Setting Up an Audio Interface

By default, Soundtrack Pro uses one of the built-in audio interfaces included with your Macintosh computer, such as a stereo headphone jack or a stereo speaker output. If you need to capture audio from a professional device, you may need a separate third-party interface. You can connect a third-party audio interface to your computer, and then connect microphones and musical instruments to the audio interface for recording. You can also connect output devices, such as monitors or speakers, a mixer, or an amplifier to the third-party audio interface.

Built-in Audio Interfaces

The following audio interfaces are included with your Macintosh computer (though some computer models may not include all interfaces).

Built-in Analog Audio

You can capture and output audio using the built-in audio ports on your computer. The built-in audio interface on your computer usually uses a stereo miniplug connector. If necessary, you can buy an adapter to connect the miniplug connector to two stereo RCA or 1/4" tip-ring-sleeve (TRS) connectors, which can then be connected to your video or audio device.

Figure. Illustration showing a computer connected via stereo miniplug and RCA connector to an analog audio or video device.

For basic mixing, you can connect the built-in audio output on your computer to a pair of external speakers. This gives you two output channels, which can be configured for dual mono or stereo playback.

Built-in S/PDIF Digital Audio

Some computers have built-in S/PDIF optical digital ports (sometimes called TOSLINK) that you can connect to some CD players, DAT recorders, and other digital audio devices.

Figure. Illustration showing a computer connected via TOSLINK to an audio or video device.

DV FireWire

If your post-production project uses a DV codec, you can output audio via the FireWire port on your computer. This allows you to use a DV deck, DV camcorder, or DVtoanalog converter as an audio interface. In this case, you connect your FireWire cable to your DV device and then connect the audio outputs of the DV device to external speakers or a television monitor.

Choosing a Third-Party Audio Interface

Before you purchase a separate audio interface, consider that most third-party video interfaces may have enough audio connectors to meet your requirements. You should consider a separate audio interface when:

  • You are trying to capture or output more audio channels than your built-in audio interface or third-party video interface supports.

  • You need to capture or output in a format not supported by your existing interfaces (for example, if you need to capture audio at a sample rate of 96 kHz but your video interface only supports a sample rate of 48 kHz).

Benefits of a Third-Party Audio Interface

Third-party audio interfaces can provide many more features than your computer’s built-in interfaces, such as:

  • More than two audio channels—eight channels is common, but some interfaces have 24 or more input and output channels

  • Professional connectors such as XLR or 1/4" TRS

  • High-quality analog-to-digital and digital-to-analog converters supporting sample rates as high as 192 kHz and 24 bits per sample

  • Support for analog and digital audio formats

  • Stable, “jitter-free” digital audio clocks

Tips for Selecting a Third-Party Audio Interface

When you select an audio interface, make sure it has the following:

  • Connectors that match your audio equipment, such as XLR, 1/4" TRS, RCA, or TOSLINK

  • Support for audio signal formats that your audio equipment uses, such as AES/EBU, S/PDIF, or ADAT Lightpipe

  • Enough audio inputs and outputs to connect your equipment

  • Sample rate and bit depth at least as high as your audio equipment. For example, if you have an audio device with a sample rate of 96 kHz and 24 bits, your audio interface should at least match this. Soundtrack Pro supports input from digital audio interfaces up to a maximum sample rate of 96 kHz and a maximum bit depth of 24 bits. If you connect an interface that uses sample rates or bit depths outside the range supported by Soundtrack Pro, an alert message appears, telling you that data from the audio interface is not compatible with the application.

  • Mac OS X compatibility. When choosing an audio interface, check the manufacturer’s specifications to make sure the interface is compatible with Mac OS X.

  • Up-to-date driver software, if needed. If the device requires a driver, make sure an up-to-date driver is included with the device, or is available from the manufacturer.

Important: If you are considering purchasing an interface, make sure it supports Mac OS X Core Audio. Soundtrack Pro supports any audio interface that is compatible with Mac OS X Core Audio.

