l=buzzing+vinyl when

[dex Otaku]

The simplest way I've ever found to remove narrowband resonances is with a narrowband filter.

Use a 4-band parametric or paragraphic EQ with a very high Q value; set all 4 filters to maximum attenuation and to centre at 60Hz. If necessary, run the same filter again. It may also be necessary to filter harmonics of 60Hz, i.e. 120, 240, 480Hz.

Pattern-based noise reduction algorithms can work with material like this, but I personally prefer to avoid them because of the audible artifacting they cause. If the noise you need to get rid of is a single resonance or specific range of harmonics, narrrowband notch filters almost always work better in the end.

[1kyle]

I had a problem and raised a thread on the forum about noise when recording from vinyl with AC power connected, I was advised to try a ground loop isolator. I didn't go down this route, I just record with the battery but it might be worth looking at as it is quite annoying. I just didn't fancy spending the money on something I wasn't sure would work and I get by okay with battery only for vinyl recording.

Here is the thread with a link to the ground loop isolator

http://forums.minidisc.org/index.php?showt...l=buzzing+vinyl when

The Hum will most cerrtainly come from the Vinyl turntable / playback combination rather than the MD unit --can almost 100% guarantee that.

One of the problems with recording from Vinyl unless you have a really top notch deck aka old Marantz or similar is that unless the impedance from the cartridge (magnetic --very low gain and highly non linear equalisation or ceramic --bit more like tape and hence more linear equalisation) is matched to the pre-amp of your recording device you'll pick up not only the rumble from the turntable (remember a cartridge converts vibration -- hopefully most of it from the record groove and not the extraneous equipment) to the analog sound wave that goes into your recording device) but also any stray hum due to the 50 / 60 HZ mains --unless the metal parts of your equipment are perfectly earthed or grounded. The 50 / 60 Hz hum is often picked up from vibration and stray magentic fields generated by the mains transformer in the turntable / amplifier gear. You shouldn't have ANY probs with mains units on MD recorders BTW but using a battery is fine for MD recording in any case.

A good way generally to record from vinyl --especially if you have an amp with an optical out-- is to play the vinyl with all "Tone settings" at FLAT or LEVEL and record from the optical out into your MD player --the Amp on which you are playing the vinyl should already have the "proper equalisation" set up for the cartridge you are using on the vinyl.

This will usually produce "Hum Free" recordings

If you have a Soldering Iron and are using a Ceramic (not magnetic) type of cartridge you can also solder two leads direct from the rear of the cartridge. At the other end a mini stereo plug can go straight into your MD recorder --there's enough gain on a ceramic cartridge to do this and the linear equalisation is similar to a tape so it should work just fine. Won't work satisfactorily however with magnetic type of cartridges.

Cheers

-K

Edited March 14, 2006 by 1kyle

[dex Otaku]

One of the problems with recording from Vinyl unless you have a really top notch deck aka old Marantz or similar is that unless the impedance from the cartridge (magnetic --very low gain and highly non linear equalisation or ceramic --bit more like tape and hence more linear equalisation) is matched to the pre-amp of your recording device you'll pick up not only the rumble from the turntable ....

Phono cartridges require phono preamps. I personaly have never seen a standalone recording device with a phono preamp built-in. The preamp not only matches the impedance of the signal from the cartridge to an amp or recorder by amplifying the signal to line level, it also de-emphasises [equalises] the signal from the cartridge. Without de-emphasis, the frequency response of the signal from the cartridge is completely whacked. You did mention this in your post, but then seemed to completely forget about it immediately after.

Jerry-rigging connections directly from the cartridge to the recorder simply will not work.

Also, while they are more linear response-wise, my experience is that ceramic cartridges are fairly rare except in exceedingly cheap [read: absolute crap-quality] turntables [the kind that come with cartridge/stylii combos that are heavy and large enough to actually plow grooves and almost completely destroy an LP in a few plays]. I could be wrong about this, but I'll note that my opinion is based on having actually serviced turntables for a few years, and having followed audiophile magazines for a number of years; I've yet to see a single decent turntable come with a ceramic cartridge, or ceramic cartridges being sold separately for decent turntables. The only places I've ever seen ceramic cartridges commonly used were on school A/V turntables [not built for fidelity by any means] and extremely cheap home sets such as the kind that often had 8-track decks built-in [also of dubious fidelity at best].

As for the hum from turntable recordings, the mainly problem is usually electromagnetic interference with the cartridge and cables. This is why most turntables and phono preamps use a cheap version of balanced connections [which functions rather poorly compared to professional balanced connections].

