Feeding Audio to your Computer

By Bruce Bartlett 2003

You have a computer with software installed for recording, editing and mixing. So you need a way to get audio signals into and out of your computer. An audio interface does the job.

The simplest form of interface is a sound card, which plugs into a computer user slot. A sound card has a single unbalanced mic input (mono or stereo), line input and line output, available as 1/8” phone jacks. The card’s sound quality and connectors are good but not up to professional standards.

The next step up is an analog sound card with TRS (Tip-Ring-Sleeve) connectors (Fig. 1) or XLR connectors on cables. It offers balanced inputs, high-quality mic preamps and 2 to 8 inputs. A digital-only sound card is a better choice if you work only with digital signals in these formats: S/PDIF, TASCAM TDIF or Alesis ADAT Lightpipe.

Fig. 1. Digital Audio Labs CardDeluxe (image from their web site)

A more convenient setup is an outboard audio interface (Fig.2), in which all the connectors are in a common chassis. Since the mic preamps are outside of the computer, they pick up less computer clock noise than analog sound cards do.

Fig. 2. M-Audio FireWire 410(image from their web site)

An outboard audio interface accepts analog mic and line signals, and converts them to PCI, USB or FireWire format to feed a DAW (a computer with recording software). Some interfaces are also controllers: they have faders, knobs and switches that adjust the DAW’s virtual controls that you see on screen. Every outboard interface includes input and output connectors, and most offer level controls and meters. If the interface does not have level meters, you must use the meters in your recording software.

Some popular outboard audio interfaces are made by Aardvark, Digidesign, M-Audio, Metric Halo, PreSonus, Tascam and MOTU (Fig. 3). Typical prices (in 2003) are $500 to $2495.

Fig. 3. MOTU Audio 896HD (image from their web site)

When deciding which interface to purchase, here are some features to look for:

Analog inputs

First decide how many inputs you need. If you are recording only yourself, or are recording only in stereo, 2 mic inputs might be enough. Four-to-eight inputs cost more but let you record with more microphones. Also decide what type of input connectors you want. XLR connectors mate directly with the XLR connectors on mic cables. TRS connectors cost less but require an XLR-to-TRS adapter cable. (A TRS connector is a ¼” phone jack with tip, ring and sleeve terminals for balanced signals).

Analog outputs

These outputs can feed an analog mixer that is used for monitor mixing. Or they can feed the analog inputs of a multitrack recorder. In this case, you use the interface as a multichannel mic preamp. Choose the number and type of connectors that mate with your mixer or recorder.

Computer interface format

You have a choice of PCI, USB or Firewire. Interfaces with the PCI format plug into a PCI user slot in your computer. Interfaces with USB or FireWire formats are much more convenient: they plug into a USB or FireWire port in the back of your computer, so you don’t need to open up your computer to use the interface. Also, they are compatible with Mac or PC. Running at 400 Mbps, FireWire is faster than USB, so FireWire allows more simultaneous tracks of audio.

CardBus FireWire adapters usually work better than built-in FireWire ports on older computers. The latest versions of software drivers and Windows provide the best compatibility with FireWire interfaces.

Digital I/O

Your format choices here are S/PDIF, AES/EBU, ADAT Lightpipe and TASCAM TDIF. S/PDIF (coaxial or optical) and AES/EBU connectors work with digital devices such as DAT recorders. ADAT Lightpipe or ADAT with S/MUX support sends signals to and from an ADAT multitrack recorder. The Sonorous S/MUX protocol allows sample rates of 88.2 or 96 kHz to be carried by ADAT optical Lightpipe interconnects. It uses two optical connectors per input and output to get 8 channels of I/O at those high sample rates. ADAT Sync makes the interface digital signals synchronize with ADAT machines. TASCAM TDIF connectors work with TASCAM DA-38, DA-88 and DA-78 modular digital multitracks.

Sampling rates

Available rates are 32 to 192 kHz. Recordings made with higher sampling rates give better sound quality but consume more hard-drive space. CD-quality audio has a 44.1 kHz sampling rate.

MIDI ports

Some interfaces have MIDI ports so that you can send MIDI signals from a keyboard to your computer sequencing software. If your interface has a MIDI port, you don’t need a separate MIDI card in your computer.

