Although Vinyl LP still has some enthusiastic users in the Hi Fi fraternity, so far as many professional engineers are concerned it has been regarded as a bit of a dead duck for a decade or two. As a result, very little original research seems to have been published on the basic properties of LP systems in recent years. However a trawl through the archives yields some interesting results which seem to have been largely forgotten. Perhaps the most interesting of these are a couple of reports written by engineers at CBS [1] and RCA [2] that report measurements carried out on the characteristics of lacquers produced on LP cutting lathes, and the performance limits of vinyl LPs pressed from stampers made from these lacquers.
In terms of background noise, their findings are illustrated in Figure 4. This shows the power spectrum of the background noise of carefully made LPs pressed using excellent vinyl material. For comparison, the Figure also shows the nominal noise spectrum for CD-A noise/dither with a flat spectrum. Note that in each case the plot shows the noise power per Hz. To obtain the total background noise power we have to add up the noise contributions across the audible range.
The results shown here represent the optimum or ‘best’ results indicated by the CBS/RCA test pressings when played via the standard RIAA system. The performance of the cut lacquers is better than shown, but degradation occurs during the process of making stampers and then pressing actual LPs. Also, the results for some (many?) commercial LPs may well be worse than shown here. Alas, anyone who recalls the era when LP was the standard commercial medium for music will probably recall that all too many LPs had faults which the above results overlook!
The spectra shown in Figure 4 indicate that the overall noise level of CD-A can be expected lower than LP. However before we can draw any conclusions about DR we have to take some other points into account:
- The spectra have different shapes. Since human hearing is more sensitive to background noise at some frequencies than others, we should take this into account in deciding how different the noise levels may be in audible terms.
- With CD-A the 0dB reference level is normally regarded as the maximum possible signal level that is allowed. However the 0dB reference level for LP is not a strict limit. Higher level signals may be (and often are) recorded onto the LP. Hence we have to allow for this when trying to compare DR ’s for the two media.
- The above assumes that the noise/dither on CD-A has a flat spectrum. This is not always the case. So this may also need to be taken into account.
In audio measurements the noise level is usually quoted either as as a ‘flat’ result which gives all the contributions over the 20Hz - 20 kHz range equal weight, or the “A Weighted’ value which tends to emphasise the effects of noise at frequencies in the range between a few hundred Hz and a few kHz, and reduces the effect of noise at higher or lower frequencies. The flat value indicated the total noise in the audible range. The A Weighted value indicates the relative audibility. If we work out these values for the CD-A and LP examples shown in Figure 4 we get the results shown in the table, below.
| Total Noise Level
|
‘flat’
20 Hz - 20 kHz
|
A Weighted
|
| CD-A
|
-92 dB
|
-94 dB
|
| LP
|
-46 dB
|
-61 dB
|
CD-A can do better than shown above by employing a technique called Noise Shaping. However we will ignore that in this article and assume the CD-A noise or dither has a uniform or ‘flat’ spectrum of the kind shown above.
Maximum levels and distortion
With vinyl LP systems we find that as the signal is increased we tend to get rising levels of distortion. At the standard 0 dB reference level used for LPs it is quite common for the LP and cartridge to produce distortion levels between 0·5% and over 1% (i.e. equivalent to levels from -46dB to over -40 dB). As background reading for this article I checked back over a number of old reviews of cartridges and those that quoted values tended to show two general distortion properties for the LP/cartridge system.
- The distortion levels tended to be around 4 times higher for vertical (‘difference’ or L-R) modulation than for lateral (‘mono’ or R+L) signals.
- That the distortion levels tend to rise rapidly once the signal level is increased beyond the 0dB level.
A nominal advantage of the LP system over CD-A is that it makes use of the ‘headroom’ above the LP 0dB level to increase the DR it can offer. CD-A does not normally do this as the 0dB level for CD-A is conventionally regarded as a strict upper limit that should not be exceeded.
By referring to reports of work in various places like Polygram [3] and Shure Bros [4], the CBS/RCA work mentioned earlier, and the results in many cartridge reviews, we can say that that nearly all good-quality LP/cartridge combinations seem to show distortion levels that rise in similar ways. In part this is to be expected as many of the distortion mechanisms involved are essentially ‘geometric’. (One example being Linear Tracking Error which Keith Howard has recently examined. [5])
Figure 5 illustrates how the noise + distortion levels tend to vary with the signal level for a ‘good’ LP system (represented by the blue line) and a CD-A system which is dithered with white noise (red line). The 0dB level represents the maximum permitted level for CD-A, and the 5 cm/sec reference level for LP. The red line assumes that CD-A levels above 0dB are ‘forbidden’ so this is represented by the vertical abrupt cut-off. The solid blue line assumes we are considering frequencies around 1kHz or less, using a good stylus/cartridge with an optimum alignment. At higher frequencies, particularly near the central part of the disc, the distortion levels may be somewhat higher. Also, above about 10dB the stylus may begin to mistrack and cease to follow the groove. These effects are indicated by the solid light blue region with the broken line boundary.
