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Dynamics Processor
By Hugh Robjohns

Although XTA are perhaps better known for their live sound processors, their new SIDD dynamics processor provides world-beating dynamics processing which is just as useful in the studio.

Although not particularly well-known in studio circles, British company XTA, based in Stourport-on-Severn in the West Midlands, are one of the leading lights of the live sound industry throughout the world. The DPA324 SIDD is a dual-channel digital dynamics processor that was launched two years ago at the AES in 2000, but to call it simply a dynamics processor is to do it a huge disservice — by any standards, this is an awesome machine designed to provide some of the most powerful and competent dynamics control I have ever come across in a single unit, combined with comprehensive EQ and various other functions I'll come back to later.

XTA DPA324 SIDDPhoto: Mike CameronAlthough primarily designed to meet the needs of the live sound industry, the SIDD also has applications in the music studio, mastering room, or broadcast centre — anywhere where comprehensive and precise dynamic control is required. However, it probably goes without saying that nothing comes for free. The down side of a hugely comprehensive dynamics processor like this is the complication of setting it up and using it, but XTA have tried, with some success, to overcome this inherent problem in two ways. The first is to offer several levels of basic and advanced operating modes — much like the Go and Pro modes favoured by the larger Lexicon reverbs. The second is a PC remote-control package, which provides a very attractive and usable graphical user interface — see 'Siddcore Remote Control Software' box for details.

The Mechanics

The SIDD is housed in a 1U rackmount case which sports a subdued blue paint scheme on the front panel, with gold legends. The four prime dynamics processes for each channel (dynamic EQ, expander/gate, compressor and limiter) each have individual access buttons and LED gain reduction meters to show what is going on. There are also separate input and output bar graphs for each channel. The left side of the panel contains a 2 x 24-character backlit LCD, and just six buttons and three velocity-sensitive rotary encoders are provided to configure this monster!

XTA DPA324 SIDD £1757
  • Simply the best and most comprehensive dynamics processor available.
  • Multiple levels of operational complexity.
  • Transparent operation and excellent sound quality.
  • Flexible additional features, including EQ, delay and harmonics generation.
  • Siddcore remote control program.
  • Complex to set up, especially from the front panel.
The mother of all dynamics processors with far greater flexibility and control than any processor I have ever come across. In addition to the independent compressor, limiter, expander/gate and dynamic equaliser (all with side-chain EQ and look-ahead facilities), the box includes six bands of parametric EQ, valve simulation, delay and multiple outputs.


There is also a PCMCIA card slot to accept 256K SRAM memory cards which can be used to carry user programs (the unit also contains 256 internal preset memories), or software update information. MIDI, RS232 and RS485 serial ports are provided on the rear panel to facilitate remote control. The RS232 link is intended for connection with a PC running the remote control software, while the RS485 is a more robust serial link which can be conveyed using standard mic cables over considerable distances, and is intended to link several SIDD units together in large, complex PA systems. As standard, the unit is equipped with two analogue balanced inputs and four balanced outputs — two main and two auxiliary — all on XLRs. The aux outputs can be used for a variety of purposes, including using them as the auditioning output for monitoring selected dynamics side-chain signals, or as a separately delayed and level-controlled output for feeding a distributed PA system, for example.

The unit's spec sheet claims a 20Hz-20kHz audio bandwidth (±0.5dB) and an impressive unweighted dynamic range of better than 110dB. In addition to the standard analogue I/O, AES-EBU digital inputs and outputs can be factory-fitted as an option, and isolation transformers fitted to the analogue I/O.

Fully Featured

The XTA SIDD offers a huge amount of high-quality signal processing. Besides the fully comprehensive dynamics, each channel boasts a six-band parametric equaliser with separate high- and low-pass filters, two short delay lines (enabling automatic double tracking effects or output time correction) and a harmonics generator to add 'valve-like' warmth. The dynamics elements are extremely well-specified and, being digital, the system has been designed to incorporate a look-ahead facility, where the signal path is delayed (up to 60µs) relative to the side-chain detection circuits. This enables the gain reduction to be changed prior to the arrival of the signal itself, solving the problem of overshoots and damaged transients.

The complement of dynamics processes includes a compressor, limiter, expander/gate, and dynamic equaliser. The compressor is a variable-knee design with two bands of parametric side-chain equalisation, while the output limiter can be configured as a true zero-overshoot, brick-wall limiter with two further bands of parametric side-chain equalisation. The expander/gate section enjoys both high-pass and low-pass filters and two parametric EQ bands in its side-chain, and the look-ahead facility can be used to open the gate slightly in advance of a signal, so that the initial transient of the wanted signal is not damaged.

The numerous independent side-chain equalisers obviously allow a degree of frequency-conscious dynamic control, and while a section is being adjusted, the side-chain signal can be auditioned to aid in setting it up, either through the main output or the auxiliary outputs, depending on how the unit is configured. A dedicated dynamic equaliser is also provided and can be used to compress or expand only signals in specific frequency regions.

Accessing any of the four prime dynamics processors for manual adjustment is as simple as pressing the relevant front-panel button. The other processing functions are adjusted by pressing the A or B button below the corresponding channel input meter, and then calling up the required parameters on the LCD window with the Next and Back buttons. When the required set of parameters is displayed, the three encoder knobs are used to adjust the corresponding parameter (up to three per menu page). While this is a little laborious, it is logical, although life is far easier using the remote control program!

