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This article is based on a PreSonus manual and are used with the kind permission from the people at PreSonus.

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The original manuals can be found at PreSonus's Technical Support Website.

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FEATURES

The following is a summary of the ACP-88’s features:

Eight Compressors/Limiters

Each channel of the ACP-88 contains an audio Compressor that can also be setup as a Limiter by varying the ratio of compression. You have broad control over Threshold, Ratio, Attack and Release for each processor. You can select between Auto or Manual Attack and Release curves and Hard or Soft Knee compression types. The Compressor is useful in many situations such as recording instruments or vocals that vary in loudness, or setting it up as a limiting device before the digital recorder to prevent distorting the digital recorder’s inputs. Live sound system processing is another great application where the ACP-88 can really take control.

Eight Dynamic Noise Gates

Each channel of the ACP-88 has a separate dynamic noise gate that can be used to Gate an entire drum kit, clean up a noisy tape machine, isolate an instrument or separate a vocal from background noise. Each noise Gate provides control over Attack, Threshold, Release and Gate close Range. The Gate close range can be useful in creating a more natural sounding blend or mix when gating many instruments at once.

Comprehensive Channel Linking

Using the Link function allows you to combine any combination of ACP-88 channels effectively forming a subgroup. When linked, all processors follow the setting of the Master processor which is always the processor furthest to the Left in a Link group. For example, you could Link together channels 7 and 8 to form a Stereo pair, channels 3, 4 and 5 to form a subgroup of processors, while channels 1, 2 and 6 remain available for independent applications such as compression, limiting, or gating as the situation dictates.

Separate Bypass and Gain for Every Channel

Each Channel has a separate Bypass for auditioning a signal ‘before and after’ processing with the compressor, limiter, or the Gate and a Gain control to make up any loss in signal level resulting from the amount of compression being applied.

Compressor Sidechain Jack on Every Channel

Each channel of the ACP-88 was designed with a special jack for spectral processing, compression keying and ducking applications. Sidechaining is useful for removing annoying sibilance from vocal tracks (de-essing) or automatically ducking tracks behind a narrator for ‘auto mixing” a service, broadcast or performance. When multiple channels are Linked together, the Send of the Sidechain jack of the Master channel contains a mix of all the channels in the Link: a feature allowing control over multiple channels of processing with one Sidechain send/return.

Separate Gate Sidechain/keying Jacks for Each Channel

The ACP-88 also includes a separate jack on each channel for Gate sidechain/Keying. This is useful for synching an external sound to a snare track, for example, or putting equalizers or filters before the Gate key to enable Gating only the lower frequency of a kick drum, etc.

Balanced/Unbalanced Inputs and Outputs

The Acp-88 accepts either balanced or unbalanced inputs and outputs using tip-ringsleeve (TRS) connectors.

+ 4dBu or – 10dBV.

The internal operating level of the ACP-88 can be switched between +4dBu (pro levels) to –10dBV (line levels), making it possible to use in virtually any application.

CONTROLS & CONNECTIONS

Front Panel Basic Layout

Notice that the front panel is divided into eight identical sections. These are the eight signal processing chains of the ACP-88.

Each channel contains:

• Compressor/Limiter
• Noise Gate
• Gain Makeup
• Link & Bypass Control

Compressor Controls

Threshold

The Compressor Threshold sets the level at which compression begins. The below and above LED’s over the Threshold knob indicate whether the input signal is below or above the Threshold setting. When the signal is above the Threshold setting, it becomes ‘eligible’ for compression. Basically, as you turn the Threshold knob counterclockwise, the input signal is compressed. (If you have a ratio setting of greater than 1:1.)

Ratio

Ratio sets the compression slope. This is defined as the output level versus the input level. For example, if you have the Ratio set to 2:1, any signal level above the Threshold setting will be compressed at a compression ratio of 2:1. This simple means that that for every 1dB of level increase into the compressor, the output will only increase * dB, thus producing a compression Gain reduction of 0.5 db. As you increase the Ratio, the compressor gradually becomes a limiter. A limiter is defined as a processor that limits the level of signal to the setting of the Threshold. For example, if you have the Threshold knob set at 0 dB, and the Ratio turned fully clockwise, the Compressor becomes a Limiter at 0 dB. This means the signal will be limited to an output of 0 dB regardless of the input signal.