In Information Theory terms we can say that our ability to resolve small details will eventually be blocked by the presence of noise. At higher levels the presence of distortion may also affect our ability to recognise details of the signal pattern. To represent this, the plot in Fig 5 shows how the level of noise plus distortion varies with signal level. In conventional engineering terms the DR value indicates the ratio of the largest ‘undistorted’ signal level to the background noise. However in any system some small amount of distortion might be present at any level. Hence in functional terms we may have to define the meaning of ‘undistorted’ to mean an amount of distortion that may easily be measurable, but which is felt to be acceptable in use.
For the sake of example, I’ll assume we can accept distortion levels of around 3%. This implies we can allow for +10dB peaks for LP. Combining this with the noise level of around -60dB therefore implies that LP would potentially have a useable dynamic range of around 70dB. Choosing a higher level for the permitted distortion might increase this towards about 80dB. However this means allowing distortion levels of perhaps 10% or more, and may involve risking stylus mistracking on replay, so it is questionable if this would be acceptable under most people’s definition of ‘high fidelity’. In contrast, the CD-A system shows a dynamic range of just over 90dB – i.e. over 20dB greater than LP.
Given the above we would have to conclude – on the basis of the standard engineering definitions of DR – that LP cannot really be argued to be equal or surpass CD-A in terms of the ability to resolve tiny details. Hence many of the apparently ‘technical’ claims about superior resolution of LP which you may read about this in various places have to be treated with scepticism. But is that the end of the story?...
At this point I’d like to refer back to a couple of articles I wrote in HFN some months ago [6] [7] that looked at the possible implications of recent discoveries about the non-linear properties of the physiology of human hearing. These may tie up with suspicions that various people have had for many years about some of the characteristics of analog systems like LP.
In terms of genuinely tiny details of the original signal, CD-A may offer better waveform resolution and DR than LP. But is this really what fans of LP are talking about when they discuss LP performance? Perhaps the answer is ‘no’ and they are using the same words as employed in conventional audio engineering, but to describe something that is subtly different. To illustrate this, consider a situation where both an LP/analog and CD-A/digital recording are made of a musical event. In each case the recorded waveforms are nominally identical within the engineering limits shown by Fig 5. In each case the waveform may contain details which - although tiny - are large enough to be above the noise+distortion levels of both systems. Now consider the possibility that in themselves, some of these details might go un-noticed by those listening to the music, but the systematic analog distortions+noise alter the details in a way that sometimes makes them more audible. In such a case the result might be that some small details of the music become noticeable with the ‘distorted’ LP/analogue version, but remain un-noticed with the CD-A/digital version.
When people refer to being able to hear tiny details on LP, or it having ‘better dynamics’ maybe they aren’t really meaning that it has the ability to record smaller details than CD-A or literally provide a higher DR. Perhaps what is really happening is that the background noise or distortion that occur with LP act upon the complex physiology of hearing and allow some people to notice details in the music which otherwise go un-noticed. These details might well be large enough to be recorded both on LP and CD-A. But perhaps on CD-A they may sometimes simply pass unnoticed. Dither or noise can be quite useful in improving the performance of CD-A, and the results may depend upon the details of the patterns used – as had been indicated in some articles by Keith Howard (see for example [8] ). Might it be that, under the right circumstances, noise or distortion enhances perception in some ways? At this point this can only be a speculation. Some relevant experiments would be needed to test the idea. But this alternative view might square the circle of the claims some people make about LP with its measured performance expressed in conventional engineering terms. Maybe both sides are ‘right’ in their own way after all, and have just been arguing using different meanings for the words they use, but without noticing. Perhaps LP is more ‘dynamic’ precisely because it has a smaller DR than CD-A!
Jim Lesurf
1st Dec 2004
2900 words
References for this page:
[1] The Dynamic Range of Disc and Tape Records. D. W. Gravereaux, A. J. Gust, and B. B. Bauer, JAES V18(5) pp 530-5 October 1970
[2] Performance Characteristics of the Commercial Stereo Disc. J. Eargle, JAES V17(4) pp 416-22 August 1969
[3] An Investigation of the Increase in Non-Linear Distortion Products from Virgin Tape to Disc Playback. O. Stephani and B. Bluthgen, AES 62nd Conv. March 1979
[4] Psychoacoustics, the Determining Factor in Stereo Disc Distortion. J. E. Jacobs and P. Whitman, JAES V12(2) pp 115-23 April 1964.
[5] Straight Line to Nowhere? Keith Howard, HFN pp 82-85 December 2004
[6] Now Hear This. J. Lesurf, HFN pp 72-75 April 2004
[7] Ringing in the Ears. J. Lesurf, HFN pp 68-71 September 2004
[8] The Dither Habit. Keith Howard, pp 76-79 HFN February 2005
