Soundware Guide to Cables, Leads and Connectors

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Soundware Guide to Cables, Leads and Connectors
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Soundware Beginners Guide to...


...Cables, Leads and Connections.

What's the difference between balanced and unbalanced cable?

Types of Cable:

  • Unbalanced
  • Balanced
  • Shielded

Types of Connector:

  • XLR
  • Jacks
  • Phono
  • MIDI
  • USB
  • FireWire
  • Optical

Types of Cable:

Unbalanced

Traditional unbalanced audio cables are made up of four parts - a conductive core, an insulating layer known as a "dialectric", a shield made of foil or wire braid, and an outer insulating layer. The conductive parts of the wire are known as "lines" - so unbalanced cable uses two lines. The core is known as the "hot" line, since this is the part that transmits the current. The outer shield is the "earth" line.



The earth line shields the core by intercepting unwanted electromagnetic interference and running it to ground, so that it cannot reach the core and affect the audio signal. However, this system is not perfect, so an audio signal will pick up noise caused by electromagnetic interference from other power sources when it is passed through an unbalanced cable. The longer the cable, the more noise will be picked up, so when working with unbalanced cables it's best to keep the lengths as short as possible.

Balanced

Balanced cables work differently - a balanced cable has a twin core with "hot" and "cold" lines. Both the hot and cold lines carry the audio signal, but the voltage on the cold line is inverted, such that when the hot line carries a positive voltage, the cold line carries a negative one. The cold line always carries the exact opposite charge to the hot line - it is in effect "turned upside-down". This allows for a form of noise reduction called "common mode noise rejection".



Common mode noise rejection uses the idea of "phase inversion". This means that signals of exactly opposite voltages will cancel each other out. For example, a positive signal of +5V and a negative signal of -5V appearing together will create a total voltage of +5V - 5V = 0. The two opposing signals will cancel each other out, and neither will register. The two voltages are said to be "180 degrees out of phase".

So, if the cold line of the cable is always carrying the exact opposite of the hot line, the two lines are 180 degrees out of phase and if the two signals are combined and played they will make no sound.


The hot and cold lines are 180 degrees out of phase

But if electromagnetic interference enters the cable as the signal is passing through, it will enter both cores at once, and the noise will have exactly the same voltage in both lines - it will not be inverted.


When noise enters the cable, it is not out of phase.

So, in the cable we have the desired signal 180 degrees out of phase across the two lines, and the noise "in phase", as it were. If we combine the signals at this point and play them, we will not hear the original signal because the phase inversion cancels it out, but because the noise is in phase, we will hear it. What's the point in that? We can hear nothing but the noise the signal has accrued along the way.

This is the clever part. At the end of the cable, the cold line is inverted again. This turns it upside-down again and inverts the phase on both the original signal and the noise. So now the original signal, which was 180 degrees out of phase, is back in phase. But the noise, which was in phase, is now 180 degrees out of phase and will cancel itself out.


When the cold signal is inverted, this brings the signal back into phase but puts the noise out of phase.

In this way, noise picked up by the signal as it passes throught the lead is cancelled out. This makes balanced cables much better for long lengths, or where electrical interference is high.

Shielded

As described above, the shield is an earthed conductive layer wrapped around the core. This intercepts electrical interference and runs it to ground before it can reach the core. All unbalanced cables are shielded, and many balanced cables also have a shield as an extra layer of protection against interference.

Types of Connector:

XLR

XLR connectors are mainly used for balanced signals - in particular with microphones and mixing desks. XLR plugs have three pins - Pin 1 carries the earth (or shield), pin 2 carries the hot (positive) signal and pin 3 carries the cold (negative) signal.

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Jacks

Jack connectors come in two sizes - 1/4" and 1/8" (or "Minijack"). Both sizes are wired the same, and come in mono or stereo types. A mono jack is split into two sections - the tip (at the very end) and the sleeve (the rest of the plug). The tip is the hot (positive) line, and the sleeve is the cold line or the shield. Mono jacks are used for instrument cables like guitar or microphone leads.

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A stereo jack is split into three sections - a tip and sleeve like a mono jack, with a ring between the two. For this reason stereo jacks are also sometimes called TRS (tip, ring, sleeve) jacks. Stereo jacks can be used to carry balanced or stereo signals. In a stereo jack being used to carry a balanced signal, the tip is the hot line, the ring is the cold line and the sleeve is the shield. If the jack is used to carry a stereo signal, the tip is the left channel, the ring is the right channel and the sleeve is the shield. Stereo jacks can be used as speaker cables carrying balanced signals, or as headphone leads carrying stereo signals.

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Phono

Also known as RCA connectors, Phono connectors can be used for audio or video signals, and are used a lot in hifi and home cinema systems. The centre pin is the hot line, and the outer ring is the shield.

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MIDI

MIDI leads are used for communication between MIDI devices like keyboards and synthesizers. They use 5-pin DIN connectors - these are German standard audio connectors. The pins are numbered - 1 and 3 are the two outermost, 4 and 5 the next two in and 2 is the centre pin. only 2, 4 and 5 are wired specifically - pin 2 is the shield, pin 4 is the hot line and pin 5 the cold. Pins 1 and 3 have no specific use, but some MIDI equipment uses them for other functions.

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USB

USB cables are used to connect computers to peripheral devices like printers, external mixers and hard drives. USB cables have a different connector at each end - an "A" connector (the big one) connects to the computer, and a "B" connector (the small one) connects to the peripheral device.

The USB protocol started as a relatively slow connection method - USB 1.0 was only capable of transfer rates of 12Megabits per second (1.5 Megabytes) - not nearly fast enough for audio recording. This has given rise to the impression that USB is too slow for use with audio interfaces. However, USB 2.0 (the most recent protocol used by newer computers) is capable of 480Mbps (60 Megabytes). The vast majority of USB audio or MIDI interfaces use USB 2.0, and this provides a fast enough data transfer for low-latency recording.

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FireWire

Firewire is used in much the same way as USB - to connect computers to peripheral devices. It provides fast rates of data transfer (up to 800Mbps) for recording multiple tracks at once.

Firewire (also known as IEEE 1394) cable connectors come in several different types depending on the type of Firewire port - using 4, 6 or 12 pins.

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Optical

Optical (or TOSlink) cables transfer digital data using optical fibre and pulses of light from an LED. Mostly used to transfer data between digital audio devices like CD and Minidisc players, optical connections can have very high data transfer rates. It is limited, however, by the quality of the cabling - the fibre optic material can have a huge difference on the quality of data transfer, so it's important to use the best-quality leads you can when using optical devices.

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