Many audio-only devices such as CD players and digital-to-analog converters offer one or the other, so depending on what inputs you have available on your receiver you'll want to understand those differences. Options include Toslink -- short for Toshiba Link, an optical audio connection originally developed for Toshiba components -- and digital coaxial cable. Both Toslink and digital coax transmit trouble-free audio from source to receiver. Learn about the certain considerations for each that may apply to your specific installation.
Either digital audio format is capable of any audio format, as long as it fits within its bandwidth. This refers to the amount of data the cable can support, which in the case of Toslink or digital coaxial begins at mono all the way through Dolby Digital and DTS. Neither optical nor coaxial cables have the bandwidth needed to play uncompressed audio sources like DTS Master Audio or Dolby TrueHD, found on Blu-ray disc and supported by HDMI. However, given the choice of a digital connection or stereo RCA, the digital option is more versatile supporting not only stereo but surround sound.
Noise Rejection and Jitter
Toslink cables do not use a direct electrical connection from source to receiver as digital coaxial does. Electrical isolation is achieved by converting the signal from an electrical pulse to light and back again. As a result, noise from spinning computer hard drives or a faulty ground is inaudible when connecting an optical cable for audio. This advantage can be countered by optical's increased amount of jitter -- caused by the same process that eliminates noise. Digital cable's slightly higher bandwidth and direct electrical connection means that there is less manipulation of the signal, resulting in potentially clearer sound. However, jitter is not generally considered audible until just before the point where the signal is entirely lost.
Digital coaxial cables are constructed similarly to conventional RCA cables, with a single solid or twisted copper conductor and ground. Surrounding these conductors is a braided metal shield and outer jacket. TOSLink, in contrast, uses a plastic or glass fiber construct, easily fractured if the cable is tighter than its radius. In either case, care must be taken when bundling the cables using zip ties or straps, because the potential for crushing the shield or fiber is present. Toslink is almost impossible to repair without training and expensive tools. In contrast, digital coaxial cables can be replaced with a standard 75 Ohm coaxial television cable with screw-in RCA fittings if necessary.
Prewiring a Home
Prewiring a home for audio and video can future-proof your wiring to a great extent, anticipating the cabling needs for current and future devices. Many times these cables travel significant distances and are sealed in walls, so choosing carefully the first time is paramount. Digital coaxial cable can carry audio much further without signal loss compared to TOSLink. The latter is limited to about 75 feet, while digital coaxial can travel up to 200 feet in most cases without issue. Also, digital coaxial cables are virtually identical to composite video cables and are thus more versatile. Toslink cables are restricted to audio only. A third option, HDMI, carries all audio formats and HD video over one cable, but is also limited to 75 feet without a repeater. Adapters are available that convert Toslink to coaxial. When you need to send signals across longer distance, adapters provide a handy solution for devices that are only equipped with optical output.
Latency is the gap between when a signal originates and when you hear or see it. In the consumer audio world, this is known as the dreaded "lip-sync error," caused by the difference in time it takes for televisions and A/V receivers to process the audio and video. Both Toslink and digital coaxial cables potentially suffer from this problem, and is especially prevalent when using HDMI for video while using either of the other two options for audio. For this reason, most A/V receivers and some televisions offer a manually adjustable lip-sync correction function.
When using a digital interconnect instead of standard RCAs for a CD player for example, differences in sound quality may exist. This phenomenon, however, is not due to the cable, but rather to how well the source device and receiver handle the information. Much is made about the quality of the digital-to-analog conversion technologies in audio devices. Better receivers and sources use better DACs, reducing the harshness and "edge" often associated with digital sound. With audio-only sources, the determination of TOSLink, digital coaxial or RCA cables is based on which device performs the superior conversion. Connecting a CD player to a receiver digitally means the player simply spins the disc and sends the data to the receiver. Consequently, if you have a quality receiver you can spend less on the audio source. However, if the opposite is true and the CD player has better DACs than the receiver, you're better off opting with RCA. With RCA, conversion takes place in the player.
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