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Publication Date: 05/1/2009
Archive >  May 2009 Issue >  Tech Watch > 

Understanding Digital Interconnects

There are two digital audio interconnecting schemes in common use in consumer electronics today. These two are the TOSLINK optical interconnect and the S/PDIF coaxial interconnect. Both of these formats are specified in the compact disc "red book" standard. A standard audio transfer file format, S/PDIF is found on all types of digital audio equipment from DAT machines to computer sound cards. The most common connector used with an S/PDIF interface is the TOSLINK connector. TOSLINK, also known as EIAJ optical, utilizes a series of on/off pulses of a red LED light to send the series of 1s and 0s that describe the audio waveform.

TOSLINK as an audio connection has advantages and some drawbacks. On the plus side, TOSLINK is not an electrical connection! Because of this, the use of a TOSLINK interconnect will not cause ground loops when the source and load are connected to different electrical circuits, such as may happen when interfacing a computer system with a home theater rig. Another advantage of the TOSLINK interface is its immunity to environmentally induced noise. The TOSLINK interconnect will not pick up electromagnetic interference (EMI) or radio frequency interference (RFI). You won't have the local AM radio station playing in the background of a TOSLINK connected source!

TOSLINK also has a couple of drawbacks that must be acknowledged and understood to maximize performance. First, the implementation of TOSLINK requires a change in format from an electrical to an optical medium. No such change, including similar changes from digital-to-analog or from analog-to-digital, is without cost. In the case of transforming the electrical data stream to an optical signal for use with TOSLINK interconnects, there is an increase in levels of both noise and distortion within the system. This decrease in fidelity may be especially pronounced with wide-bandwidth multi-channel signals.

Second, an optical interconnect has a maximum bend radius before internal reflections become detrimental to the signal. You can't make a tight turn with a TOSLINK interconnect without causing a measurable drop in signal level. In fact, a tight bend in a TOSLINK interconnect can permanently damage the delicate optical conductor and cause a complete failure of the signal. The best TOSLINK interconnects will boast a high purity quartz conductor which allows as tight as a three-inch bend before signal loss exceeds 0.5dB.

Coaxial Interface
The second common digital audio interface is the coaxial S/PDIF. This connection looks, acts and is, for all intents and purposes, identical to a composite video connection. It is designed to operate at a characteristic impedance of 75Ω, is immune to tight bends as long as they don't physically change the geometry of the cable, and suffers virtually no limitation at practical installation lengths. From the earliest days of compact disc to today, many audio connoisseurs have considered the coaxial S/PDIF interconnect audibly superior to the optical TOSLINK format. Improvements in optical conductors have somewhat leveled the playing field, but if the connection needs to be more than 20 feet or so in length, the coaxial solution must be considered.

Digital Video
There are two commonly used interface protocols used for digital video in the consumer electronics industry. There are, of course, more than three but most of the others are endemic to the computer world. Here, we are only focused on interface schemes commonly used between a source and an A/V Pre-Pro or consumer display device.

DVI (Digital Visual Interface) is an uncompressed digital connection scheme originally developed for the PC industry. Initially designed as a low-cost, high-bandwidth digital connection between PCs and digital monitors, DVI is now the most widely used digital display interface in the computer industry. In home theater applications, DVI may be combined with High-bandwidth Digital Content Protection (HDCP) technology to create a protected digital connection that prevents unauthorized duplication of appropriately encrypted programming. Members of the Cable and Satellite distribution industries are supporting DVI with HDCP as part of their complete digital interface solutions.

DVI is a copper-based electrical interface that is effective to 60 feet or more. If significantly longer interconnect distances are needed, a DVI "extender" device must be used. While common in the realm of PCs, DVI connections on home theater products are found primarily on projectors, monitors, advanced DVD players and some satellite receivers. All home theater applications of DVI utilize the DVI-D standard in a single link configuration. DVI-D dual link will support up to a 1080p image, but that level of performance is rare — and very expensive — in today's market.

It is important to note that a DVI-D dual link cable is completely compatible with single link applications and offers the added benefit that as you upgrade components you won't need to invest in new cables. HDMI offers significant advantages over existing analog A/V interfaces. Its uncompressed, digital format transports high-definition video, multi-channel audio and control signals between components. By combining audio with video on one cable, HDMI offers a convenient connection alternative to the maze of existing analog A/V cables. HDMI also ensures compatibility between products. Capitalizing on the interoperability standards created in CEA (EIA/ CEA-861x), HDMI assures that the best video format is always sent from source to display. HDMI is "backwards compatible" with DVI, so that an HDMI equipped source can digitally drive a DVI equipped display device.

HDMI, like DVI, is a copper-based electrical interface. HDMI is effective to more than sixty feet, and ensures full performance in almost any consumer application. A small connector, HDMI interconnects also have the advantage of being able to slide through a conduit. This interface will become a preferred candidate for all kinds of custom installations. HDMI is available on hundreds consumer electronics products from more than two-dozen manufacturers. There is little doubt that the future of home theater and high-definition entertainment will be intimately entwined with the HDMI protocol.

Clearly the world of digital signal interface has evolved in important and unexpected ways over the last few decades.

Giant leaps in technology have become so commonplace that they are nearly taken for granted. Understanding the implications of this evolution in technology will most certainly pay dividends in the form of better audio/ video performance and a longer lasting return on your investment in interconnect technology. As the old saying goes, change is the only constant.

Contact: Cables To Go, 3555 Kettering Blvd., Moraine, OH 45439 937-224-8646 fax: 800-331-2841 or 937-496-2666 E-mail: Web:

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