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In Reply to: RE: Fremer's ZENITH ANGLE CORRECTION or have we all gone crazy. posted by flood2 on February 10, 2021 at 20:40:04
My take thus far on this is quite different and only a hypothesis at this point so apply salt to what follows.
I suspect that the idea that incorrect zenith simply effects phase is flawed. The Electronic measurement I use is measuring the IMD of a 60Hz+7kHz tone with a 4:1 ratio. You then filter out the 60Hz and look at the FFT of the signal and the level of the sidebands at 6940Hz and 7060hz tell you the amount of IMD. If those levels are 40dB below the 7kHz fundamental then the IMD is 1%.
This is not a new testing concept. The Ultimate Analogue Test LP has instructions and a test track for setting VTA using this exact procedure and both the CBS STR-110 and the Shure TTR-103 lay out this test for mistracking. I stumbled upon this using Analog Magic's VTA setup and quickly realized that the zenith if slightly misaligned dominated the distortion measurements and a zenith error of 1.2° assures that a null point will never be hit on the playable record surface.
In trying to use this test to set VTA, The distortion was high and I could never obtain meaningful numbers by any amount of VTA change. I saw reference to try another alignment geometry so I twisted the zenith a bit and was shocked how much the measured distortion numbers changed. After a few I then began initially setting the Zenith first by using the VTA test outlined above. After a null in distortion was located in the vicinity of where the null point should be I them proceed to using the track to set VTA as it was intended.
here are the plots of IMD distortion vs. Zenith angle for a conical and a micro-ridge.
I have used this approach for electrically setting zenith on more than 20 cartridges and once you get the feel for it, it is a quick and effective way to get repeatably good sound across the entire side of an album.
I decided I should document this more fully as I typed this so I made a forum post about more of the details and to better document the whole process. It will be linked below and to avoid any appearance of being a shill I will say that I am a dealer for analog magik and while it is entirely possible to measure IMD by other means, I find it to be a simple and reliable tool for electrically aligning zenith. I would be all for any discussions on alternative methods of easily measuring IMD with a test record and a scope.
dave
Follow Ups:
Hi Dave
That's very interesting!
I was just having a good think about the point you raised about the issue of phase vs tracking error over the last few days.
If I may just go back to the issue of tracking error in relation to a long groove wavelength (relative to the radius of curvature for the scanning surface) and how it causes distortion - if the cantilever axis is not tangential to the groove then the movement of the cantilever is not perpendicular to the groove and this results in a distorted waveform with an FM-like distortion that is related to the groove wavelength. With a conical stylus, the points of contact are such that the line joining them is always perpendicular to the direction of the record motion and matching the cutter. I am sure that this is why a cartridge such as the DL103 has such ardent fans. With an elliptical or line contact stylus, the contact points are no longer perpendicular to the groove wall except at the null points and tracking error introduces the phase component which makes these styli far more sensitive to tracking error.
So the question is if, for a groove wavelength that is very much longer than the radius of curvature of the scanning surface, the tracking error due to cantilever alignment still dominates? It is not until the groove wavelength decreases sufficiently that the phase error due to the zenith compounds the problem. However, nulling a zenith error doesn't solve the distortion due to the cantilever tracking error and that you are still introducing an FM distortion to the signal.
I don't know the answer. Maybe zenith distortion dominates particularly for line contact styli and this IS the right approach, but I can't help thinking that "two wrongs don't make a right" and I think that shifting entirely to nulling zenith at the expense of the longer wavelength FM distortion due to cantilever tracking angle doesn't solve the problem. I have a number of styli that have a zenith error, I may do the experiment. However, I have not found setting the offset to be particularly consistent and I normally consign faulty styli/cartridges to the reject pile and move on to suitable candidates that are worth the time and effort!
The test method you described sounds a clever way to determine IMD.
I have the Ultimate Analogue Test LP and also didn't find the VTA test terribly useful given that the VTA is essentially pre-determined by the cartridge manufacturer and is also has a fixed relationship to the SRA.
One thought came to mind in relation to the position on the record for the VTA test - it is centred at a radius of around 100mm. For a typical 9" arm set to the standard 66/121mm nulls, the tracking error is about 0.9° with a estimated distortion level of about 0.5%. Therefore wouldn't adjusting zenith for the minimum distortion not be changing the alignment completely? Surely one would be trying to set the distortion to the equivalent value based on tracking error? If on the other hand, the you had a suitable test tone centred around the nulls, then perhaps your method applies directly.
