Is the earth flat or spherical? Until the 5th Century B.C, we thought it flat. It reportedly took the Ancient Greek philosophers – experts in their field in whom the rest of Ancient Greece put intellectual stock – more than 200 years to convince their fellow man that the earth was spherical. No doubt controversial at the time, it took two centuries for their fresh thinking to become accepted wisdom. Two centuries of conversations like this:
Greek dude 1: “Turns out the earth is round after all”
Greek dude 2: “No, it isn’t.”
Greek dude 1: “The thinking coming in from these philosophers is highly compelling”
Greek dude 2: “Show me a peer-reviewed white paper proving your hypothesis”
In other words, a change-pain period.
Nowadays, very few of us challenge the spherical earth model. We treat it as accepted wisdom.
What of the hi-fi world?
Digital audio is a mere thirty-eight years young. Upon the Compact Disc’s 1982 launch, we were told that digital audio was just a matter of ones and zeroes; that its signal transmission was immutable. We were told this so frequently thereafter that it became part of the audio world’s collective wisdom. Who were we to think otherwise? To challenge it was to mark one’s self out for ridicule.
The ensuing decades have given scene experts – the people who design CD players, DACs and streamers – the time to better explore digital audio’s engineering quirks. And those experts are now telling us that changing out the USB cable that connects PC/Mac to DAC, for example, can impact sound quality. PS Audio’s Paul McGowan told us only last week in no uncertain terms that USB cables do make a difference. Wavelength Audio’s Gordon Rankin, the inventor of asynchronous USB, tackled the issue on these very pages well over two years ago.
One of the most common misconceptions about USB audio that I still see, most often in YouTube comments, is that it is error-correcting. It isn’t. Due to the isochronous data handling, USB audio only offers error-checking. If an error is detected, the data packet is dropped, never to be resent. How do we know this? From talking to experts like Rankin. And McGowan. And Powell. And Vitorino.
Also MBL’s Jürgen Reis (here) – who reminds us, crucially, that not everything that is heard can be measured. A sentiment echoed by HEDD Audio’s Klaus Heinz (here), a German loudspeaker designer of forty years who says that measurements only tell him if he’s made a mistake, not how a loudspeaker will sound. Paul McGowan agrees 100%.
But USB audio differences don’t only fall to USB transmission errors. The retort that ‘my printer doesn’t produce a nicer image with a better quality USB cable’ is as flimsy and as old as a 5 1/4″ floppy disk. Your network engineering buddy of twenty years who talks of moving petabytes of data without issue is working in an entirely different field. As are the Microsoft, IBM and HP experts who stand behind it. Moving files from one hard drive to another or executing a print job over USB doesn’t invoke isochronous transfer mode (per digital audio) but ‘Bulk/Burst’ mode. Hard drive and print job data aren’t time-sensitive. Quality isn’t impacted by timing errors (‘jitter‘) as it is with digital audio.
Being told repeatedly that digital audio is “just ones and zeroes” would, quite understandably, see us fixate on the data aspect of USB audio. And yet, according to the many DAC designers I’ve spoken to over the past ten years, data dispatch and arrival is only half the story. A second issue – electrical noise – must also be addressed.
This brings us to a second common misconception: actual ones and zeroes travelling down a cable. In reality, there is no such thing as a digital signal, only an analogue representation of a digital signal. From Wikipedia: “In digital electronics, a stream of binary values is represented by a voltage (or current) waveform. However, digital signals are fundamentally analog in nature, and all signals are subject to effects such as noise, distortion, and loss.”
As we know with loudspeaker cables, electrical noise can impact an analogue signal. Inside a DAC, electrical noise can impact the unit’s analogue output stage. Prior to that, in the pseudo digital domain where we have only an analogue representation of a digital signal, electrical noise can corrupt the signal to the point of data loss.
Here’s a fresh snapshot of a vanishingly small time window from Gordon Rankin showing a USB audio data transmission error:
How does electrical noise worm its way into the guts of a DAC? A USB cable is effectively an aerial that picks up RFI from the air around us; WiFi is now a strong pollutant. We know this from talking to AudioQuest’s Garth Powell (here) and we hopefully already know what an aerial does: its send what it receives into the device to which it’s connected. In our case, a DAC.
Further, the host computer’s internal circuits create an ‘electrical firestorm’, sending its own dose of electrical noise down the USB pipe and into the DAC. This line of thinking we’ve heard from Powell but also Innuos’ Nuno Vitorino (here).
Sometimes the DAC itself can generate electrical noise. In that same interview, Vitorino explains how a USB (or TOSLINK) receiver chip can generate electrical noise as it engages additional levels of circuitry to better read jittery/noisy signals.
MBL’s Jürgen Reis elaborates: “At the USB receiver, there is a PHY chip (or PHY part in the USB chip) that regulates the level and the shape of the incoming signal. And when this PHY chip has some work to do, it produces a side-band in the jitter spectrum that can be seen in the measurement. So even the PHY chip can bring the input signal to a (different) level and shape; the data can be read without error but the “work of this chip” can produce jitter sidebands. This is similar to the work of the regulation chips for a laser lens of CD transports. Even when the regulation chips can regulate the focus of the lens so that no data read is corrupted, you can measure (and hear) the work of this regulation chip.”
From AMR’s Thorsten Loesch via an Audio Circle post (that’s well worth your time): “USB uses 1ms and 125uS signal frames, hence the creation of 1kHz and 8kHz frame noise. Below is a Stereophile test result of quite popular USB DAC from a reputable manufacturer, which clearly shows the 8kHz frame noise.”:
The hi-fi industry’s digital audio experts are repeatedly telling us that digital audio isn’t simply a matter of ones and zeroes. But are we listening?
Conspiracy theorists are often quick to point out that it remains financially advantageous for industry experts to espouse differences in USB cables (or sources). After all, a USB cable designed specifically for digital audio transmission sells for a lot more than an Amazon Basics cable.
But have these same conspiracy theorists considered that they too are subject to the same financial advantages? That slamming the door on first-hand experiences, reasoning that “it can’t possibly be true”, brings with it its own monetary gain? After all, an Amazon Basics USB cable sells for a lot less than one specifically designed for digital audio transmission.
We see a similar dichotomy when it comes to evidence. The burden of proof on USB audio differences falls to the digital audio experts but this burden of proof cuts both ways. It also falls to the conspiracy theorist to prove that vested financial interests have caused audio engineers to ignore science in favour of profit.
This week, in a wonderful piece of synchronicity, MOON by Simaudio’s Costa Koulisakis and I sat in a rooftop restaurant across from the Parthenon in Athens to discuss this very subject: do USB cables matter? His answer was an unequivocal ‘yes’:
Before you object, ask yourself: are you an expert? Have you discovered from first principles that digital audio is simply a matter of ones and zeroes? Have you spent a large portion of your adult life designing digital audio circuits? Or are you, in fact, simply paying forward the thinking of digital audio experts past?
In sampling this website readership we find that along with yours truly, the majority do not self-identify as experts in the audio engineering field.
As a non-expert, could you entertain the possibility that digital audio thinking has changed since the advent of the CD – that it is no longer simply a matter of ones and zeroes – just as the thinking changed for the Ancient Greeks and their understanding of earth’s physical form?
Are we now in the middle of a USB audio change-pain period?
To be continued…