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Frequency Response of Vinyl, Split from "To get into vinyl, or not?" (TOS #5)
almostmitch
post Dec 3 2012, 20:15
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QUOTE (botface @ Dec 3 2012, 13:49) *
I guess you can expect a factory installed arm/cartridge to be set up properly. I'd still want to check though even if only for reassurance that things haven't gone "off" in transit.


This was something I considered. I will most likely double check things, because as you implied, you never know what kind of beating the box took in transit.
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BFG
post Dec 4 2012, 00:16
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This whole conversation has me intrigued about the technical limitations of vinyl. For example, is there a minimum or maximum frequency that can be "encoded" in vinyl (akin to the ~20Hz and 44.1kHz cutoffs on digital CDs)? Does this differ for 33s, 45s, 78s? Is there a way (based on the needle tip and equipment setup) for a knowledgeable person to precalculate the bands that the most noise interference will likely be in, and thus minimize or remove it? Etc.

It's particularly pertinent as I'm planning to attempt a vinyl-to-FLAC encoding soon.

This post has been edited by BFG: Dec 4 2012, 00:17
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Axon
post Dec 4 2012, 09:01
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High frequency vinyl noise is strongly broadband, so there's little use shifting that around, per se.

The arm+cartridge resonance defines the low frequency limit. If you're lucky, it will be 8-12hz; if unlucky, 5-20hz.

The HF limit is dependent on many factors, but at the very least, CD4 records go all the way to 45khz. Typically, you'll know if you have a CD4 record. Otherwise, the useful frequency range limit might be as low as 15khz, or as high as 20-30khz.

Hypothetically, on arm+cart combinations which are too low (<8hz), it might make sense to e.g. play a 33rpm record at a much faster speed, so as to shift the signal spectrum away from the resonance, because spot frequency SNR is ludicrously low around resonance. But AFAIK, that's probably more trouble than it's worth.
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cliveb
post Dec 4 2012, 10:33
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QUOTE (Axon @ Dec 4 2012, 08:01) *
The arm+cartridge resonance defines the low frequency limit. If you're lucky, it will be 8-12hz; if unlucky, 5-20hz.

Very few records are cut with any signal lower then about 20-30Hz. Also, all LPs will have their bass summed to mono below about 100Hz to prevent extreme vertical groove modulation.

The thing about the resonant frequency is that you want it to be below the lowest signal frequency on an LP (ie. less than 20Hz) to prevent exciting the resonance. But you also want to keep it above the frequency of the micro-warps that exist on even the seemingly flattest LP (ie. above about 10Hz). This means that the arm/cartridge will ride up & down over warps - if the resonant frequency is too low then the warps will be read as very low frequency signal.

QUOTE (Axon @ Dec 4 2012, 08:01) *
The HF limit is dependent on many factors, but at the very least, CD4 records go all the way to 45khz. Typically, you'll know if you have a CD4 record. Otherwise, the useful frequency range limit might be as low as 15khz, or as high as 20-30khz.

The ultra high frequencies on a CD4 record can only be read by specialised stylus profiles (eg. Shibata). Play them with anything else (eg. a standard elliptical or (heaven forbid) a conical), and the carrier frequency is destroyed. But CD4 is a historical footnote - nobody actually uses it these days (do they?).

Mainstream LPs are often cut with a low pass filter around the 18kHz mark to prevent overheating of the cutting head. Any kind of signal that comes off an LP above 20kHz is pretty much entirely noise and distortion.

QUOTE (Axon @ Dec 4 2012, 08:01) *
Hypothetically, on arm+cart combinations which are too low (<8hz), it might make sense to e.g. play a 33rpm record at a much faster speed, so as to shift the signal spectrum away from the resonance, because spot frequency SNR is ludicrously low around resonance. But AFAIK, that's probably more trouble than it's worth.

I don't see how this would work. Play a 33rpm record at 45rpm and you'll shift the warps higher up the frequency range, and therefore still above the resonant frequency. So they will still be incorrectly read as signal. If anything, the way to avoid this would be to reduce the speed to 16rpm - but then the bass signal is in danger of dropping low enough to excite the resonance.

If your arm/cartridge combination has a resonant frequency in the wrong place, you have these choices:
1. If it's too high, you can add mass to the arm - stick a penny on the headshell. This will drop the resonant frequency. (Don't forget to reset the tracking force correctly. This is NOT the same as the old trick we did on Dansettes to stop them skipping!)
2. If it's too high, replace the cartridge with a lower compliance model.
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