Virtual record stabiliser - Disc Echo Cancellation

Virtual Record Stabiliser — Disc Echo Cancellation

"Should the archivist or collector use a record stabiliser (weight or clamp) when making needle-drops?"

The principal argument

The principal argument for record stabilisers - here paraphrased - runs something like the following. We call this Argument 1.

When playing a disc, tiny vibrations are created in the vinyl. If left untreated, these vibrations can return to the stylus, causing spurious output from the cartridge. The turntable and mat are intended to damp the LP, thus absorbing these errant vibrations.
For the damping effect of the turntable and record mat to be maximised, there must be intimate contact with the record. The intent of a clamp or record weight is to establish this tight coupling.

Other claimed advantages

A record weight increases the mass of rotating system and thereby reduces flutter
A clamp, or weight permit the flattening of warped records
Either helps prevent records slipping on the mat

Prima facie, these claims sound reasonable enough. But do they stand scrutiny? In short, should the archivist or collector use a record stabiliser (weight or clamp) when making needle-drops?

Argument 1 is dealt with here - and a radical new solution proposed.

The ancillary advantages (Arguments 2, 3 & 4) are discussed in Appendix 1. To summarise, none of the ancillary arguments stand up to careful examination; except for the last.

Disc Echo (Argument 1)

As a record turns, the energy imparted by the motor drives power into the phono cartridge via the controlling channel of the groove modulation.

We usually treat the various masses, compliances and damping materials in the cartridge as an effective impedance to be driven by the groove modulation (see equivalent circuit of the dynamic system of a phono cartridge). A typical value for the effective impedance (in terms of the force of reaction from the groove upon the stylus as it tracks the modulation) is somewhat less than about ½ g (gram); largely resistive in the midrange where the maximum energy exists.

Now, because there exists no action without a reaction, the record is also subject to an equal and opposite force, in proportion to its mass (typically about 120g). Here there lies a possible mechanism for error because we want the the record to be absolutely still relative to the cartridge¹.

In amplitude terms, the movement of the record alone would be ^0.5 ⁄ ₁₂₀ = 48dB below the magnitude of the stylus movement; a figure which is certainly audibly significant.

But that is not the end of it, to help keep the medium still, the record sits on a massive turntable held to it by gravity and friction. A typical turntable weighs around 1.5kg. This extra mass will ensure that the movement - provided the disc isn't free to move on the platter - is reduced by at least another order of magnitude. We can calculate that the movement of the disc plus 1.5kg platter (once again, magically spinning in space) will be:

^0.5 ⁄ _{(120 + 1500)} = -70dB relative to the movement of the stylus.

A level which isn't so low that this effect may be totally disregarded.

Of course, this simple model¹ in which we consider the record (and turntable) free to move in space isn't realistic. Instead, the platter is held rigidly in its bearing. It isn't free to move in reaction to the forces upon the stylus and the energy imparted into the disc will bounce around the medium. We call this energy Disc Echo.

This is the energy referred to in the arguments for record-weights and clamps, but these appear to be entirely conjectural: no evidence is offered. Does Disc Echo exist, and can we detect it?

Looking for an Echo

"Disc Echo does exist, and at a level above the noise mechanisms of a playing record. It is a legitimate cause for concern."

Devising audio experiments to test the assertion that record may produce spurious signals at the cartridge due to stored energy fed back from the medium proved difficult.

Eventually, we discovered a methodology⁵ by which we could identify and measure Disc Echo and can disclose (possibly for the first time ever) a recording of this effect. The following audio example is a recording of the spurious signal fed back to stylus from energy stored in the medium whilst playing a 300Hz tone at 15dB above standard recording level (the original signal having been eliminated). The signal level has been boosted 30dB to be easily audible.

Take a moment to consider what you are listening to here. This is the sound of the energy fed back into the vinyl as the record (and the turntable) react to the interaction with the stylus, re-transduced and recorded. This is the energy we have named Disc Echo.

Do record stabilisers (weights or clamps) help?

