19" PreSonus DCP8 & ACP8 8-channel Dynamics Processors (1997?)

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Ever fancied having a separate compressor, noise gate and fader automation on every channel of your mixing desk? The latest modules from PreSonus offer all of this and more, bringing that elusive 'SSL sound' within the reach of even the most basic consoles. DOMINIC HAWKEN puts the new units through their paces.
 

Over the past few years, a number of mid-priced mixing consoles which set new standards in sound quality and signal-to-noise statistics have hit the market. These desks have effectively bridged the gap between the high-end systems in use at some of the top studios, and the budget desks that are usually the only option for home and demo users. Silicon technology has made possible low-cost, low-noise equipment capable of producing master-quality recordings for a fraction of the price paid by the artists of yesterday. The number of hit records and soundtracks mixed on affordable consoles -- produced by the likes of Allen & Heath, Mackie and Soundcraft -- is on the increase, as the dividing line between home and professional studios becomes ever finer.

The professionals, however, always seem to remain one step ahead of the rest. Given the choice, most producers would opt for SSL, Neve or Euphonix consoles to master their recordings on, as they offer a clarity and separation that still outclasses the less expensive consoles. It is also true that if a record company has invested large sums of money in a new act, they tend to feel a lot more comfortable if the final recordings are mastered in a high-end studio, and this logic is usually rewarded with superior-sounding results that cut across the competition.

One of the main reasons that the more expensive desks tend to produce better mixes lies in their inclusion of individual signal processors within each channel. Every discrete sound entering the console can be compressed for extra punch or control, and gated so that extraneous noise can be eliminated, cleaning up the overall sound. They also have comprehensive automation facilities, logging fader movements and channel mutes into a central computer that can play back or edit entire mixes at the touch of a button. Couple these facilities with a solid equalisation section and a good stereo compressor across the entire mix -- something that the SSL, in particular, is noted for -- and you have the tools to create a sound to rival any of the productions around at the moment.

But there's hope yet for those people forced to master their recordings with a more basic setup. The lack of a comprehensive EQ can be overcome to a large degree by using good source sounds, and final stereo compression can always be left to the cutting room. The basic limitation of the inexpensive mixing desks on the market lies in their lack of individual channel compression and gating. Until now, it's been pretty impractical to buy separate outboard units to cover each input. Even if you had the space available, it would be far better to spend the money on a new computer system or keyboard and put the change down as a deposit on a house in the country!

That was until PreSonus came on the scene, with their launch of two new rack modules designed to complement any console fitted with the necessary insert points. The DCP8 is a digitally-controlled analogue compression and gating system, with signal limiting and level automation built in as standard. Its cousin, the ACP8, is an analogue version, adjusted via front-panel controls rather than MIDI information, and both units are capable of handling eight channels of audio simultaneously. At last, a cost-effective way of transforming your mix quality.

ACP8 FEATURES
Taking up only 2U of space in a studio rack, the ACP8 is controlled completely from its front panel. This is split into eight sections, each identical and covering an individual channel of sound. Unlike the DCP8, the ACP8 provides no automation facilities, and is therefore more likely to find favour in both live and studio-based environments; it would certainly be a useful addition to any rack of multi-channel outboard. Each of the eight sections offers a comprehensive compressor and gate, as well as an overall level control and a number of switching options.

The compressors each feature full control of the signal threshold, compression ratio (adjustable between 1:1 to 20:1), attack, release and gain. The gates feature threshold, release and attenuation. External connections to the unit are via rear-panel sockets, and the system accepts both balanced and unbalanced signals for input and output. There is a side-chain jack on each section, allowing spectral and effect processing (see 'Side-chain Spectral Processing' box), and a key input for each of the gates, which allows the signal to be 'cut' against a separate audio trigger. Buttons select hard/soft-knee compression, peak and auto limiting, bypass, and channel linking -- whereby any individual channel (apart from the first) can be linked to the channel on its left for smooth stereo or multi-channel processing.

Balanced signals are delivered via stereo jacks, which revert to unbalanced operation when a mono plug is used. Side-chain connections also use stereo jacks (on the same format as most standard insert sockets), and the gate key is via a mono jack.

DCP8 FEATURES
The DCP8 crams all of its functionality into 1U of rack space, and offers the same audio specifications as the ACP8 (with the exception of a side-chain option or trigger inputs). The extra cost of the unit is reflected in the inclusion of a comprehensive automation system that allows full computer-based control of all internal settings and features. Ideally suited to a MIDI-based studio environment, the unit is also designed to plug into the insert points of your mixing console, and adds compression, limiting, noise gates, and full level/muting control on each of its eight channels.

