Off The Record
It something of a surprise to me to find that – in the year 2007 – I am still wishing to play Vinyl LPs! However although the LP has vanished as a mass market carrier for music, it continues as a specialist format that is preferred by enthusiasts. I am dubious about much of what has been said in favour of LP, but one thing has become clear to me as a result of recent LP/CD comparisons I’ve done. This is that many CD re-issues of older ‘popular’ music suffer from quite serious amounts of level-compression, and clipping distortions. Thus despite the inherent potential of CD we have the ironic situation where some old LPs actually deliver a wider dynamic range and lower distortion than modern CD versions of the same original recording! With the above in mind I have recently turned my mind to checking that my old LP playing system works OK.
If you are like me you may have an turntable/arm/cartridge which is getting on in years. If not, you may have a new system. Either way, you may wish to be able to check out its performance, and perhaps adjust it, to ensure it is working properly. In my case I have a 20+ year old cartridge (Shure V15/III) with a stylus (VN35MR) that is almost as old! The problem here is that an LP stylus does wear out, and when it does it can not only sound poor, but can begin to damage the LP when played. Also, with old turntables and arms, the mechanical bearings may deteriorate and cause all the old symptoms the LP enthusiast will un-fondly remember – rumble, wow and flutter, etc.
With the above in mind, I dug out my old Test LP (HFS75) and played this, recording the results onto CDR. I was then able to load the results onto my Iyonix and analyse the results. I will say more about that in the next article, and also provide a new application to perform the analysis. In this article I want to explain the causes of the problems, how to recognise the symptoms, and then diagnose or treat them.
The LP works by placing a stylus – usually a shaped diamond tip – in the groove that has been pressed into the LP surface. As the record rotates, the undulations in the groove displace the stylus sideways. The shapes of the modulations of the groove make the stylus vibrate and this is then sensed by a small electric generator system at the other end of a small lever which is attached to the stylus tip. The result then emerges as a pair of voltages whose patterns of vibration mimic the undulations of the groove, and which represent the soundwaves.
The problems which can arise can now be summarised as follows:
Firstly, the replay arm (and the tiny lever upon which the stylus tip is placed) move in circular arcs. Whereas the LP groove was cut using linear transverse motions. This means there are some distortions which arise during replay as a result of geometric differences between the recording and replay methods. The electromagnetic generator which converts the stylus movements into electronic signals also tends to be nonlinear, and produces some distortion. In addition the stylus shape differs from that of the cutter used to create the original groove, and this shape also leads to some distortion. The details of this last factor will vary with how worn the stylus may be by use, or by any misalignment of the height or angle of the cartridge.
Secondly, the turntable system may not rotate ideally. The speed may vary, producing the effects called ‘wow and flutter’ as the replayed signal frequencies are frequency-modulated. The bearings may also grate or move irregularly, shaking the turntable and producing low frequency ‘rumble’.
Thirdly, the stylus may not be able to stay in contact with the groove movements. This can lead to what is called ‘mistracking’. In effect, the stylus loses contact with the sidewalls of the groove and then moves in a path that isn’t following the groove shape. The result is an irregular form of distortion. This can be made worse if the playing weight (vertical downward force) is too low, or if there is a large enough horizontal force. In some cases problems may be due to the forces between stylus and LP being too large, causing the stylus to dig in and damage the groove.
If you have a good quality LP replay system you will probably know the recommended playing weight for your stylus. Typically this is in the range from 1 - 2·5 grams. Using a lower weight will risk mistracking when the vertical modulation is loud. But a high weight may cause groove damage. You will probably also have an adjustment generally called ‘anti skating’ or ’bias’ on the arm. This applies a horizontal radial force that corrects some of the geometric effects and helps ensure the stylus can stay in the groove when the monophonic part of the signal is loud. Mistracking is serious as the stylus can easily then act as a tiny chisel and damage the groove as it is launched or lands back in the groove. hence the results don’t just sound bad when it occurs, the result can be permanently written onto the LP!
As illustrated above, the movements of a stereo LP groove/stylus are such that the sidewalls of the groove are at ±45 degrees to the horizontal. One wall is modulated to convey the Left channel pattern, and the other to convey the Right channel. Mono is then conveyed by moving both sidewalls to produce horizontal movements, and vertical movements convey any differences between Left and Right to produce stereo. Ideally the stylus applies much the same average pressure on each sidewall. Unless the playing weight and bias are set correctly, the force on one wall will be greater than the other. This means the responses to left and right hand signals will differ.
To complicate matters, as the LP system is used, there will be a tendency for the stylus to wear, and the suspension systems of the stylus lever and the arm pivot to change. This means that the optimum playing weight and bias force may change over the years. Hence it can be a good idea to check these in some way on a regular basis. This will allow you to get the optimum sound, minimise LP wear, and help you to decide when a new stylus, etc, may be advisable. (Assuming, of course, that replacements are still available!)
