During the creation and development of this book, the USB has progressed, since its introduction in 1996, through three iterations, and was scheduled for a fourth revision in 2019. In essence, the USB connection is a concept expressed as a written standard that describes an electro-mechanical and software protocol system through which a host computer is automatically interfaced to peripheral devices when a standardized electrical cable is plugged into each unit.

In USB 2.0 cables there are four wires for electrical transmissions and power delivery. Digital information is transferred on a twisted pair of wires known as D+ and D- (color-coded as white and green, respectively). A nominal 5V, 400 mA power is carried on the cable’s red wire, with the remaining black serving as a ground. (Occasionally, orange and white are powered with green and blue data lines.)

USB 3.0–compliant cables have an additional two insulated twisted-pair data lines in addition to the four-wire 2.0 specifications to aid in increasing the data-transfer capability of the newer specification.

Information in digital systems is carried by the high-frequency on–off binary nature of the encoding. The digital-based USB communication system is very dependable and not only connects and interfaces the host and peripheral devices but also transmits data, usually without error. However, audio and high-sensitivity electronic measurements have been reported to be compromised by high-frequency noise on the USB connection.¹ An experimental noise-reduction filtering system and its effects are detailed in the UK website, and numerous USB audio noise-suppression devices and methods are available commercially. An in-depth white paper is available as a PDF from the Wurth Elektronik company entitled “ANP024 The USB Interface from EMC Point of View.”² The thirteen-page paper fully describes the USB standard, hardware, electrical specifications, topology, and protocols for the system. The paper discusses in detail the electromagnetic compatibility (EMC) of the components and systems of the USB communications connections and details the techniques and components that can be used for controlling signals and noise on the link with various USB filters. A kit is available from the Wurth company for developing a USB EMC filter to suit a particular experimental application.

In an effort to minimize the USB noise level in the author’s computer system, an iDefender 3.0 USB isolator and noise filter, manufactured by the iFi audio company, was purchased ($100 CDN) (ifi-audio.com) and installed, as seen in Figure 8-1 on the author’s desktop computer.

Item 1 in Figure 8-1 is the isolation device that is plugged directly into the host bus port. The glow on the bottom portion of the red case is from the pilot light for the auxiliary 5V supply that is powering the downstream devices on the bus. Item 2, the white rectangular box, is a 5V supply powering the positive and ground wires in the USB cable connected to the iDefender. The USB power lines in the cables connected to the iDefender are thus drawing current from the line or mains supply and are bypassing the host computer’s internal supply and wiring buried inside the electromagnetic wave–filled environment within the host’s grounded metal case.

System noise reduction can be increased through electromagnetic compatibility considerations that include high-quality shielded USB cables fitted with ferrite bead chokes and well-regulated external bus power sources. For host computers with multiple USB ports, test each one with a high-scale expansion strip chart recorder display to find the connection with the lowest level of spurious signals, spiking, and noise.

Typical Baseline Noise and Large Signal Distortions

Ringing is depicted in Figure 8-2 and can also be seen in the black trace of Figure 8-3, recorded during the development of high-sensitivity thermistor temperature measurement.

Figures 8-4 and 8-5 show spiking at different levels, high and levered, respectively.

Noise from differential signal sources is shown in Figures 8-6 and 8-7.

Commercially Available Noise-Reduction Devices

USB audio devices are often found to have extraneous noise embedded in the audio being played. Background hum, clicks, and other noise elements are often reported as distortions that have been attributed to “ground loops” in the grounding circuitry and fluctuations in the nominal 5V power system (USB power is called VBUS). As depicted in Figure 8-1, iFi Audio (ifi-audio.com) manufactures a “plug-in” noise and power-line filter called the iDefender 3.0 that significantly reduces USB noise and provides a bypass, allowing an external 5V supply to power the downstream devices drawing power from the VBUS line. Figure 8-8 illustrates the observed baseline noise reduction provided by the isolation filter.