There are so many products (see sidebar) that it is impossible to be definitive. However, the answer appears to be ... no. In fact - at best - no. In our experiments, not only do record clamps and weights not substantially reduce Disc Echo, but they may make it worse! ⁶. Footnote 6 details the experimental data we obtained.

Virtual Stabiliser - Disc Echo Removal

In the audio example given earlier, the signal level was boosted 30dB so that the spurious signal was more easily audible. But note how this signal was easily identifiable above the groove noise.

Put simply, Disc Echo does exist, and at a level above the noise mechanisms of a playing record. It is a legitimate cause for concern.

Happily, having taken considerable pains to isolate and discover Disc Echo we have the means, in digital signal processing, to eliminate it.

The unique Disc Echo Removal signal processing (only available in Stereo Lab) is engaged in the PHONO settings tab. Think of it as a virtual record weight — one that really works and doesn't wear the turntable bearing (see Appendix).

Two short demos follow. Audio prior to the beep is standard: after the beep, Disc Echo Removal is engaged. No other DSP was applied (except RIAA EQ).

Some hardware alternatives

Weights

The manner in which record-weights attempt to improve the coupling of the disc to the tuntable is obvious enough; the extra mass simply is intended to hold the record in closer contact with the mat in the area of the label.

Some of these extra weights are substantial; as much as ½ kg or more. Some products claim to incorporate a unique dissipative internal structure.

Clamps

Clamps act in the same way as weights, but without the penalty of the extra mass (see Appendix 1). A centre hold-down clamp in the form of a screw-down "puck" on a threaded turntable spindle is a practical solution but is clearly only an option when included in the design of the turntable.

For use with more standard smooth centre pins, clamps employ a collet mechanism so that, as the top part of the clamp is turned, the chuck tightens on the centre pin. Provided downward pressure is applied to the disc as the collet is tightened, the record is held against the turntable. Michell Engineering make a product of this type for a reasonable price.

Because holding the clamp down against the compliance of a Villchur type suspension² is awkward (and potentially damaging if clumsily done), another type of clamp³ has been developed in which the clamp automatically pulls itself downward as the collet is tightened (as illustrated above). The disadvantage of this clever mechanism is that the clamps are expensive.

Ring Cycle

A few products (sometimes called periphery clamps) exist which are annular and are intended to weigh-down the edge of the record.

These offer the most effective immediate treatment for record warps. Commercial examples are heavier than they probably need to be (see Appendix 1) which is a shame because these devices add substantially to the moment of inertia of the turntable and thereby increase both the load on the centre-bearing and the load on the motor.

For obvious reasons periphery clamps, when fitted, demand great care when cueing because most of the run-in groove is below the lip of the weight. They may even compromise the ability to play modulated grooves very close to the edge of the disc depending on the cartridge clearance. These products too tend to be expensive.

Vacuum hold-down

Relatively ubiquitous in record lathes, and available in some very high-end players, is vacuum clamping or vacuum hold-down or vacuum-chuck in which the record is tightly coupled to the platter by means of a constant low-pressure vacuum maintained beneath the LP.

A vaccum-chuck has the significant advantage of applying equal force over the entire record, whereas a clamp or weight can only apply force at the label area. The drawback is very considerable expense and complexity.

The TechDAS Air Force V turntable which features vacuum hold-down is illustrated. At end of 2019, the cost of this deck (less the tonearm) is Ł12,500 in the UK.

Flutter control (Argument 2)

On this aspect of the performance of record-weights, various claims and counter claims exist from commentators and manufacturers. Whilst some claim the advantage of extra weight in reducing flutter, the principal worry of the opposing camp is the concern that the turntable bearing must support the added weight. There is some concern that the motor must have enough torque not be affected by the added load.

The concern about motor-torque is probably misplaced except where a very feeble motor is employed. The moment of inertia of a circular disc is given by the equation,

= ^M.a² ⁄ ₂ (in kg m² if all dimensions are SI), where M is the mass of the disc and a is the radius.

A 1½ kg turntable has thus a moment of inertia of ½ × 1.5 × (0.16)² = 19.2 × 10^-3 kg m². And a record weight of (let's say) ⅓ kg and a radius of 36mm has a moment of inertia of ½ × 0.333 × (0.036)² = 216 × 10^-6 kg m².

