USB Audio Gremlins Exposed: Beyond 1s and 0s, by iFi Audio

1. Bits are really just Bits?

1.1 Background
A common sentiment heard in the context of digital audio is that ‘Bits are Bits’ and as long as data makes it from A to B, different or ‘premium’ cables and other ‘tweaks’ cannot have any effect in the physical realm and are purely imaginary.

While this view has long been debunked, where SPDIF and AES-EBU connections are concerned (not least by Stereophile and others), it is often considered a tenet of faith for USB audio - especially asynchronous USB Audio - that ‘Bits are Bits’ and any differences arising from cables, operating system tweaks and other items are purely imaginary (delusionary?).

Yet, just as differences in SPDIF Cables and/or sources, as well as DACs (and their combinations), can be shown to produce not just audible but reliably measurable differences, USB audio is subject to its own set of limitations and problems.

1.2 Asynchronous USB audio to the rescue?
As far back as early 2001, computer-based audio with asynchronous USB audio proved it was possible to outperform SPDIF and/or AES-EBU based ‘cookie cutter designed’ systems. However, asynchronous USB audio is also far from perfect.

Over time as we gain more experience with USB Audio, we find room for improvement for asynchronous USB audio; just as sufficient experience showed that SPDIF implementations needed care.

1.3 What about alternative systems like Firewire or Ethernet?
Lest someone suggest that if SPDIF and USB are fraught with problems, we should instead use Firewire, Ethernet or some other as of now unknown protocol - all these protocols are developed as general purpose systems, not as something dedicated to audio (SPDIF was originally a ‘debug’ port using video signaling) – each are subject to their own set of problems - there is no silver bullet/panacea.

2. A Matter of Class

2.1 Audio vs. Data transfer
It must be understood that USB audio devices (and USB video streaming) have much stronger requirements for USB hardware and software layers than any other USB devices, such as printers, hard drives or flash drives.

Further, USB Audio Class 2 devices (2 Channels@32Bit/768khz) have even greater requirements due to the much higher throughput at high sample rates compared to USB Audio Class 1 devices (2 channels@24Bit/96kHz). They must run at the much higher speed even if only streaming lowly CD standard signals. The key issue is that USB Audio Class devices use ‘Isochronous’ transfers while other devices use ‘Bulk/Burst mode’ transfers.

2.2 What is Isochronous mode?
"A sequence of events is isochronous if the events occur regularly, or at equal time intervals."
Source Wikipedia

Isochronous mode is used for media streaming because it guarantees bandwidth on the USB bus by scheduling one transfer per available frame. By comparison Bulk or Burst transfers make use of ‘leftover’ bandwidth and may be ‘choked off’ if higher priority isochronous data transfers saturate the USB Bus.

Isochronous transfer mode uses error-checking but includes no re-transmission in case of Cyclic Redundancy Check (CRC) errors. Electrical noise on USB signals causes CRC errors and thus data loss, as does poor signal integrity. In mild cases, this leads to audio signal distortions. In the worst cases, clicks and dropouts. It means that a USB audio device can work correctly only if USB signal quality is excellent and no CRC errors occur.

Note: Do not confuse ‘asynchronous USB’ with ‘Isochronous,’ an asynchronous USB system still uses Isochronous mode to transfer audio.

3. Of Mice and Fans (and Curley’s Wifi)

3.1 Why is my pet mouse so well-behaved?
Most other USB device types (e.g. USB mouse, USB hard drive, USB flash drive, USB printer, USB Fan and yes, Wifi Sticks) are based on bulk transfer mode, which uses automatic re-transmission in case of errors. These kind of devices are much more tolerant of USB signal distortion.

For this reason, it is possible that a USB mouse, USB keyboard, USB flash drive, USB printer etc. all work well on a given USB port and using a given cable, while an audio device does not work with the same port or cable.

3.2 Which way did USB audio go, George; which way did it go?
A partially faulty hardware component, e.g. out of spec USB cable or USB port (which is more common than one might suspect), may not have an impact on standard USB devices such as a USB flash drive but can be catastrophic for an USB audio device.

A USB Audio Class 1 Device (no driver required on Windows, 2 channels@24Bit/96kHz) may function perfectly, but a USB Audio Class 2 may not, even if using the same port or cable and a low sample rate.

3.3 Encountering unexpected Cyclic Redundancy Check (CRC) Errors
In 2014, audiophile and technologist Fred Mak (known more commonly by ‘Fmak’ moniker) showed that with some combinations of cables and USB devices, asynchronous USB audio streaming can produce disturbingly high levels of CRC errors.

Though his observations were under-appreciated, we at AMR can concur as we found in 2010 that in some cases, active USB cables were able to cut the USB audio error rate to zero. Using cables with a tight impedance specification also helped greatly to eliminate such errors; leading ultimately to the introduction of iFi USB audio improvement devices and cables.

