Hof Dynamic Master

Paul White studio tests a compressor whose main claim to fame is that you can't hear it working.

Is transparency merely an attribute of the Emperor's new clothes, or is this unit something rather special?Today's marketplace is full of compressors that deliver a punchy sound, or a warm sound, or a vintage sound — but where do you look when you need a compressor that doesn't change the sound of the music at all? For example, if you're mastering an album and you need to moderate the dynamic range of the music without changing the feel or the overall sound, you need a compressor that's effectively invisible. The same is true if you're producing a TV commercial that needs to sound as loud as possible on air, yet shouldn't sound compressed. Aphex are probably the company most commonly associated with subtle gain control, and their Compellor is used in mastering suites and broadcast studios worldwide, but now they have a challenge from Austrian manufacturer HÖF Audio, who claim to have made some advances in this field.

Technical details on exactly how the HÖF Dynamic Master works are deliberately vague, as the designer is more than a little concerned about being copied (he even scrapes off some of the Integrated Circuit numbers). However, from what I can tell, the overall process is very similar to that used in the Aphex Compellor, but implemented in a different way.

The Dynamic Master is essentially a full‑band compressor, but it also includes a leveller for keeping the long‑term signal level constant, a soft‑knee limiter, and a fast peak limiter (called a Peaker) to prevent the output from exceeding a set maximum level. As I understand it, the Peaker limiter includes a clipper, which permits very short spikes to be clipped rather than subjected to overall gain reduction, and this is a principle that Aphex have used and refined in their Compellor. Experiments show that the human ear is incapable of registering very short periods of clipping, so by allowing up to, say, 1ms of clipping (the actual duration used here is not divulged) before the limiter brings in gain reduction, you can achieve much greater perceived loudness while still preventing overshoots. Apparently, clipping becomes more audible if several peaks are allowed to clip in quick succession, and Aphex have an intelligent circuit that reduces the amount of time for which clipping is permitted when the peak density is high. I would imagine that a similar strategy is used in the Dynamic Master, because even deliberate abuse fails to make clipping audible. The Peaker may be switched out of circuit when not required — when mastering to analogue tape, for example — though the soft‑knee limiter will remain active.

A further stage of processing comprises a 1:2 expander to keep the noise down during signal pauses, and because the Dynamic Master is designed specifically for working on stereo material in a mastering situation, the two channels are permanently linked and are controlled from a single set of front‑panel controls. All the dynamic control functions in the unit have a soft‑knee characteristic to make gain control as unobtrusive as possible, though I assume that the Peaker is a hard‑limit device. The manual includes a discussion of compressor characteristics without actually saying which ones are employed by the Dynamic Master, but reading between the lines I think it likely that this model uses a multiple‑step release system, allowing it to cope with both short‑duration dynamic events and long‑term signal averaging.

As you'd expect, the compressor has a user‑adjustable threshold. Depending on the dynamics of the programme material, the leveller is brought into play automatically, the balance between short‑term compression and long‑term levelling being set using the Density control. Because it would be undesirable for the leveller to bring up the system gain during pauses between tracks, or other gaps, there's an automatic hold circuit that detects gaps and maintains the gain reduction at the previously set value until playback resumes. Amongst other benefits, this prevents the compressor from bringing up background noise during pauses.

If a continuous test tone is applied to the input and the Peak mode switch is operated, the circuitry puts the machine into Setup mode, allowing the user to calibrate the output potentiometer positions to the limiter action. The whole procedure is described step‑by‑step in the manual. During calibration, the Peakers, compressors and levellers are bypassed and the soft‑knee limiter operates at the Peaker's threshold. A green Test LED shows when the calibration/test mode is active.

