You are here

BSS Opal DPR944

Parametric Compressor/Gate By Hugh Robjohns
Published August 1999

BSS Opal DPR944

BSS are well‑known in the live‑sound market, but some of their well‑specified gates, EQs and compressors are equally at home in a recording environment. The DPR944, for example, has two gates and two parametric compressors, with independent ins and outs. Hugh Robjohns is impressed.

A decade ago I used to think of BSS as a company that made excellent DI boxes and system phase checkers. Today, the company produces a substantial range of sophisticated professional audio equipment mainly intended for the live‑sound market, such as the Varicurve equaliser/analyser and Omnidrive systems to optimise and protect, respectively, a PA system. They also offer a range of frequency‑dividing processors, crossovers, and active signal distribution units for multi‑band systems, and even delay units for time‑alignment of distributed PAs.

However, of more relevance to Sound On Sound, BSS manufacture some interesting gates, compressors and dynamic equalisers. The Opal DPR944 parametric compressor/gate under review here is, in effect, a combination of the gate technology from the DPR522 advanced dual gate unit, and the subtractive compression technique employed by the DPR422 dual compressor de‑esser. The DPR944 is, like the other units in the Opal range, a 1U rackmounting unit, and provides two separate (but stereo‑linkable) gates with external key inserts, plus a pair of 'parametric compressors' (also linkable). The device measures roughly 170mm front to back, and the case is painted in a durable black finish with an attractive opal‑green front panel and clear white legends.

The ergonomics of the machine are delightfully simple, and every push switch is illuminated to make the operating status of the machine very obvious. Normal operating modes are illuminated in green, whilst potentially dangerous states are yellow and positively disastrous ones are red — all very logical and helpful.

The rear panel is straightforward, with just four pairs of electronically balanced XLRs for the processor inputs and outputs. Labels adjacent to the inputs indicate that Pin 1 on all the female XLRs is ground‑lifted to minimise earth loop problems. There are also two quarter‑inch TRS jack sockets, one for each gate, providing key signal inserts. Mains power is connected via a standard IEC socket but there is no mains isolator switch on either the front or rear panels. A mains voltage selector and fuse are both provided and accessible from the rear.

In technical terms, the DPR944 boasts good specifications, with a noise figure of ‑95dBu for all processors, and distortion at 0.04 percent for the gate. The compressor exhibits a superb distortion figure of 0.005 percent when the signal is below threshold; this rises to a still‑very‑good 0.01 percent when applying 10dB of gain reduction at 1kHz. Input impedances are sensibly set at 10kΩ and output impedances are usefully low at 50Ω, so driving long cables shouldn't be a problem. Frequency response is flat within a quarter of a decibel between 20Hz and 20kHz and crosstalk between processors is below 85dB.

Shut The Gate

BSS Opal DPR944

The two independent gate sections occupy the left‑hand side of the panel, whilst the two compressors are on the right. Taking the gates first, the main parameter controls are a red illuminated 'wine‑gum' lozenge button to switch the processor in or out of circuit, a threshold control (down to ‑50dBu) and a release control (1mS to 4 seconds). The release control is quite clever in that it follows a convex curve, initially falling very slowly with a built‑in hold period of around 20 percent of the total release time. This allows the programme signal to exhibit its natural decay dynamics but, as the release period nears the end, the attenuation is applied much more steeply, chasing the background noise down before it becomes obtrusive.

The Opal DPR944 is a useful machine, equally at home in the project studio rack or live sound flightcase.

Neat oval buttons switch the gate depth (range) between ‑80 and ‑20dB, provide a slow attack mode (2mS instead of 40µS), and a pair of LEDs to indicate when the gate is open (green) or shut (red). Reducing the gate depth to 20dB allows some background spill to bleed through and often sounds a lot more natural than the artificially 'dead' sound of an 80dB switch‑off. It also allows the gate to open faster, as it is obviously quicker to remove 20dB of attenuation than 80dB! Normally the fast attack mode is preferred in gating applications, but with some delicate signals this can often damage the starting transient and produce a click. In such circumstances, the slower attack time offers an alternative.

So far, these are all standard facilities for a gate, but the interesting part is the key filter section. A third control knob alters the centre frequency (60Hz to 12kHz) of a three‑octave band‑pass filter in the side‑chain which modifies the gating control signal, making it more or less sensitive to specific parts of the frequency spectrum. The idea is to remove signals not required to control the gating action, hopefully making the gate more reliable in operation, with less false triggering. An additional button allows the filter bandwidth to be reduced to just half an octave, making it extremely selective if required. To assist in the tuning of this filter another button allows the side‑chain key signal to be auditioned through the main signal path (this is illuminated red when active, since it is a dangerous operating mode).

The final control is a stereo link button (illuminated in yellow) which commons the side‑chain key signals of both channels so that a suitable trigger from either will open both gates identically. All of the controls of Gate 2 are disabled in this mode, Gate 1 providing control of all parameters.

The rear‑panel key insert socket allows an external processor to be used to trigger the gate. The insert jack's send signal is just a buffered version of the main audio input, while the return is routed into the side‑chain signal path. The section of the handbook dealing with connector wiring suggests the output signal is taken after the band‑pass filtering, but this is not the case. In a gate such as this, where a decent side‑chain filter is already provided, it is far more likely that the key insert would be used to provide an external trigger input to the gate, rather than to use external key‑signal processing. It is therefore extremely inconvenient that the return signal is on the ring connector of a TRS plug, and a special 'bodge' lead would have to be constructed to access this facility.

