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October 2010 issue - Page 4

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October 2010 issue
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Page 4
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Page 6 - Mac Mini
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ANALOGUE AND DIGITAL

Mr. Noel Keywood has never been less than stimulating and insightful in his monthly OpEd pieces for Hi-Fi World, and nine times out of ten I find myself nodding in agreement at the paragraphs of logic and common sense he writes. However, I feel strongly that his piece in the June edition of the magazine was written either with his tongue in his cheek or was consciously disingenuous. As usual, Mr. Keywood treats us to an apparently impeccable and irrefutable argument: “since all signals are analogue until converted to digital, it’s impossible for digital to be better than analogue...” In making this statement Mr. Keywood conveniently overlooks the fact that in an analogue recording process, signals – which start as sound waves passing through the air – are converted into a grove embedded into a piece of vinyl, which is a pretty radical transformation by any measure.


The process of achieving this transformation involves many separate stages. In the analogue world, each of these stages - recording on to master tape, mixing a master copy, cutting a press tool, making a physical pressing – are all subject to imperfections which, in this domain, are additive and so all will be present in the final analogue representation of the original sound. These imperfections may take the form of noise, distortion, resonance and so on, so that the final representation is the original sound plus the extraneous effects of the analogue process steps. While efforts are made to minimise these effects, they cannot be entirely eliminated.


Some manufactures went to greater lengths than others to reduce these imperfections. Decca for example used a proprietary vinyl compound and a pre-production sintering process so that the vinyl material was pre-stressed before going into the press tool. Decca believed this contributed to the final “Decca sound” and it also meant that Decca’s discs were not prone to warping and the other pressing defects of certain of its rivals (up the road in Hayes especially).


On the whole, industry adopted digital to overcome the additive defect problem present in analogue, and although I would admit that the initial digital standards may have been too low to offer the “perfect sound” claimed, it was undoubtedly the right decision, as anybody who had seen the horrors of record pressing in Hayes in the 1970s would agree.

It is also worth pointing out that the analogue process is not entirely analogue as, for example, the cutting lathes used to make disc masters have been CNC (i.e. digitally controlled) for decades, while the much praised analogue FM stereo radio of the BBC relies on digital transmission over the miles of land lines between studio and transmitter.

 

sacd--blu-ray-discs

The SACD disc at left sounds different to the PCM disc at right. But why?

 

The other objection I have to the purist view of analogue is that an orchestral recording made with a dozen or more microphones, which is then skillfully mixed down to two channels, is no analogue of reality. It is instead an artificial construction of reality, just as digital encoding and decoding is an artificial construction of reality, and it seems to me that one form of artificial construction of reality has no inherent superiority over any other. I personally think that the best we should expect from recorded sound it that it properly represents the timbre of voices and instruments and, in my opinion, the latest digital recordings do that (you guys may be able to spot the difference between digital and analogue, but can any of you spot the difference between a Steinway and a Bosendorfer?).


Also, while it is generally claimed by the analogue purists that digital is not as good as analogue at “imaging”, imaging is part of the artificial construction of reality created by the sound engineers at their mixing desks, and regular attendance of live performances will confirm that much praised imaging of hi-fi systems has no analogue in reality. I think you purists need to get out more.


Meanwhile it seems to me that the BBC must be on to something if they can send digital signals to analogue transmitters without anybody noticing: they must have cracked the “digital problem” sufficiently to make the most ardent analogue supporter happy with digital.


Best regards, and congratulations on producing the best and most interesting hi-fi magazine, even if I do not always agree with everything you write.

Keith Hodgkinson

 

michell_tecnoarm

The holes in a Michell TecnoArm act as wavebreaks. It is also damped internally.

 

Thanks Keith. I don't really disagree with anything you say, but there are problems with the detail. Analogue's additive distortions were a lot easier on the ear than digital's. I recall reading an account of early digital development by a Philips engineer, I think it was, where he admitted that they knew quantisation worked at sub-multiples of the sampling frequency, but they had no idea what happened in-between! This meant that most of the music got mashed, and no one quite appreciates this. But you can hear it.


Higher resolution PCM does seem to do the trick, but it remains the case that SACD sounds different to 24/192 PCM and no one knows why, as far as I am aware (I don't accept that out-of-band noise is somehow responsible).


Imaging a contrivance engineered at the mixing desk? Er, I don't think so. Sound location is an acute ability in humans, as William Yost's researches at the Parmly Hearing Institute so clearly show. Get his book 'Fundamentals of Hearing' from Amazon to read more, or direct from Loyola University, Chicago.


I do sometimes feel that digital better conveys analogue nasties as well, and gets lumbered with them. Many microphones have rising treble and quite a rough top end, and they feed into poor IC preamps, often incorporated into mixing desks. This gives audio a poor start in life and helps explain, I suspect (as does violinist and audiophile Rafael Todes) why strings recorded digitally commonly sound coarse and timbrally anaemic. As impressed as I am by, for example, by the very dedicated 24/192 PCM and SACD recordings being issued by 2L I am still not quite convinced perfection has been reached. But blaming digital is, as you suggest, likely incorrect. NK

 

DENON SOUP

I thought my last e-mail was the last word on the subject of ‘souped up Denons’. However I found that the more I listened, the more there was still a bit of a problem of ‘spit’ and surface noise and a slight edginess and glare to the sound that I could not get rid of. Fitting the metal body to the standard DL 103 cartridge did lift the top treble a bit, but it stayed in balance. However with the Paratrace stylus it could sometimes be edgy and fatiguing. I played around with tracking weight, alignment and electrical loading without success, so I went back to basic engineering principles.


I had noticed that the cartridge and arm seemed to have become more microphonic than previously: when tapped with a screwdriver it clinked and clanked. Maybe the metal body was transmitting high frequencies into the headshell and arm too well and there was not enough damping or energy dissipation?


If this was the problem, then in theory sandwiching a very thin layer of suitable material between the cartridge and the headshell like a gasket could provide a break in the energy path and act as constrained layer damping to dissipate excess high frequency energy. However the material would need to be very thin and reasonably stiff, with good self-damping so that it would not decouple the cartridge body from the headshell and affect mid- and low frequencies.


After a little thought I settled on standard 80g photocopy paper as a good starting point. I carefully cut a piece to fit, tightened up the mounting screws and sat down to listen. My hunch about the cause of the problem must have been correct, because it worked exactly as theory predicted: there was no loss of midrange or bass definition but the ‘glare’ and ‘spit’ had gone and the treble now had excellent definition and clarity.


It seems that the ‘paper gasket’ is a simple and effective solution - and it could help other metal-bodied cartridges where treble ‘spit’ and glare is a problem. Before you scoff too much at the idea, it is based on sound engineering principles, it works, it costs nothing and it isn’t half as drastic or wacky as the Cartridge Man’s Isolator. It is worth considering (and trying?) as a cheap and effective alternative to the latter.

yours sincerely,

Alasdair Beal

 

Scoff? Not at all Alasdair. Our accelerometer measurements of pickup arm vibration clearly show there is a lot going on around 6kHz and moving the accelerometer to different locations on an arm suggests the headshell is responsible. However, Karl Heinz Fink, who uses laser interferometry to assess arm behaviour, tells me these modes exist in the arm tube. Whatever, something is going on. One solution is to wrap a plastic damping sleeve around the arm tube; another is to drill the tube with holes that act as wave breaks; another would be to add ribs, which also dissipate wave motion. So you are likely spot on with your solution! NK



 

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