Tuesday, November 24, 2015

Heretical Digital Cable

Why Heretical?

Every digital audio coax (SPDIF) cable out there claims that it is "75 ohms" (whatever that means). The Mad Scientist Heretical Digital makes no attempt to conform to this standard. This post explains it all..

What is this "75 Ohms"

In the SPDIF standard (Sony/Phillips Digital Interface Format), the coax electrical interface is defined to have a characteristic impedance of 75 ohms.

Characteristic Impedance however is not so easy to understand or picture as regular impedance, say like a loudspeaker is 8 ohms, or an amp might have an input impedance of 50kohms. These are simple resistances, and you can measure them easily with a multimeter.

On the other hand, Characteristic Impedance is all to do with transmission lines. You also can't measure it with a multimeter. Here is the formal definition, from Wikipedia:

The characteristic impedance or surge impedance (usually written Z0) of a uniform transmission line is the ratio of the amplitudes of voltage and current of a single wave propagating along the line; that is, a wave travelling in one direction in the absence of reflections in the other direction. Characteristic impedance is determined by the geometry and materials of the transmission line and, for a uniform line, is not dependent on its length.

So why does that matter?

The crucial point is this : If you have a 75-ohm transmission line (like a digital cable) and you have some part of the system that is NOT 75-ohms, then you get signal reflections. The signal can bounce up and down the cable, in some cases dozens of times. These reflections can and do alter the original signal in hard-to-predict ways. This mechanism is the underlying cause of jitter in digital cables. And jitter is one of the chief culprits of 'digital nasties'. Not the only culprit, for sure, but a very important one.

There have been articles written about the optimum length for a digital interconnect, making it the right length so the reflections avoid the critical points where the detector is trying to detect a transition. However you need to know things like the rise time and propagation time to figure out the correct length for your system; as far as I can see there's no length that would work for everything. The effect that jitter produces is a horrible mixture of noise and distortion - not easy on the ear at all. But it accounts for the differences in sound of digital cables.


The amount of jitter that high quality digital clocks produce is getting to be very small - measured in a few tens of picoseconds. Now that figure is meaningless to most people, me included. So how's this : Light travels about 3mm in 10 picoseconds. And as you may know, light is really really fast. This small amount of jitter can be dwarfed by other sources like cables.

Heretical Digital Cable

The Mad Scientist Heretical Digital takes a different approach. It does not attempt to conform to a 75-ohm characteristic impedance. It does however have a significant amount of resistance as the main conductor is made from treated carbon fiber.

Two things happen with a carbon fiber conductor:

  • Skin Effect is very very small compared to metal conductors. The square waves that make up the digital data are sent at a  few megahertz - fairly slow by digital standards. However, the harmonics that make up the square wave go much higher, into the tens or even hundreds of megahertz. At these frequencies, copper has a skin depth of a few micrometers. By comparison the skin depth for carbon fiber is still as few millimeters.

This is important as the correct transmission of all the component harmonics is crucial for the correct transmission of the whole wave.

  • Resistance soaks up the reflections. This is probably more important than skin effect. The resistance of the conductor, being of similar magnitude to the 75-ohm loading means that reflections are not going to be able to do much damage - they will be turned into heat.

If you imagine the digital link being like a light tube, with flashes of light being the data pulses; A normal cable has silvered parts and so you get glare and reflections. Our Heretical Digital is like filling the tube with slightly darkened glass - only the bright flashes get through, reflections are absorbed.

The Heretical Digital Cable has a resistance of about 37 ohms - half the 75 ohm characteristic impedance. But all SPDIF inputs are terminated with 75 ohms. This means that a reflection that reflects off the DAC end of the cable/plug will travel towards the source where it can interfere with the data. But the resistance will tend to turn the energy into heat, so reflections are dissipated very quickly.

What it takes to develop cables - a small selection of prototypes

What Actually Happened

This all might sound like it was planned. But that's not quite how it happened. As is often the case in science, an ad-hoc experiment showed some interesting results which lead to further research, theorizing, more experiments and so on.

After the carbon fiber interconnects were developed, I thought I'd try one as a digital cable. I wondered whether it would work at all and I wasn't expecting it to sound good.

But it did sound good, which surprised me. So a new project was born. I needed to figure out why this was producing such good results. Also I needed to figure out what the best sounding design was.

It turns out that a much simpler design than YANAM/TORFORB is needed. Out with the multiple bundles of carbon fiber, now just a single, thicker conductor is used. Also a cheaper plug seems to work better.

As with the interconnects, I was trying to make the best digital cable that I could. It would also have been ideal if I could come up with a range to suit all pockets. But the result of the listening and design process showed that the simple one is the best. If anyone wants a "signature" version, I can sign it with a silver pen if you like ;)

How Does It Sound

Usual disclaimer about how I am biased, etc. But those that know me might want to take note..

The Heretical Digital Cable astonishes me. I've tried a good number of digital cables in my time, conventional and not-so-conventional. This one shines, being the best one I've heard. Not "best for $99" but "best for any money". The things that leap out at me are:

  • analog-like sound but with digital crispness - so a lush sound but the leading edges are still fast
  • very pure treble with lots of air and very fast top end
  • wide and fast dynamics
  • intelligibility - I'm hearing lyrics much more easily. Also makes complex passages easier to parse

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