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Digital coaxial cable

What is a digitized audio signal?

Many video and audio are offered digitally via DVD, Blu-ray disc or streaming services over the Internet, for example. A digital signal has some important advantages over an analog signal. It is much less sensitive to outside influences and suffers little from signal attenuation in cabling. If the signal level remains above a certain threshold, the digital video or audio signal is decoded flawlessly by the receiving device. For transmitting digital video signals with associated audio, HDMI is the best-known connection standard. For transmitting digital audio signals, S/PDIF [Philips Sony Digital Interface] is the most widely used connection standard in the consumer audio world. It can be used to transmit an uncompressed stereo audio signal or to transmit compressed 5.1/7.1 surround sound. This makes S/PDIF suitable for transferring a high-quality digital stereo signal from CD and DVD, but also suitable for transferring the surround sound audio signal from a movie to a home cinema set. This has made it a widely accepted and widely used digital audio connection.

Two types of digital audio connections

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S/PDIF supports two types of connections: the electrical connection and the optical connection. One advantage of the electrical connection is that it keeps equipment costs down. The digital signal is already available in a device and only needs to be buffered before being fed out. The electrical S/PDIF connection uses the tulip connection commonly used in the audio world. When using the electrical connection, it is important to note that a standard audio tulip cable is not adequate for signal transmission. With a normal audio signal, the frequency range of the signal runs from about 20 Hz [Hertz] to about 20 KHz [kilohertz]. Such a signal can be sent via a properly shielded cord without audible degradation of the signal. The digital S/PDIF audio signal has a bit rate of several megabits per second [Mb/s]. Transporting this signal requires a much higher bandwidth than an analog audio signal.

What is a digital coaxial cable?

Because a digitized audio signal has a bit rate of several Mb/s, it requires cabling with sufficient bandwidth so that the digital signal arrives at the receiver without much distortion. In theory, the bandwidth of the cabling should be at least twice the bit rate per second in Hz. In practice, at least three times is used for this, but the higher the bandwidth of the cabling, the easier the digital signal is to be recognized by the receiver. A standard audio cable has insufficient bandwidth, which is why a coaxial cable is used for S/PDIF. Standard commercial coaxial cable has a bandwidth of a few hundred megahertz, making it an excellent transport medium for an electrical signal. Digital signals with a frequency of a few megahertz behave like high-frequency signals, creating a fixed ratio of voltage to current. The ratio is called impedance and depends on the construction of the cabling. To transport an electrical S/PDIF signal, coaxial cable with an impedance of 75 ohms is used. The tulip plug, well known in the audio world, is chosen as the connection.

The right cable

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When ordering a digital coaxial cable, it is important to know that you are ordering a true coaxial cable and not a shielded analog audio cable. A digital coaxial cable can be recognized by its cable thickness. It is about twice as thick as a standard tulip audio cable. Because the digital audio signal contains both stereo channels or, in the case of surround sound, all surround channels, you also only need one digital coaxial cable to connect a digital audio signal. This is in contrast to the standard stereo signal where both channels have their own connection. To ensure that the digital signal remains neatly enclosed in the cabling so that it does not cause interference in other cables or equipment, a digital coaxial cable is often chosen to use a cable with double shielding. Here, the first shield often consists of aluminum foil and the second shield consists of a braided copper screen. For signal transmission quality, it is important that you do not cut, shorten or lengthen the cable yourself. Therefore, first measure what length you need and then order a cable of an appropriate length.