What is a Compellor? In short it is a Compressor-Leveler-Limiter. The device is specifically designed for the transparent control of audio levels.
It operates as a stereo processor or as a two-channel (mono) processor supporting independent channel control.
The device includes 3 interactive gain controllers:
– Frequency Discriminate Leveler
Additional features include a Dynamic Release Computer (DRC), Dynamic Verification Gate (DVG), and a Silence Gate.
The original device (model 300 Stereo Compellor) was released in 1984. The product line evolved and culminated in 2003 with the release of the 320D. Through the years the Compellor has been widely used in professional broadcast, post houses, recording studios, and live venues.
In 2004 I purchased a used model 320A from a radio station. The “A” reference indicates it’s analog circuitry. I’ve used the 320A for countless audio file and tape transfers, post production processing, Telephone/Skype recording sessions, and monitoring. The device provides three selectable Operating Levels … +8dBu, +4dBu, and -10dBV.
Recently the complex level and gain reduction metering for the right channel failed. I replaced the faulty 320A with a 320D. This version features digital and analog I/O with common selectable (analog) Operating Levels (+4dBu, and -10dBV).
At some point my faulty 320A will be shipped out to Burbank California for authorized service.
320D – Automatic Processing and Detection
As noted Aphex classifies the Compellor as a Frequency Discriminant Leveler. It responds slower and less aggressively to low frequencies. In essence low frequency energy will not initiate gain reduction.
A Dynamic Release Computer (DRC) instantiates program dependent compression release times.
The Dynamic Verification Gate (DVG) computes the historical average of peak values and verifies whether measured values exceed or are equal to the historical value. When the signal level is below the average, leveling and compression gain reduction is frozen.
The device Drive control sets the preprocessed VCA gain. Higher settings yield a higher level of gain reduction (VCA refers to Voltage Controlled Amplifier).
The Process Balance control allows the operator to fine tune the Leveling and/or Compression balance and weighting. Leveling is a slow method of gain reduction. It maintains transient retention and wider dynamics. The Compression stage works faster and acts more aggressively on inherent dynamics. The key is by combining both modes, the processed output will be very consistent
A Rate (speed) toggle option is provided: Fast, suitable for speech/voice, or Slow, suitable for program material such as produced TV and/or Radio programs.
The device Output control normalizes the processed audio to 0VU.
Silence Gate: Aphex stresses – this is not an audio gate! It is a user defined threshold parameter. When the signal drops below the threshold for 1 sec. or longer, the Silence Gate freezes the VCA gain. This prevents the buildup of noise during pauses and/or extended passages of silence.
The device Limiter features a very fast attack and high threshold. It is designed to prevent occasional high transient activity and overshoots.
A Stereo Enhance mode is available on the 320A and 320D models. When activated it widens the stereo image. It’s effect is dependent upon the amount of applied compression.
The 320D Compellor features three, bi-color (red, green) LED metering modes: Input, Output, and Gain Reduction. For Input/Output metering – the red LED’s indicate VU/average. Green LED’s indicate peak level.
When the meter is set to display gain reduction (“GR”), the green LED’s indicate total gain reduction. Depending on the Process Balance control weighting – a floating red LED may appear within green LED instances. The floating red LED indicates Leveling gain reduction. If Leveling gain reduction is in fact occurring, the total gain reduction will be indicated by the subsequent green LED(s).
Below are 4 examples:
Example 1 displays Input or Output metering with an average (red) level of 0VU and a peak (green) level of +6dB. This translates to a +4dBu average level and a +10dB peak level (analog OL set to +4dBu).
Example 2 displays 4dB of Leveling Gain Reduction and 8dB of Total Gain Reduction.
Example 3 displays 12dB of Leveling Gain Reduction.
Example 4 displays 10dB of Compression Gain Reduction.
**Notice the position of the Process Balance control for examples 2, 3, and 4.
The 320D is essentially an analog processor utilizing standard XLR I/O jacks. The device also includes AES/EBU XLR jacks along with an internal DAC for digital I/O. The Input mode and/or Sample Rate is user selectable.
When implementing digital I/O – the Incoming audio is converted to analog as it passes through the device. The audio is then converted back to digital and output accordingly.
The digital input is calibrated internally and matches -20dBFS to 0VU on the Compellor’s meter. The +4dBu/-10dBV Operating Level options only affect the analog I/O.
The Aphex Compellor is a long standing, highly regarded, and ubiquitous audio processor. It has been an integral multipurpose tool for me for 12+ years. My newly purchased (used) 320D is in near mint condition. In fact it looks and feels as if it was hardly used by the previous owner.
My system includes additional Aphex audio processors (651 Compressor, 109 EQ, 622 Expander/Gate, and a 720 Dominator II Multiband Peak Limiter). As well, a Mackie Onyx 1220i Mixer, Motu I/O, dbx 160A Compressor, dbx 286A Mic Processor, Marantz CF Recorder, and a Telos One Digital Hybrid. All components, with the exception of the 286A – are interfaced through a balanced Patchbay.
A typical processing/monitoring chain will pass system audio through the Compellor, followed by the 720 Peak Limiter. The processed audio is ultimately routed to the system’s Main Output(s). This chain optimizes playback of poorly produced Podcasts, VO’s, live streams, or videos. The routing is implemented via Patchbay.
A typical audio processing chain will route Pro Tools audio out via hardware insert (or bus, alternative output, etc.) through the Compellor (or a more complex chain) and returned in Pro Tools. In this scenario I use a set of assignable interface line inputs/outputs. The routing is implemented via Patchbay. I document the setup and use of hardware inserts here.