Attack

Attack sets the speed at which the compressor ‘acts’ on the input signal. A slow attack time (fully clockwise) allows the beginning envelope of a signal (commonly referred to as the initial transient) to pass through the compressor uncompressed, whereas a fast attack time (fully counterclockwise) immediately subjects the signal to the Ratio and Threshold settings of the compressor.

The attack control is only active when the Auto button is not pushed in. When the Auto button is pushed in, the compressor automatically determines the appropriate Attack time for compression.

Release

Release sets the length of time the compressor takes to return the Gain reduction back to zero (no gain reduction). Very short Release times can produce a very choppy or ‘jittery’ sound, especially in low frequency instruments such as bass guitar. Very long Release times can result in an overly compressed signal, sometimes referred to as ‘squashing’ the sound. All ranges of Release can be useful at different times however and you should experiment to become familiar with the different sound possibilities. (Refer to the applications section of this manual for some ideas.)

The Release control is only active when the Auto button is not pushed in. When the Auto button is pushed in, the compressor automatically determines the appropriate Release time for compression.

Soft

The Soft button selects Soft Knee and Hard Knee compression curves. When this button is pushed in, Soft knee compression curves are used, otherwise hard knee compression curves are used. With Hard knee compression, the gain reduction applied to the signal occurs as soon as the signal exceeds the level set by the threshold. With Soft knee compression, the onset of gain reduction occurs gradually after the signal has exceeded the Threshold, producing a more musical response.

Auto

When pushed in, the Auto button places the compressor in automatic attack and release mode. The Attack and Release knobs become inoperative and a pre- programmed Attack and Release curve is used.

Gate Controls

Attack

The Gate attack control sets the speed at which the gate opens to allow signal to pass through it. This control is variable from 10 microseconds to 100 milliseconds. It is advisable to use slower Attack times when gating vocals or quieter instruments to avoid what is often described as gate ‘clicking’. This phenomenon is not made by a mechanical device in the noise gate, but is rather, an audible manifestation of ‘no signal’ to the presence of signal as the gate opens. The fastest setting is fully counter-clockwise (left) to the slowest setting all the way round to the right (clock-wise).

Threshold

The gate Threshold sets the level at which the gate opens as indicated by the ‘OPEN’ LED above the Threshold knob. Essentially, all signals above the Threshold setting are passed through unaffected, whereas signals below the Threshold setting are reduced in level by the amount set by the Range switch. The ‘CLOSE’ LED above the Threshold knob indicates when the gate is closed (the signal level is below the threshold). If the Threshold is set fully counter-clockwise, the Gate is turned off (always open), allowing all signals to pass through unaffected.

Release

The Gate Release time determines the rate at which the gate closes. This is indicated by observing the OPEN and CLOSED LED’s. As the Release time is lengthened (clockwise), you will notice that the CLOSE LED reflects the close time. Release times should typically be set so that the natural decay of the instrument or vocal being gated is not affected. Shorter Release times help to clean up the noise in a signal but may cause ‘chattering’ in percussive instruments. Longer release times usually eliminate ‘chattering’ and should be set by listening carefully for the most natural sounding Release for the signal being processed.

Range Switch

The Gate range is the signal level reduction that occurs when the gate closes. Therefore, if the Range switch is set at 15 dB, there will be a slight change in the signal as it crosses the Threshold. If the Range switch is pushed in, the signal will be Gated (reduced) by 60 dB. The CLOSE LED above the Threshold knob indicates this by changing brightness relative to the Range amount that has been selected. When the Range is set to 15 dB, the CLOSE LED will show half illumination. When the Range is set to 60 dB, the CLOSE LED will illuminate to it’s brightest level at the end of the release time.

(REMEMBER: Release time is set by the Release control.)

Gain

Gain

When compressing a signal, Gain reduction usually results in an overall reduction of level. The Gain control allows you to restore the loss in level which occurs due to the amount of compression used. (like readjusting the volume.)

Bypass & Link

Bypass

Activating the Bypass effectively removes all processing being performed by the ACP88 and returns the signal to unity gain. You should use Bypass often when setting up the ACP-88 to compare the ‘before and after’ results of the signal processing effecting the audio signal. Bypass affects both the Gate and the Compressor. When Bypassed, the Link function is interrupted for that channel and essentially breaks the link in the linking ‘chain’.

