We are far beyond the time that we allowed DSP audio processors prove themselves. There is no doubt that the power available through DSP has been the best thing that ever happened to audio processing. Yet, for the most part, most of the DSP-based audio processors available up to this point have been beyond the economic reach of many stations in smaller markets. Broadcast Warehouse, an English company has introduced the DSPX, the first DSP-based audio processor designed and fabricated in the United Kingdom. It has most of the features of the more expensive entries to the field, yet it is more in line with smaller market budgets.
The unit itself occupies one rack unit of space, with a depth of 7.5". The analog inputs and outputs, along with the AES in and out all use XLR connectors. All processing adjustments are made on the front panel via a control knob and three soft-keys. There is a convenient headphone output located on the front panel as well. Navigation of the menu tree is possible via a front panel LCD display. The processing parameters are also available via computer control, either via a serial connection or an Ethernet connection. All the critical processing activity can be viewed at the same time by a quick study of the front-panel LED displays, which are the blue ones.
The basic processing chain is straightforward and should be familiar to audio processing veterans. Adjustments to the processor are easily done because the control features are intuitive and will be familiar to anyone that has used an LCD display and menu tree combination.
Performance at a glance
|1RU DSP-based audio processor|
Analog and digital inputs
Analog, digital and composite outputs
Front-panel LCD menu-tree
Dual audio control paths
19 factory presets
Communication via serial or Ethernet
The top of the menu tree consists of four adjustment menus: input, process, output and system. By rotating the knob to highlight the menu and then pressing the knob, you open that menu, allowing access to more specific functions. For example, the process menu allows access to the parameters of the wideband AGC, the low frequency enhancement, the multi-band AGC, the multi-band limiter and the virtual mixer. Escaping to the last step up the menu tree is accomplished with one of the soft keys.
The low frequency enhance menu controls the low-frequency processing in the DSPX. Low-frequency shelving (a 12dB/octave shelf with up to 12dB of boost) and a peaking bass amplifier are available, which is essentially a parametric EQ. There are four choices for the peak frequency, four Q settings and variable gain that can be changed in 1.5dB increments from 1.5dB to 6dB. Each of the four bands in the multi-band AGC section has the following adjustments: drive, attack speed, release speed, compression ratio, gating level and RTR level and speed. RTR simply means return to rest, which is the point that the AGC will seek when gated. Each of the four bands in the multi-band limiter section has the following adjustments: drive level, threshold, peak attack, peak decay, average attack, average decay and hold. My experience with the DSPX is that the multi-band limiter threshold adjustments are the most effective means by which distinctive changes in the sound of the unit can be made.
Getting to know it
One of the important features of the DSPX is its second peak-control path known as DR (for digital radio). This is the preferred output path to use in the event that the DSPX is used to drive a bit-rate-reduced audio codec. Instead of clipping a signal, the DR path uses look-ahead limiting, eliminating distortion products that waste bits in the output data stream. The first peak control path, called FM, uses conventional techniques and always feeds the stereo generator. The user chooses which peak control path is used to feed the analog outs and the AES out.
While testing the unit, I was informed that a new version of its software was available, and I downloaded it with a link supplied by the manufacturer. In the event that future software updates are made available, they can be uploaded in to the DSPX easily and rapidly via the front-panel RS-232 connector.
All potential users want to know how the DSPX sounds, so I set up a standard test just to judge it for myself. I took the composite output of the DSPX, ran it through a coaxial switch and out to a late-model analog exciter. I then sampled the RF output with a standard run-of-the-mill FM tuner, and took the output of that tuner to an amp, driving both speakers and headphones. The other input of the coaxial switch was the composite out of a famous, high-end audio processor. The music source was a classic-rock format from one of our stations in Seattle. Bar-graph metering on the exciter allowed me to match the composite levels exactly, thus affording me an apples vs. apples test.
I found that the DSPX is easy and quick to adjust, but I also found the best results were achieved when I started with one of the 19 factory presets. By doing some EQ tweaks, and moving the thresholds of the multi-band limiters, I achieved a respectable and competitive sound with this unit. It was close, in practice, to the sound of a much more expensive unit. Its dual outputs are useful in this day and age of streaming audio and IBOC. If you are in the market for a DSP-based audio processor, this unit deserves your attention.
Irwin is director of engineering for Clear Channel Radio in Seattle.
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