All digital audio interfaces can be susceptible to latency, a noticeable delay between the time the audio signal is produced and the time you hear it. When connecting an audio interface, you should connect the interface directly to the computer, rather than connecting it through a hub or daisy-chaining it through another device. Connecting an audio interface through a hub or an intermediary device can cause an unacceptable amount of latency, particularly with slower protocols such as USB.

FireWire for Digital Audio

FireWire is a professional and consumer standard for both audio and video equipment. The combination of fast data-transfer rates, high storage capacities, and plug-and-play connection makes FireWire an attractive choice for working with digital audio files. FireWire is included on most current Macintosh computers, and a number of FireWire audio interfaces are available. These third-party interfaces are not DV devices, but merely devices that use FireWire as a means of transferring digital audio data. FireWire interfaces are more convenient to switch between computers than PCI Express cards, and they can be used with both desktop and portable computers. A FireWire interface typically supports ten or more inputs, and at least eight outputs.

Figure. Illustration showing a computer with a FireWire connection and a breakout box.

Important: Although a FireWire bus supports a very high data rate, connecting too many devices, such as a DV video device, a FireWire hard disk, and a FireWire audio interface, could potentially exceed the bus speed, resulting in audio with missing samples.

Figure. Illustration showing a computer connected via FireWire to a breakout box, and the breakout box connected to an analog or digital VTR via XLR.

USB Audio Interfaces

There are a wide variety of USB audio interfaces available. USB supports plug-and-play operation and the ability to connect several devices in sequence (daisy-chaining). USB is included on all current Macintosh computers.

Figure. Illustration showing a computer with a USB interface and an audio interface.

Most USB audio interfaces support two or four audio channels at one time. USB audio interfaces vary in quality considerably, so take some time to research them before you purchase one. The connectors on USB interfaces vary; some interfaces have RCA connectors and others have both XLR and 1/4" TRS connectors. Very inexpensive USB audio interfaces may only have a stereo miniplug.

Figure. Illustration showing a computer connected via USB to an audio interface, and the audio interface connected to an analog or digital VTR via RCA connectors.

Make sure you don’t exceed the total bandwidth of the USB bus. USB 1.1 has a fairly low data rate, so be careful not to add too many USB items to the bus. Ideally, USB audio interfaces are always directly connected to your computer, not connected via a hub or to the computer’s display, keyboard, or another peripheral. If you are having audio problems with a USB interface connected to a hub, try removing USB devices and eliminating the hub from the connection.

USB 2.0 audio interfaces are also available. Because USB 2.0 devices can handle high data rates (similar to FireWire 400), you can treat them similarly to FireWire audio interfaces (see FireWire for Digital Audio).

PCI Express Audio Interface Cards

Peripheral Connect Interface (PCI) Express interfaces, unlike FireWire and USB interfaces, require that you install a dedicated sound card in your computer. Of all the audio interfaces, PCI Express audio interfaces provide the maximum transfer speed and can support many channels with high sample rates. Many PCI Express audio interface cards come with a separate breakout box where the audio input and output connectors are located.

Figure. Computer with a PCI audio interface card.

PCI Express Audio Interface Card with Built-in Connectors

Some PCI Express audio interface cards have audio connectors attached directly to the card. Because there is limited space on a PCI Express card to mount audio connectors, RCA connectors are typically used.

Figure. Illustration showing a computer with a PCI card connected to an audio or video device via RCA connectors.

PCI Express Audio Interface Card with Breakout Box

The majority of PCI Express audio interface cards have a breakout box with a large number of XLR or 1/4" TRS connectors. The breakout box is attached to the PCI Express card via a long cable with a multipin connector. The long cable allows you to place the breakout box in a convenient location, so you can easily connect and disconnect your video and audio devices without going behind your computer.

Figure. Illustration showing a computer with PCI card connected to a breakout box and the breakout box connected to an analog or digital VTR via XLR connectors.