To put this in perspective, simply placing a (120g) record on the turntable increases the moment of inertia by ½ × 0.12 × (0.16)² = 3.0 × 10^-3 kg m².

In other words, the record weight only adds just over a percent to the moment of inertia of the turntable alone. (The record itself adds 16% to the moment of inertia.) The weight adds an insignificant amount and is unlikely to cause problems to the motor.

Correspondingly, adding just over a percent to the moment of inertia of the turntable shows that the flywheel effect of the turntable will be hardly changed and a positive effect on wow or flutter performance is very unlikely.

We took measurements with a standard, direct-drive turntable with and without a centre hold-down weight of ⅓ kg. The measured results are presented in the table below: they demonstrate that the addition of the weight has virtually no effect on wow or flutter; however it is measured.

Configuration	Wow & flutter (DIN)	W & F (Unweighted)	Wow (½ Hz to 6 Hz)	Flutter (6 Hz to 100 Hz)
No weight	RMS 0.06%; Peak 0.10%	RMS 0.07%; Peak 0.15%	RMS 0.07%; Peak 0.12%	RMS 0.05%; Peak 0.09%
With weight	RMS 0.06%; Peak 0.12%	RMS 0.09%; Peak 0.13%	RMS 0.08%; Peak 0.09%	RMS 0.05%; Peak 0.10%

Carrying extra weight

The concern of the extra weight on the thrust bearing is not easily dismissed. The lifetime of bearings is a complicated subject, but simplified calculations relate lifetime (L) in terms of rated load (M) to working load (W) raised to an exponent, such that,

So, if the working load is an appreciable proportion of rated load, a change in working load can have a dramatic effect on working life. If the relation M/W is halved, lifetime is reduced by eight times. And, even if the bearing is very conservatively specified (for example where M/W = 10), then doubling the working load on the bearing will still reduce life by 30%. Worse yet: we simply never know the value of M/W and cannot easily ascertain or measure it.

In conclusion, the likelihood of the reduced life of the centre thrust-bearing precludes advice to add more weight - except where the turntable has been designed from the outset to incorporate such a component. Geometrical considerations reveal that any weight added in the form of a label-sized centre hold-down has such a minute effect on increasing the moment of inertia of the turntable that an improvement in flutter performance is very unlikely. Measurements confirm this.

Warp control (Argument 3)

In some specific cases, it is easy to see how a centre clamp or weight would help to flatten a warped disc. For example, in the case in which a disc has acquired a dished form and the side to be played is the lower face of the dish. The record thus presents a dome shape when on the turntable. Clearly force exerted in the centre region will help to flatten the dome.

But what if the inner face of the "bowl" is to be played? It is not evident here that a centre clamp will help much - if at all. And what about a pinch-warp, the most common form of physical disc distortion? A very simple experiment is illuminating.

We clamped a record with a central nut and bolt and fixed the bolt in a vice. A vertical ruler was installed so that it was possible to measure the deflection of the edge of the disc. We then applied force (via a series of small weights) at the periphery of the record and at the edge of the label. A 2 oz. (57 g) weight at the periphery was required to cause a deflection of 4mm which we took to be the maximum tolerable warp amplitude.

Moving the weights to the edge of the disc label (actually in the dead-wax of the run-out groove area), 8 oz. (227g) was required to give 2mm deflection at the edge. And a 1lb. weight (454g) was required to give a deflection of 4mm.

So, for any great levelling to be accomplished with a centrally placed device, a very great deal of force (weight) is necessary to have a significant effect.

In conclusion, it is not possible to generalise the advantage of centre weights or clamps on flattening warped records: it depends on the distortion on the medium. The restriction of the effect of any clamp or weight to the centre label section of the disc limits what is possible. Annular weights may offer a possible solution in certain situations but these have other significant issues (as discussed in the side-bar).

Slip control (Argument 4)

To test this, we devised a simple experiment. We rested a standard 12" LP record on a commercial, graphite-rubber turntable mat and determined the force required to get it to slip against the mat using a precision balance clipped to the run-in groove area of the record.