The front panel features a number of buttons, and an LCD display to show the current status, as well as a data wheel and a separate program number LED to show the currently-selected patch. Up to 100 audio 'scenes' can be named and stored within the unit for instant recall -- very useful for live situations. It is also possible to access and adjust all the internal parameters without the need for a computer, by using the control buttons on the front of the unit. To change a gate threshold, for instance, you press the Gate button, cycle through the available channels using the Up/Down buttons, and then adjust the value with the control wheel.

Where the DCP8 really excels, however, is with its automation facilities. Each parameter can be adjusted remotely via MIDI controller messages -- easily sent from virtually any MIDI sequencer. Mixer maps are already available for both Steinberg's Cubase and Emagic's Notator Logic, and it's possible to customise your own automation screens within most software packages. Running the unit in this combination, with the sequencer locked to timecode and synchronised with a multitrack, the user has a complete mix automation system at their disposal.

The processing is done entirely in the analogue domain, with levels adjusted via a high-resolution VCA with 4096 steps of resolution. This means that any level can be set to one of 4096 different steps for smooth and comprehensive control -- compare this with the standard SSL console, which has a resolution of only 128, and you begin to realise the true power of the system.

 

"Both these units perform their tasks quietly and efficiently, and boast a sonic quality that is quite superb for systems of this price."
 

Fader grouping is a common feature of most automation systems, and the DCP8 is no exception. Multiple faders can be controlled by one master for simple signal management -- for example, two faders controlling a stereo signal can be grouped so that their relative levels remain consistent. The DCP8's grouping feature is adjusted using the Level control. After setting up a group, adjusting the level of any fader within it changes the levels for all the others. Up to four groups can be set, and grouping information is stored as part of the current program within the unit, for easy recall.

In addition to fader grouping, the DCP8 goes one stage further, with the inclusion of stereo linking. Using this facility, pairs of channels can be linked so that their compressor thresholds, ratios and gate thresholds remain exactly the same. Altering the settings of one channel in the group correspondingly alters the settings of the other, for true stereo consistency. Levels are not automatically linked in this way, but level linking can be achieved using the grouping function discussed above, which has the added advantage of allowing different relative levels to be set for stereo source sounds.

DCP8 MIDI AUTOMATION
Automated control of the DCP8 is achieved by using MIDI program and Control Change information, and setting up the unit to achieve this is a relatively simple affair. Pressing the Setup key and selecting 'MIDI Setup' allows the user to assign a MIDI receive and transmit channel to the DCP8, on which controller information can then be sent or recorded. These channels can be set to identical values, or different ones can be used to facilitate the recording of new adjustments on the transmit channel, whilst previous information is played back on the receive channel. Entire audio 'scenes' can also be loaded using MIDI program change data, in the same way that new patches are selected on a master keyboard. Whenever a new scene is selected using the front panel of the DCP8, a program change command is also sent via the MIDI output of the unit, which can be recorded by a sequencer ready for playback.

MIDI Control Change data is used to adjust the settings of individual parameters within the unit, so that the compression and gate settings can be adjusted in real time when mixing. Overall channel muting can also be set using these controllers; to take full advantage of the automation system, a special mixer map template is needed for your current sequencer. As mentioned earlier, programs like Cubase and Logic both support mixer maps, and copies for these particular sequencers are already available from PreSonus. As the MIDI receive channel of the DCP8 can be set internally by the user, up to 16(!) units could be used in combination on any single MIDI port, and each would then be capable of playing back its own unique mix information.

IN USE
Patching in either of the units in a studio environment is a simple affair, and is achieved by running jack cables to the unit from the insert points on each selected channel of the desk. Audio is then automatically routed through the system prior to entering the main signal path of the console. Both the ACP8 and DCP8 can be switched to operate at either +4dB or -10dB levels -- on a per-channel basis -- to correctly match your existing system. This is an especially useful feature if your console is operating at pro level (+4dB), and your recorder is operating at standard line level (-10dB). The unit can then be configured to run all channels at line level, dropping the audio by just the right amount to interface correctly with your recorder.

