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The Korf Blog

The inside story: our research,
development and opinions

14 April 2022
Antiskating, Part III
Korf Audio has a tradition of doing practical studies of turntable and tonearm behaviour. We've done Tonearm Geometry, Azimuth Adjustment, and Tonearm and Cartridge Matching. Now we're exploring antiskating, and this is the third and final part where we share our ways of setting it right.

Setting the Correct Antiskating Force
In our previous posts, we have demonstrated that the skating force is a function of stylus drag and its geometric projection. It can be quite simply calculated, but unfortunately, the skating compensation ("bias") markings on the tonearms and turntables are hopeless. What are we to do?

As usual, there's an easy answer and the hard one. The easy one is simple: because the antiskating setting is so approximate, adding just about any is fine. Set the tonearm's scale to the same number as your downforce and forget about it.

That's an acceptable solution, but I guess many of you want to do better. For me personally, knowing for sure that any mistracking is definitely not caused by a skating force is worth going through the slightly more elaborate ritual.
~
But first, let's put aside the methods that definitely don't work.

  • The most ridiculous one is watching the cantilever from the front to make sure it's exactly in the middle when the LP is being played. Even if it's a perfect disc, and you have eagle-like eyesight, this is still ridiculous. A typical horizontal cartridge compliance of \( 25 \cdot 10^{-6}\) cm/dyne means that the stylus moves approximately 0.245 mm when 1 gram-force is applied to it. This means that a 0.2 gf error in antiskating force—a difference between "none" and "enough"—will displace a cantilever by 0.05 mm. Five hundredth of a millimeter. You think you can see that?

  • Next, "centering the stereo image". It assumes the playback volume is a straight function of the tracking force. Generally it isn't. There's some tentative relationship, but it's largely confined to very low frequencies. Humans are quite terrible at reliably placing those in the stereo field.

  • Blank disc. That's not as bad as the previous two. But remember, skating force is a product of stylus drag. The actual LP grooves "brake" the stylus more than a flat sheet of plastic. Hence, antiskating set with a grooveless disc might be lower than necessary.

  • Watching for equal total harmonic distortion in left and right channels. If you have a test record with a constant 10 kHz (or higher) signal, this is a valid if labour-intensive way. But the highest steady tone most test records have is 3125 Hz. This is not enough to achieve a reliable setting.
~

Magnified CD-4 disc grooves. These tiny ripples are the 30 kHz carrier signal

What we need is a test LP.

Ideally, it should have the signal level typical of the commercially recorded LP. But also, enough very high frequency information to induce mistracking when the skating force is not compensated right. Do such test records exist?

In a way, yes. They're called CD-4 quadraphonic discs.

CD-4 has nothing to do with compact disc, and stands for Compatible Discrete - 4 (channels). It is a 1970s all-analogue true surround sound format. The clues on how to extract rear channel information are encoded with a 30 kHz carrier signal in the same grooves.

When your antiskating is set right, you will get uninterrupted carrier in both channels on the whole disc. If your antiskating's wrong, the 30 kHz signal will flicker on and off in the channel that's not getting its fair share of downforce.

Audio Technica AT14S in a very pretty white/gold colour scheme, sold under the JVC brand as a dedicated CD-4 cartridge.
Note: PSpatial Audio, makers of StereoLab software RIAA phono correction, wrote an interesting primer on CD-4 decoding.
Obviously, we can't hear the 30 kHz carrier (that's the whole point of it being so high). But the signal is clearly visible on an oscilloscope or in a computer recording made with a sampling frequency of 96 kHz or higher. This is what a steady carrier signal might look like:
You're not likely to get something as pretty as above, but it's farily easy to see when the carrier is there. For example, here the carrier signal is flickering in the right channel (red):
Knowing that the right channel is encoded on the outside wall of the LP's groove, this points to too little antiskating. Add a bit more. Conversely, if the left channel's carrier signal is erratic, we need to dial the skating compensation back a bit.

A good idea is to start with zero antiskating. Keep adding it until the carrier in the right channel is stable. Then check that the left channel's carrier reproduction hasn't suffered.
~
CD-4 quadraphonic disc is a very precise method. However, it's not without its problems. First, many simpler styli (conical, spherical, larger or bonded elliptics) are unable to consistently reproduce this 30 kHz signal. If your favourite cartridge doesn't have an advanced stylus, this method is not for you.

Second, if your tonearm and stylus are really good, the carrier might be stable over the whole range of skating compensation.

Third, not everyone has an oscilloscope or a sound card connected to their stereo.

And, last but not least, CD-4 records are hard to find in some parts of the globe. Other quadraphonic discs (SQ, QS etc) will not do.

In these cases, we have no choice but to fall back on commercially available test records. Unfortunately, their antiskating setting tracks are often the exact opposite of what is needed! Instead of hard-to-track high frequencies at normal levels, they feature an easily trackable 300 Hz signal, recorded at torture levels of +14 dB or more. This creates stylus drag that is simply not present on any actual record.

We can back calculate from antiskating needed to track this very high level to a normal one. Have a look—here's the relationship between recorded level and drag. It very much depends on the cartridge used.

Fortunately for us, it's mostly a function of compliance. And we have everything to decode this chart. Our old buddy Ortofon SL-15 is "SPU Lite", Goldring 800E and ADC 1 are low compliance MMs, and the ADC 10E is high compliance. And the Decca is a Decca. If you're lucky enough to have one, you don't need to do any calculations.

Say, you have set your antiskating using the +15 dB bands of ubiquitous "Hi-Fi News and Record Review" test LP. Your cartridge tracks those perfectly with antiskating set to "2.5" on your tonearm. With a modern MM cartridge, you'll need to dial it back a bit less than a half, to "1.5" or so.

Here's a little table showing how much you need to decrease the antiskating from one that tracks +15 dB perfectly. We have re-checked these with a CD-4 disc where it was possible, and the resulting settings were very close.
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27 April 2022 update: after I have published this page, our reader Jouni Heikkinen from Järvenpää, Finland, wrote us a short letter:
I have found a perfect way to evaluate the need for the force. I have made a short wire in which the right channel is out of phase. Then I attach this extension cord between the player and the amplifier and I listen to the sound of a good quality mono record. Theoretically, (in the optimal case) I should hear nothing. Sure I do, but the less I hear, the closer the adjustment is to optimum. I have compared this method to the others and found this one satisfying.
Jouni's method is ingenious. It only relies on our remarkable hearing to equalize reproduction in both channels. No tools needed, no computers, oscilloscopes etc. The required cable is very simple to make, and doesn't have to be fancy. But you need a "mono" button to be able to sum the channels.

Switch the phase in one channel, and that's it
It is now firmly my favourite way of setting up antiskating. Just make sure that the grounds of both channels do not meet at the tonearm side.

The Summary
By tradition, we end our small exploration of antiskating with a summary:
1
The skating force is there, and it's real. It's about 1/10th to 1/5th of downforce (as "seen" by the stylus mounted on a 9" tonearm)
2
It's nice to have something to counteract it. The presence of the skating force compensation is more important than its extent—this isn't a precision setting by any means
3
Turntable/tonearm antiskating scales are often useless (but that's ok)
4
If you're obsessed with getting your antiskating exactly right, invert a phase in one channel, put a mono record on and press the "mono" button on your amp. Then adjust the antiskating till it's really quiet
5
Watching a 30 kHz carrier signal from a quadraphonic CD-4 disc is another way to set it precisely
6
If you only have a test record like a HFNRR test LP, adjust using +15 dB bands and dial the antiskate back as described above

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