Jitter, or “why digital audio interfaces are analog”

Confused by the title?  Most people probably are, and that’s the point.  We constantly hear that “digital is perfect” and “there cannot be differences between transports” etc. We heard this from engineers, computer scientists and armchair experts. All three are wrong, but the engineers really ought to know better.

Let’s start with some basics.  Most musical instruments from the human voice to a guitar or piano, are analog. Our ears are analog.  And the sound waves between  the two must be analog.  God did it. Don;t argue with God.

Digital is a storage method.  It can only occur in the middle of this chain, with sound converted to digital and then back.  The goal is 100% transparency – or, more accurately, transparency so good we cannot tell the difference.  While that sounds like a cop out, its not. Analog records are also intended to be 100% transparent and fail miserably. CD, DSD, or whatever need only fail less to be an improvement.  My opinion is that done right, it DOES fail less and is potentially superb. Its that word “potentially” that trips us up.

While there are many points along the chain where we can lose fidelity I want to talk about one in particular:  Jitter.  I want to talk about jitter for two reasons:

  1. It has a huge impact on sound quality in real life systems today.
  2. No one talked about it until recently, and very few understand what it is or why it’s a problem.

To understand jitter, first we need to understand CD playback. I will use the CD example simply because I have to pick one and it is the most common High end format. CD music is digitized by measuring the music signal at very tiny increments. An analogy would be Pixels on your screen, and they use a lot of “pixels” — 44,100 samples every second. The height, or “voltage” of each sample is represented by a number we debate endlessly: the bit depth. CDs use 16 bits which means 64,000 shades of gray.

     
Illustration of the height and spacing of music samples;  courtesy: Wikimedia.org.

But there is another characteristic that is equally important to sound quality in fact, mathematically it is part of the same calculation, and nobody talks about it. That characteristic is the time between samples. Think about height and time like a staircase; each step has a height and tread depth — the two together determine the steepness.   Similarly the analog output of “pulse code modulation” (which CD is) is determined by the height (limited by 2^16 or 64000 levels) and the time between samples. That time is assumed to be precisely 1/44,100 second. But we live in an imperfect world and that fraction of a second varies some, in the variation, which is random, is call Jitter.

Because it is random, it is not harmonically related to the music; and therefore in musical terms dissonant (or lousy sounding).  So while bits are in fact bits, there is much more on the interface between the transport and a DAC, than bits.  There is also jitter and noise, and noise causes jitter.

So well the engineers telling you that transports we’re streaming servers, where the digital signal cannot impact sound quality failed to read Shannon’s (and Nyquist’s) work.  It’s that simple guys.

For those of you who don’t much care, and just want your music to sound good, it gets both better and worse 🙂

There are two ways to send a digital signal between a source and a DAC.  The traditional method is by an interface called SPDIF.  It’s the little yellow RCA jack on the back of your CD transport. The problem with SPDIF is that a) it is Synchronous and b) the source is the master clock, and therefore determines jitter.  So when you do something logical like buy a fancy DAC to make your cheap CD player sounds better, you get the jitter of the cheap CD player and as we noted above that’s half the story.

There are more problems- primarily related to electrical noise but I think they are less severe than jitter and certainly are another topic.

I hope this has shown you that sound differences from transports and digital signals are neither snake oil nor mysterious, only annoying.  I will make only one recommendation: make sure that SPDIF connection is a true 75 ohm cable. It need not be a fancy audiophile cable. It can be amazon basics. It can be cheap Schiit (I think they call them that).  But it must be 75Ω.

Now if we could only fix crummy digital mastering, but that’s out of our control.

All the best,

 

Grant

CEO Sonogy Research LLC

Comments

    • And I’m glad that people are getting value from it. I have great hope for digital, but am learning a lot as I dig into all the dark corners. We’ll see what kind of, if any, product(s) comes out if it 🙂

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