There are no dumb questions

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hsgw
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Re: There are no dumb questions

Post: # 41084Unread post hsgw
Fri Mar 04, 2016 8:46 am

Hi All.

I am thinking about making a lathe and a cuttinghead.
I have a question about cutting amplifier.

Many DIY cuttinghead is driven by higher power than maximum rated power of the speaker/transducer.
example: http://lathetrolls.com/viewtopic.php?f=15&t=5945&start=60
His amplifier is 85W or 300W, but the tranducer is looks like maximum 6W....

Why is not burnt out coil?
I think average power is lower then maximum rated power.
Is this right?

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Sillitoe
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Re: There are no dumb questions

Post: # 41086Unread post Sillitoe
Fri Mar 04, 2016 8:57 am

Hi hsgw,

There is a great explanation by Markrob in this thread. http://lathetrolls.com/viewtopic.php?f=15&t=6346

Cheers
James

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hsgw
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Re: There are no dumb questions

Post: # 41088Unread post hsgw
Fri Mar 04, 2016 9:11 am

Hi James.

I am grateful for let me know!
I can resolve my confusions.

Takuya

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JJZL
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Re: There are no dumb questions

Post: # 44221Unread post JJZL
Tue Oct 04, 2016 8:43 am

Dear forum members, I am trying to understand the processes of record cutting and have a ouple of questions.

If I am in the wrong thread or forum please let me know.

I understand that many records are cut at "fixed pitch" where lateral displacement of the cutting head goes at a constant pace. And for that some rules are used if I understood correctly this thread:

http://www.lathetrolls.com/viewtopic.php?f=9&t=824

My questions are two.

When a record is cut at "variable pitch" (changing lateral displacement of the cutting head in order to increase time per side or optimize dynamic range of music cut or trying to get a balance of both) is this process performed for each song? Is it performed for each turn of the record? Is it preformed continuously?

And, after searching in the forum and out of it and getting no clear answers, are there any formula that calculates lateral displacement of the cutter for a given combination of frequency and intensity in the music signal? I understand that this could vary for each head but I suppose there are some rules as to the maximum lateral and vertical displacements before cutting heads start having trouble or discs become unplayable on most systems.

Thank you for your patience.

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markrob
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Re: There are no dumb questions

Post: # 44222Unread post markrob
Tue Oct 04, 2016 9:30 am

Hi,

You can control the pitch manually in real-time adjusting the pitch as the record is being cut (this assumes you know what is coming). The other option is to use an automatic pitch controller that looks at the incoming audio in advance of being cut and changes the pitch as needed. In this case the audio to be cut is delayed by a fixed amount (in some systems one revolution), and the controller reacts to the the real-time audio (a preview) making the pitch decision in advance of the audio being cut. This is a simplification as there are many things to consider, but that is the basics of it. Hope this helps.

Mark

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JJZL
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Re: There are no dumb questions

Post: # 44230Unread post JJZL
Tue Oct 04, 2016 5:15 pm

Thanks a lot.

From your answer I understand that usually "variable pitch" is appliead in a continuous way and not in a discrete manner.

The second question remains unanswered. Are there any formulas relating music contents with lateral needle displacement? I suppose so as variale pitch devices wouldn't be possible without some kind of alogorithm. Maybe equipment related?

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markrob
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Re: There are no dumb questions

Post: # 44231Unread post markrob
Tue Oct 04, 2016 5:38 pm

Hi,

You have to apply pitch changes in a continuous manner. In fact, if you try to change the pitch too quickly, you will end up modulating the groove and the playback stylus will render the changes as audio.

You can determine the lateral excursion to be cut based on the audio information. That requires knowledge of the recording characteristics (velocity recording, RIAA Eq, etc). It has been done and is not rocket science. Typically, the largest lateral excursions when cutting are due to low frequency information.

Are you looking to design a pitch control system?

Mark

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JJZL
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Re: There are no dumb questions

Post: # 44238Unread post JJZL
Wed Oct 05, 2016 1:16 pm

Thank you again.

We are not really trying to design a pitch control system.

A friend of mine and I are trying to create a "vinyl cutting simulator" so we would need to know what parameters are used in order to calculate lateral displacement of the cutting "needle" form a perfect spiral.

Not that we wouldn't love to design our own record cutting console including pitch control.

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markrob
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Re: There are no dumb questions

Post: # 44239Unread post markrob
Wed Oct 05, 2016 1:32 pm

Hi,

I see. So you need to know how the audio information translates to lateral movement of the stylus. You should be able to figure that out once you view the RIAA recording EQ curve and understand the difference between constant velocity vs. constant amplitude recording. Is this for a school course project or do you have some other purpose in mind. I don't want to do your homework assignment for you.

