The word “interface” has become very fashionable in recent years. The Spanish Royal Academy's dictionary defines the Spanish term interfaz as a “physical and functional connection between two independent devices or systems.” That definition can apply to a great many pieces of equipment, so it's no surprise to find many different kinds of interfaces: network access, graphics, voice.
In our case, audio and MIDI interfaces are devices that let us route audio signals or MIDI data to and from a computer. Although there are a great many of these interfaces on the market today, choosing the one that fits our needs doesn't have to be complicated. Below we go over some of the concepts you'll commonly run into on interface spec sheets. Understanding them can be a big help when deciding between one unit and another.
Key Concepts
Audio Inputs
The number of independent inputs available can limit our recordings. We need to decide how many we need and exactly what type. In general, current interfaces tend to offer three types of input:
- Mic input: for connecting microphones. If we want to use condenser mics, the input needs to provide phantom power, generally 48V.
- Line input: this is where line-level signals are connected, generally coming from external gear such as a guitar amp, a mixing console or a mic preamp.
- Instrument input: for plugging in instruments such as electric guitars or basses directly. These are also known as high-impedance or Hi-Z inputs.
Audio Outputs
The number and type of outputs should also be taken into account:
- Line outputs for connecting to other gear such as mixing consoles.
- Monitor outputs for connecting to an amplifier or powered monitors.
- Outputs for headphones, auxiliaries, sends...
Balanced and Unbalanced Lines
This concept applies to both inputs and outputs. Without getting into the details, balanced lines are more resistant to external interference than unbalanced ones because they carry one more conductor for the signal:
Balanced lines: Hot, Cold and Ground.
Unbalanced lines: Hot and Ground.
Using balanced inputs and outputs together, connected with balanced cables, is advisable for weak signals (such as the one generated by a mic) or for long cable runs.
Connectors
The most common connectors in audio are:
- XLR: also known as “Cannon” or “mic” connectors, they are typically used for balanced audio connections. You'll also find them on AES/EBU digital connections.
- TRS: short for Tip Ring Sleeve, these connectors are better known as “phone plugs” or “stereo phone plugs.” They are used for both balanced and unbalanced connections (in the latter case, TS, or Tip Sleeve, connectors, also called “mono phone plugs,” can be used as well).
- RCA: the audio connector on consumer equipment, also used for S/PDIF digital audio inputs and outputs.
- BNC: you'll find these on AES/EBU connections and Word Clock sync signals.
- Toslink: connector for optical lines. It is used for ADAT-type connections, although it's also used for S/PDIF.
With MIDI, there is generally only one type of connector, the 5-pin DIN.
Digital Connections
Another way to get inputs and outputs on our interface is through digital audio coming from other gear. The most common connections of this kind are:
- S/PDIF: this connection carries two channels of audio (one stereo channel), generally at 44.1 kHz and 48 kHz. It usually uses RCA or Toslink connectors.
- AES/EBU: also known as AES3, it is the professional version of S/PDIF. It usually uses XLR connectors, although BNC is used for long runs.
- ADAT: used in Alesis's popular recorder, this connection carries 8 channels of audio at 16 bits and 44.1/48 kHz. The usual connectors are Toslink.
Preamps
The signal from inputs such as the mic input is very weak, so it must be amplified before it can be processed further. Preamps take care of this. These devices have a major influence on the quality and “color” of the unit's sound, which is why it's not unusual to find devices dedicated exclusively to this task that can cost several thousand euros.
A/D and D/A Converters
A/D (analog-to-digital) converters sample the analog signal to turn it into a digital one. The number of samples per second is determined by the sample rate, and the possible values those samples can take are determined by the number of bits per sample.
The higher the sample rate, the more faithful the digital signal is to the original, but we'll also need more bandwidth to transmit it to the computer and more hard drive space to store it.
The number of bits per sample, or resolution, affects the signal's dynamic range. The higher the resolution, the greater the dynamic range but, again, the more bandwidth and hard drive space required.
Although an A/D converter essentially takes certain information and discards the rest, converter quality can have a big effect on the resulting signal. As a result, it's also common to find units dedicated solely to A/D or D/A (digital-to-analog) conversion that cost several thousand euros...
Synchronization
When using devices that are connected to one another via digital audio (ADAT, S/PDIF, AES/EBU...), they need to be synchronized. This can be done in several ways, one of which is to use a word clock, a device that emits a signal of regular pulses so the rest of the gear can sync to it. Sometimes the signal itself is also called word clock.
Host Connection
The buses most commonly used today to connect interfaces to a computer are USB, FireWire and PCI. The first two are more convenient and allow “hot plugging,” that is, connecting without having to shut the computer down. PCI connections don't offer that convenience, but they enjoy greater bandwidth.