The analogue ACP8 makes excellent use of LED indicators to show compression levels and threshold points. Each channel has its own bargraph to display the amount of compression currently in use, and two further indicators light to show whether the current input signal is above or below the threshold point. High-quality rotary controls are featured, and the overall feel is of a professional and solid product, perfectly capable of withstanding the rigours of a modern recording environment.

The audio quality produced by the unit is superb. There is no discernible increase in background noise levels when the ACP8 is connected and working, and the controls have been tailored to cover a wide operational range that is capable of processing virtually any signal you're likely to be working with. The compression system is accurate and punchy, with an adjustable ratio that is capable of squashing sounds to an almost ridiculous extent when set at its maximum limits. The attack control is also more capable than most available on units within this price range -- setting it to 0.1 milliseconds really does cause the compressor to act on the first available transient, and some excellent effects can be achieved.

The individual gates also perform well, with no obvious noise or flutter. Drum and percussion tracks, in particular, benefit from being gated to quieten the channels between sounds, and it's quick and easy to set up the ACP8 to perform this function. During my tests, the instruments processed by the unit took on a new dimension and quality, seeming to be separated from those using the unprocessed channels, and with their perceived positions within the mix pulled forward. Sounds began to 'jump out' from the monitors when they had previously sounded leaden and relatively uninspiring. It was a simple matter to tighten up a kick drum without losing any of the bottom end, and hi-hats and shakers took on a new clarity and cut.

The DCP8 offers the same sonic quality and functionality as the ACP8, although the lack of a side-chain option is a shame. Gate trigger inputs are also not available, although users can overcome this lack to some degree by using MIDI automation to adjust gate or level settings remotely. As with the ACP8, the unit is virtually noiseless in operation, and when automating mixes, the quality of the VCAs are immediately apparent, with none of the 'zipper' noise associated with some other systems on the market. With some units, it's often possible to hear the steps of a VCA as it is adjusted, either as small jumps in level, or as quiet clicks as the values are changed. Thankfully, none of this is apparent on the DCP8, and level automation is achieved with the same accuracy as is possible when using a high-quality analogue fader.

Although PreSonus didn't send any MIDI mixer maps with the review unit, I found that it was easy to design custom ones for a sequencer. Both Logic and Cubase offer a 'learn' facility, which monitors any incoming data and assigns control information to custom sliders and buttons accordingly. As the DCP8 sends out the appropriate MIDI information whenever any changes are made via the front panel, it's a simple process to set up a screen full of controls, and define their MIDI functions automatically. Having done this, remote operation of the DCP8 is easy, and the recording and playing back of control changes is smooth and efficient, with no discernible delay or errors in communication.

Although it must be said that I still prefer the physicality of adjusting real controls on outboard equipment, rather than computer-based ones, operating the DCP8 remotely soon becomes second nature. The power offered by the system is excellent, and having such fine control over fader levels makes mixing vocals especially easy. Unlike the ACP8, however, there is no on-board level indication, which makes setting the initial levels somewhat harder. To overcome this problem, PreSonus have also launched the MB8 Meter Bridge, a visual display unit that packs level indication for each of the eight channels into another 1U rackmount module. The meter bridge connects to the DCP8 via a single interface cable that also supplies power to the system. For anyone currently considering purchasing the DCP8, the MB8 is very useful addition and makes the system easier and faster to use. It would also be useful if PreSonus could make available a switching unit, to allow a single meter bridge to function with more than one DCP8, thus keeping the purchase costs down.

CONCLUSION
Both these units perform their tasks quietly and efficiently, and boast a sonic quality that is quite superb for systems of this price. The quality of the DCP8's level automation in particular, with its 4096 steps of resolution, is stunning, and makes it a worthy competitor to some of the more expensive consoles on the market. Adding the luxury of individual compression and gating across specific console channels has transformed the quality of the mixes coming out of AL Digital's studio by a noticeable degree. To combine this facility with level and mute automation (not to mention the overall automation of the compressors and gates as well), and to then release the results in a package that retails at well under £1000 is a remarkable achievement. The ACP8 offers a cost-effective method of adding punch and clarity to your desk, across channels that do not need heavy automation (keyboard inputs are ideal). The DCP8 is a truly innovative product, and highly recommended if your current mixes lack that indefinable sparkle.

Dominic Hawken is a songwriter and programmer, whose recent successes include tracks by East 17 and Ant & Dec. He is also a director of AL Digital, whose current projects include the development of a Digital Satellite Radio Station for Sony and Time Warner.

