Fig. 1. Typical settings in Slate Digital Virtual Tape Machines for mastering. JUST LIKE their analog counterpart, tape-emulation plug-ins can greatly enhance a recording—or irreparably mar it, if misused. This article will show you how to get the most out of your reel-to-reel impersonator.
I’ll use the Slate Digital Virtual Tape Machines plug-in (VTM)—to my ears, the most accurate tape emulator currently available in Audio Units format—to illustrate my points (see Figure 1). But first, a little background on how tape machines work will help you understand what the plug-in’s controls do.
Are You Biased? When calibrating a professional open-reel tape recorder, a specific amount of AC bias—an inaudible high-frequency signal typically between 40 and 150 kHz—is added to the audio signal to correct for magnetic tape’s nonlinear response at low signal levels. The bias moves the audio signal into the tape’s linear-response region, reducing distortion.
Unfortunately, there’s no free lunch. As you increase the bias, the tape recorder’s high-frequency response begins to decrease at progressively lower signal amplitudes for the audio signal as a whole. Reducing the bias allows high frequencies greater headroom, but it also increases nonlinearity for the wideband audio signal; the net effect is greater total distortion and saturation, and reduced dynamics and transparency overall.
To complicate things further, the optimal bias setting—a compromise between rendering the least amount of distortion and preserving the most highs—depends on the tape formulation and tape speed used. The best tape emulators—including VTM—not only offer a virtual control for changing the bias but also provide a different bias control range for each type of virtual tape formulation and tape speed you select in the plug-in.
So why would you want to change the bias from its optimal setting? Depends on your application. For example, to warm up a bite-y electric guitar track, try using a high bias setting to soften the pick strike’s highs. If, on the other hand, the electric bass guitar track sounds thin and clinical, using a low bias setting will help produce the saturation and distortion you need to make it sound bigger and more lush. When mastering, you’ll probably want to use the normal bias setting— unless you need to take off some edge, in which case high bias might be the ticket.
Tips for IPS A tape recorder’s tape speed specifies how fast its tape moves along on the machine’s transport in inches per second (ips). As tape speed increases, the high-frequency response becomes progressively extended, the bass-frequency response becomes less extended, dynamic range increases, noise decreases, wow and flutter diminish slightly and the inherent head bump moves to a higher center frequency in the bass band. Head bump is an inherent boost in bass-frequency response—typically 1 to 4 dB in amplitude and 1 to 1.5 octaves wide—that occurs in a tape machine’s playback mode. For most but not all tape recorders, head bump is most accentuated at 15 ips tape speed. Doubling the tape speed (for example, from 15 to 30 ips) moves the head bump an octave higher.
Despite the higher roll-off in bass frequency response at 30 ips compared to 15 ips, professional 2-track machines nevertheless generally have great response down to at least 40 Hz—which is about as low as most masters need to go before tailing off. Couple this fact with 30 ips’ extended highs, greater dynamic range, lower noise, and typically milder head bump, and it’s easy to see why this higher tape speed is usually your tape-emulator plug-in’s best setting for mix-bus and mastering applications.
Fig. 2. The Bass Alignment slider is lowered in VTM’s Settings window to moderate the head bump. On the other hand, if you want to beef up your bass guitar or kick drum track’s sub-bass punch, select the 15 ips setting to move the bass bump an octave lower. If the head bump is in the best-sounding frequency range for your purpose but sounds too overwhelming, click on VTM’s Settings button and lower the Bass Alignment slider (see Fig. 2). On bass guitar, take advantage of the slower tape speed’s lower dynamic range—crank the plug-in’s input to saturate the signal and add girth and luster.
Just Your Type VTM offers models of two different types of tape: Ampex 456 (dubbed FG456 in VTM’s GUI) and Quantegy GP9 (VTM’s FG9). FG9 offers greater dynamic range, making it the logical choice for mastering. FG456 saturates at lower signal levels, so use it to add lustrous harmonic coloration and girth to electric guitar, bass, and drum tracks.
Putting It All Together Different applications suggest combining different settings in your tape-emulator plug-in. For example, to give a bass track the greatest saturation and biggest bottom, you’d likely want to select low bias, FG456 tape, and 15 ips tape speed in VTM. For mastering, normal bias, FG9 tape, and 30 ips should lend the most open, dynamic, detailed, and cleanest sound (while still imparting subtle analog-like smoothness and enhanced midrange girth).
But there are no hard and fast rules. Maybe your bass track already sounds saturated enough and needs its dynamic range preserved, like a higher bias setting would do. Perhaps your client wants his or her master to have more of a vintage sound, like FG456 would lend. Armed with a deep understanding of how tape machines and VTM work, you can get there.
Michael Cooper is a recording, mix, mastering, and post-production engineer; a contributing editor for Mix magazine; and the owner of Michael Cooper Recording (soundcloud.com/michael-cooper-recording) in Sisters, Ore.