Link

When the Link button is engaged (pushed in) the LED labeled LINK becomes active indicating this channel has become the slave of the channel to its immediate left (The Link LED is only active when the signal is present in the Linked channel). All of the controls for the Linked channel become disabled and metering occurs by way of the Gain reduction meter of the channel to the left (Exception: the LED meter of the Slaved channel is still useful to indicate the presence or lack of signal). Essentially, the left channel is still the Master and the channel with the Link button pushed in is the Slave channel in a stereo linked pair. If multiple Link buttons are pushed in, then the channel farthest to the left of the multiple Linked channels (without its link button pushed in) becomes the Master for the multiple Linked channels. In this case, all metering should be referred to the Master channel’s meter.

IMPORTANT: When a channel is Linked (link button pushed in), all of it’s controls are inactive. The Link LED is active. Also, even though the meters on the Linked channel are still operational (presence or lack of signal), all metering should be referred to the Master channel’s meter for the Gain reduction level of the Linked channels!

Patch Panel (Back)

Input

The Input jack accepts balanced tip-ring-sleeve or unbalanced tipsleeve connectors. The Input can handle up to +24 dBu unbalanced or up to +18 dBu balanced signal levels.

Output

The output jack accepts balanced tip-ring-sleeve or unbalanced tip sleeve connectors. The output will deliver up to +24 dBu in signal level, balanced or unbalanced.

+4/-10 Switch

This switch adjusts the internal operating level of the ACP-88 when it is connected to line level (0 dB = -10 dBU) gear. With this switch in the ‘-10’ position, the signal is adjusted by 11.2 dB so that it can be processed at the lower noise floor of the ACP88’ s internal circuitry. The signal level is lowered on the way out to match with the line level gear’s input. When the switch is in the ‘+4’ position, the signal is not changed since this matches the optimum internal operating level of the ACP-88.

Compressor Sidechain

The Sidechain jack on each channel interrupts the signal that the compressor is using to determine the amount of Gain reduction to apply. When no connector is inserted into this jack, the input signal goes directly to the compressor’s control circuitry. When a connector is inserted into this jack, the signal path is broken. If you have inserted a 1/4 inch tip-ring-sleeve (TRS) connector, the input signal is sent back out of the ACP-88 via the ring of the connector. This signal can then be processed by an equalizer for example to reduce sibilance (de-essing) in a vocal track. The signal is then returned to the unit via the tip of the connector. The signal sent via the ring could be that of a narrator or vocalist. In this application, the audio that you are passing through the compressor will automatically ‘duck’ when the narrator speaks or vocalist sings.

Gate Sidechain/Key Insert

The Gate sidechain jack accepts a tip-ring-sleeve connector and is used to open the Gate from either a modified version of the signal passing through the Gate or some other external source. For example, the Gate may be Keyed from a version of the kick drum’s signal with all of the high frequencies rolled off. This set-up could be useful to stop the kick drum’s gate from opening during a cymbal crash, for example.

• NOTE: The Key and Sidechain are in the signal path of the gate and compressor, respectively. These inserts require periodic cleaning as you would a patch bay to insure proper operation. It is not uncommon for condensation to form a film on these contact points. Failure to clean these inserts can result in a loss or degradation of signal. Should a gate or compressor fail, inserting a 1/4 inch connector several times into the insert points commonly restores normal operation.

Power

Power Connection

The power jack on the ACP-88 accepts a standard IEC cord like those found on most computers and professional recorders. The ACP-88 contains a custom built; internal power supply.

BASIC SETUP & APPLICATIONS

Patching

Inserting into the mixers insert points

Between multi-track recorder inputs/outputs to mixer outputs/inputs, unbalanced

Applications

Example setup compressing a voice or vocal track

Probably the most common use of a compressor is to control the dynamic range of a vocalist during a live performance or when recording. It is almost always necessary to compress a vocal take during a recording session. Most vocalists have a very wide dynamic range, and if the vocalist does not have ‘studio’ experience, controlling the dynamic range becomes even more challenging due to the lack of proper microphone technique.

This wide variation in level makes it difficult to properly record a vocal take, especially on digital multi-track recorders which have definite limitations in headroom. Compressing a vocal also makes it easier to place the vocal track in a mix so that it remains ‘in the face’ throughout the track.