XTA DPA324 SIDDPhoto: Mike Cameron

Moving on to the other processing elements, each section of the fully parametric six-band equaliser can have its centre frequency tuned between 20Hz and 20kHz. In addition, separate second-order (12dB/octave) high- and low-pass filters are included. A short delay line (300ms maximum) is provided, along with an ADT (automatic double-tracking) module. The Harmonics Generator creates second and third harmonic distortion, the amounts of which can be controlled precisely.

The two channels can be set up to operate in ganged and linked modes and here a word of explanation might be useful. The ganging mode makes one channel the master and the other the slave, so that any parameter changes made to one channel affect the other equally. However, the side-chains of the various dynamics elements are not automatically linked to ensure a stable stereo image when the channels are ganged — linking is done separately on a section basis. While this may initially seem a little odd, it affords considerable flexibility in situations where the two channels are related in terms of their set up and parameters, but are not handling stereo information, and therefore do not required linked side-chains.

So, a lot of processing power with a huge degree of flexibility in a very small box — hardly a recipe for the most ergonomic and controllable of products. Nevertheless, XTA have done a very good job in their user interface and, while a lot of button pushing, menu diving and knob twiddling is inevitable during the set-up phase, once the system is doing what you want, fine-tuning is very easy and immediate. Favourite patches can be stored in the user memories, of course, to make subsequent use a lot faster.

 Siddcore Remote Control Software 
 XTA DPA324 SIDD dynamics processor.The XTA SIDD's Siddcore remote control software package will run on any PC using a Windows operating system from Windows 95 onwards. The latest software version available from the web site is a beta of v1.07, although this will only run with SIDD units updated to v1.80. A standard RS232 serial link via a nine-pin D-Sub connector is used to link the PC and SIDD. The control data is bi-directional so that changes made through the Siddcore software occur in real time and changes made directly to the unit are reflected on the PC screen immediately. The main screen provides separate windows for each processing module of each channel, and the arrangement of these windows may be customised to show only the wanted information — preferred configurations can be stored in one of eight memories.

A channel overview mode shows simultaneously all the windows associated with a specific audio channel, and if the SIDD is adjusted manually, the relevant parameter window is automatically brought to the front of the PC screen to highlight the alteration. Other windows include real-time metering of both input and output levels as well as the gain reduction of each connected unit, as well as EQ and dynamics windows. The EQ frequency plots (for both main and side-chain EQs), and the dynamics transfer curves, can all be adjusted by clicking and dragging the appropriate line on the screen.

In the dynamics windows, the transfer function curve shows how the compression or expansion slope is set up, and a 'bouncing ball' shows the audio level relative to the curve (two different coloured balls are shown when linked for stereo). This configuration makes it very easy to visualise the ratio and threshold settings. For the Luddites, there is also a screen showing a traditional VU meter displaying the gain reduction.


Using The SIDD

The SIDD is designed to be used either as a processor for individual channels or groups, helping to control and refine sources and submixes, or as a stereo unit across the final stereo output of a console as a mastering process. The EQ and Limiter algorithms have been borrowed from some of XTA's other digital products, but the rest of the dynamics processing is new. The processing algorithms operate with double-precision signal processing and a 40-bit internal data path. Being a digital product, each parameter is not continuously variable, but has a large number of predetermined values. For example, the equaliser band centre frequencies are adjusted in 1/36-octave steps providing 368 settings between 20Hz and 20kHz — more than fine enough for normal usage. Similarly, the EQ bandwidths can be set to 100 different Q values between a very broad 0.4 (2.5 octaves) and a surgically precise 128 (tight notch). Any EQ band can also be switched to have a shelving response.

The EQ is very precise, but sounds less digital than many. The dynamics are all astounding, remaining extremely transparent even when driven hard. Most of the familiar side effects of heavy dynamics control can be introduced if required, but this unit is really intended to effect complete control without drawing attention to itself, and it does that remarkably well. The look-ahead facility is a key factor, enabling the machine to reduce the gain in advance of a transient peak, or to open the gate fractionally before a drum sounds — both situations preserving the shape of the transient perfectly. This is something most compressors and gates fail spectacularly to do, although that is part of their character of course!

The harmonics generator is very effective in thickening the sound and making it both seem louder and 'warmer' — the relative amounts of second and third harmonics afford a lot of variety in the quality of the sound. The ADT effect is inherently limited and does not bear comparison to the state-of-the-art solutions presented in units like TC Electronic's Voice One (reviewed in SOS August 2002), but then this function is a freebie in the SIDD, rather than its raison d'être.

I can understand why the SIDD has become as popular as it has in the live sound market. It is phenomenally powerful, comprehensively equipped, extremely accurate, and sounds very clean. In applications where the same settings tend to be used repeatedly, it is an ideal solution, since it can be set up and more or less left alone. However, where the configuration of which processing elements are used and how they are set up is likely to have to be changed on a regular basis I fear the SIDD's sheer complexity will make it a little less ideal. In these kinds of applications the Siddcore program is essential to provide a much slicker and easier interface. Certainly, I found driving the unit from its front-panel controls rather wearing after a couple of intensive hours' use!

I think for the average user the SIDD is probably too flexible and too well-equipped, and consequently too complex for most small studio applications. Nevertheless, this is a truly professional tool which is thoroughly impressive and extraordinarily competent in what it does. If you have a dynamic control problem which cannot be solved with more conventional equipment, the SIDD is almost certainly the solution you require. Remarkable!

 Test Spec 
  • XTA SIDD OS v1.74.
infop.gif £1756.65 including VAT.
infot.gif Beyerdynamic +44 (0)1444 258258.
Published February 2003