Meridian Unplugged - Powered by MrTechGuy.co.uk Forums Discussion HiFi & HT Chat Can sound quality be measured?

In the interest of avoiding leading people astray and creating confusion, I think it's important to set some context around the following statement:
The matter of what one hears as a result of changing cables is certainly a matter of debate and always will be, as we all respond differently to different things in music.

But the idea that an electromagnetic signal can travel down a conductor and be absolutely unchanged is profoundly wrong. Even in the most basic terms of signal transmission, be it analogue or digital, several factors are at play that have a significant impact on the signal. If we keep it simple and analogue, we're looking at inevitable attenuation and group delay (different parts of the frequency spectrum moving at different speed down the cable), among other things. The impact of analogue cables on sound is well-established and many of us have experienced this.

Digital signals are more important and relevant in this debate. I'm sure some people are aware of this, but I'd guess that many aren't: Digital signals, when transmitted, travel in the analogue domain as electromagnetic waves, meaning that the signal contains both electrical and magnetic energy. Therefore, the energy in these signals is therefore subject to all the external and internal electrical, magnetic and mechanical influences that affect analogue cables and equipment. Of course, it’s a matter of extent, and a signal that is modulated digitally is fundamentally more tolerant of noise than an analogue signal, before degradation becomes noticeable

If we take a an Ethernet signal and simplify it, you can think of a square wave running at a steady base or 'fundamental' frequency (not quite true in practice, but all the principles hold true). This signal, when transmitted, is carried as a series of analogue sine waves, known as a Fourier series. In Ethernet, the fundamental frequency is 31.25MHz. The Fourier series (effectively the spectral makeup of the overall signal) will consist of a series of analogue sinusoidal signals, with the firs sine wave running at the fundamental frequency of 31.5MHz and at the same amplitude as the base square wave. Then we have a series of further sine waves at odd harmonics of 31.25MHZ (i.e. 3x31.25MHz, 5x31.25MHz, 7x, 9x and so on). The amplitudes of the 3rd harmonic (93.75MHz) will be a third of the fundamental amplitude, the 5th harmonic will be at a fifth of the amplitude and so on.

Here's a good link that gives a really nice 'plain English' explanation of what all this looks like. The video also shows how all the component sine waves add up (in a theoretically ideal world) to re-form a something approximating a square wave at the receiving end of the cable.

So, we're now in an physical reality in which our digital signal is actually made up of a long series of sine waves, at increasing frequencies and decreasing amplitudes. Here's a useful reference point that shows clearly how these basic principles of digital transmission are being applied by manufacturers of bog-standard CAT6 Ethernet cables: If you want to transmit faithfully 95% of the total Ethernet signal energy, you need a cable which can do a pretty decent job of carrying the base frequency, right out to the 19th harmonic. If model a Fourier series, you'll see that the frequency of the 19th harmonic is just under 600MHz. You may have noticed that CAT6 standards require a signal bandwidth of 600MHz - now you know why. If you look at the text printed on the sheath of a CAT6 cable, you'll probably see a reference to 600MHz.

What's the point of all this? Well, if we were to take a very fast oscilloscope and attach the terminals to the negative and positive-going wires from one of the four twisted pairs in an Ethernet cable (let's say it's 10 or 15 meters long, as is often the case in a streaming application), we'd see a surprisingly messy signal. The reason for this is that all the component sine waves have been exposed to the ravages of cable transmission characteristics. At these frequencies, attenuation is a major problem (due to skin effect) and group delay is a big factor. All this leads to a very messy signal at the receiving end.

The good news is that, in an Ethernet signal, the combination of the receiving devices and TCPIP do an incredibly good job of recovering the original square wave.

Provided specified distances for a given cable standard aren't broken, the receiving network devices and TCPIP can handle a situation where only a fraction of the original signal energy contributes to re-constituting short sections of the 'flat tops' of the original square wave.

Here's the bad news. Let's say that 25% of the original signal energy is 'seen' by the receiving network device as the wanted flat tops (and bottoms). What happens to the other 80% of the original signal energy ...?

This is what buggers our systems. All the distorted harmonics are still in there and they're all firmly in the band of nasty RFI frequencies. For all it's cleverness, is utterly transparent to RFI.

So we have a situation in which Ethernet cables contribute positively to RFI noise in a hi-fi system - you could call it self-noise. As for why higher quality Ethernet can sound better in the real world, it's down to better design and materials that minimise attenuation, group delay and several other unwanted factors, meaning that there is less overall diction and less self-noise.

Sorry for the long post. I've taken some time and effort to write this and I've done it because I hate seeing the wrong message going out and putting people off. Without a shadow of a doubt, cables can and do make a difference. They may not have a 'sound' as such, but they most certainly can influence sound. To argue that this is wholly impossible in an analogue scenario would literally require Ohms law to disappear.

The idea that cables can't make a difference in the digital world is much more understandable, but the basic physics of transmission described above mean that digital cables really can make a difference, and an unexpectedly big one at that because of the frequency range that Ethernet operates at (and, unavoidably, all higher frequency harmonics that go with it).

By the way, if you're in doubt about the validity of what I've said, there's a plethora of information on the web about digital signal transmission, both generally and in the context of Ethernet. This is all based on long-established signal transmission principles.

Rolski, I mean no offence at all and all credit to your friends in the space and automotive industries: what they say about deign and fitness for purpose is absolutely true. But remember, in the audio world, the end outcome isn't purely about moving packets of data from A to B - it's about music and music is affected heavily by things like RFI that don't matter (at least not as much) to cars and space ships ... :-)