07 May 2013

What do we lose when we go digital? Featured

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What do you lose when you go digital What do you lose when you go digital RedShark/Shutterstock

Index

My article last week about whether you could build a camera in a shed sparked off discussions in all directions, but a persistent one was whether you lose something when you go for analogue to digital, and in particular, whether audio vinyl disks will always sound better than digital reproduction.This is a fascinating subject that, seemingly, won't go away. But it should have by now, surely? It also has a lot in common with film vs digital video recording

We can now sample audio and video at such high resolutions that there should be no question that it will look and sound better. Is there anyone who would prefer scratchy, dusty spinning record to a pristine 192 Khz sampled 24 bit recording?

Well, yes, there is. And it's just possible that I might be one of them.

Perhaps I'd better qualify that.

I’ve spent the last 28 years working with digital audio, but in fact, "Going Digital" has been going on for a lot longer than that. Incredibly, some of the digital processing techniques that are in use in today's studios and edit suites were invented in the 1920s and 30s. They were only theoretical then, and it was only in the 60s and 70s in forward-looking research establishments like IRCAM (Institut de Recherche et Coordination Acoustique/Musique) that computer music composition programs started to produce real results - even if it took a week of number-crunching on those early computers to generate a few seconds of synthesised sound.

So I know what digital audio sounds like; good and bad. The sad fact is that most of it - compared to how good it could be - is actually pretty bad. Psychoacoustic compression like MP3 hasn't helped to maintain quality (unless you're talking about restricted bandwidth, in which case it has vastly improved the quality of audio that you can push, say, down a copper telephone line).

Unwanted consequences

The basics of digital audio sampling are pretty simple to understand. It's the unwanted consequences that can be harder to explain and to eliminate. I'm not going to go through this stuff in detail here because it would make for an extremely long article, but what I will say is that some of the most basic assumptions about sampling at the very least fail to tell the whole story. Suffice to say that the generally held notion that if you sample at twice the highest wanted frequency then you'll get all the frequencies you need reproduced is only true if you look at it edgeways. There's all manner of stuff that happens as a consequence of sampling and it's not impossible - indeed it's quite likely - that two digital audio devices playing the same material will sound discernibly different.

But sampling is only one part of the digital audio equation. The other part is the number of bits used to describe the audio.

CDs use 16 bits and, used properly, can sound pretty good in terms of the dynamic range they can reproduce. It's quite rare - and potentially harmful to your loudspeakers - to need more than 16 bits to describe any recording, but if you focus on the quiet sections, they don't sound as good as the loud ones. This shouldn't matter, because they'll be so quiet that you won't notice, but in practise, most recordings don't use anything like all 16 bits - if they did it would have been a miracle when they were being recorded that they didn't over-modulate and cause some nasty digital distortion whenever there were unexpected peaks.

Think of it like this. If you use all 16 bits, you have over 65,000 levels to describe the amplitude of a sound. But quieter sounds may use only 8 bits, which means that there are only 256 levels. You can hear the difference. The worst case is if you have, say, a sustained piano chord which might take almost 30 seconds to die away to nothing. As you get towards the tail of the sound, what you'll hear is mostly distortion, as the digital system struggles to reproduce the waveform with as few as two or three bits of resolution. A silky-smooth sine wave can easily be reduced to nothing more than an unpleasant sounding staircase.

This type of artifact, called Quantization Noise, is particularly unpleasant, but it is intrinsic to any system that is based on sampling. The best you can do is minimise it by using more bits, and 24 bits seem to do a very good job at the same time as being a practical proposition for modern equipment and recording mediums, except that CDs remain stubbornly 16 bit and 44.1KHz sample rate. (Higher quality versions such as Super Audio CD and DVD Audio never really found a mass market, and seem incredibly niche products in a world where the average consumer thinks that 128 Kbit/s MP3s sound good).

There’s another issue as well when going from analogue to digital: the quality of the conversion.

 

DAT



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David Shapton

David is the Editor In Chief of RedShark Publications. He's been a professional columnist and author since 1998, when he started writing for the European Music Technology magazine Sound on Sound. David has worked with professional digital audio and video for the last 25 years.

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