by Hugh Robjohns

The Focusrite Liquid Channel has been one of the most keenly awaited products of the year (or last year for that matter!), and at recent trade shows the Focusrite booth has always been jam packed with people keen to see what all the fuss is about for themselves. It is the result of three years of joint research and development between Focusrite and Sintefex. I previewed of the product back in SOS December 2003, but the completed product has only now been launched. Part of the delay in bringing this product to market has been the sheer time it has taken to create all of the necessary convolution data required for the 80 simulations of mic preamps and dynamics units. The company have also been working hard to complete the remote control software.

Another issue which is still to be fully addressed, but which has slowed progress slightly, is that of copyright. A convolutional processor is, in theory, able to faithfully recreate every sonic nuance of any chosen product. It could be argued, therefore, that by replicating a manufacturer's product with a convolution processor, their copyright may be infringed. Looking at it another way, had that manufacturer's product been reverse engineered and the circuitry copied component for component, the breach of copyright would be fairly obvious — yet the convolutional approach, in theory, produces indistinguishable results. It's certainly a thorny and untested area of law, and it will be interesting to see how things resolve in the future.

Of course, this is wholly irrelevant to the user — you buy the box, dial up the effects you require, and get on with making music — but there are implications as far as publications are concerned. In reporting on direct comparisons between the Liquid Channel's simulations and the original hardware units, legal advice suggests that it is prudent for us only to identify by name units which are out of production or which are from Focusrite's own stable. Having said that, it is logical to infer that the accuracy (or otherwise) of any one emulation should translate to all the others...

These effects play a very significant role in creating the overall sound character, as anyone who has played with a switchable-impedance preamp can confirm, and they cannot easily be created by convolution technology. To do so would require each and every preamp to be measured with each and every microphone, which is clearly an impractical thing to do. The solution which the Focusrite engineers came up with was to simulate the preamp's front-end characteristics in hardware — although even this means that considerable additional measurements are needed to analyse the interface properties. The result, though, is that when the user dials up a preamp Replica the convolution processing takes care of the preamp's own sonic characteristics — the effects of discrete solid-state circuitry, valves, op amps, and so on — while the configurable hardware ensures that the microphone 'sees' exactly the same input impedance and properties as if it were connected to the actual product being simulated. The overall sound character and tonality should therefore be exactly the same as if the identical mic were connected to the original preamp.

Making a configurable input stage is not simple, and goes against the normal practice of keeping the microphone signal path as short and simple as possible. Focusrite have had to come up with a complicated arrangement of very high-quality relays to switch various inductors, capacitors, and resistors into the input circuits, in order to provide the appropriate impedance characteristics. Furthermore, a special custom-made transformer can also be switched into the input circuitry when necessary, because it was found that some microphones react with the transformer in a way which can't be captured by convolution.

MORE THAN JUST A PREAMP

Replicating the characteristics of a reverberant space, equaliser, or compressor are far from trivial, as already discussed, but once the measurements have been made it becomes a well-defined problem to solve. Replicating the characteristics of a preamp seems, at first sight, to be equally straightforward... until, that is, you consider the interaction between the microphone and preamp.

Equalisers and compressors are generally line-input devices, and the signal levels and impedances for line-level signals are well defined. Provided the device has a reasonably high input impedance, and the source has a reasonably low output impedance, the actual interface plays a negligible part in the sonic character. The same is not true for microphone preamps, where the relationship between its input impedance and the output impedance of a microphone is more complex, with far greater variations between products.

Whereas line inputs tend to be predominantly resistive, microphone circuits are inherently more reactive, with significant capacitive and inductive components to consider, particularly where the preamp has a transformer input circuit. As a result, the same microphone may sound different when connected to alternative preamps, and vice versa.