Applying Mastering Hardware and Psychoacoustics
A Respected Manufacturer's First Live Product
By Garrett Soden
From Live Sound International magazine
The MaxxBCL, which features a bass extender, compressor, and maximizing limiter in a 2U rack-mount unit, is the first product developed by Waves for the live sound market, and was envisioned as a unit that could be put across the master buss of a live board, or individual channels, to solve a number of problems.
Of the three processors used in the unit, the bass extender stands out, as it doesn’t fit in any standard class of processors. The technology is the third generation of MaxxBass, a processor that has been previously marketed in software and earlier hardware products. It provides bass extension, but is not an equalizer and does not increase bass frequencies. Instead, it takes advantage of a psycho-acoustic phenomenon known as the “Missing Fundamental” effect.
This phenomenon is the perception listeners have of a lower tone, when related higher harmonics are present. The effect was discovered by pipe organ builders in the Middle Ages who needed deep bass notes (the better to thrill their audiencesome things never change) but who didn’t always have the space or budget to build the massive pipes needed. So instead of building a 32-foot pipe, for example, they would build two smaller pipes (a 16-foot pipe and a 10-foot, 8-inch pipe in this case), which would be sounded together to deliver the impression of the fundamental tone, which was actually missing, hence the name.
Here the phenomenon is put to use on a more sophisticated level. The algorithm analyzes each fundamental tone and generates a set of related harmonics, in real time, that produce the perception of the lower fundamental. The amount of the effect can be dialed in using the Intensity potentiometer, while any amount of the original low frequencies can be retained or removed by adjusting the high-pass cut-off frequency in a range of 20 - 120 Hz. An 18dB/octave crossover is used to split the input signal into low and high signals, with processing then only applied to the low signal.
For example, if the frequency is set to 60 Hz, frequencies above 60 Hz pass through unaltered, while frequencies below 60 Hz have harmonics generated and added back to the final signal as set by the Intensity control. By engaging the High Pass filter switch, the original bass frequencies can be removed entirely, the result being that they are replaced by the generated harmonics. There’s also a Bypass switch, so users can compare the altered sound to the original.
This technique of replacing the bass frequencies can help solve several bass issues. If a line-array system, for example, is too light on bass, the low end can be improved without putting up more subwoofers, because the frequencies generated to make the improvement (the harmonics) are reproduced through the midrange drivers.
If there is bass build-up in corners or near walls, the low frequencies that are causing the problem can be eliminated, while the overall sound is augmented with the generated harmonics to keep the bass sounding full. And in a venue with a low frequency rumble that causes low frequency oscillation and/or feedback, the processor can eliminate these problematic low frequencies by replacing them with higher frequencies that are above the rumble.
And there are other benefits. For sound companies with several systems out at the same time, the MaxxBass processor can help supplement the low end at smaller shows, so that subwoofers and amps can be allocated to larger shows as required. It also can be argued, that the reduced need for low frequency means the likelihood of less blown drivers and more efficient amplifier performance. In addition, for installations, the unit may be able to help stretch budgets by reducing the need for larger amps and additional subwoofers.
The L2 Ultra-Maximizer
It’s always been a challenge to live sound to overcome the distance between the system and the audience to bring the music “up front.” The L2 Maximizer is often used in mastering houses to create this “presence,” so having the ability to strap this same processor across the master buss of a live console gives mix engineers access to the same technique. Moreover, the processor is also a brick-wall limiter which can stop peaks even when very little, or none, of the maximizing effect is used.
Another use, discovered when MaxxBCL prototypes first went into the field, was that the L2 can be used to improve monitoring. In one application, a monitor engineer used two MaxxBCLs to create a vocalist’s in-ear mix. One L2 limited the vocals; the other limited the rest of the mix. With compression applied to the backing music and vocal separately, the singer was able to hear a music mix that included everything, but which could be adjusted against the level of the vocal. And in addition, more audible bass could be included in the monitor mix.
