Variable pitch cutting - bass pre-emphasis?

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audadvnc
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Variable pitch cutting - bass pre-emphasis?

Post: # 29650Unread post audadvnc
Sat Apr 12, 2014 8:20 pm

I've designed and built a dc motor based carriage drive for our Scully lathe, which originally (and until this year) always took its feed from the turntable drive shaft. With the Arduino-based drive controller taking a rectified DC feed of the audio, the motor receives PWM power through a Darlington switching transistor and rolls merrily on its duties.

I am not familiar with the variable pitch systems used in commercial lathes such as the Neuman. Is anyone familiar enough with their system to answer this question:

Should the audio feeding the system receive bass pre-emphasis to guarantee sufficient groove land with strong LF program? I am thinking a 6-10 dB bass boost to 40 Hz, rolling off to flat around 250Hz. OTOH, perhaps the normal program feed provides the correct source for the rectified amplitude feed, as bass energy is going to predominate.

Maybe I'll put in a switch to allow either...

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opcode66
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Re: Variable pitch cutting - bass pre-emphasis?

Post: # 29654Unread post opcode66
Sat Apr 12, 2014 10:42 pm

I think after some test cut you could dial in that gain value. Leaving a safety pocket makes sense, no?
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audadvnc
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Re: Variable pitch cutting - bass pre-emphasis?

Post: # 29656Unread post audadvnc
Sun Apr 13, 2014 3:05 am

I know the Westrex audio chain puts in a strong bass pre-emphasis, but I believed it is a compensation for roll-off in the cutting head. OTOH, running LF tone (20-40Hz) through the system creates a groove with lots of excursion relative to similar level signals at mid frequencies. I think I'll make some test tone cuts and see for myself...

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markrob
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Re: Variable pitch cutting - bass pre-emphasis?

Post: # 29657Unread post markrob
Sun Apr 13, 2014 8:14 am

Hi,

Here is my take on the problem. If you use the average value (e.g. rectified and filtered) of the raw lateral audio (L+R), you are not taking into account the RIAA and the fact that your cutting system is flat vs. velocity (within its feedback range). If you forget about the RIAA for a minute, you find that the cut amplitude decreases as frequency increases (-6db/oct roll off). That is why bass frequencies take up so much room when cut. If you now add in the RIAA, you have 3 turnover points to deal with (50hz, 500hz, and 2122hz). It seems to me you need to do the following:

1. Take the audio after the RIAA so that this is taken into account. This is velocity that will be cut. If that is not possible, just add you own RIAA filter mimic the system.
2. Integrate this signal so that you convert from velocity to amplitude. Velocity is change in amplitude vs. change in time so you must integrate velocity to obtain amplitude.
3. Use this rectified and filtered signal as the measure of cutter excursion.

You probably don't want to use a true integrator. Instead a "leaky" integrator would be better. This is just a low pass filter with a a cutoff at a low frequency (say 5-10hz). This avoids the DC drift that happens with a true integrator.

Mark

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audadvnc
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Re: Variable pitch cutting - bass pre-emphasis?

Post: # 29662Unread post audadvnc
Sun Apr 13, 2014 6:38 pm

Thanks for your input. I tested our system today. I made up a series of test tones from 440Hz down to 20Hz, and ran that through the lathe. The Westrex electronics pre-emphasize the LF with a significant bump at 20Hz, but using a Westrex 3B head my cut groove showed pretty much flat response, so I don't need to create a bass pre-emphasis for the variable pitch drive.

Interestingly, I had tested our Westrex 3DII head last year and saw a bass bump in cut grooves. Perhaps our Westrex electronics are properly calibrated for the 3B, and I'll need to adjust them when I put the 3DII on again.

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smithadamm
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Re: Variable pitch cutting - bass pre-emphasis?

Post: # 31929Unread post smithadamm
Wed Oct 22, 2014 11:46 am

audadvnc wrote:I've designed and built a dc motor based carriage drive for our Scully lathe, which originally (and until this year) always took its feed from the turntable drive shaft. With the Arduino-based drive controller taking a rectified DC feed of the audio, the motor receives PWM power through a Darlington switching transistor and rolls merrily on its duties.
I'm messing around with something similar as an upgrade from the real simple LM741/2N3055 carriage drive motor control I have now. Did you end up incorporating any kind of feedback from the motor into your design, or is the torque pretty stable with your PWM setup?

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