Arcam Delta 170 CD Transport Into the Optic?
Sidebar 3: Into the Optic?
In a single week this past month, no fewer than six products with fiber-optic inputs or outputs have entered my listening room/test laboratory. This trend is a harbinger of the future as optical technology assumes an ever-increasing role in audio equipment. Since all three products reviewed here feature optical jacks, I thought this would be a good opportunity to provide a short primer on optical transmission of digital audio.
In a piece of copper wire transmitting digital audio, electrons carry the information in the form of pulses from one point to another. Fiber-optic cables, however, transmit information by sending photons through glass (or plastic). Fiber optics offer many advantages over wire: complete immunity from electromagnetic interference, wide bandwidth, no crosstalk, and low propagation delay. Immunity from electromagnetic interference is especially important when the digital signal is unbalanced, as is a CD player output. In addition, fiber-optic connections isolate the grounds of the transmitting and receiving components (a CD player and outboard D/A processor, for example), preventing noise from the player's servos and digital electronics from reaching the analog section. Hum-generating ground loops are also eliminated. A ground loop occurs when connected ground points are at a slightly different voltage (caused by resistance in wire or PCB traces), resulting in a small current flow through the ground. Since there is no electrical connection between grounds with fiber-optic connections, these problems are avoided.
Another advantage of fiber-optic transmission is that all frequencies travel at the same speed through glass. This is contrasted with an electrical conductor where speed varies with frequency: low frequencies travel more slowly than high frequencies. This differential is significant even within the relatively limited bandwidth of analog audio (20kHz). When an electrical conductor is asked to carry the wide bandwidth of a digital audio signal, the problem is magnified. Some digital transmission formats contain a DC component along with squarewaves. Squarewaves possess substantial high-frequency harmonic energy. A 1MHz squarewave still has significant harmonic content at 100MHz. When these harmonics are skewed in time, the rising edge of the pulse (where the transition containing binary information is detected) becomes sloped instead of vertical, especially with long cable runs. This phenomenon introduces jitter (time-axis variations) in the signal which may cause audible problems. This problem is compounded when the transmitted serial bit stream contains the clock information, such as the S/PDIF output from a CD player or DAT machine. According to some, clock jitter affects imaging. Optically coupling the transmitting and receiving components removes this potential source of sonic degradation.
Most audio components with optical inputs/outputs use the EIAJ (Electronic Industry Association of Japan) standard jack and cable. This cable is alternately referred to as a Toslink connector. Most optical cables supplied with CD players and outboard digital processors use a plastic light conductor, not glass fiber. High-purity glass, with 1/15 the transmission loss of plastic, is much better at transmitting light. The quartz glass used in optical fiber loses half its intensity in about 15 miles of cable. By comparison, ordinary window glass loses half its intensity in one inch.
A basic optical cable consists of the light conductor called the core, cladding to reflect light back into the conductor, and a buffer or sheath for protection. A complete optical transmission system consists of an optical modulator to convert an electrical signal into light pulses, a transmission medium (cable), and an optical receiver to detect and decode the signal.
Optical cables do have some limitations, however. They are more prone to damage by sharp bends and kinking than their electronic counterparts. In addition, there must be precise alignment between the cable plug and jack for transmission. Another problem is the difficulty of splicing cables. Optical interconnects carry light so well because of the lack of obstruction in the fiber. Any roughness or discontinuity introduced at a junction may prevent transmission, not to mention the difficulty of working with a strand the size of a human hair. This is why the dummy plug found in optical jacks should remain in place unless a cable is plugged in. The light in an optical cable can also cause eye damage. Never look into the end of a connected optical cable!
These few disadvantages, however, are minor compared with optical fiber's benefits. There is little doubt that as digital audio technology improves and proliferates, fiber-optic transmission will become integral to music recording and reproduction. You may one day regale your amazed grandchildren with stories of the days when electrons carried audio down strands of metal.Robert Harley