This article is a tutorial-style guide to setting up your system correctly, and eliminating any problems to get the best results from your studio. The two most popular home setups are likely to be similar arrangements to the following:
Fig. 1: Setup 1
Fig.1 shows vocals or instruments being processed through a Focusrite channel strip of some kind and being fed to a desk, which is possibly receiving signals from a keyboard or other devices. Additional Focusrite units may be in use, as inserts, with the desk. The desk then sends to monitors and a recording medium of some kind, most likely a hard disk- or analogue/digital tape-based format. The desk and recording medium may be substituted by a digital multitrack in some cases.
Fig.2 is probably the most common configuration, and illustrates a computer-based setup running Pro Tools/Logic or another audio platform. The vocals or instruments are being processed through the Focusrite unit (and possibly being converted by the A/D card) and then going to an interface (Mbox/Digi 002) connecting to a computer. The interface(s) (Audio and MIDI) may be a soundcard within a computer as opposed to an external device. Additional Focusrite units may be in use, as inserts, with the interface. The computer might be sending MIDI information to other equipment (through the same or a separate interface) which is then also feeding the interface. Depending on the front-end Focusrite unit in use, audio may be returning from the interface to the Focusrite device in order to make use of Zero Latency Monitoring.
Fig. 2: Setup 2
The main consideration with either setup is whether audio is to be transmitted in analogue or digital format.
ANALOGUE: If analogue is chosen, then the obvious problems are to do with signal degradation, interference and noise.
Where does noise and interference come from in an analogue system?
Providing the components and circuitry within a studio's equipment produce no unwanted sounds or distortion, the problematic noises on an audio signal are normally caused by inadequate wiring. The main reason for this is that wires are essentially antennas, and are therefore likely to pick up external fields caused by RF sources, electrostatic events or magnetic fields. This effect is reduced by shielding cables with braiding or foil. One of the most irritating interfering currents in larger studios (with long cable runs) comes from the mains supply. These power lines radiate a lot, but fortunately the 50Hz pickup is minimal with small cables, and so shouldn't cause significant problems in most home studios. However, a common annoyance in a variety of audio applications is caused by the use of differing ground sources, which create hum. The cable's shielding that serves to protect against high frequency interference can compound this hum.
All systems need to have a ground reference (0V), which is used to compare with the input signal to amplify and process the difference. This reference, generally connected to the chassis or grounding pin of the mains plug, will have all voltages inside the unit measured against it. Problems occur when two pieces of equipment are linked because their references, that are also joined, must then agree. With the cable shield often picking up hum and/or carrying current that results in a voltage drop (unavoidable if it's part of the signal path - as with unbalanced interconnects), there can be a difference in value between the two references. An input circuit may interpret this variation as the signal and amplify it. You may think this would be insignificant, but although the voltage may be small, the impedance is tiny and this, combined with Ohm's law, can make the current high. The hum that results, despite being at 50Hz, is not easily filtered due to the upper harmonics (100Hz, 150Hz...) generated.
More complex studio arrangements (especially post-production houses) can develop problems as a result of multiple ground paths combining to produce a loop antenna, which picks up 50Hz noise. These "ground loops" can be difficult to solve, as the internal structure of equipment is not always easily worked out.
Balanced cabling was developed in professional audio applications to be immune to these unavoidable occurrences. Probably the most common source of confusion is on the subject of connecting balanced and unbalanced equipment. Many users seem unsure of compatibility and, if compatible, the method of wiring two devices together. So here's a rough guide:
What do the terms 'Balanced' and 'Unbalanced' mean?
The terms 'Balanced' and 'Unbalanced' (or 'single-ended') are used to describe the kind of electrical interface (and hence cabling required) between devices. The balanced circuit has its electrical midpoint grounded whereas one side is grounded in unbalanced. Unbalanced cables only have two contacts and the 'common' conductor (the shield) is used as a return path for the signal. The chassis/ground references of two pieces of equipment are likely to have different voltages (see above), causing a ground noise current to be carried by the shield and hence be added to the audio signal. This produces undesirable noise or hum. (This isn't generally much of a problem in home studio arrangements with shorter cable runs and less equipment/power lines...)
A balanced cable, however, uses two dedicated conductors for the forward (+) and return (-) paths of the signal. The voltages of the two conductors, at any point in the balanced cable, are equal in amplitude and opposite in phase (180 deg out of phase). Any external interference, causing identical noise/distortion in the two signal carrying conductors, will be ignored at the balanced input stage by a differential amplifier, which only processes the difference between signals. With a 'twisted pair' cable, the two conductors are intertwined and wrapped with the shield to further prevent distortion from RF sources, electrostatic discharge or magnetic fields. The two chassis are then linked together using the shield, which may be an antenna for attracting hum but is not a signal-carrying conductor. So, providing the internal grounding of each bit of equipment is properly designed, no undesirable noise is created.
Fig. 3: Balanced cable
Fig. 4: Unbalanced cable
Ideally, the perfect system should have balanced interconnects between all devices, with cable shields tied to the metal chassis on entry at both ends of the cable. This will guarantee hum-free results and the best possible protection from radio frequency interference and noise.
Can Balanced and Unbalanced equipment be connected?
Yes, but it isn't ideal and must be done carefully as "blindly" connecting unbalanced with fully balanced can lead to audible problems. As most Focusrite outputs are balanced, on XLR or TRS jack, the following guidelines relate to these instances (balanced output to unbalanced input).
The main issues regarding the correct cabling procedures are to do with the nature of the balanced output; whether it's transformer or electronic. With an electronic output, the connecting technique is less important but a transformer output will almost certainly require some cable modification to work. Sending the forward signal (+/live) from pin 2 of the XLR, or the tip of the TRS jack, to the tip of the phono plug or mono jack is always necessary. Connecting the return signal (-) from pin 3 of the XLR, or the ring of the TRS jack, to the shield of the phono plug or mono jack, will give a signal if the output is electronically balanced (Fig 5). However, this signal will be reduced by 6dB. If it's a transformer output there'll be no signal at all.
Fig. 5: Balanced (not transformer) to unbalanced
Fig. 6: Balanced (transformer) to unbalanced
As most outputs are cross-coupled electronic or transformer, the best configuration is Fig. 6. Connecting the shield of the TRS jack, or pin 1 of the XLR, to either the ring of the jack or pin 3 of the XLR (as shown) will result in a signal from the transformer output that is reduced by 6dB, and no loss of level from the electronic cross-coupled output.
All TRS Jack balanced outputs of Focusrite gear (which are electronically balanced and cross-coupled) can be connected to an unbalanced jack/phono input of a desk or interface, using a standard mono jack to mono jack/phono cable. This may cause the unbalanced input to be overloaded as the balanced signal output from the Focusrite will be at line level. To overcome this, either :
ï reduce the output level
ï disconnect pin 1 from pin 3
ï use a -10 unbalanced jack output where available
If connecting an unbalanced output to a balanced input, the connections are more straightforward. Simply join the tip of the phono plug or mono jack to pin 2 of the XLR, or the tip of the TRS jack, and then the shield of the unbalanced connector to pin 3 or the ring, as follows:
Fig. 7: Unbalanced to balanced
If the input happens to be transformer balanced, then either the ring of the TRS jack or pin 3 of the XLR should be connected to the shield of the cable.