The original Ortofon (0001 and 0002) test discs have a difference-tone test stepping down from 20/19 kHz down to 8/7kHz which enable stylus condition to be monitored using the level of IMD as a metric. These are still available on discogs and I can recommend it to you as I find it the most useful of all my test discs (which includes the CBS STR-110 and 112 as well as the Telarc Omnidisc). In particular it has a white noise test tone which makes doing frequency response checks a breeze!
Regards Anthony
"Beauty is Truth, Truth Beauty.." Keats
> In particular it has a white noise test tone which makes doing frequency response checks a breeze!
Wouldn't pink noise be the type of test required for phono cartridge frequency response? I thought white noise would have a 6-dB per octave increase in amplitude when played with a velocity responding magnetic cartridge.
Thanks!
John Elison
If you use a flat gain amplifier then a pink noise test tone would be applicable. However, most consumers would likely be using a conventional phono stage with RIAA EQ in which case the output signal being measured will have the pink 10dB/decade drop requiring the user to either interpret the response as is or apply a 10dB/dec gain to analyse the deviation from flat.
In the case of the Ortofon test disc, the test signal is pre-emphasised for RIAA therefore the output of a phono stage would be a flat signal within the tolerancing of the phono stage EQ.
In practical terms, it isn't validating the phono cartridge in isolation, it is only validating the combination of the phono stage and cartridge response, but most decent phono stages have gain topologies and fine component tolerances to give an RIAA response typically better than ±0.5dB which is better than most cartridges so the white noise output can reasonably be taken as being that of the cartridge.
Regards Anthony
"Beauty is Truth, Truth Beauty.." Keats
White noise has equal intensity at different frequencies. In other words, the energy of white noise in-between 10-Hz and 20-Hz is the same as the white noise energy in-between 1000-Hz and 1010-Hz. Because of this, a phono cartridge will have a 6-dB per octave increase in amplitude when reproducing white noise.
Pink noise, on the other hand, has equal energy per octave. This means that the pink noise energy between 10-Hz and 20-Hz is the same as the pink noise energy between 1000-Hz and 2000-Hz. Therefore, pink noise produces a flat frequency response when reproduced by a velocity sensitive phono cartridge.
RIAA equalization is different from either of these. The RIAA recording curve and the RIAA playback curve are pictured below.
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I thought my reply to your question was very clear, but I will repeat the key sentence but this time with capitalised words of importance in case you missed them the first time around...
"In the case of the Ortofon test disc, the test signal IS PRE-EMPHASISED FOR RIAA therefore the output of a phono stage would be a flat signal within the tolerancing of the phono stage EQ."
Secondly, as I pointed out, the test discs in question assume that a conventional phono stage is being used, therefore pre-emphasis is typically applied to the signal whilst proper test discs made to the DIN standard are linear cuts without pre-emphasis at a reference amplitude of 11.25um for 0dB.
Pink noise is typically used for testing loudspeaker sensitivity and subjective response balance because the 1/f spectral noise power is closer to that found in natural systems to which the human auditory system is attuned to. Hence, sound engineers use pink noise to evaluate room response.
White noise is an equal amplitude signal and is therefore ideal for analytical measurements not involving the subjectivity of the auditory nerve centre. Pink noise is derived by filtering white noise with a 10dB/decade attenuation characteristic.
Your statement that a pink noise test tone will give a flat response with a magnetic phono cartridge is incorrect for the commonly available consumer test discs such as the HFNRR and AP Ultimate Analogue Test LP which assume that the user will play back through a standard phono stage.
Using the pink test signal from the AP Ultimate Analogue Test LP, the spectrum on the left is with flat gain and the spectrum on the right is the RIAA equalised signal with the frequency range zoomed into just below 1kHz to just above 10kHz. You will note that with flat gain the response is up ~4dB at 10kHz with respect to 1kHz. When RIAA EQ is applied, the response drops by the expected 10dB/decade and the cartridge tested shows a fairly linear response of the Pink Noise test signal
The RIAA emphasised white noise test signal yields a flat response as expected from the "equal amplitude" characteristic of white noise.
Regards Anthony
"Beauty is Truth, Truth Beauty.." Keats
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