Mixing on AIR part 4: BF76

While not technically part of the new AIR collection of plug-ins, the Bomb Factory BF76 compressor does come included with every Pro Tools installation and is the only ‘vintage’ style dynamics processor that ships with the standard configuration. Those in the know will quickly recognize the interface of BF76 as a virtual recreation of the famous Urei 1176 peak limiter, but because of its less than intuitive controls many new users forgo this little gem in favor of the more straightforward Digrack dynamics package. This week at the Pro Tools corner I will fill you in on some of the history behind the BF76 and show you how to integrate this great vintage modeled plug-in into your mixes.

Dynamics Review:

Before we can get into the specifics of BF76, it is useful to review the basics behind dynamic range and processing. The term “dynamic range” is widely used in audio engineering and is actually quite easy to understand. Simply put, a signal’s dynamic range is the difference between the softest and loudest parts of that signal’s amplitude. Dynamic range can be measured over a very short period of time, like the difference between the transient peak of a snare drum and its ringing decay, or over longer periods, like the difference between the soft and loud words of a vocal phrase. The job of a dynamics processor is to work within this realm of dynamic range by reacting to the variations in signal level that occur over time.

For example, a compressor or limiter (such as BF76) reacts to the louder portions of an audio signal by attenuating or turning the signal down by a specified amount (or ratio), with the goal of reducing the overall dynamic range of that signal over time. Think about a vocal with a wide dynamic range. Left unprocessed, many words will pop out over the rest of the mix, sounding awkward and disconnected, while others will be lost beneath the mix completely. Using a compressor or limiter the engineer is able to automatically turn down these louder words while simultaneously bringing up the softer ones, effectively reducing the overall dynamic range of the vocal. In this case, using a compressor to reduce the dynamic range allows the vocal to sit in the mix without poking out or getting lost, all without extensive volume automation.

Mixing with BF76

The bomb factory BF76 is designed as a plug-in model of the famous Urei 1176 peak limiter, developed in the late 60s by Bill Putnam and still being manufactured today by Universal Audio. The 1176 is a FET (field-effect transistor), solid state dynamics processor with a very unique sound, known for retaining the brightness and clarity of a signal that other compressors often take away. Because of its extremely fast attack and release times, the 1176 is a very versatile processor that works well on almost anything, from evening out a vocal or bass, to creating punchy snare drums, even master bus processing.

The Interface

The interface of BF76 is a near replica of its real world counterpart featuring a limited number of parameters. At first these parameters may seem counterintuitive to what you are use to seeing on other compressors, but once you learn what they do you may find that it is actually easier to use this processor.

Input: You may have noticed that the BF76 does not have a ‘threshold’ parameter like most compressors do. Many vintage compressors, including the 1176, feature a ‘fixed’ threshold that is driven by an input control, so think of the input parameter as the threshold parameter on the BF76. To increase the amount of gain reduction (or compression), turn the input counterclockwise towards 0 while watching the GR meter until the desired amount of compression is achieved. Remember, the threshold of a compressor defines the level at which the compressor begins to act on or ‘compress’ the incoming signal, in a sense it defines the “what is loud, what is not” line that the compressor uses to turn on and turn off, enabling it to control dynamic range.

Output: The output of the BF76 is used to return the signal to unity gain after compression. After achieving the desired amount of gain reduction (compression) use the output control to return the signal to its pre-processed level. It can be useful to use the plug-in’s bypass button to determine the correct output setting, as a general rule try to match the signal’s level to the bypassed state. This can help in evaluating the actual processing, avoiding the “it’s louder so it must be better” approach.

Attack: As with most compressors/limiters, ‘attack’ defines the amount of time the unit takes to grab onto the signal once a threshold breach is detected. Think of it this way, when you watch your favorite program on TV and an insanely loud commercial comes on, breaching your ears threshold of “too loud,” your “attack” would be the time it took for you to reach for your remote and turn the volume down.

The attack time on the BF76 is variable from .4 ms to 5.7 ms, which is quite fast even at its slowest setting. ‘7’ or 100% clockwise is the fastest attack while 1 or 100% counter-clockwise is the slowest attack. This is counter intuitive for most people, as one might assume that ‘7’ would be slower than ‘1’, but remember these numbers do not represent millisecond settings like most compressors, as a general rule just remember the BF76s attack/release controls are backwards. Slower attack times allow more of the signal’s transients through and can be great for putting a sharp ‘thwack’ on the head of a snare or kick drum while faster attack times can soften a signal’s attack. Because the range of attack time on the BF76 is so small, changes to this parameter can range from very subtle to inaudible, depending on the program material.

Release: Release defines the amount of time the unit takes to recover after the signal falls back below the threshold. Going back to our TV example, this would be how long it takes you to turn the television back up after the loud commercial break was over.

The release time on the BF76 is variable from 60 ms to 1.1 seconds. ‘7’ or 100% clockwise is the fastest release while ‘1’ or 100% counter-clockwise is the slowest release. Setting the release control close to ‘7’ can really help bring out the sustain of a signal for a super aggressive sound, but be careful as it can go from 0 to insane sustain and pumping with the tiniest tweak. Because the 1176 design has program dependent attack/release characteristics it is best to use your ears when setting these values rather then consuming yourself with millisecond values (notice how these values aren’t even labeled in milliseconds on the units controls). Be aware that the release control provides a much wider range than the attack and is much more sensitive to small changes, setting the attack and release times too fast can result in distortion.

Ratio: The ratio buttons define the amount of gain reduction in correlation with the threshold. For example, a ratio of 4:1 would attenuate a signal ¾ dB for every 1 dB over the threshold, so if a signal’s input is 4dB past the threshold only 1 dB will reach the output, 8dBs past the threshold at input would yield 2 dB at output. Ratios of 12:1 and 20:1 act more as limiters. Try shift clicking one of the ratio buttons to engage the famous “all buttons in” mode, dramatically changing the compressor knee and the character of the compression.

Metering: BF76 provides gain reduction (GR) and output metering modes (-18 and -24). In meter mode -18, the meter is calibrated so that -18dB FS equals 0 VU while in meter mode -24, -24 dB FS equals 0 VU. Because the less than ideal ballistics of this virtual meter don’t always help me personally, I often set it to ‘off’ and work just by ear.

Tips for using BF76

  • A great starting point for most material is the default of 3 attack and 6 release, or “10 and 2 o’clock.” Set the ratio to 4:1 and adjust the input until the gain reduction (GR) starts diving a bit. Use the output control to balance out the signal, using the plug-in’s bypass as a guide. This method is great for sitting a vocal or acoustic guitar in the mix.
  • Use BF76 as a parallel processor on snare or overhead tracks (duplicate the track and run one track dry and one track with BF76). Over compress the duplicate track using a very fast release (try 6 or 7) to really bring out the sustain of the processed track then blend the over compressed duplicate track with the original to taste.
  • Try processing a mono room mic with the “all buttons in” mode.
  • Try using it on the master bus with a ratio of 4:1 or 8:1, just kissing the tops of the transients (1-3 dBs of gain reduction). You will probably have to reduce the input from the default level to achieve this.

In closing

While some will argue that there are better third party plug-in recreations of the 1176 available for Pro Tools, and they might be correct, as a professional I like to be able to get the job done with just the stock plug-in set when I have to. Being familiar with all the stock Pro Tools plug-ins, including BF76, prepares you for any recording/mixing scenario, regardless of the plug-in availability on the machine.