4. Ports, Cables, Packets and Noise

4.1 Being ‘PC’ (Port Correct)
On some PC main boards (or laptops) the signal quality can be insufficient for isochronous streaming from poor performing USB ports. The cause could be the quality of cabling or connectors used to connect the external USB port with the main board are insufficient, or the internal cables are placed too close to the power supply or other sources of electrical noise.

External USB ports (mounted on a front panel or elsewhere in the PC case) are a possible source of USB signal distortion. Quality of cables or connectors used to connect the external USB port with the main board could be insufficient, or internal cables are placed close to the power supply or other sources of electrical noise.

Here is what an interesting Pro Audio article said on the technical performance of different USB ports:

"Native Instruments thinks that the issue is serious enough to include recommended USB ports for Mac users in all it’s (sic) soundcard manuals in the Troubleshooting section.

"Hercules also have it in their FAQ. Serato do not go into specifics as to which port to use, but acknowledge that one port is 'good' and the other 'bad'."

4.2 USB cables
Quite often, the USB cable (or its connectors) is the main cause for USB signal distortions. Some cables (regardless of cost) available in the market do not adhere to the correct 90Ω impedance and are not suited for USB 2.0 high-speed communication (480 Mbps). Also the maximum allowed cable length of 5 meters should not be exceeded.

The following eye pattern succinctly illustrates what is desirable/undesirable when it comes to signal integrity.

Source: www.edn.com

Some special USB cable offerings, despite being optimized for audio, or cables which include additional functionality such as status LEDs, can cause poor signal integrity. Cables for USB Audio Class 2 devices should be certified for USB 2.0 use and follow the formal USB specifications closely.

4.3 USB packet noise
We also find factors like the noise and jitter levels induced by the packetized nature of the USB Protocol. For example, Full Speed USB (12MbpS - USB Audio Class 1, 2 Channels@96kHz), transfers one packet of data every 1 millisecond, giving rise to a 1kHz frame rate. This may manifest itself as noise at 1kHz.

For High-Speed USB (480MbpS - USB Audio Class 2, Two Channels@768khz), transfers one packet of data every 125 microsecond, giving rise to an 8kHz micro-frame rate. This may manifest itself as noise at 8kHz.

Due to limits on the data contained within each frame, additional noise components may be generated, usually at inaudible frequencies, which nevertheless can translate into jitter that will fold back into the audible frequency ranges.

All of this applies before we even account for such factors as the quality (noise levels) of the USB bus power that is used to power many less expensive USB DACs.

5. Measure > Test > Develop. Circum et circum. Advancing USB audio.

Given the issues and limitations designed into USB audio, it should come as no surprise that USB devices may be created that are found to improve the sound quality, even if we have yet to find a way to reliably and consistently measure their impact using standard test gear (e.g. our Audio Precision System).

At iFi/AMR, we have, in fact, been able to use the Audio Precision System to measure such factors as USB power noise and USB frame noise, though it often requires new tests to be programmed and set up and to be validated. These tests are not as trivial as knowing just where to place one's probes.

iFi is an ardent supporter of improving all aspects of USB audio, from USB power (iUSB Power - introduced in 2012) to signal integrity (Gemini and Mercury USB Cables & iPurifier - introduced in 2013).

With the latest range of USB audio focused devices (such as the iPurifier2, the nano iUSB3.0 and the micro iUSB3.0), we have updated the iFi product line to address the full range of known issues to maximize the quality of USB audio.

Computer audio certainly does not stand still, it develops dynamically and with some speed.


References

Stereophile. "Bits are Bits"
Stereophile. "A Transport of Delight: CD Transport Jitter"
Stereophile.The Jitter Game"
EDN Network. "Eye Diagram Basics: Reading and applying eye diagrams"
Pilmat. "MacBook USB Port Inequality"
Wikipedia. "Cyclic Redundancy Check"


About iFi
iFi Audio, part of AGL, is headquartered in Southport, UK and also owns the HiFi brand Abbingdon Music Research (AMR). AMR designs and manufacture high-end audio ‘home-based’ components. iFi Audio designs and manufactures portable and desktop ‘ultra-fidelity’ audio products. The combined in-house hardware and software development team enables AMR and iFi audio to bring to market advanced audio products.

COMMENTS
Doak's picture

Or hope for? **
**An interface purpose designed/implemented to transfer high resolution audio.

The audio industry is quickly moving into purpose built "digital audio appliances" and moving away from using general purpose computers - just makes good sense. Even so we remain saddled with the problematic USB interface - a leftover from an earlier phase of "computer audio."

Any chance for change?
Worth the effort/expense?

Paul E.'s picture

Michael, your post is perfect timing as I was shopping this past weekend for a DAC and audition the new Marantz NA8005. I brought along my 64 GB Flash-drive and 320 portable USB drive with WAV and AIFF files. Went through the gamut of Diana Krall tunes other jazz standards with both drives. The sales person asked if I was planning to use the current USB cable that came with my 320 GB portable drive, out of reflex I said yes. He then proceeded to swap the USB cable with one (high end cable from company AQ costing $100)
He said it'll open up the music, give it depth and increase the sound stage, frankly I didn't hear the difference and if their was it may have been perceived as minuscule thinking for the $100 for the cable I could purchase 2 x 120 GB high speed flash drives and go directly into the DAC. We didn't try any High-Rez files and wasn't sold on expensive USB cables for now

Michael Lavorgna's picture
The issues discussed here apply more directly to a USB link which is used to connect to a USB DAC. I have not tried switching out a USB cable used to connect USB storage to a player so cannot speak to this use.

