The Secret Society of Lathe Trolls

blacknwhite · Joined: 24 Apr 2008 Posts: 105

Question: What is the official technical terminology used for when you process the signal to a stereo cutterhead to limit excessive out-of-phase signal (vertical movement)?

If I were looking for a rack-mount analog signal processor to perform this task, or a digital software plug-in, what would it be called?

Thanks
- Bob

cuttercollector · Joined: 11 Jun 2006 Posts: 307 Location: San Jose, CA

Others may be better qualified to answer the question, but here goes.
I don't really know of an analog outboard "box" to do this although some old school audio preamps had some of these features back in the day.
Stereo information is by nature differential information between the the Left and Right audio channels. If there were no differential information the signal would be mono - both channels identical. If you took the phase of this mono signal and reversed the phase to one channel you would have full differetial information and all vertical stylus movement with no lateral movement.
Real program material in stereo has some in phase information and some differential or disimilar information between the 2 channels. A vocal panned center and a stereo drum mix with the kick in the center and other mics panned to a degree would be examples.
What is key for disc cutting is to not have too much out of phase info., especially at low frequecies. Too much out of phase info is bad anyway if the material is ever summed to a mono signal no matter what the playback media is. You get odd sounding cancelations and frequency response alterations.
As far as requirements for a disc cutting box are concerned a "seperation" knob that allowed you to blend the 2 channels together from full stereo to full mono would be ideal. A further refinement would be if you had some way of making this knob have a variable frequency adjustment so you could make just the bass as mono as you needed to while leaving full stereo seperation for the mid and high frequencies.
Perhaps there are software DSP tools for this.
But being aware of phase as you record is a good thing too. As I said, making something mono by combining large amounts of of out of phase material will lead to odd sounding results. Out of phase mic pickup used to be the major issue but today it could be artificially generated synthesized stereo sounds as from a keyboard, guitar effect etc.

drdub · Joined: 28 Nov 2005 Posts: 48 Location: AUSTRIA

> or a digital software plug-in, what would it be called?

if there is REAL problems we use the monofilter.

http://www.nugenaudio.com/monofilter.php

works fine
_________________
satan spins vinyl

*** www.drdub.com ***

cuttercollector · Joined: 11 Jun 2006 Posts: 307 Location: San Jose, CA

Very cool product. I note some of the reasons they stated using it were some of the same ones I mentioned.
It probably allows vastly more control and display of channel phase than the old school analog way I was thinking of some home built analog box with pots and R/C filter networks and a lissajous pattern on an oscilloscope for display.

blacknwhite · Joined: 24 Apr 2008 Posts: 105

Thanks for the responses, Cuttercollector & Dr. Dub.

Yes, very cool looking plugin, seems highly "intelligent", as in digitally recognizing patterns in waveforms.

I was originally, as cuttercollector was saying, pondering the design of an analog circuit to do something similar i.e. "shrink the signal towards mono" the more out-of-phase the material becomes, sort of like an analog dynamic compressor but with the trigger being "channel1 + inverse of channel2" and the result being bleeding together of left & right on output, but the plugin looks much more capable.

- Bob

motorino · Joined: 24 Aug 2005 Posts: 212

similar module http://www.etec.dk/disc-cutting.html

drdub · Joined: 28 Nov 2005 Posts: 48 Location: AUSTRIA

Hamilton · Joined: 18 Sep 2006 Posts: 17 Location: Argentina

Hi fellows.
An old records cutter man in USA told me a couple years ago to avoid this mess the easy way is to keep one channel a little lower than the other and is done no misterys or reserches.

Take cae and keep on cutting.
Very Happy

Hamilton.

cuttercollector · Joined: 11 Jun 2006 Posts: 307 Location: San Jose, CA

How could that help? Only in the case of strong out of phase low frequency material could that do anything. Any change from identical signals in both channels (including level) by definition introduces some "stereo" signal. Consider a mono signal fed to both channels at equal level = mono groove. Now, reduce the level to the left channel all the way. Now you are cutting a right groove only, which produces vertical modulation of the cutter. Think about it. Smooth blank on one wall - signal on the other. The only mono groove cut with a stereo cutter has to be identical left and right channels - even in level. Anything else is some form of 2 channel information. It is a misnomer to say "in phase stereo". Stereo by definition introduces out of phase information. It is just a questuion of how much, how loud, and at what frequency.

The devices we are talking about make the bass mono.

Aussie0zborn · Joined: 11 Mar 2006 Posts: 82 Location: Australia

Thats a nice piece of software Dr Dub. The Neumann SP series of mastering consoles have an eliptical equaliser built in for this very purpose.

drdub · Joined: 28 Nov 2005 Posts: 48 Location: AUSTRIA

motorino · Joined: 24 Aug 2005 Posts: 212

Dub Studio · Joined: 20 Jun 2006 Posts: 51 Location: Bristol

Yeh I guess people refer to an elliptical EQ when they mean a specially designed unit to "mono" certain frequencies, but essentially I think these EQs work using the same process as the M/S mic technique.

Brainworx do a pretty awesome plugin called the bx_control which allows you to set up an M/S matrix, and then you can use any plugin you like to control stereo width, even a multiband compressor if you like.

If you prefer analogue you can make your own matrix really easily.

diamone · Joined: 09 Aug 2008 Posts: 21 Location: Silicon Valley