Parametric Compression

The two parametric compressor sections of the unit each comprise five rotary controls, three buttons and a dual function LED bar‑graph meter. The same style of wine‑gum lozenge switches each compressor into circuit, and two smaller oval buttons provide a side‑chain listen facility (illuminated red) and a fast release time option. All the usual compression controls are provided, including threshold (out to ‑30dBu), ratio (up to infinity:1), and gain make‑up (‑20 to +20dB). The release operation is a programme‑dependent auto mode which exhibits a fast release for transient peaks with a slower recovery for less dynamic changes, thus avoiding obtrusive pumping or breathing effects. The fast release button reduces the time constants by a factor of ten, making the compressor more appropriate for percussive sounds, for example.

The metering is unusual in that the bar‑graph array is actually two meters end‑to‑end. The right‑hand side consists of seven red LEDs showing the gain reduction applied, first in 3dB then 6dB steps, down to ‑24dB. The left‑hand side initially looks like a normal input‑level meter, but closer inspection reveals that it actually shows the input signal level in relation to the threshold set on the relevant control knob. The bottom four green LEDs show the signal level down to 20dB below the threshold in 3, 6 and 8dB steps, whilst the top oval yellow LED indicates that the threshold has been reached or exceeded. Consequently, as the signal level increases, the LED bar‑graph shows a growing row of green lights, followed by the yellow threshold light, at which time the red gain‑reduction LEDs start to illuminate in a decending row back towards the threshold lamp. This distinctive system is both attractive and effective, and I grew to like it a lot.

A fourth button activates the stereo link mode (yellow). As with the gates, the side‑chain signal of both channels is combined and processed by the side‑chain filter, threshold, ratio and release controls of channel 1 before being applied equally to both channels. However, the make‑up gain controls remain independent for both channels, as do the filter bandwidth controls. The compressors also offer a useful frequency‑selective mode — see box, below.


The Opal DPR944 is a useful machine, equally at home in the project studio rack or live sound flightcase. The combination of a pair of gates and subtractive compressors in the same box covers many applications more cost‑effectively than separate units because of the shared expense of the power supply. However, I am not too happy about the complete absence of a power switch, which might be seen as an advantage in a live sound application, but could be a drawback in a project studio. Also, needing to return an external key signal to the gate on the ring contact of a TRS jack is inconvenient. These are both relatively trivial complaints, though, and may well hold no relevance for a lot of potential users.

The sound quality of this unit is very good, the gates working well and resisting false triggering to a good standard on real instruments in difficult situations. The compressors are also quite excellent as broad‑band units, with a very transparent sound quality even when working hard. However, their real strength is in the frequency‑selective subtractive technology, which allows troublesome regions to be controlled without affecting the rest of the audio signal. Overall, then, a very powerful and useful tool.

The Selective Approach

Like its gates, the 944's compressor provides frequency‑selective side‑chain processing, which is operated via a pair of rotary controls, with a button to audition the side‑chain signal. The frequency control determines the centre frequency of a band‑pass filter tunable between 60Hz and 12kHz, whilst its bandwidth is set by the adjacent control over a range from 10 octaves (ie. full audio bandwidth and effectively out of circuit) down to a narrow two fifths of an octave. So far, this is very similar to the side‑chain processing used in the gates, but there is a very significant difference in how the processing is applied.

Most compressors operate on the entire frequency spectrum at once. A loud bass signal, for example, will cause the entire audio programme to be reduced in level. Some compressors include side‑chain filters to reduce, for example, susceptibility to dominant bass signals by filtering LF out of the control circuit. However, any peak signal will always cause the entire spectrum to be attenuated. Multi‑band compressors overcome this problem entirely by independently processing three or more separate frequency bands, but the necessary band‑splitting, parallel processing and recombining makes for an expensive and complex machine. The DPR944 provides a simpler facility, but one which is still extremely effective in a wide range of applications. Instead of reducing the level of the entire audio spectrum when a peak is encountered, it only applies attenuation to the selected frequency band, leaving the rest of the audio spectrum unaffected.

For example, consider a vocalist troubled by excessive sibilance. By auditioning the side‑chain, the frequency and width controls can be optimised to highlight just the sibilance region. The threshold and ratio controls are used to determine the amount of gain‑reduction applied to signal peaks in this frequency region. This level‑controlled and band‑limited signal is then subtracted from the main, full spectrum, audio signal so that only the sibilance frequencies are reduced. This is a very powerful technique which BSS implement on all of their current range of compressors, and it is extremely effective.

The subtractive technique has a wide range of applications besides de‑essing, and I tried out another example suggested in the handbook to balance the level of a guitar between picking and strumming, reducing the powerful mid‑frequency energy generated by strumming. The technique has to be applied carefully as it obviously alters the tonal balance, but I found it very effective within a complex mix.


  • Ease of use.
  • Attractive styling.
  • Excellent sound quality throughout.
  • Broad‑band or frequency‑selective compression.


  • No mains power switch.
  • External key signal input on ring of TRS plug.


A good‑sounding and ergonomically well designed unit, with the useful ability to perform frequency‑selective compression.