The limiter automatically increases makeup gain by the same amount that the Threshold is lowered (Threshold range is from 0 to -18dB). The Out Ceiling potentiometer sets the absolute level of the highest possible peak. The Link button links the Threshold and Out Ceiling controls together, so that rotating either of the controls will change both values, while retaining their relative settings (from the time Link was turned on). This allows the engineer to increase the “in-your-face” sound without changing the overall gain. The maximizing aspect of this limiter also offers the practical benefit of getting louder sound from smaller systems.
The Renaissance Compressor
The third processor in the MaxxBCL is the Renaissance Compressor, which is designed to work with a minimum of controls. The release time is handled by the proprietary ARC(Automatic Release Control), which adjusts to the program material in real time, matched to the response of the human ear. As such, RMS and peak transients are analyzed and reacted to differently, as in general, the release is faster for peak transients and slower for the overall RMS level. For instance, with a relatively constant compression of 4 dB, peaks beyond this need faster release times. ARC does this, varying the release time to fit the ear’s expectations while increasing RMS, and without creating distracting artifacts. In this way, the compressor can simultaneously serve as a leveler and as a fast compressor.
Although automatic, ARC offers two compression behaviors for different “flavors” of compression: Electro and Opto.
In Electro mode, release time becomes increasingly faster as the gain reduction approaches zero, but only when gain reduction is less than 3 dB. So when the Electro mode is used with moderate compression, Electro produces a great increase in RMS level. When reduction is more 3 dB, the release time becomes slower, so the compressor behaves more like a leveler when used in high gain reduction situations. The Opto mode is the inverse of Electro. As such, Opto slows down the release time as the gain reduction approaches 0 dBFS (dB Full Scale) when gain reduction is less than 3 dB. Additional controls include Bypass, Attack (0.5, 1, 2, 5, 10, 15, 20, 50 ms), Ratio (1:1 1:12), Threshold (0 60 dB), and a switch that allows the user to put the Renaissance Compressor before or after MaxxBass, allowing for a significant difference in sound. (The L2 is always at the end of the signal chain.)
The unit features transformer-based analog input/output stages (using Jensen transformers) and AD/DA conversion using 24-bit converters, which are second-generation versions of those used in Waves hardware boxes marketed to mastering houses. Internally, all digital processing is done with 48-bit precision (that is, double precision), while a proprietary IDR(Increased Digital Resolution) algorithm is employed for dithering and noise-shaping, to re-quantize the signal to 24-bit, and to feed the DA converters for analog output. In addition, digital output can be set to 16-bit for direct real-time recording into CD or DAT recorders.
Working Under Fire
The MaxxBCL offers a large range of input and output levels, which can be set separately, with a range of + 9, 12, 15, 18, 20, and 24 dBu. Input and output levels can also be adjusted via an extra 12 additional 1 dB steps of trim, per channel, via the front panel. Inputs and outputs include XLR/TRS combo connectors for balanced or unbalanced inputs, XLR connectors for balanced outputs, and TRS jack connectors for balanced or unbalanced outputs. Digital inputs and outputs include both AES-EBU and S/PDIF capabilities at sample rates up to 96 kHz.
A major consideration at the design stage was ease of use during a show, and as such, the unit features four preset buttons, configured so that a short press recalls a setting, and a long press (2 seconds) stores a setting. The peak holds for all meters can operate as momentary (2 seconds) or infinite, and are reset by pressing the Peak button briefly. It is perhaps, the top potentiometer of each processor that will receive most attention during a show, as it controls intensity for MaxxBass, ratio for Renaissance Compressor, and out ceiling for the L2 limiter. In addition, displays and meters are visible under any lighting, including direct sunlight. Transformers on both the inputs and outputs provide protection against shorting, and for power and/or unit failure, the analog inputs and outputs both have bypass relays. These allow the signal to continue through the unit, defaulting to the last preset used when power returns.
To sum up, the designers of the MaxxBCL claimed to have done their utmost to appreciate the needs of live sound engineers in the field. And, since Waves is not yet well known in the live sound community, it was extremely important that this product achieve the performance and endurance standards that sound reinforcement industry professionals demand.