 
COMPRESSION & GATING
• COMPRESSION: In essence, compression is an audio process that reduces the output level of an audio signal by a fixed ratio, relative to the input. Effectively, if a loud signal is sent into a compressor, it pulls down the level sent to the output to keep the signal within reasonable limits. The higher the compression settings on the front panel, the greater the reduction in output level.

This is useful for controlling the dynamic range of an instrument or vocal, making it easier to record without distorting the input of the recorder. It can also assist in the mix process, reducing the amount of level adjustment needed for a particular instrument. How severely the compressor reduces the signal level is determined by the compression 'ratio' and 'threshold'. A ratio of 2:1 or lower is considered mild; ratios in excess of 10:1 are considered 'hard limiting'. Limiting, in this case, refers to the point at which the signal is restrained from going any louder at the output. The level of input signal at which the output is reduced is determined by the compression threshold. As the threshold is lowered, more and more of the input signal is compressed.

It should be noted that using too much compression will destroy the dynamic response of a performance, rendering it bland and flat. There are, however, exceptions to this rule, and some engineers use masses of compression on individual sounds as an effect, often with great results. Just take a listen to the drums on the last Prodigy record for a good example of this technique.

• NOISE GATING: This is the process of automatically turning down or muting the output signal when the input signal is below a certain level. The level at which the gating occurs is known as the 'gate threshold'. Noisy guitar tracks are often gated to remove unwanted buzzing and clicking between wanted parts of a performance, and in this case the gate threshold is adjusted so that it is higher than any background noise, but lower than the sound of the played guitar. Care should be taken when using noise gates across sounds that decay slowly, as the gate may kick in before the sound has had enough time to finish, resulting in an abrupt ending. Once again, this potential limitation can also be utilised to produce interesting effects, the classic Phil Collins-style gated drum sound being a good example.

 
SPECIFICATION
• Input Impedance 13K(omega), balanced
• Maximum Input Level +18dBu
• Signal to Noise 92dB
• THD + Noise, non-weighted, 0dB 0.01%
• Dynamic Range >115dB
• Gate Threshold Range -90dB to +20dB
• Gate Attenuation 85dB
• Gate Attack Time 100µs• Gate Hysteresis 1.5dB
• Compressor Threshold Range -50dB to +20dB
• Compression Ratio 1:1 to 20:1
• Compression Attack Time 200µs
• Compression Release Time Programme dependent
• Mute Attenuation (DCP8) 95dB Mute
• Rise/Fall Time 600µs
• VCA Level Control Range -85dB to +10dB
• Level Scaling Logarithmic
• Fader VCA Type THAT4301
• Nominal Output Level +4dBu or -10dBu
• Output Impedance 51 Ohms, balanced
• Maximum Output +22dBu
• MIDI Connections (DCP8) In, Out, Thru
• Memory Size 100 Programs
• Digital Control Resolution 12-bit (4096 steps)

 
SIDE CHAIN SPECTRAL PROCESSING
The inclusion of a side-chain insert on each of the channels of the ACP8 opens up a number of processing possibilities. One common use for this facility is to configure the unit as a de-esser, for removing any nasty sibilance from an audio track without having to use overall equalisation to remove the brightness across a whole channel. This is particularly useful when recording or mixing vocal tracks.

The side-chain socket effectively provides an insert point into the signal chain, feeding the input of the compressor but not interrupting the actual audio signal passing through the unit; it's thus possible to modify the sound feeding the compressor, without modifying the source sound that is being processed. If you route the side-chain signal through a graphic equaliser, then boost all the high frequencies and cut the lower ones, no frequency changes will be made to the sound passing through the compressor, but the actual compression system will be fed by the signal from the graphic, causing It to compress only when high frequencies are present in the original source signal. The system will thus compress any high-frequency transients and ignore any low-frequency ones, effectively de-essing the source sound. The same principle can be applied to other frequency ranges, simply by adjusting the settings of the graphic equaliser.

 
pros & cons
PRESONUS DCP8 £749 & ACP8 £599
pros
• Excellent sound quality.
• Smooth and quiet automation with the DCP8.
• DCP8 has capacity for controlling multiple inputs, to cover more than eight tracks.
• External gate/trigger side-chain option on the ACP8.
• Switched line level on each channel.
• Great value for money.
• Very compact.

cons
• No external gate trigger/side-chain option on DCP8.
• Level indication only available as an optional extra on DCP8.

summary
Both these units are excellent accessories for any studio with aspirations to professional-sounding recordings.

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