HUM Audio LAAL

This all‑analogue feed‑forward limiter has the sort of look‑ahead function normally found only in digital devices.

Established in 1999 in Poland, the HUM of HUM Audio Devices is an acronym for Human Unhindered Maker, apparently. The company currently offer a small but growing range of products, comprising three ribbon microphones (two active models, one with a one‑inch and the other a two‑inch ribbon, plus a remote‑controlled stereo mic), a mic preamp (also with remote control), an ambitious modular analogue console launched at GearFest UK in the Summer, and the subject of this review, the LAAL, which is a rather unusual mastering limiter. LAAL stands for ‘look‑ahead analogue limiter’, so before diving into the details of the LAAL itself I thought it would be useful to provide a brief overview of analogue look‑ahead limiters more generally.

Delay‑line Limiting

One of the inherent problems of all dynamics processing is the accidental introduction of unwanted distortions as side‑effects of modifying a signal’s instantaneous dynamics. There are many causes for these distortions, such as non‑linearities in the gain reduction circuitry, or the way in which the side‑chain time constants relate to the signal’s component frequencies. For example, a very fast release time can often result in ‘envelope modulation’ whereby the amount of gain reduction varies according to the instantaneous amplitude of a low‑frequency waveform. Another well‑known issue is the way a very fast attack time constant results in misshaped transients, creating clicks or transient distortion.

This last issue is particularly pertinent to protective peak limiters that are designed primarily to prevent transient overshoots from overloading the signal path. Controlling fast peaks inherently needs a very fast response time, but a conventionally fast attack inevitably ‘reshapes’ the initial transient in a way that often creates audible distortion. Consequently, a slower compromise must usually be found in standard limiters, balancing absolute transient control with acceptable audible artefacts.

Figure 1: in a delay‑line limiter, the main audio path is delayed a tiny amount (less than 1ms) by a network of all-pass filters, so that the control path ‘sees’ the signal fleetingly before it arrives at the gain reduction stage.A better way to overcome this problem of transient control is to anticipate an upcoming signal peak and thereby introduce the appropriate amount of gain reduction in advance of the peak’s arrival (see Figure 1). In a normal feed‑forward limiter, the input signal is split in two, with one part feeding the gain‑reduction element (GR) and the other part feeding the side‑chain processing (SC). The side‑chain can therefore only respond to signal peaks at the same time as they arrive at the gain‑reduction element. In order to anticipate the peak, the side‑chain needs to become aware of the signal before it reaches the gain‑reduction element, and in practical terms that requires the signal to be delayed slightly before it reaches the gain‑reduction element, to give the side‑chain time to react and reduce the gain before the peak arrives.

In the digital world short delays are very easy to implement, and many advanced DAW limiter plug‑ins work in exactly this way, using an approach often called ‘look‑ahead limiting’. Achieving the appropriate delay in the analogue world is nowhere near as easy — but it’s not impossible. In fact, the delay‑line limiter concept was first implemented in the analogue domain back in the late 1960s for the critical protective limiters that were necessary in broadcast radio transmitters. If you’re interested, one of the first such designs, the BBC AM6/7, is explained in a lovely 1967 BBC Monograph that’s available online: www.bbc.co.uk/rd/publications/bbc_monograph_70.

So now you’re wondering how a high‑quality, analogue audio delay can possibly be created for the main signal path through the limiter, and the solution adopted in that BBC design (and others) was a chain of ‘all‑pass’ analogue filters. I explained the concept of the all‑pass filter in detail in my article about how phaser effects pedals work back in SOS August 2021 (https://sosm.ag/how-phasers-work). But in case you haven’t read that yet, an all‑pass filter is essentially a special form of audio equaliser. Normal equalisers are designed to change the signal amplitude at different frequencies and, as a (usually) benign side‑effect, the phase response changes too. An all‑pass filter is a special case where the frequency‑amplitude response remains completely flat and unchanged, but the frequency‑phase...