HÖF Hardware

Now that I've dispensed with the basic theory, I can reveal that the physical embodiment of these diverse processes and principles is a stylish but fairly conventional 1U package, with balanced XLR audio connections and a relay‑operated hard bypass. Rear‑panel switching is available to select between +4dBu and ‑10dBV operation. Separate control is provided over the input and output levels of both channels, via precision pots, presumably so that imbalances in the programme material can be compensated for, and for all the internal complexity of the compressor, the only controls are for Threshold and Density, with the expander section having Threshold and Range controls.

With so many gain‑reduction processes going on at the same time, it's important to be able to keep track of exactly what the unit is doing, and here the designers have hit on the elegant solution of using tri‑coloured LEDs for the metering: these show red for limiting, green for compression and orange for levelling. The 10‑section meter shows the gain reduction in 2dB steps, but a multiplication button allows this to be changed to 1dB steps if desired.

While the degree of compression can be controlled using the Threshold control, the limiter's upper level is preset and independent, so it is possible to apply less compression and more limiting by increasing the compressor threshold and increasing the overall input gain setting.

Compression Density is related to the release time of the compressor, but a degree of parameter interactivity makes this less simple than it might first appear — even the soft‑knee limiter is linked in somehow. When the control is set fully anti‑clockwise, the main function is levelling, so the majority of changes are in long‑term level control. As the control is moved clockwise, the compressor starts to react to shorter‑term events, so you get less levelling and more compression/limiting. The result of a higher Density setting is also to increase the average energy of the mix, by reducing the dynamic range between signal peaks and the average signal. A green LED shows when compression is taking place, and a yellow Hold LED comes on during programme pauses, to show that the gain‑reduction amount has been frozen.

In Use

Because the whole idea of the Dynamic Master is that you shouldn't hear it working, it's important that you have a fast PPM meter somewhere at the end of your audio chain, so that you can verify exactly how much dynamic range compression is taking place. A lot of what goes on happens during brief signal peaks, so with slow meters you might think you're gaining less than you really are. I would imagine that in most normal situations you might only want to skim a few dBs off the signal peaks, but you can can actually apply a huge amount of gain reduction with the Dynamic Master before the process becomes at all noticeable, and even then it stays remarkably musical. Especially impressive is the fact that you can really level out a voice‑over but still leave it sounding quite natural. The expander is also extremely smooth on vocals.

When mixing music, you can use the Dynamic Master to enable you to get the maximum possible level onto a digital master recorder but with the absolute certainly that clipping won't occur. It also keeps the stereo image fully intact, which isn't true of every stereo compressor. Compression and levelling can be applied as required, but I'd recommend you monitor the output of the Dynamic Master when mixing, rather than simply setting up a mix and then compressing it, because with some types of material heavy compression can change the apparent balance of the mix, even though the sounds themselves seem unchanged. This is true of any compressor, though, and I have to say that the Dynamic Master manages to remain incredibly transparent under the most trying of circumstances.

The three‑coloured metering makes it a lot easier to understand what's going on, and because you can juggle the amount of limiter action against the compressor/leveller action, it's possible to achieve audible compression for use as an effect, if you run the limiter harder than you might do normally. However, the real strength of the Dynamic Master is its ability to control levels without displaying obvious side‑effects, so if you like to hear your compression working, this isn't the unit you should be looking at.

Summary

The HÖF Dynamic Master is a specialised device that's most useful when mastering, especially to a digital medium such as DAT. Despite its complex internal workings, the user interface is very friendly, with excellent metering. Applications include both digital and analogue mastering, broadcast limiting, disc cutting, and even cassette production. If you're making a series of cassettes for in‑car use, for example, you can hold the levels nominally constant without taking away the apparent dynamics of the original programme material. Obviously, the dynamic range is being reduced, but because the other audio cues are being preserved the sound doesn't appear stifled or compressed.

This is one of the best analogue mastering compressors around, but it's expensive by comparison with most regular compressor/limiters, and not dissimilar in price to the Aphex Compellor, with which it obviously competes. However, anyone with a cassette duplication system or mastering suite would do well to check out this machine, as it offers some very real advantages.