Although the test LP I used is an old one, newer ones are available. For example, ‘Hi Fi News’ magazine sell one. These LPs generally have a series of ‘test bands’ on them, and there will be an explanation of each with the LP. Until recently, most home users would have had to rely on their ears to judge the results because test instrumentation would have been too expensive for use outside professional/lab users. However if you have a RO computer and a way of sampling the LP replay, then you can now use the computer as your test and analysis instruments, as I will explain in detail in the next article.
As you might expect, there is a tendency for the levels of distortion to rise with the signal level. Less obviously, the distortion level, and the risk of mistracking, also rise as the signal frequency increases. This is for two basic reasons.
Firstly, the signal level for LP is defined in terms of modulation velocity, not displacement. The standard reference level for LP is 5 cm/sec. If you look at the specifications for many cartridges they say what amount of output voltage will be produced for this ‘0dB’ reference level when the signal is a 1 kHz sinewave. However acceleration is the rate of change of velocity. It follows from this that at high frequency we are having to change the velocity more quickly than at a lower frequency. This means that a 5cm/sec sinewave at, say, 2 kHz requires the stylus to experience double the level of acceleration than at 1 kHz. Since the stylus will have inertia, this means that the force the groove wall has to exert to move the stylus also has to double. Hence the mechanical stresses and forces tend to rise with frequency.
The second reason is that LP signals are recorded with a deliberately chosen modification to the frequency response. This reduces the level of bass signals, and boosts that of treble during recording. The replay system has to correct for this modification of the frequency response. During the 1950’s when the LP system was new, various competing ‘correction curves’ were proposed and used by various makers. Fortunately, this led to an agreement by the companies to adopt what has come to be known as the RIAA curve (which also became a British Standard). The shape of this curve is shown below.
The above graph shows the RIAA correction curve in terms of how the gain of the disc replay preamp should vary with signal frequency. Imagine using an LP replay system where you’d adjusted the volume so that a 5cm/sec 1kHz sinewave gave a given output signal level after RIAA correction.
The graph above shows what stylus velocity would be required to give the same output sound level at various sinewave frequencies. Looking at this we can see that much higher velocities are required for high frequency sounds. By the time we reach 10 kHz the stylus velocity would need to be over 24 cm/sec to give a sound at the same level as at 1 kHz. The implications of this are shown below.
The graph above shows how the amplitude of the LP sidewall displacements required vary with frequency for the same output level.
The above graph shows the required accelerations the stylus has to endure. Note that the vertical scale of the acceleration graph is logarithmic as the range covered is so vast.
The above graphs are all plotted for the 0dB reference output level. Looking at the acceleration graph we can see that this means a stylus acceleration of around 32 times the Earth’s gravitational acceleration at 1 kHz, but over 6000 gravities at 20kHz! This would demand enormous sidewall-stylus pressures to force the stylus to accelerate at these levels.
The result is an increase in distortion at high frequencies, and eventually the stylus can no longer follow the groove, or the vinyl material permanently deforms under the pressure. If you look at the specifications of a cartridge or stylus you can sometime see a value for ‘tip mass’ quoted. This represents the inertia the stylus assembly presents to the sidewall accelerations, but quoted in terms of a mass. The most obvious implication of this can be shown with a simple example.
Consider a stylus whose tip mass is 1 milligram (mg), and which is being played with a ‘playing weight’ (downforce) of 1 gram. This means that the downforce being applied is such that it would accelerate a 1 gram mass by 1 gravity if the the upward pressure of the LP was not preventing this. Thus the same downforce would accelerate a 1 mg mass at 1000 gravities. It follows from this that in such a case we can’t expect even a perfect stylus to stay in contact with the groove if there is a vertical modulation with accelerations in excess of 1000 gravities. Loud high frequency vertical signals would literally throw the stylus out of the groove!
In practice, excellent cartridges/stylii may have tip masses smaller than a milligram, and be designed to play with tracking weights in the range of a 1 - 2 grams. This means that they may just be able to track stereo signals on the LP with accelerations around a few thousand gravities. But unless the tip mass is very low - or the playing force is high - in practice it is essentially impossible to play stereo signals as loud as the reference level at 20kHz without serious distortion, mistracking or damage. This is regardless of how easily the same system can replay sounds at lower frequencies at this level. Unfortunately, manufacturers and magazine reviews often fail to specify the effective tip mass, or show the results of trying to play such signals. So it can be hard to decide if a given cartridge will cope.
At low frequencies there is also a risk that the larger sideways movements required may lead to higher distortions or mistracking. Excellent stylii should be able to track the order of 80 microns displacement at a few hundred Hz, but this may be affected by the careful optimisation of the playing forces employed. If the above were not all worrying enough, we also need to bear in mind that the chosen level of 5cm/sec at 1 kHz is just a reference. It does not represent the maximum level present on an LP! Some ‘hot cut’ LPs may have much higher levels than this, and can represent what Shure used to call an ‘obstacle course’ for the stylus.
With the above in mind, I’ll continue in the next article to explain how to diagnose the symptoms of a poorly adjusted system, and how to use the results of analysis to improve performance. So stay in contact.
11th Feb 2007