AMS RMX-16 Digital Reverb
The device under review here is an up-market, highly professional unit and this is reflected in its £4500 plus price tag. The purpose of reviewing such devices is not to tempt you with what you can never afford, but rather to show you just what a 'state of the art' machine is capable of.
Another reason for looking at this instrument is that the underlying technology is constantly becoming less expensive, and already less sophisticated machines are appearing on the market at a price which is realistic to the home recordist. The first of such machines is the Yamaha R1000 which offers considerably less variation than the AMS and at a justifiably lower price.
Traditionally, reverberation has been produced by spring or plate methods, but to simulate a natural-sounding reverb this way is both difficult and expensive. The cheaper spring units suffer from an unnatural response to transients making them all but useless for percussive sounds, whilst the more sophisticated units cost almost as much as a good plate reverb.
The plate on the other hand is capable of truly excellent results on almost all types of signal programme but, apart from being expensive, it is large and needs to be acoustically isolated, as the plate itself is extremely microphonic.
At this point, enter digital technology. Standard delay techniques cannot produce a convincing reverberation effect and to understand the reason for this, we must examine the principles of reverberation itself.
Imagine that you are in an empty hall and someone claps their hands. What happens? Well, first you jump out of your skin because the hall is supposed to be empty... Seriously though, you'll initially hear the direct handclap and your binaural interpretation of it will help you to locate the source of the sound. The sound of the clap then radiates in all directions and on encountering a solid surface such as a wall or floor, it will be reflected, albeit at a lower volume and properties of the reflecting surface. It is these early reflections that are heard next, and these give the brain information to estimate the size of the room and, to some extent, the type of wall material.
These reflections continue to rebound from the walls, all the time diffusing and losing energy until they eventually die away altogether.
Using a normal delay or echo unit, we can produce repeats which are all the same distance apart (temporally speaking) which, when using a short delay time with feedback, produces a metallic, resonant effect which bares little resemblance to natural reverberation.
The digital method currently employed to produce realistic reverberation involves digitising the incoming audio signal and then performing a series of arithmetic manipulations on the stored data before reconverting it to an analogue signal. Spectral filtering of the decaying sound is also employed to simulate the effect of different environments.
As might be expected, these arithmetic manipulations are under the control of a microprocessor in order to achieve the speed necessary to produce the effect in real time. Furthermore, by altering the algorithm to which the manipulation is performed, different types of reverberation can be produced including some unnatural special effects.
The RMX-16 from Advanced Music Systems digitises the incoming signal into 12 bits plus 2 bits of range data which are then converted into a linear 16 bit format and placed on the data bus. This 16 bit information is stored in RAM where it is later read and manipulated by an array of arithmetic processors which operate according to the 'program' algorithm.
A 64 kilobyte RAM memory provides the necessary time delays and 'scratchpad' memory. Two 16 bit 'words' emerge from the arithmetic processor and are converted into left and right output signals via the usual low pass filters to remove quantisation (conversion) noise and giving a 90dB dynamic range.
When a new reverb 'program' is selected, the microprocessor instructs the arithmetic processor to work to the appropriate algorithm, and at the same time mutes the outputs for a short time to avoid a burst of 'rubbish' being discharged onto the audio lines as the memory is flushed out.
The machine comes in a well-constructed, fan-cooled, 2U rack unit and is supplied with nine reverberation programs which reside in non-volatile memory. Nine further user-definable settings may also be stored in memory and, if the optional remote terminal is used, up to 99 user-definable programs may be stored.
A useful aspect of this system is that the settings stored within the remote terminal, being non-volatile, can be implemented using any RMX-16 (or DMX-15 delay) in any studio throughout the world.
The front panel contains a calculator-style keypad for entry of parameter values or the recall and storage of reverb programs whilst the parameters themselves may be accessed via a row of LED illuminated pushbuttons below the numeric display.
Having decided to alter a specific parameter, for example, pre-delay time, the preset value may be changed by inputting the new value via the keypad or it may be incremented or decremented using the up/down 'nudge' buttons.
If an illegal parameter value is entered, an 'E' appears in the display to inform you of your sins. All parameters are constantly displayed on the front panel whilst the title of the current reverb program is indicated to the right of the numeric display.
'Delay time' may be entered via the keypad, changed by means of the nudge buttons or assigned to a pot on the front panel (left) for continuously variable adjustments.
As the control program and system parameters reside in EPROM memory, the system may be easily updated in the event of any software improvements. There is even provision for the addition of a bar code reading pen for immediate software updates from printed paper sheets.
Input and output levels are adjustable to accommodate a variety of signals, and four coloured LEDs between the level pots permit fine adjustment of the input signal for optimisation of the signal-to-noise ratio. Line outputs are balanced with XLR connectors, but may be used unbalanced if required.
As the name suggests, this is intended to simulate the characteristics of a reverb plate device which it does with surprising authenticity. Sound diffusion is rapid and the effect is flattering to percussive sounds. Pre-delay of up to 300 milliseconds may be selected and the 'Decay' time varied from extremely fast to unusable - with some excellent effects to be found in between. Both 'High' and 'Low' Decay filter settings may be modified to simulate the effect of different acoustic surfaces ie. amount of absorption of sound.
This setting is useful for the general enhancement of a variety of programme material and the sound is a blend of plate and hall-type reverberation. Pre-delay up to 200ms is selectable and again the Decay Filter is active.
This simulates a 'live room' rather than a hall consequently the effects of standing waves and colouration may be heard at longer decay times. Used sparingly, this setting is useful to take the edge off 'dry' (unreverberated) material. Pre-delay up to 300ms is again permissible.
Characterised by its strong initial reflections, this setting adds presence to vocals, and again, both filter profiles may be modified.
This is a pure delay program of very high quality with a maximum delay time of 810ms. The Decay control may be used to provide feedback in this mode so that multiple echoes may be induced. This effect, however, is mono.
This is a subtly different alternative to Plate B1, again very realistic with just the right amount of colouration. Pre-delay up to 300ms plus Decay filtering.
Similar to Hall C1, with the exception of the early reflections which are less predominant in this case. Pre-delay up to 300ms plus Decay filter.
This is a 'special effects' program and any resemblance to any acoustic environment whether live or dead is purely unintentional.
The sound does not decay for the first period set by the Decay control, but then dies away rapidly. Pre-delay may also be used. Both signal outputs differ when in this mode, output 1 being more diffuse than output 2.
I found this effect most suitable for enhancing handclaps and producing modern 'gated reverb' type drum sounds, but only if used in moderation.
In this mode, the envelope of the reverberation 'tail' is reversed, creating an eerie backward tape effect to the sound.
This treatment is particularly effective on vocals and percussion, if the Decay time is not set too long. Excessive delay times on percussive sounds produces a raspy croak which probably has little artistic usefulness. Again pre-delay may be utilised to render the effect even more bizarre.
Any user-modified versions of the previous program settings may be stored for instant recall and the non-volatile memory means program settings are not lost when power is turned off. Memory is maintained by means of a rechargeable battery which is trickle-charged from the power supply when the unit is in case.
The quality of engineering design and sound, is up to the highest standard, whilst the 18kHz bandwidth and 90dB dynamic range can only be gazed upon with awe.
The price is high, but not unreasonably so when compared to other industrial electronics of a similar complexity.
A good reverb plate system would undoubtedly cost more and it is unlikely to offer the flexibility, bandwidth and good noise performance of the RMX-16. The only problem I can see with this machine is that I've got to give it back!
Review by Paul White
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