Word Clock I/O

Some interfaces offer Word Clock connectors, which control the timing of digital signals. To reduce clock jitter, it’s best to use a single Word Clock to drive all the digital devices in your studio with a common clock signal.

Driver support

An audio driver is a program that allows recording programs to transfer audio to and from an audio interface. Interfaces are usually sold with several drivers in various formats. Be sure that your interface has drivers that match those of your software.

The most popular audio driver formats are ASIO, DAE, Direct I/O, GSIF, MAS, SoundManager, Wave, and WDM.

Here’ a brief description of each driver format:

ASIO — (Mac, PC): Audio Streaming Input and Output. A very popular driver developed by Steinberg. Allows multiple channels of simultaneous input and output, and low latency with software synthesizers.

DAE — (Mac, PC): Used only with Digidesign audio interfaces. It’s a multi-channel driver and audio engine application. DAE runs with a compatible host such as Pro Tools, Logic, and Digital Performer. Using DAE lets you use RTAS and/or TDM plug-ins in a host application.

Direct I/O — (Mac, PC): Works with Digidesign interfaces as a multi-channel driver only. Does not let you run RTAS or TDM effects.

GSIF — (PC): Permits very low latency when playing samples from hard disk with TASCAM’s GigaStudio software sampler. Latency is the signal delay through the driver and interface to the monitor output. This can be a problem in overdubbing, in which the monitored signal in the insturment you’re overdubbing is heard later than the pre-recorded tracks. Some drivers have less latency than others.

MAS — (Mac): Developed by Mark ofOffers resolutions up to 24/96 and multiple simultaneous input and output channels. It’s also a plug-in format for realtime audio effects.

SoundManager — (Mac): Macintosh’s standard audio driver. It lets you record and play mono and stereo files up to 16-bit and 44.1 kHz. Has a moderate amount of latency.

Wave — (PC): The PC standard audio driver. Wave can be used with a variety of audio interfaces (likeSoundBlaster-type sound cards) to record and play mono or stereo audio. Has a moderate amount of latency.

WDM — (PC): A multi-channel driver. Allows low latency with WDM-compatible audio hardware and DXi software instruments. Also, DirectX audio effects can be used live on input signals, not just during playback. This lets you monitor and record effects in realtime while you play.

If you have multiple drivers installed, they may conflict. Then your computer might crash or the recording software might not access the audio interface.

Other options

Listed below are several features that some interfaces offer, along with a brief description of each.

  • ProTools compatible: the interface works with ProTools recording software and hardware.
  • Built-in control surface (Fig. 5): Has faders, switches and knobs that adjust the “virtual controls” you see on screen when using a recording program. Easier to use than a mouse, but adds cost.

Fig. 5. TASCAM FW-1884 (image from their web site)

  • Motorized faders: The faders in the control surface are motorized, so they move like the virtual faders on screen.
  • Standalone mixer mode: You can use the control surface as a regular mixer.
  • Footswitch jack: Accepts a footswitch for punch-in/out. The footswitch works only if your recording software supports this function.
  • Surround sound: Provides 5.1 or 7.1 surround sound monitoring.
  • Powered by FireWire bus: The FireWire cable powers the interface; you don’t need another power supply. This is convenient, and allows on-location recording with a laptop.
  • Battery powering: This makes the interface portable for on-location recording.
  • 192 kHz playback: Playback can be at a very high sampling rate of 192 kHz for state-of-the-art sound quality.
  • Aux send/return: Allows you to connect an external analog signal processor.
  • Supplied recording software: The interface is packaged with recording software, so you might not need to buy other software.
  • A/D/A converter mode: The interface can act as a realtime analog-to-digital and digital-to-analog converter.
  • SMPTE sync: The interface will synchronize with a SMPTE time code signal.
  • Insert jacks: An insert jack in series with each channel’s signal allows you to plug in an analog compressor or multitrack recorder.
  • MIDI routing and merging: Allows control of MIDI signals.

As we’ve seen, there are a wide range of features and connectors among different interface models. A good interface is well worth the price because of its top-quality sound, convenient connections and easy control of audio signals.

Recording engineer Bruce Bartlett is the author of Practical Recording Techniques 3rd Ed. published by Focal Press.