Further thoughts on tracking ability and tracking tests

We entirely agree with, J. Walton who wrote in an important article in 19671.

It is necessary to concentrate mainly upon this factor of "trackability" as the first essential in gramophone pickup performance. All other measurements (including that of frequency response) depend first of all upon proper tracking - that is, the continuous and consistent contact between stylus and groove wall.

The tracking perfomance of a phono cartridge is the manifestation of the battle between the downforce of the gravity upon the cartridge in the groove and the accererations due to the groove modulation acting on the dynamic mass of the stylus assembly (which create a "lift" force). The point at which the lift forces due to the modulation outweigh the downforce of gravity is the point at which the stylus begins to lift out of the groove and mis-tracking results. But this simple analogy is greatly complicated by the consideration that the frequency of the forces imparted to the stylus range over a span of some twelve octaves!

Tracking theory

The moving assembly of a phono cartridge has one, apparently simple, duty: to transfer the motion of the stylus tip at one end of the cantilver to the moving elements of the electrical transducer at the other end.

These three parts (stylus tip, cantilver and the moving elements of the electrical transducer) constitute the mass of moving system and one of the goals in cartridge-design is to minimise the mass of this assembly; and especially its inertial effect at the stylus point. The pivot of this mass is supported with a flexible, elastomer bearing which, like all springy things, has a compliance2.

Now there is one inevitable truth concerning any physical system comprising a compliance and a mass: that it possesses a dependence of how it behaves when it is excited by vibrations of different frequencies.

And that is the real issue with the design of phono cartridges for good trackability: that there exist conflicting requirements in achieving good tracking ability simultaneously in the low, mid, and high frequency regions. Compliance of the moving system is the controlling factor at low frequencies, effective tip mass ultimately limits the ability of the stylus to follow high-frequency groove modulation and the damping of the moving system is the key to tracking in the midrange frequencies.

To quote from a Shure technical document3,

The demands upon the elastomer bearing are different for each part of the total spectrum. In the low-frequency region (below approximately 1,000 Hz) where recorded amplitudes are of main concern, the stiffness of the bearing limits trackability and, therefore, it should be highly compliant. To track all audio frequencies below about 4,000 Hz, the damping properties of the bearing should be low, because of high recorded velocities. Above 4,000 Hz, the ideal bearing would have a large amount of damping to enhance trackability as the stylus resonance frequency is approached. Additional bearing stiffness may also be desirable to raise the stylus resonance beyond the audio limit of 20 kHz. Also to be considered is the need to limit compliance for sub-audible or warp performance. It is apparent that because of these conflicting requirements the bearing cannot, be ideal over the entire spectrum.

Later in the same seminar4, the point is put more forcefully still,

The material qualities [of the elastomer bearing] can... be defined...... the material should be stiff in the subaudible region, compliant in the low- and mid-frequency region, and then stiff again in the very high-frequency region - not an easy requirement to satisfy!


How do you test for tracking?

Now, it's one thing to claim that tracking matters more than anything else, but another to devise a good method to test for it!

The industry standard test consists of taking the cartridge through a series of Labours of Hercules in the form of 300Hz test tracks of increasing displacement (and thus of velocity and acceleration).

The initial band in this test is usually about +6dB relative to standard recording level of 7cm/s (peak, equivalent to 5cm/s RMS)5.

All decent cartridges should be able comfortably to track the first few bands of this test sequence up to a level which is +12dB relative to standard recording level; a modulation of 90µm peak-to-peak (usually referred to as 45µm peak). Indeed, this is the band on which harmonic distortion tests are usually performed.

The bands of 300Hz modulation then increase through various levels to a final modulation of +18dB relative to reference level; a physical peak-to-peak modulation of 180µm (90µm modulation peak).

Testing for tracking involves the simple process of lowering the stylus on these bands of increasing modulation and listening for the point that the stylus mistracks. This is betrayed by a change in the tone so that - to the pure note somewhere between D and D# above middle C - a rasping, octave buzz is added.

The cartridge's ability to track is defined by the peak modulation of the last band in which the stylus negotiates the groove without mis-tracking. Thus we see a tracking specification such as: Tracking ability (300Hz): 80Ám lateral.

Limitations

The problem of these tests at 300Hz is that they only test the tracking performance of a cartridge in the low-frequency region where compliance dominates the proceedings. The test thus tends to favour high-compliance over the two other important parameters (effective mass and, particularly, damping). Some impression of the artificiality of this test is given by plotting the groove velocity of the final test track (90µm peak) on the famous curves derived by Shure of theoretical and measured groove velocities (right)4 & 6.

That this test is well outside the standard (grey) operating-envelope of the cartridge is obvious. But this test is also misleading and its wide adoption of a measure of tracking ability potentially allows a great many cartridges with poor HF tracking to escape detection. An especially serious situation because HF mis-tracking is known to cause groove damage3.

Better tests?

So how might wideband tracking be tested? The remedy is not simple. The obvious solution is to include middle- and high-frequency tests for tracking too, but these are not widely available on standard test records. In addition, HF mis-tracking cannot be determined by ear in the same way as the 300Hz tests, because the harmonics produced in the process are often above the hearing-range.

Shure Brothers Incorporated (who deserve enormous credit for fully investigating tracking and trackability of phono cartridges forty years ago) produced a very important test record (TTR-103) in 1979 which remains tremendously useful. This 45RPM LP hosts a series of very carefully recorded bands to test LF, MF and HF tracking. It was widely used in consumer reviews at the time and, why this disc has fallen from widespread use, is a puzzle. Although, it is fair to say that the data it produces is fairly difficult to interpret; especially in real-time. With the aid of a modern Digital Audio Workstation or DAW, so that a needle-drop can be recorded and analysed post-recording, these tracks reveal a great deal about wideband tracking with great precision.7

Listening for tracking problems.....