Mark

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Gridlock
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Re: There are no dumb questions

Post: # 44241Unread post Gridlock
Wed Oct 05, 2016 4:43 pm

Sounds almost like that "digital vinyl" hogwash that showed up for a split second a while back
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Recordette Sr.......Presto K-8

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JJZL
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Re: There are no dumb questions

Post: # 44242Unread post JJZL
Wed Oct 05, 2016 6:33 pm

Thank yoy all.

I am glad you think I am an student but regrettably I am a little bit older than that.

I am working at this with a friend of the university, curiously both of us biologists, as a hobby project.

I think I understand the behaviour of RIAA curve and its inverse, at a theoretical level (decrease lows while recording and enhance highs and the inverse during reproduction). But I don't kno not how does a given sound signal translates into microns.

I have searched trough the forum and found no detailed information.

If someone could point me to a manual, book, text, url... or any other place where I could study the subject, I would be grateful.

Greetings

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markrob
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Re: There are no dumb questions

Post: # 44244Unread post markrob
Wed Oct 05, 2016 8:36 pm

Hi,

If the audio were recorded in constant amplitude mode, the stylus excursions would be a direct copy of the audio waveform. That would imply that the stylus moves with the same peak to peak excursion at 20 hz as it does at 20 Khz if the same audio level was apply to the cutter head at each frequency. At low frequencies (down to the 50 hz RIAA turnover point), this is the method used, but its clearly not possible to do this at 20 Khz because the G forces needed to accelerate the stylus mass would be astronomical.

The solution is to transition from constant amplitude recording to constant velocity at some point. This happens at the 500 Hz turnover in the RIAA curve. From 500 Hz to 2122 Hz, the recording characteristic is constant velocity. This reduces the excursion of the stylus at a -6db/oct rate in that region and keeps the stylus acceleration down to reasonable levels. At 2122 Hz there is pre-emphasis (+6db/oct) applied to overcome the noise floor of the medium. In effect, the recording characteristic returns to constant amplitude. This is doable because normal program material has less energy at the higher frequencies. Below 50 hz, the recording becomes constant velocity based to keep the ultra low frequency excursions from getting out of hand.

If you do the math, you discover some interesting things about the size G forces and how tiny the smallest excursions become (think wavelengths of light). Note that the typical published RIAA curves for both record and playback are velocity-centric. Most modern pickups are velocity responsive and cutter heads are also typically flat in response vs. velocity, so it makes sense. You can easily view the system as amplitude responsive if you integrate the velocity.

If you assume a velocity based cutting system is in place, you convert it from velocity to amplitude by integrating the waveform. If you then apply the RIAA EQ, the resulting waveform should be an analog of the excursions traced by the cutting stylus. To determine the actual movement, you need to know the typical recording reference level used to cut. A good place to start is 3.54 cm/sec RMS at 1Khz (5 cm/sec peak). To determine the excursion at this frequency use the following relationships:

Velocity
V(t) = V * Sin(wt) Where w = (2 * Pi * F) and V is peak velocity

Integrating you get:

Excursion
S(t) = v/w * Cos(wt)

Given peak velocity = 5e-02 meters:

then peak excursion at 1 Khz = 5e-02 / ( 2 * Pi * 1000) = 7.95e-06 meters (15.9e-06 peak to peak)

Finally, most modern cutter heads have peak mechanical limits of about +/-150e-06 meters

Hope that helps.

Mark

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Gridlock
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Re: There are no dumb questions

Post: # 44246Unread post Gridlock
Thu Oct 06, 2016 12:47 am

...but hey, ya ask a silly question, ya get a silly answer...

-Tom Lehrer
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Recordette Sr.......Presto K-8

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dubcutter89
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Location: between the grooves..

Re: There are no dumb questions

Post: # 44247Unread post dubcutter89
Thu Oct 06, 2016 2:24 am

Hi!

Mark said it all!
So if you're able to integrate sine/cosine etc. and do some fourier/laplace transformation (or your computer does) then you are already there - 90% done and exactly what would be needed to transform audio input into 3D cad file for prototype printing, lasering HD-Vinyl...

Just a writing mistake I've found:
Below 50 hz, the recording becomes constant velocity based to keep the ultra low frequency excursions from getting out of hand.
True, below 50Hz it's velocity based again BUT therefore ultra low will get out of hand and need more space (=amplitude)! This turnover was introduced to get rid of mechinacal rumble etc. on the playback side (no extra boost of ultra-low-frequency)...

typo: velocity in meters per second

also there is the geometry of the groove: for a mono system the above is true (lateral or vertical modulation). If you want to calculate the groove of a stereo disk then you would have to look at each signal seperate, and rotate them by 45 degrees. Then you get a lateral as well as a vertical modulation, which are both the same amplitude for a single channel but of course divided by sqrt(2)... The channels (left, right) are orthogonal to make them independent.

