Stereo cutter heads and cutting low frequencies

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Stevie342000
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Stereo cutter heads and cutting low frequencies

Post: # 32121Unread post Stevie342000
Tue Nov 04, 2014 3:40 am

It has occurred to me that for those that have Neumann or similar cutting set-ups for stereo cutting will have this built in.

But those who are developing their own cutter heads and complete cutting lathes may be missing this if they are cutting in stereo.

My knowledge is limited so hence the questions, which resolves around cutting bass frequencies.

1. At what frequency are the bass frequencies summed to mono?
2. Am I correct in assuming that the system is based around a filter and summing/post summing mixing unit?
3. Is there anything that I am missing here?
4. Summing to mono at bass frequencies should be ahead of Inverse-RIAA equalisation?

At this point in time I will be cutting in mono but at a later date I may wish to cut in stereo, some of my sources may be stereo therefore I need a unit which will combine both channels to mono. But from what I have read there are issues with cutting low frequencies in stereo, the whole method of cutting is a number of trade-offs. Trackability is just one, bass effects the number of grooves per inch is another, excessive excursions in the bass are the main issue when cutting in stereo as is phase. For cutting with a mono head you would use switch positions 7 or 8 and for cutting a stereo sum or difference bass signal you would use switch positions 1 or 2.

I found a suitable circuit in Audio August 1958 Page 24 here is the link: http://www.americanradiohistory.com/Audio-Magazine.htm

As for the filter this unit might be of use: http://www.bonavolta.ch/hobby/en/audio/5881_2.htm See the Crossover section.

This unit will allow me to sum to mono at this stage and a later stage by the process of lets say e.g. a filter cutting all frequencies above 150Hz and one that passed all frequencies above 150Hz. The below 150Hz is then passed through the compatibility translator and then remixed with other signal. This would then go on to InV-RIAA and on to the power amplifier prior to the cutting and cutting level meter.

I think I am seeing it right as the translator would take care of any phase issues in the lower register?

Why am I using such out-dated technology? Because I understand it, I have taken the modular approach to my system so that it can be upgraded and used in a number of ways, no one unit is tied to one use, they can be used for many uses by being patched in as and when or where I need them. I may very well run into to issues with hum loops but a ground lift resistor should solve that in each unit.

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Stevie342000
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Re: Stereo cutter heads and cutting low frequencies

Post: # 32152Unread post Stevie342000
Wed Nov 05, 2014 3:45 am

I see from the Boden book that the crossover frequencies are 30, 70, 250 & 700 cycles would the above set up work.

I understand the principle but exactly what an elliptical equaliser is I am at loss to understand. To me it is just some fancy name to make it sound like it does something outside of the norm so the developer can give it some mystique and charge a high price.

The Stereo Compatibility Translator includes a phase switch and you could check the phase with the aide of an oscilloscope. In my mind the set up of the translator, preceded by the crossover network followed by a 2-1 input mixer should cover the process for cutting stereo discs and to minimise out of phase signals in the lower frequency band.

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Greg Reierson
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Re: Stereo cutter heads and cutting low frequencies

Post: # 32157Unread post Greg Reierson
Wed Nov 05, 2014 9:47 am

1) Use of any filtering is program dependent. There is no filter that's always in the circuit. It's not built in to the cutting system per se. It's part of the mastering signal chain that feeds the cutting system. The cutting amps will try to cut what you feed them. Knowing what to feed them is the trick :)

2) It's an EQ/filter acting on the difference (vertical) channel. Same as a mid / side EQ just working on the side channel. Nothing fancy beyond that. No summing / mixing going on. Just about any plug-in EQ with M/S capability can do it.

3) I think it's simpler than you imagine. Excessive LF in the vertical can cause the needle to jump up and potentially out of the groove. It can also cause the cutting stylus to leave the surface of the lacquer which will create a non-continuous groove. That's bad. Reducing that LF energy reduces the chance of those things happening. It's only a problem when the vertical LF energy is excessive. Beyond the cutting system's ability to keep the stylus in the lacquer and the playback system's ability to keep the needle in the groove. It's not something you worry about on normal program material. It's a good idea to monitor the difference channel to hear what's going on down there. Then those processing decisions can be made. Looking at a meter or scope has never been helpful to me.

4) All of your processing happens before the RIAA network.

None of that matters if you're cutting mono.
Greg Reierson
http://www.RareFormMastering.com
VMS70 :: SAL74B :: SX74

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Stevie342000
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Re: Stereo cutter heads and cutting low frequencies

Post: # 32160Unread post Stevie342000
Wed Nov 05, 2014 12:49 pm

That makes sense to me but in the past bass frequencies were often summed to mono to avoid the problem of the vertical element causing a non-continuous groove. In addition does it not to an extent limit lateral movement as well? As excessive movement in either direction results in a non-continuous groove or you run into problems with clashing grooves.

Yes I am with you on the oscilloscope and visual monitoring.

I will probably stick with mono, I may however fashion a stereo Connoisseur cutter from the patent and not worry about the lower frequencies. As most of my material will be the human voice there should not be too much of the lower frequencies to worry about.

The Connoisseur head differs in that it cuts at 0/90 degrees not 45/45 as in standard Westrex Stereo cutters. The translator may be of use in this case and yes agreed any processing such as this would be ahead of Inverse RIAA.

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