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Cheat Sheet: Computer Audio Interfaces

May 20, 2008
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ZERO-LATENCY MONITORING

Passing a signal through a computer results in some delay (“latency”), so monitoring a signal at the computer’s output will be delayed compared to real time. An interface with zero-latency monitoring can monitor the signal going into the computer, thus avoiding latency. However, you will not hear any processing added by plug-ins inserted in the computer.

MIXER APPLICATION

Many interfaces include a software mixer application for routing signals within the interface itself, and between the interface and any hardware connections to the “outside world.” This is also where you’d likely set up zero-latency monitoring.

SOUNDCARD LATENCY INACCURACIES

Soundcards report their latency to the host program, which displays this figure. However, note that some interfaces give the latency of a signal going into the computer (“one-way latency”), while others report the “round-trip” latency—in and out. Furthermore, some audio interfaces do not report latency accurately, and might be off by dozens or even hundreds of samples.

SAMPLE RATES

Almost all interfaces support 44.1/48kHz and most add 88.2/96kHz support. 176.4/192kHz are rarely used but included in some high-end interfaces. Higher sample rates can mean lower latency, but the interface may not be able to handle as many inputs simultaneously, or exhibit other limitations.

MEASURING LATENCY: MILLISECONDS VS. SAMPLES

Latency is given in milliseconds or samples. To translate samples to milliseconds, first determine a sample’s length. With a 44.1kHz sampling rate, each sample is 1/44,100th of a second long—about 0.023ms. So a soundcard running at 44.1kHz with a latency of 256 samples equals a delay of 256 X 0.023ms, or about 5.8ms.

ONBOARD DSP

This usually means that the interface includes digital signal processing, such as dynamics control, EQ, or reverb. These processors do not require any CPU power from your computer, and can often process the signal(s) being recorded on their way into the interface.

DRIVERS

The Mac’s Core Audio is a complete audio subsystem with drivers that most audio interfaces can access. Windows interfaces often include custom drivers, but always check the manufacturer’s website for updates. Follow any driver installation instructions exactly—some audio interfaces require installing the driver software before connecting the interface, while others require the reverse. With Windows, an interface may offer several driver choices. ASIO and kernel-streaming WDM are best, while DirectSound and MME are useable but have relatively high latency; avoid any mode that reads “emulated.” For best results, use a driver that includes the name of the audio interface’s manufacturer (e.g., MOTU ASIO).

EXTERNAL AUDIO INTERFACES

These connect to your computer via USB 1.1, USB 2.0 (faster; important for running lots of channels, or using high sample rates), or FireWire. With FireWire, check the interface manufacturer’s website to make sure your computer’s FireWire chip set is compatible (TI chip sets are popular). If there is audio interference (e.g., audible clicks or pops) when other USB or FireWire devices are on the same bus, install a separate USB or FireWire card and dedicate that to audio. Caution: Most manufacturers advise against combo USB/FireWire cards.

INTERNAL CARD INTERFACES

These cards insert into a slot in your computer’s motherboard, and provide slightly higher performance compared to using an external USB or FireWire box. The card’s backplane (the side that faces out from the computer) will typically have numerous connectors for analog and/or digital I/O. Some cards instead have a multipin connector and a “breakout” cable with various connections. Cards must be well-engineered to avoid picking up noise from the computer.

INTERNAL CARD + EXTERNAL BOX

The internal card still does most of the work, but instead of mounting connectors on the backplane or using a breakout cable, a multiconductor cable runs from the card to an external box containing the various connectors, controls, etc. This gives the performance advantage of a card-based interface, but keeps sensitive electronics out of the computer.

INSTRUMENT INPUT

Electric instruments like guitar and bass typically lose level and high frequencies when driving a line input directly. An instrument input accommodates the needs of electric instruments. However, if you use a guitar-compatible electronic processor (multieffects, stomp box, etc.) prior to feeding the interface, an instrument input isn’t necessary.

HOT-SWAPPING

Although in theory you should be able to plug/unplug FireWire and USB devices from a computer while the power to either or both is turned on, there have been isolated reports of motherboards failing from having powered-up peripherals plugged into them. It’s prudent to make connections to peripherals with both the computer and peripheral powered-down.

POWER ISSUES

A bus-powered interface can receive its power from the voltages provided at a FireWire or USB port. However, not all ports can deliver enough current for all interfaces, so you may need to use an external adapter. FireWire devices with 4-pin connectors do not receive a supply voltage, and require an external adapter.

+48V PHANTOM POWER

Not all interfaces generate a full +48V (some produce considerably less). While this usually isn’t a concern, if a condenser mic requiring phantom power doesn’t work properly with an interface, check the phantom power voltage at the mic’s XLR connector with a voltmeter.

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