To setup for recording a vocal, insert a channel of the ACP-88 into the same channel as the microphone on the mixer. If the mixer has an insert point, insert the compressor there (-10 dBu on most mixers). If you don’t have an insert point you will have to place the compressor after the mixer’s bus output or whichever output you’re using by coming out of the mixer to the input of the ACP-88; then out of the ACP-88 to the input of the recorder. If you’re using a stand alone mic preamp; patch the ACP-88 after the preamp and then to the recorder. It is important to utilize the compressor last in the signal path or positioned in line so that the input level of the compressor is stable. (Remember: adjusting the signal level being sent to the ACP-88 will require a readjustment of the various parameters of the compressor.)

Turn the Threshold of the channel’s compressor you’re using completely counterclockwise. If you’ve gotten the cables right, you should see the LED’s above the Threshold knob of the channel you’re using flashing when you talk into the microphone.

Make sure that the channel on the ACP-88 has the following setup:

• Link button is out
• Bypass is out
• Auto is in
• Soft is out
• Gate Threshold knob is fully counter-clockwise
• Compressor Ratio knob is set to 1:1 (fully counter clockwise).

To set the compressor: turn the Ratio to the 2:1 mark on the legend. With the vocalist yodeling into the mic, turn the compressor’s Threshold until the gain reduction meters read about –7dB. Now you will probably need to boost the compressors’ output by turning the Gain knob clockwise so that the mixer or multi-track meters read 0 dB (in the case of some digital recorders this is –14 dB).

This would be a basic, very simple but useable setting for recording a vocal. Things to vary to suit the taste might be: adjusting the ratio and threshold for more/less dynamic range. Push the Soft button in and check out the Soft Knee compression curve. Take the compressor out of Auto mode and adjust the Attack and Release times to taste.

Compressing and gating drums or percussion instruments

Limiting the dynamic range of a drum kit may or may not be something you want to do to the drum tracks, but there are some other features of the ACP-88 that can help you get that "in the face", “bigger than life” drum sound.

If you consider the sound of a snare drum, you will notice the sound has a beginning loud sound (transient) followed by a sound of decreasing intensity (decay). Patch a channel of the ACP-88 for compressing the snare drum as described above and we can explore some dramatic changes that compression can have on the snare drum's sound.

For a simple setup, make sure that:

• Soft button is out
• Auto button is out
• Ratio is set to 4:1 on the legend
• Attack is fully counterclockwise
• Release is straight up at the 12:00 o'clock position
• Gate Threshold is fully counter-clockwise
• Link button is out and the Bypass button is out.

Adjust the Threshold until you see about -lldB on the gain reduction meters. At this point, the snare's volume is simply lowered by lldB. Now slowly raise the Attack time by turning the Attack knob clockwise. You should notice that the beginning 'transient' of the snare sound is starting to jump out of the speakers and slap you in the face. This becomes even more noticeable when using digital reverberation on the snare.

Now you can adjust the Gate on the snare to stop those other drums from 'bleeding' through the snare mic: Begin by adjusting the Attack time of the Gate to .01(10 microseconds) by turning the Gate Attack control knob fully counter-clockwise. Turn the Gate Release knob to about the middle position, set the Gate Range to 60dB (Range Button pushed-in), adjust the Gate Threshold slowly clockwise until you begin to hear those other drums disappearing. Too high of a Threshold setting might disrupt the snare drum's natural sound, a Threshold setting that is too low will let the sound of the other drums open the Gate. You need to adjust the Gate Threshold and Release to suit the drumming taste.

This simple application applies to any percussive instrument, of course. Experiment.

Limiting

Limiting is defined as compressing with a Ratio of infinity to one. This setting acts like an imaginary ceiling for the level of a signal. On the ACP-88, Limiting occurs when the Ratio knob is fully clockwise. At this point, the Threshold knob essentially becomes the 'ceiling' knob for the channel. You will most likely want a virtually instantaneous Attack time while Limiting.

To achieve this: Make sure the Auto button is out, Turn the compressors Attack time knob fully counterclockwise and set the compressor's Release time for the most natural sounding decay for whatever input signal you are using. (Warning: Improper settings of the Release time have been known to cause pumping, breathing and coughing in limiters. Consult the ears when fine tuning.)

Compressing bass guitar

Very similar to compressing vocals, the bass guitar is almost always compressed or limited during recording. Start with the setting described above for vocals. Vary the Compression Ratio, Attack and Release to suite the taste.