Shure also produced a series of test records back in the 1970s entitled An Audio Obstacle Course to demonstrate the superiority of the Shure V15 range. These contain various music tracks recorded at gradually increasing velocities which were intended for qualitative investigation of wideband tracking. With care (and, once again, with the help of a modern Digital Audio Workstation), certain tracks may be employed systematically to investigate HF tracking. Sadly none of these discs is produced today.

As an example of how the Shure discs may be used, the Orchestral Bells track on TTR 101 includes four, short, close-miked recordings of a glockenspiel tune organised so that the peak velocities in the final version reach 25cm/s in the band above 10kHz.8

By comparing the four versions of the glockenspiel tune, it is possible to investigate tracking-ability of real, musical sounds (not just test-tones!) in the extreme HF range over a four-fold range of velocities.

To make this this very technical issue "come to life", a short audio file is available below in which a small phrase of the glockenspiel tune from the first version of the track and the last are inter-cut so that they may be A/B compared. The levels have been adjusted according to Shure's calibration so that the judgement is made a precisely the same intensity. (This is a good example of how modern digital-audio editing enhances the usefulness of these test tracks.) Note how these two versions; one played mp and the other played ff, sound very similar. It's only the variation in the surface noise which reveals the version which has been boosted in level by 400%.

Glockenspiel phrases mf & ff inter-cut.

For wide is the gate and broad is the way that leads to destruction

When wideband tracking tests are performed on various cartridges as we have done at Phædrus Audio, the unpalatable truth emerges that some, apparently capable, cartridges demonstrate high-frequency mis-tracking at elevated velocities. And neither a high-price, nor an exotic stylus shape appear to be guarantee that tracking-performance has been thoroughly investigated. In fact, hyper-elliptical / line-contact styli often provoke mis-tracking because of the greater amplitude of movement and the greater accelerations involved.9

It seems to us quite possible that many audiophiles are unaware of this due to concentration on the standard 300Hz tracking tests in which compliance alone counts above all else.

Both the PHLUX-II active MM cartridge which Phædrus Audio designed in conjunction with Pspatial Audio and the new displacement-sensitive DisC cartridge are designed to offer good trackability throughout the whole audio range (see the PHLUX II specification). Indeed the glockenspiel examples given above were recorded with the PHLUX-II.



References & Notes

1. Measuring Gramophone Pickup Performance, J. Walton WIRELESS WORLD, DECEMBER 1967

2. Compliance is defined as the distance someting springy is compressed, extended or deflected for a given force. Today, one ought to express this in SI units, but that doesn't seem to be the way phono cartridges are spoken about. In this field, c.g.s. units still rule the day. So compliance is expressed as deflection in cm due to a force in dynes where one dyne is equal to 10 micro-Newtons (10 × 10-6 N).

3. Getting The Signal From Tip-To-Terminals, Frank J. Karlov. Part of a Shure Technical Seminar in 1978 High Fidelity Phonograph Cartridge. Available from Shure.

4. Design Considerations of the Vl5 Type IV Phonograph Cartridge, L. R. Happ. Op. cit.

5. Standard recording level is equivalent to a displacement of 11.2µm peak of a sinewave modulated groove at 1kHz. Velocity can be calculated using the formula, v(t) = 2 . π . f . a, where a is the peak amplitude and f is frequency. Thus,

    2 × π × 1000 × (11.2 × 10-6) = 0.07m/s = 7cm/s peak or 5cm/s RMS

In this respect referring a 300Hz test track to a 1000Hz reference (as does the documentation to CBS STR-112 - the disc we use for these tests) is somewhat confusing.

6. Right is a micrograph of the +18dB (90µm peak) 300Hz test band on a standard test record. The artificiality of this track is demonstrated by the fact that the grooves are triple spaced according to standard groove spacing. And, were it not for the choice of 300Hz with a period of 3.3333mS, inscribed on a disc with a rotation periodicy of 1.8 seconds so that the crests of one rotation of track will only very rarely coincide with troughs in an adjacent groove, the master would be severely over-cut. Passing this "tracking torture test" (as it is sometimes called) is therefore somewhat academic. Right click on the image to see a bigger version.
7. Shure's documentation for the TTR-103 test record includes the advice to look at the MF and LF tracking test tracks using a Lissajous display as illustrated here to diagnose mid-frequency mis-tracking. The fail in the right-hand trace is obvious and easy to diagnose. Once again, the DAW is invaluable in performing these tests in non-real time (and in providing the Goniometer). It is true that this is by far the best display to investigate tracking performance of these recorded signals.

8. We can verify Shure's claim: if the track is high-pass filtered with a steep filter with a turnover at 9kHz, the peak velocities do indeed reach 25cm/s.

9. There is thus a rather complicated relationship between the advantages for reduced scanning-loss with narrow-profile styli and the disadvantage of the increased risk of high-frequency tracking problems. The reduced mechanical amplitude-response of (for example) the simple, conical stylus to small-wavelength detail on the record has one advantage in that it acts like a low-pass filter. This reduces the excitation of the cantilever and thereby on the requirement for the appropriate "tuning" of the mass-compliance-damping ratios of the cartridge's mechanical system at high-frequencies. Isolated from damping considerations of the elastomer bearing, finer stylus shapes offer no guarantee of better high-frequency reproduction.



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