Lukas
Wanted: ANYTHING ORTOFON related to cutting...thx

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markrob
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Re: There are no dumb questions

Post: # 44249Unread post markrob
Thu Oct 06, 2016 7:54 am

Hi Lukas,

Thanks for the correction on the 50 hz turnover and typo.

Mark

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Ember
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Re: There are no dumb questions

Post: # 47302Unread post Ember
Thu Jul 06, 2017 8:46 pm

Hi All!
I'm new to the cutting pressing world. Like so new I don't even know what to call it, haha.

Is there a glossary of terms or some good YouTube videos you all could direct me to? I know so little I don't even know what or how to ask for help. I'm looking for a place to get started.

Thanks!
Em

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boryo
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Re: There are no dumb questions

Post: # 53599Unread post boryo
Sun Jul 07, 2019 1:53 pm

Take a look at this: https://www.lathetrolls.com/viewtopic.php?f=22&t=8328
Ember wrote:Hi All!
I'm new to the cutting pressing world. Like so new I don't even know what to call it, haha.

Is there a glossary of terms or some good YouTube videos you all could direct me to? I know so little I don't even know what or how to ask for help. I'm looking for a place to get started.

Thanks!
Em
Best,
Bob

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Dogtemple
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Re: There are no dumb questions

Post: # 58290Unread post Dogtemple
Sat Apr 10, 2021 5:41 am

Hi got a dumb question…

Been reading through this site for months now and seen lots of talk about feedback heads but I’m not clear on what they do exactly. I get the jist of it but not a proper understanding. Can anyone explain please?

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dmills
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Re: There are no dumb questions

Post: # 58302Unread post dmills
Sun Apr 11, 2021 1:27 pm

So the cutting stylus should move with a velocity proportional to the voltage output by the IRIAA network to ensure that what gets read back comes out flat.

Now cutting heads have some very lightweight linkages and various things about them that are less then rigid, so if you just hook a massively underdamped head up to a power amp and apply IRIAA output to it, you will find that the resulting frequency response is all over the place, with typically a honking great resonance at a kHz or so give or take and a rather noticeable fall off at high frequency if you are lucky, there may be some other resonances as well.

In a head designed to be run open loop deliberate damping is introduced to try to control this, and between that and some fairly extensive eq you can cut a record that way.

What a feedback head does is to add a set of doings that sense the actual velocity of the (Typically) linkage just in front of the drive coil, and by subtracting that from the desired motion produce a velocity error signal that is amplified and used to drive the drive coil to make the linkage track the desired velocity. There is a whole subject called "Control systems theory" that deals with feedback systems and their stability which is a really DEEP rabbit hole.

This is much better because you can underdamp the mechanics which makes for a more efficient conversion of electrical power into scratch, and the feedback loop means that differences in compliance, frequency response errors and such get compensated automatically. The other MAJOR advantage is that the feedback tends to improve the crosstalk on the disk due to imperfect mechanical summation.

Traditionally the drive amplifiers are voltage output, which complicates things a little, force being proportional to current not voltage, and hence velocity being the integral of the current, ideally a current drive would be better, but it was difficult to do back in the day.

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Dogtemple
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Re: There are no dumb questions

Post: # 58308Unread post Dogtemple
Mon Apr 12, 2021 3:05 am

dmills wrote:
Sun Apr 11, 2021 1:27 pm
So the cutting stylus should move with a velocity proportional to the voltage output by the IRIAA network to ensure that what gets read back comes out flat.

Now cutting heads have some very lightweight linkages and various things about them that are less then rigid, so if you just hook a massively underdamped head up to a power amp and apply IRIAA output to it, you will find that the resulting frequency response is all over the place, with typically a honking great resonance at a kHz or so give or take and a rather noticeable fall off at high frequency if you are lucky, there may be some other resonances as well.

In a head designed to be run open loop deliberate damping is introduced to try to control this, and between that and some fairly extensive eq you can cut a record that way.

What a feedback head does is to add a set of doings that sense the actual velocity of the (Typically) linkage just in front of the drive coil, and by subtracting that from the desired motion produce a velocity error signal that is amplified and used to drive the drive coil to make the linkage track the desired velocity. There is a whole subject called "Control systems theory" that deals with feedback systems and their stability which is a really DEEP rabbit hole.

This is much better because you can underdamp the mechanics which makes for a more efficient conversion of electrical power into scratch, and the feedback loop means that differences in compliance, frequency response errors and such get compensated automatically. The other MAJOR advantage is that the feedback tends to improve the crosstalk on the disk due to imperfect mechanical summation.

Traditionally the drive amplifiers are voltage output, which complicates things a little, force being proportional to current not voltage, and hence velocity being the integral of the current, ideally a current drive would be better, but it was difficult to do back in the day.
Excellent reply! Thanks for taking the time to do that, it’s really helpful

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