4. Line Level Audio Leads
This is a term used to denote the standard strength of an audio signal transmitting analogue sound information between components such as CD and DVD players, hi-fi amplifiers, synths, effects processors, mixing desk line-inputs etc. Line level is defined as follows:
|Domestic/Consumer:||-10dBV||which equals a nominal voltage of 0.3162V RMS|
|Professional:||+4dBu||which equals a nominal voltage of 1.2280V RMS|
This difference need not concern us when choosing audio leads as the requirements are exactly the same in each case (although connector types are usually different see below). Be aware however, that feeding signals from consumer to pro gear may yield weak and possibly insufficient signal levels at the receiving end.
The currents involved are normally very small, as in audio equipment, for minimum distortion and best frequency response, you generally expect a low output impedance for the sender and a high one for the receiver. A rule of 10 is often applied in audio electronics, whereby for optimal fidelity, the input impedance of the receiving device is ten times the output impedance of the transmitting device.
Cabling is not super-critical for line levels, and you will often find that consumer gear comes supplied with suitable leads to get you up and running (usually a lapped screen). These are not likely to be of particularly great quality though, so you might want to consider an upgrade. Three different types of connector are commonly used to terminate cables carrying line level audio:
¼ Inch Jack Plug (or Phone Plug)
The jack plug is sometimes called a phone connector because it was used for many years by telephone operators to patch phone lines together. These 1/4-inch plugs have either a tip and sleeve, or a tip, ring and sleeve (TRS). A TRS connection is either used for balanced audio lines (see the section on mic leads), or for unbalanced stereo sound, or for insert points (where a single socket provides both an input and output from a piece of equipment, e.g. to insert a compressor into the signal path). The mono variety (tip and sleeve only) is very common on musical instruments and other devices such as effects pedals, mixing consoles, active speakers, and amplifiers. They also come in 3.5mm and 2.5mm diameters, although these smaller sizes are only commonly used for headphone sockets, or for audio transfer where panel space is limited.
RCA or Phono Plugs
RCA or Phono its the same thing. RCA sockets are found universally on hi-fi and home entertainment systems, as well as on a huge range of studio equipment. Youre more likely to find RCA connectors on devices which can be left wired in permanently, whereas jacks are a better choice for live gear where frequent connection and disconnection is likely. The RCA plug and socket mate very securely with good electrical contact. Jacks on the other hand sometimes need to be cleaned or at least wiggled in the socket to fix a noisy or intermittent connection.
Whilst these are usually associated with mic cables, they are of course also found carrying balanced line level signals (+4dBu) between professional audio devices. Unlike jack and the RCA plugs, XLR plugs come in two sexes male and female. These will interface with corresponding male and female sockets on the device. In this sense we can think of XLR cables as polarised. This ensures that input to input and output to output connections are unlikely if not impossible - foolproof! (For those in doubt, the male connector is the one with parts sticking out, the female has the holes )