In the end some of the best solutions I know of are:<ul><li> keep the turntable as far from all EMR sources as possible</li><li>build a faraday cage around the turntable, including the lid, and ground the cage</li><li>make sure the polarity of the cartridge cabling is correct [matched for both sides]</li><li>make sure the grounding wire is actually connected to your preamp</li><li>try replacing your cartridge wiring with a shielded/balanced cable, turntable cabling can be purchased from electronic retailers and is made small enough to run inside your tonearm. </li></ul>

And lastly:

<ul><li>buy a better cartridge/stylus, and turntable, one with proper balanced wiring out to a preamp or with a built-in preamp</li><li>buy a better preamp [noting that the preamps built into even decent home A/V amps are often absolute garbage, as are the kind built into the cheap "plug & play" turntables sold these days, as are the inexpensive standalones sold at places like Radioshack]</li></ul>

A good way generally to record from vinyl --especially if you have an amp with an optical out-- is to play the vinyl with all "Tone settings" at FLAT or LEVEL and record from the optical out into your MD player --the Amp on which you are playing the vinyl should already have the "proper equalisation" set up for the cartridge you are using on the vinyl.

I somehow missed this on the first pass. My apologies, 1kyle. I'll add, though: the recording outputs of a preamp/switcher/home AV amp almost always bypass any built-in EQ or tone control.

[1kyle]

I should have mentioned the output from the Pre-amp -- I didn't mean from the actual cartridge itself --you'll probably break it if you try and take a connection from the actual pickup.

The gear I was also referring to were like those cheap and nasty units such as "Dansettes" --anybody remember them --they are probably worth a little bit in the collectors market today however.

Some ceramic cartridges were actually quite ok provided the weight of the playing arm wasn't too great-- I've still got an old Lasky's radio catalog giving some specs on those devices --in those days London's Tottenham Court Road / Edgware road areas were full of "self build" electronic and "Hi-FI" shops. --Remnants are still there today --Tottenham Cour Road is still a great place for the latest gizmos etc.

If you've got a high end turntable the chances are that you'll have gear to match and the "Hum problem" won't have raised it's head.

I can remember messing around with some of the old "Heathkit" stuff years ago ( great electronics self build kit which were brilliant for teaching purposes) where you could actually take an output from the cartridge itself before it went into a preamp. This was so you could feed the signal into an Oscillascope -- as an instruction on the sort of response a cartridge would yield.

Cheers

-K

Edited March 14, 2006 by 1kyle

[dex Otaku]

Bwahaaa. Clarity. Coolness. Cheers back.

[blobby113]

The noise seems to be much more broad-band than a pure 60Hz tone.

Attached is a noise sample (17sec, 2.8MB, WAV) with some talking and instrument sounds. I could not find a period of pure noise when nobody was making other sound, which is part of the problem when trying to filter the noise with Audacity. (I should try to get a pure-noise sample at home, though I'm not sure whether my AC has the same noise profile.)

-Jeff DLB

I removed the 60Hz by blocking the sound into small chunks and removing the sinusoidal component by regression. The problem is that the 60Hz hum is getting amplified, so needs to be filtered locally in time. I'm attaching the corrected sound, which doesn't have a 60Hz component. If you look at the spectrogram, you'll also see that frequencies very near 60Hz have also been removed, but this can't be helped. This technique won't touch any harmonics of the 60Hz, but these can also be removed in a similar fashion. Needless to say, the technique is best used offline (i.e. postprocessing).

Barry[attachmentid=1507]

line_noise_mod.wav

[blobby113]

I removed the 60Hz by blocking the sound into small chunks and removing the sinusoidal component by regression. The problem is that the 60Hz hum is getting amplified, so needs to be filtered locally in time. I'm attaching the corrected sound, which doesn't have a 60Hz component. If you look at the spectrogram, you'll also see that frequencies very near 60Hz have also been removed, but this can't be helped. This technique won't touch any harmonics of the 60Hz, but these can also be removed in a similar fashion. Needless to say, the technique is best used offline (i.e. postprocessing).

Barry[attachmentid=1507]

Or you could just use "dehum" in Nero Wave Editor! I haven't tried it, but everything else in Nero works fine.

[Top Cat]

If you manage to get a pure noise profile (same equipment, same settings, same location etc.), you can try to substract that from the original recording (open both files, invert the pure noise, match it to the original recording and mixdown).

I suspect this'll only work in case you manage to get sample-to-sample exact second recording of the noise, which is likely improbable. More probably you'll get the noise amplified by 3 dB on average