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 Post subject: Noise Reduction --- How Is It Done?
PostPosted: Dec Thu 07, 2017 5:04 pm 
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Joined: Feb Sat 12, 2011 2:29 pm
Posts: 477
Location: Fayette County, Pa
I have a stack of old 78s that are about in what would be considered average for their age. (Most are very early 1900s - 1920s.)

I visited the website listed elsewhere on this forum and some are the same copies as what I have. When I download them in many cases they are clean; virtually no surface noise at all yet maintain their frequency response within what would be expected for the time. Even the surface noise that was present when new had been eliminated. Obviously they were done by professionals to obtain this level of quality. My question is how is it done? What steps are taken to obtain this level of restoration? Obviously it goes beyond just running them through a computer program like Audacity, etc. , I am curious what additional steps they do to obtain this clarity to something that never was this good even when new?


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 Post subject: Re: Noise Reduction --- How Is It Done?
PostPosted: Dec Fri 08, 2017 3:52 am 
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Joined: Sep Tue 30, 2014 6:08 am
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Location: The Old Dominion VA 23518
Back in 2012, I saw a display at the Smithsonian Museum of American History where lasers were "reading" a vinyl disc that was also "mapped by another laser that found the best area to read, so perhaps a laser was used. According to the display, the use of lasers was brought about to make new masters where an LP existed, but no master disc or tape could be found. The whole thing was sponsored as a "future tech" feature in each room of the exhibit. Apple, Sony, and either BMI or EMI sponsored the sound exhibit.

Aside from lasers, modern digital signal processing can "clean up" just about any dirty waveform, and with only femto-second delays. With a laser "map" of the un-damaged areas of a record groove, you could read the disc with a laser, send it through all the filters (scratch, rumble, etc) and yield an ear-clean recording.

I'll admit this is as far as my interest can take me. Being hearing impaired, I have no need for hifi or audiophile quality anything.... :(

_________________
Brian
"Capacitor Cosmetologist since 1979"
USN Retired 1984-2006 (Avionics/Cal)


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 Post subject: Re: Noise Reduction --- How Is It Done?
PostPosted: Dec Sun 24, 2017 2:15 pm 
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Joined: Feb Mon 06, 2012 7:24 pm
Posts: 2377
Location: Gold Country, (Stanislaus National Forest) California 95235
Between that and CAREFUL/SPARING use of any number of noise reduction algorithms - and the right size AND SHAPE stylus - people (well the public anyway) is always amazed at what can be done.

The general principle is there's three kinds of noises - crackle/scratches, hum and hiss.

Crackles and scratches have an attack several times magnitude faster than any music - so the first pass gets rid of most of that - and then for stragglers you go in with your DigiPen and either edit out the offending crackle one at a time and use auto-smoothing or vice versa (auto-edit and manual smoothing).

The second pass is usually for hum or buzz. For buzz it's fairly easy as the buzz is usually either a square wave or triangle/sawtooth wave or some such - so the parameters are reset to distinguish sine waves (music) from other waves and lifted out.

For hum it's a little harder - but only on wide-range recordings. For acoustic 78s that rarely go below 80 Hz - it's a simple act of either lopping off the bass that's not there anyway or doing spectral editing to zero in on just that frequency and maybe a few surrounding frequencies and use the digital equivalent of a notch filter to get rid of it.

Now you have music with surface hiss but few if any of everything else.

So you highlight a section of hiss by itself (in the lead in or lead out groove - or sometimes in a silent passage between sections on a record) sample it for about 2 seconds or so - create a file for it - use the file to auto-set the parameters and then GENTLY and SPARINGLY apply the effect.

You can tell if you ``slid over too far'' in the effect percentage because the resulting file will have a mumbly and dingly effect left behind as a digital process artifact. Lighter applications will minimize - but not completely erase - the surface hiss - and therefore leave the music all but intact.

Before you can do all of that through - either your laser or optic fiber pickup must be calibrated to ignore previously worn sections of a groove - such as that worn out and destroyed by your average conical stylus of the period - or you must have the correct size and shape stylus if doing it by hand.

And then the last thing is to take the Center Channel Extraction. Most noise of any capacity is spread out randomly across both groove walls - so the monaural material is recorded with a stereophonic pickup and stylus size/shape appropriate to the groove in question to be able to pick up as much of the groove walls as possible.

