Audio Interface

Chapter 3. In the Digital Domain

3.2. Audio Interface

Now that we have converted our signal into discrete (separate) bits of binary information, we are ready to start playing with it, right? Right – but where does this conversion physically happen? Where is the AD converter? That is where the audio interface comes in (if you are using outboard AD converters, you should not be reading this book :-)).

What is an audio interface?

The audio interface is simply a bundle of audio components, most of which we have already discussed. However, because they are so pervasive in home and project studios, and because their accessibility (both in terms of price and required technical expertise) is a large part of what has made audio more widespread, audio interfaces deserve a section of their own.

Figure14 Audio interface

Figure 14 Simple Audio Interface Internal Logic A microphone (M) picks up incoming sound. The cable connecting the microphone to the audio interface transports the analog signal into a preamplifier (Pre). The signal is converted into a digital signal by the AD converter, then comes out of the audio interface and is sent to the DAW for processing. Finally, it goes back into the interface into the DA converter before being sent out of the interface to a speaker (S). Note how the audio interface carries its name well: it bridges the analog (dark) and digital (light) domains.

An audio interface can either take the form of an audio PCI-type card to insert into your favorite computer, or of a separate hardware unit connected to a computer via USB, Firewire or some other means. In its simplest form, the audio interface is an AD converter and a DA converter. In higher-end models, it can also contain components such as mic preamps, EQ filters, compressors, MIDI input and output connectors, etc.

If we look at the figure above, we have a single microphone connected to the audio interface (white box), usually via an XLR connection. Within that audio interface, a microphone pre-amplifier adds gain to the signal so that it can be properly processed by the AD converter. Out of the AD converter go the bits corresponding to the analog signal captured by the microphone, and into the output of the audio interface (the DAW), usually via a USB or Firewire connection. Once the signal is stored, it can be played back by reconstructing the analog information from the digital information in the DA converter, and then out of the audio interface into the speakers, usually via an XLR or jack connection.

I want to buy an audio interface, help me!

As you can see in the figure above, the audio interface is the central device in this example. It acts as the go-between between the analog domain (dark grey) and digital domain (white). You will find in the table below some famous audio interfaces for home or project studios. The pros usually use larger units such as consoles.

Vendor Model Price Conn. A/D PP MIDI Input Output
Apogee Duet $600 USB 24/192 Yes Via USB 2xC 2x¼” (SPK)

1x¼” (HDP)

Avid FastTrack Duo $140 USB 24/48 Yes No 2xC

2x¼”

2x¼” (SPK)

1x¼” (HDP)

Focusrite Scarlett 2i2 $150 USB 24/96 Yes No 2xC 2x¼” (SPK)

1x¼” (HDP)

Lexicon Alpha $50 USB 24/48 No No 1x¼” (INS)

2x¼”

1xXLR (MIC)

2xRCA

2x¼” (SPK)

1x¼” (HDP)

M-Audio M-Track Plus II $150 USB 24/96 Yes No 2xC

 

2x¼” (SPK)

1x¼” (HDP)

Mackie Onyx Blackbird $500 FW 24/96 Yes No 2xC (INS)

6xC (MIC)

2x¼”

8xADAT

6x¼” (SPK)

2x¼” (HDP)

8xADAT

MOTU Ultralite MK3 $550 USB

FW

24/192 Yes In, Out 2xCombo

6x¼”

1xS/PDIF

8x¼” (SPK)

2x¼” (HDP)

1xS/PDIF

Presonus FireStudio Project $400 FW 24/96 Yes In, Out 2xC (INS)

6xC (MIC)

2x¼”

1xS/PDIF

12x¼” (SPK)

1x¼” (HDP)

1xS/PDIF

Roland Quad Capture $250 USB 24/192 Yes In, Out 2xC

1xS/PDIF

2x¼” (SPK)

1x¼” (HDP)

1xS/PDIF

Steinberg UR 22 $150 USB 24/192 Yes In, Out 2xC (1 INS) 2x¼” (SPK)

1x¼” (HDP)

Table 9 Audio Interfaces

In the table above, famous audio interfaces are listed. The fourth column (“Conn.”) indicates with which type of cable the interface can be connected to a computer (note that the Apogee Duet is Apple only); FW stands for Firewire, the name given by Apple to the IEEE1394 norm. The “A/D” column specifies the characteristics of the A/D converter in terms of its bit depth (in bits) and sampling rate (in kHz), in that order; see section 3.3 for details. The “PP” column indicates if the interface provides phantom power or not. The “Input” and “Output” columns list the inputs and outputs for the interface: INS = Instrument input (high impedance), MIC = Microphone input (connected to preamp); ¼” = ¼ inch TRS, SPK = Speakers output, HDP = Headphones output, C = Combo connector, allowing both ¼” TRS and XLR

For each microphone (XLR or combo) input connector there is a corresponding microphone preamp in the unit. The quality of the preamps is one of the factors influencing the price of the unit, but not the only one: other factors include, in no order, the number of connectors, the quality of the AD and DA converters, the presence of digital meters and the inclusion of a MIDI input and/or output connector.

 

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