But the most important thing here, imo, is that you tried it and decided it wasn't worth it. No harm, no foul.

Cheers.

PAR's picture

" I have not tried switching out a USB cable used to connect USB storage to a player"

Try it andd see what you think. I have and it does make a difference. I tried it initially, not for any direct audio reason, but the drive that I was using had generated several reviews on Amazon complaining about slow writing speed. One reviewer said that he had changed the supplied USB 3/micro cable to another and that the writing speed increased significantly.

So, having read the reviews, and being bored on a wet afternoon I swapped the supplied USB cable to one transferred from another make of drive. Worthwhile audible improvement. So I bought another USB cable. Nothing special just a generic one costing around the price of a cup of coffee. Even better audio performance. At least I think so. Others should try their own little experiment. Of course it may be that the original USB cable was just crap, period.

I also found that portable drives powered via the USB link sounded worse than those with their own wall wart power supply (IMO). That might fit with i-fi's comments on electrical interference.

Michael Lavorgna's picture
I do not a USB drive to store my music, I use a NAS.
VK's picture

Some manufacturers (like iFi) say that USB cables make difference (maybe to sell those USB "tweaks"), and others like Benchmark say it doesn't matter.
I think that Benchmark CEO said once that it depends on how good is the implementation of the USB receiver in the DAC (i have to look where i read that... but it was something like this...). If it is well constructed, the cable doesn't matter.

So... i don't know man. But good article anyway!

Best regards!

Michael Lavorgna's picture
...you'll live forever ;-)
VK's picture

Is someone really thinking that way? That's some false advertisement! But the world is full of it, so... let's keep our cheap vegs ;)

Michael Lavorgna's picture
It was meant to illustrate the notion that anyone can say anything and if you're willing to believe it, you're good to go. Reality typically intrudes.
VK's picture

joking too, haha ;)

But it is true: people say whatever they want, and it's up to us (i mean, in a particular way) to filter what's true and what's nonsense.

Well, time to go to bed here, it's almost midnight.

Best regards!

PAR's picture

Using that logic if any alternative cable sounds better than the first then the first must not be well constructed. There need be no other explanation.

Of course if another cable somes along which sounds better than the first and the alternative then neither of them is well constructed. And so on.

It doesn't help advance either science or art as what is "well constructed" is a movable feast.

VK's picture

He doesn't mean it in that way. That CEO only said that they (Benchmark) doesn't endorse the use of expensive USB cables. A simple Monoprice will do the job perfectly, and a better cable won't change anything.

Best regards!

draenor94's picture

another nice piece AudioStream. So is USB audio the best we can hope for? just wish the industry could agree to move forward on common audiophile standard but would be like the European Union comming to agree on things. good luck and may the force be with us all. you guys do really good pieces. other magazines should take note of Audiostream!

monetschemist's picture

... to AMR for putting this together and to you for putting it on your site. A very informative article!

Thank you

Yogawa-L's picture

very insightful technical information. keep up these kind of pieces from other manufacturers. Bravo.

Carl.twostreet's picture

Personally, I do not see anything new, exciting or useful in the article. It is marketing. It tries to create a suggestion of a host of problems in people's minds so that iFi can "solve" it for them. But, it has offered no real proof of any of the evils of USB it tries to suggest. I therefore consider it slippery and unreliable. Any time a manufacturer suggests or alludes to measurements he has made, here even to measurement equipment he uses, but then does not show us the actual measurements, that speaks volumes. I put them in my "do not waste your time list". That list is growing, now including AudioQuest, Uptone, etc.

I am quite happy with USB, myself. Given a decent, well isolated DAC, it sounds great to me. I have not heard a "tweak" that makes any meaningful difference. I am still waiting to be convinced by actual evidence, not hand waving and pseudo science that USB is rotten at the core and needs "fixes" or a replacement.

Here is an anecdote to consider. I formerly used an Oppo player with hi def Blu-Ray video/hi rez Mch audio from a hard drive via a generic USB2 cable. There was never one iota of visual or audible difference between that and playing the silver BD disc directly. Several friends had exactly the same experience. And, the bandwidth requirements for video are huge compared to audio. So, where are all these problems with USB?

Michael Lavorgna's picture
"Industry Voice" is where this is published and it's the section of AudioStream that provides industry members a voice. Some have found this post useful, even you, albeit for various reasons.

The Uptone Regen, Schiit Wyrd, AudioQuest JitterBug, and iFi micro - iUSB 3.0 have all proven to be effective -- based on listening. Since you feel comfortable sharing listening-based anecdotes, there are many supporting these claims of an audible improvement.

Cheers.

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