The term "Digital interconnect" is used to describe electrical or optical interconnects which connect audio equipment. This can include coaxial cables with RCA connectors, coaxial cables with BNC connectors , shielded twisted-pair cable with XLR connectors and an adapter cable with different connectors on each end. Optical interconnects are typically the Toslink format, and can also include Toslink to 3.5mm Mini adapters.
Digital interconnects basically fall into three sub-categories:
- Balanced (or “differential”)
Balanced interconnects typically take the form of 110 Ohm shielded twisted-pair cable with XLR connectors on both ends. Cables of this type carry relatively high level differential AES/EBU professional format signals with a voltage of 2-7 Volts p-p. Connections of this type are designed for cable lengths up to 100 meters and offer excellent noise rejection. Signal transformers in both the transmitter and receiver allow simple "wired" adapters to be used for connection to most S-PDIF equipment with short cables, and in unusual situations where ground-loops are an issue, to disconnection of shield “ground” connections to eliminate ground loops.
There are two types of unbalanced interconnects commonly used in digital audio:
- RCA cables
- BNC cables
(1) "RCA" cables are most commonly used in consumer digital audio systems and are constructed with 75 Ohm coaxial cable. Most connections of this type carry relatively low- level “S-PDIF” signals with a signal voltage of only 0.5 - 0.6 Volts p-p. The relatively low signal voltage in combination with the unbalanced connection limits this type of connection in length and noise immunity. It is recommended to keep cable length as short as is practically possible; 1-2 meters is good, 3-5 meters is OK, with a maximum of 10 meters. Cables longer than 1-2 meters should be a low capacitance type such as RG-59 cable used commonly for video connections or AES3 digital audio.
(2) "BNC" cables are most commonly used in professional digital audio systems and are constructed with 75 Ohm coaxial cable. AES3 digital audio connections of this type carry signals with a signal voltage of 1 -1.2 Volts p-p. Cables are typically RG-59 and can have a length up to 150 meters.
Another type of BNC interconnect is very similar and carries Word Clock signals, which are a one cycle per sample “square waves” typically “TTL” 5 volt level. In a similar manner; 75 Ohm BNC interconnects are also used for AES sync connections, which are electrically similar to AES3 digital audio signals.