Compressing acoustic guitar

It is sometimes easy to over compress an acoustic guitar because the first impression is that compression sounds awesome on acoustic instruments. If you're adding the acoustic instrument to a mix that already includes a lot of other instruments such as distorted guitar, compressing the life out of the guitar will help you later in the mix. If the guitar is recorded as a solo instrument or part of an acoustic ensemble, you should experiment with less compression because you don't want to severely limit the natural acoustic dynamic range of a good sounding guitar. Start by using the setup described in the vocal section...

De-essing using an inserted equalizer into the Sidechain

A common annoyance when attempting to place a vocal track in a mix is that as you boost the higher frequencies of the track to 'breathe life' into the vocal, all of the 'SSS's of the dialogue or lyrics tend to become louder than the rest of the track. A proven method of reducing this problem is to de-ess the track with 'spectral' compression.

If you don't have an external equalizer, you can use a spare channel of the mixer by sending the track to both the spare channel and the original channel. Take the direct out of the spare channel and send it to the Sidechain input of the ACP-88 (signal on the tip of the tip-ring-sleeve connector). Once patched, setup the compressor as a limiter as described above. How this works is that when the Sidechain has an inserted signal, the Compressor generates the Gain reduction from the Sidechain return signal only. Now, if you boost those frequencies associated with sibilance (SSS), such as 2kHz to 8kHz, the Compressor will apply the Gain reduction more to the sibilance because it is now 'seeing' more signal around the frequency of the sibilance. You will need to adjust the frequency of the equalizer to locate the source of sibilance more precisely. Parametric equalizers are most effective for this type of compression but you can get by with most any equalizer.

Stereo Compressing/Limiting

The Linking feature of the ACP-88 will allow you to have up to four stereo Linked pairs of Compressor/Gates. As you may have read in the section in Chapter Two on the Link function, this is easily achieved by pushing in the Link button on the right-hand channel thus creating a Linked Stereo Pair. The channel on the left becomes the Master for the Linked stereo pair and all functions on the Right channel (with the exception of the Bypass switch) become inoperative. Meters on the right channel will behave as before but have no significance since the Gain reduction for this channel is being derived from the Left channel.

To avoid confusion, it might be useful to turn the Compression Ratio to 1:1 on the channel with the Link button pushed in (the right channel). This will eliminate all Gain reduction and effectively turn off the Gain reduction meters. The Gain reduction is of course being derived from the Master channel.

The Link LED indicates when a signal is being passed from the Linked channel to the Master channel. It will ONLY light up when signal is applied to the Linked channel.

TECHNICAL

Specifications

Number of Channels:8

Dynamic Range :>115dB

Signal to Noise Ratio : >95dB

Headroom:+24dBu, Unbalanced; +18dBu Balanced

Frequency Response :10Hz to 50kHz

Crosstalk: >82db @ 10kHz

Compression Threshold Range: 40dBu to +20dBu

Compression Ratio :1:1 to 20:1

Compressor Attack Time: 002ms to 200ms

Compressor Release Time: 05sec to 500sec

Auto Attack and Release: Program Dependent

Gate Attack Time: 001ms to lOOms

Gate Threshold Range: Off to +20dBu

Gate Release Time: 002sec to 2sec

Gate Attenuation Range: -15dB or -60dB

Input Impedance: 10kOhms

Output Impedance: 5I Ohms

THD + Noise: <002%

Output Gain:20dB to +20dB

Compression Curve Types: Soft Knee or Hard Knee

Compressor Metering: Above and Below Threshold, Gain Reduction

Gate Metering: Open and Closed

Sidechain Output Impedance: 5I Ohms

Sidechain Input Impedance: 10kOhms

Gate Input Impedance: 10kOhms

Internal Operating Level: +4dBu = 0dB

Input Range: +4dBu or -10dBV, Switchable

Input Connectors: 1/4", Tip Ring Sleeve, Balanced or Unbalanced

Output Connectors: 1/4", Tip Ring Sleeve, Balanced orUnbalanced

Sidechain Connector: 1/4", Tip Ring Sleeve

Gate Key Connector: I/4", Tip Ring Sleeve

Power Supply: Internal, Linear Supply

Power Requirements: 100VAC to 120VAC, or 200VAC to 240VAC

Weight: 12lbs

Rack Size: 2U

Block diagram

Categories: PreSonus Manuals | Manuals