- USB (Universal Serial Bus): These connections are the most widespread. Version 1.1 has a theoretical transfer rate of 12 Mbps (megabits per second), and the newer version 2.0 reaches 480 Mbps.
- FireWire: this connection was designed for professional audio and video applications, with speeds of 400 Mbps (IEEE 1394 standard) and 800 Mbps (IEEE 1394b standard). It isn't as widespread as USB yet, but it has been gaining ground in recent years.
- PCI: this is the fastest way to connect devices, with speeds of 1056 Mbps on 32-bit, 33 MHz PCI slots and up to 4224 Mbps on 64-bit, 66 MHz PCI slots.
Audio Interfaces
Now that we know the characteristics that typically define an interface, let's take a look at some of the options on the market today.
Up to Four Channels
Among audio interfaces with up to four channels, the standouts are Digidesign's Mbox2, Mackie's Spike system and Steinberg's Cubase System.
All three systems use USB 1.1. The first two offer two mic/line/instrument channels with 48V phantom power (24-bit/48 kHz on the Mbox2 and 24-bit/96 kHz on Spike's XD-2), one stereo S/PDIF I/O channel and MIDI I/O ports. The MI|4 interface in the Cubase System also has S/PDIF I/O and MIDI I/O, but it offers four audio inputs (two with preamps and 48V phantom), at resolutions up to 24-bit/96 kHz.
The Mbox2 comes with Pro Tools LE 7 and a very complete software bundle (for more details, there is an article on the Mbox2 in this same issue of ISP). The Spike system includes Tracktion recording software and an integrated chip for dynamics and EQ processing on the input channels. The Cubase System comes with the powerful Cubase SL.
Along the same lines we find Edirol's UA-25, Alesis's IO|2 and M-Audio's Audiophile USB interfaces.
More Than Four Channels
Once we go past four channels, we generally move to FireWire or the still somewhat recent USB 2.0.
Supplied with the same software as the Mbox2, the Digi002 has eight audio inputs (four with preamps and 48V phantom power), eight balanced line outputs, ADAT I/O, S/PDIF I/O, one MIDI in port and two MIDI out ports. 24-bit/96 kHz A/D and D/A converters and an integrated control surface round out this FireWire interface, which is also available in a version without the control surface called the Digi002 Rack.
The Edirol UA1000 was one of the first audio systems to use USB 2.0. It allows simultaneous recording and playback of up to ten channels at 24-bit/96 kHz and has four mic preamps with phantom power, S/PDIF I/O, ADAT I/O, MIDI I/O ports, Word Clock I/O and 4 sends.
Mackie offers the Onyx 400F in the FireWire 400 interface range. Eight input channels, four of them with preamps and 48V phantom power, 24-bit/192 kHz A/D and D/A converters, 8 balanced line outputs, Word Clock I/O and S/PDIF I/O are some of this unit's features.
The Motu 896HD FireWire system has 8 mic/line/instrument input channels with preamps and 48V phantom power capable of working at 24-bit/192 kHz, 8 balanced outputs, ADAT I/O, AES/EBU I/O and Word Clock I/O, and it can be programmed to work without a computer as a mixer in live situations. In addition, several units can be linked together to increase the number of I/O.
MIDI Interfaces
As we've seen, many audio interfaces also include MIDI ports. In any case, if we need a dedicated MIDI interface, the choice probably won't be as difficult. MIDI interfaces don't need mic preamps or A/D and D/A converters (remember that a MIDI signal is data organized in digital frames), so what we really need to look at is the number of In, Out and Thru ports we need and how we're going to connect them to the computer.
Recapping
OK, so how do I choose the interface that's right for me?
We can start by looking at the number and type of inputs we're going to need. To record a drum kit, we'll need several mic inputs with preamps, or several line inputs plus external preamps.
If we want to record the signals coming from a mixing console, we're probably looking for an interface with multiple line inputs; and if we also want to use the console for mixing, we'll need as many line outputs as channels used, to send the audio from the computer back to the console.
We shouldn't forget about digital audio options. Our interface may not have many mic inputs, but if it has an ADAT input, we can use mic preamps with ADAT output, which give us eight extra channels.
The next step can be to define the outputs we need: line, headphone, monitor, sends, and so on. If we already have other gear, a look at the types of ports and connections it has can give us ideas about the kinds of inputs and outputs to look for on the new interface.
The host connection is important, but it shouldn't limit our decision. USB 1.1 is more than enough for interfaces with only a few input channels, so we shouldn't rule these interfaces out on the assumption that the newer USB 2.0 ones are necessarily better. Nor should we worry if the interface that suits us best is FireWire or USB 2.0 and our computer doesn't have those kinds of ports. There are PCI cards that will provide FireWire and USB connectivity.
In general, a realistic assessment of our present and future needs is the ideal way to define the features to look for in an interface, so that we not only cover today's needs but also allow for possible expansion of the setup down the road.