We then repeated the test with a 333g record-weight placed on the centre label. The dimensions and construction of the (stainless-steel) weight are illustrated.

The results were a great surprise. In repeated tests, the force required to move the record was 6 to 7 times greater with the weight resting on the record label then when it lay on the mat with no extra weight.

In conclusion, the claim that a weight/clamp reduces record-slip is justified. Experiments confirm that the addition of a weight or clamp increases friction between the record and the turntable mat over and above the the ratio of the weight added, so the weight actually changes the coefficient of static friction⁴ between the surfaces.

That said, we have never experienced a 12" LP record slip under normal (or even abnormal!) conditions. And measurements of wow & flutter do not indicate a slip mechanism between record and mat (see. Argument 2 above).

The exception to this is the RCA designed 45 RPM "single" record. Club DJs report that these do slip sometimes and we can provoke it too - especially when these discs are used with the large 38mm centre hole. Remember that the 45 RPM record was conceived so that a stack of discs could sit on an autochanger deck. By means of a mechanical release in the central spindle, records were designed to fall down and play sequentially; each disc sitting on top of its predecessor.

A record stabiliser is an obvious solution if confronted with a 45 "single" which slips. It should only be used when necessary (see conclusons to Argument 2), and, remember to choose a weight or clamp which fits within the smaller label format of this format. The label diameter of a 7-inch, 45 RPM record is 3⁵⁄₈" (92 mm) against the 4-inch (101mm) label of the LP.

Notes and References

1. Rather than think of our recorded medium as rigidly fixed and try to calculate the effect of internal vibrations within that structure, we've simplified this to think in terms of the medium spinning in space with no contraints, the reaction force on the record is thereby translated into free movement of the medium itself and its movement as the spurious signal.

2. A New Turntable-Arm Design. Villchur, E. Audio Sept & Oct 1962. This article introduced the suspended sub-chassis type design pioneered by Villchur for Acoustic Research, Inc. of Cambridge Mass. and which was widely adopted by the industry in the 1960s and 1970s; for example in the famous Linn Sondek. A concern is sometimes expressed that, should the turntable employs a Villchur suspended chassis² any extra weight (such as that proveded by a record weight) will upset the suspension alignment. However, if the record weight is planned to be used every time, there would be scope for re-aligning the suspension to incorporate the extra mass. Almost certainly, the change to the suspension natural frequency would be beneficial because it will be lowered. So this is probably a bit of a red herring.

4. The coefficient of friction is a dimensionless ratio of the force needed to move one object against another as a ratio of the force pressing them together. If a 1kg object sitting on the floor needs 9.8N of force to get it to move, we say that the coefficient of static friction (µs) is equal to 1 because the same force is required to move it as the force holding it to the floor (in this case, gravity). If it requires 9N to keep the object moving, we say that the coefficient of kinetic friction is (9/9.8), or µk = 0.9.

5. We originally thought it would be useful to have a test disc recorded with a series of high-level signals which stop abruptly, so that the "reverberant" energy could be measured (as in a room reverberation test). But the results of tests of this type were very difficult to interpret. The problem is that Disc Echo is a subtle effect and way below the excitation level. Eventually we developed the two tonearms method.

This involves fitting an extra tonearm on the record deck; complete with cartridge but with no amplification. All this tonearm does is to track the record and provide the reaction of the stylus impedance. The original tonearm is used as the echo pickup. A record is required in which a track of high-level tone is adjacent to a silent track. The test involves recording the output of the main tonarm as it scans the silent track whist - at the same time - the second tonearm scans the adjacent modulation track. The results in footnote 6 are all derived from this method.

The results of our experiments with record-weights on a standard direct-drive turntable are below. The figures were obtained in 4 conditions:

Measurements were taken of the level of the the 300Hz remnant signal induced into the live pickup due to the 300Hz excitation of the stylus of the passive tonearm relative to the level when the active tonarm scanned the modulation track. 96kHz, 24-bit needle-drops were recorded and RIAA EQ'd and rumble-filtered in Stereo Lab. The results were analysed using the spectrum analyser in Adobe Audition.