This means instead of the conical stylus mentioned above - special elliptical or hyper-elliptical styli must be manufactured to the specs of the original groove - which varied from 2.5 - 3.5 mils for commercil acoustic 78s (some electrical 78s in the late 50s e.g. King Records used a 1 mil mono LP groove compared to the 0.7 mil stereo groove).

Just like the CD-4 quadraphonic hyper-elliptical styli (developed in the 70s to pick up the UHF carrier frequency upon which the remaining two channels were modulated) plays stereo records extremely well - especially those worn out from overuse by a conical stylus - the many hyper-elliptical custom styli in use for transfer of 78s can oftentimes achieve the same results - minimizing the need for digital processing.

After this is done, the algorithms can be programmed to only save material that is common to both channels i.e. most of the music and very little noise. Some restorationists prefer to do their center channel extraction first and then proceed as above - but a lot of guys find that the stereo field helps with most if not all of the other processes as well.

When you are done, you will usually have 3 channels, a left, a right and a center from what you just extracted. Between those three - you go in manually second by second and see which channel is both clearest as well as having the fewest noise or digital artifact elements and then splice in and out of all three of those accordingly to get your final mono waveform.

The best jobs have all those processes done multiple times, and the 3-channel result edited together manually after each process before going onto the next - so if you wondered why it takes months or years to do one song - now you know.

Film restoration is performed with video versions of the same thing.

_________________
2 kinds of men/tape. Low Noise/Wide Range.
LN=kind. WR=abrasive. Engineers=same thing.


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 Post subject: Re: Noise Reduction --- How Is It Done?
PostPosted: Jan Mon 08, 2018 2:30 am 
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Joined: Jun Wed 08, 2011 2:33 am
Posts: 6592
Location: Ohio 45177
Wasn't there a device back in the 70s called an autocorrelator that eliminated pops and clicks from records for taping or listening? Pre digital signal processing in the modern sense, I believe. Sold as a hi fi accessory for cleaning up LPs but I suppose it could work on a 78 too. You can buy special mono phono cartridges nowadays for playing 78s but they are fairly expensive like a better quality stereo cartridge. As for me, I learned to live with the noises back in the day and an occasional pop on an old LP does not seem strange or overly disturbing as such artifacts might to a digital generation listener.


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 Post subject: Re: Noise Reduction --- How Is It Done?
PostPosted: Jan Tue 09, 2018 12:42 am 
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Joined: Feb Mon 06, 2012 7:24 pm
Posts: 2377
Location: Gold Country, (Stanislaus National Forest) California 95235
wazz wrote:
Wasn't there a pops and clicks eliminator back in the 70's?
SAE Clicks and Pops Assassin http://www.swingmusic.net/sae.html was the most well known.

It along with it's brother the Thomson Vocal Eliminator used the rear half of any number of multi-channel matrices in addition to its' impulse noise processor.

While great on mono sources since the only thing there NOT common between the channels is usually noise you are trying to get rid of - playing a stereo source through there also eliminates a great deal of the ambience from the recording - which if you use yours in the modern day as I do mine - will have to be put back digitally in order to maintain at least a somewhat pleasing sound.

Even with all the modern tools in e.g. Diamond Cut Pro or any of the other e.g. Sound Forge etc programs - these old analog units do surprisingly well. I put all my banged up mono sources through there (actually three set to different parameters) before recording all 6 resulting tracks into the computer and choosing or combining what sounds best prior to restoring the ambience (especially on the odd stereo record I pipe through there).

_________________
2 kinds of men/tape. Low Noise/Wide Range.
LN=kind. WR=abrasive. Engineers=same thing.


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 Post subject: Re: Noise Reduction --- How Is It Done?
PostPosted: Jan Wed 10, 2018 3:36 am 
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Location: North Central Iowa
Several years ago I used the software from Diamond Cut Productions to clean up some of my records and then burn them to CD's. The software did an amazing job of removing clicks, hiss, etc. but with each pass of the filter it also took a little bit of the recorded material as well. After awhile I decided that I preferred to listen to my records just as they are...scratches and all.


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 Post subject: Re: Noise Reduction --- How Is It Done?
PostPosted: Feb Fri 09, 2018 3:15 am 
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Joined: Jun Wed 08, 2011 2:33 am
Posts: 6592
Location: Ohio 45177
State of the art version of the old analog concept, I guess. If you got 1500$ lying around and need to archive to digital. http://www.sweetvinyl.com


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