Measurements were taken of the left/right signal level, the lateral (L + R)/2 signal level, and the vertical (L - R)/2 signal level. It was felt that the vertical and lateral signals might give insight into whether the internal vibrations were longitudinal waves in which the particle motion in the medium is parallel to the direction of the wavefront: or shear waves in which the particle motion is perpendicular to wave direction (see diagram and data in footnote 7.)

In our experiments, the excitation was purely lateral (mono) and the waves thereby produced were, in all likelihood, originally longitudinal. However, when sound travels in a solid material, longitudinal waves can be transformed into shear waves (and vice versa) in a process called mode conversion which happens when a wave encounters an interface between materials of different acoustic impedances and the incident angle is not normal to the interface - a situation which is bound to happen in a disc of plastic.

Results are given in the table. Note that the magnitude of the measured echo signals relative to the recorded modulation are very close to the level predicted in our theoretical model.

Condition	Echo level L/R relative to excitation	Echo level Lateral relative to excitation	Echo level Vertical relative to excitation
Record on turntable	-62dB	-68dB	-64dB
⅓ kg weight	-60dB	-62dB	-62dB
Dissipative weight	-62dB	-66dB	-66dB
Record not supported	-60dB	-64dB	-68dB

As the measurements clearly indicate, fitting a ⅓ kg record weight, doesn't reduce the amplitude of either lateral vibrations (longitudinal waves) or vertical vibrations (shear waves). In fact, the "echo" is considerably worsened compared with the standard arrangement of the record lying on the turntable under its own gravity load. The level of the longitudinal waves is doubled with the weight fitted.

Perhaps this increase in longitudinal mode echo is what people hear when professing to prefer the sound of a clamped or weighted record. The echo levels are consistent with perceivable effects under quiet conditions and attentive listening. Unfortunately, audible though the changes may be, they are actually in the "wrong" direction and represent a deterioration rather than an improvement in the quality of the needle-drop.

Our own, experimental, dissipative Sorbothane weight proved hardly effective at all. Perhaps there is a marginal tendency to suppress vertical vibration in favour of an increase in lateral vibration. A doubtful advantage and, in any case, a very small effect.

The fourth condition, in which the majority of the record was spinning unsupported produced interesting results. The idea behind the configuration was to attempt to simulate the initial condition in the theoretical model. The expectation was for very much more vibration than that obtained in any of the other configurations because contact between the record and the turntable mass was expected to reduce Disc Echo.

Counterintuitively, shear wave energy is reduced in a configuration in which most of the disc is free to move vertically — a real surprise. Perhaps less of the originating longitudinal wave energy is being converted to shear waves. At any rate, the data suggest that closer coupling between the turntable and disc is not the panacea advanced in the rationale of Argument 1.

One result stands out in the data above. Whilst the Sobothane weight and the Heath Robinson arrangement with the unsupported record, resulted in a shift of vibrational energy from vertical to longitudinal modes (or vice versa), fitting the weight resulted in more returned energy in both modes. The results using the added weight are thereby - and by a considerable margin - the worst in all the experimental arrangements.

7. Table 1. Shear waves have a slower velocity and shorter wavelength than longitudinal waves of the same frequency in most materials — including PVC. Here are the longitudinal (L-Wave) and shear wave (S-Wave) velocities of sound in a few materials relevant to discs, turntable, mat and weight designs.)

Material	L-Wave velocity m/s	S-Wave Velocity m/s
Aluminum	6375	3130
Brass	4394	2120
Glass (plate)	5766	3430
Nylon	2692	1090
Plexiglas	2692	1270
Rubber, vulcanized	2311	—
PVC	2388	1060
Stainless Steel	5664	3120
Teflon	1372	6350

The speed of sound in PVC given in the table, and the relation λ = ^v ⁄ _f , ought to remind us that there will be very little wave-like motion in the disc except at very high frequencies. For example, the longitudinal wavelength of the 300Hz test signal in PVC will be nearly eight metres.