ACMT - Applied Computer Music Technologies Ltd

ACM70SA Product Operating Guide
 
Access to plug-in parameters is provided by a fully resizable graphical user interface. Your host application may also provide access to parameter adjustments and additional configuration.
 

 
Graphical User Interface
 
The plug-in settings are adjusted by clicking and dragging on the rotary controls or buttons. Some controls may have detents – these manifest themselves as areas in the rotation where the reluctance to move is increased such that you have to drag them a bit harder. They are intended to behave similarly to real controls which may have a 'click stop' at 0dB for example.
 
You can also adjust the controls by placing the mouse pointer over them and using the scroll wheel. In this case the detent has no effect.
 
Control Modes
 
The control mode determines the way in which the rotary controls respond to mouse movement. This can normally be configured via the host application preferences. Please refer to your host application documentation for details.

• Circular. In this mode, clicking on a control will move it immediately to the mouse pointer’s angular position. To adjust the control, drag the mouse pointer in a circle or an arc.
• Relative Circular (Default). This is the default mode unless changed by host application settings. Similar to circular mode, however moving the mouse will adjust the control relative to its current setting.
• Linear. In this mode, the control responds to vertical mouse movement. Drag up to increase the value, turning the control clockwise, or down to decrease the value, turning the control anticlockwise.

In all modes, double clicking the centre of a control will return it to its default position.
 

Control Functions
 
The ACM70SA is a stereo compressor / limiter, split into two sections which can either operate independently, or linked. For simplicity, only one channel, the 'Left' or 'Lateral' (see 'Lateral / Vertical' operation later) is described here.
 
1. In
 
The 'In’ switch places the plug-in in the signal path or implements a clean bypass by ‘powering down’ the emulation when deselected, and allowing audio to pass through unaffected. As the emulation powers up, there may be a slight delay before the audio begins to be processed by the plug-in, however there should be no interruption to the audio during this time.
 
2. Meter Select
 
Each channel has a VU meter which shows the amount of gain reduction being applied. The three position switch determines the meter source. In the 'zero' position, the meter will show a maximum of approx 20dB gain reduction, deflecting to the left from the '0' point with increasing gain reduction. In each 'Bal' position, the meter will show the signal current in one half of the balanced variable-u stage. In a hardware implementation this would be used when adjusting or recalibrating the circuit.
 
3. Channel Input Gain
 
Each channel has a 20 step input attenuator labelled 'Input Gain'. With the control turned fully anticlockwise, the input signal is reduced by 20dB, progressing in 1dB increments to 0dB with the control fully clockwise. As the variable-mu amplifier stage contributes some gain when there is no limiting, the input control has a default setting of -14dB to ensure unity gain through the plug-in.
 
4. Channel Threshold
 
Each channel has a threshold control which determines the onset of compression / limiting. With the control fully anticlockwise, no signal passes into the side-chain and the plug-in functions as a conventional amplifier. As the control is turned clockwise, the threshold at which the side-chain begins to reduce the AGC gain becomes progressively lower.
As the design provides a gradual transition from the onset of gain reduction to maximum limiting, the concept of a precisely defined threshold or ratio is not as it might be in a more modern compressor. The result is that the compression ratio increases as the plug-in is driven with higher signal levels, providing exceptionally transparent gain control.
 
5. Channel Time Constant
 
The channel time constant switch selects the side-chain attack and release times. The attack settings are fixed at 0.2 - 0.4ms with the release times becoming progressively longer as the control is advanced through the first four positions.
In positions 5 and 6, the release time is program dependent, which means that for short periods of high level the release time will be fast, however for longer more sustained periods at high level, it will be progressively longer, automatically adjusting the time constant for the prevailing dynamics.
 
6. Mode Select
 
The mode switch provides three distinct modes of operation:

• Left / Right - In this mode, the plug-in operates as two completely separate compressor / limiters, with the left channel adjusted using the upper set of controls, and the right channel using the lower set.
• Link - In Linked mode, the plug-in operates as a normal stereo processor, with the same amount of gain reduction applied to left and right channels simultaneously, preventing any stereo image drift. This was a popular modification to original vintage 670 compressors and is the recommended configuration for stereo processing. In this mode, the other controls should be kept as closely matched as possible between the two channels (unless a specific effect is desired).
  If different time constant settings are selected, the side-chain response will be the average of the two channels.
• Lat / Vert - Fairchild 670 limiters often found uses in preventing over modulation of disc-cutting lathes during the creation of master recordings for record pressing. The 'Lat' or 'Lateral' refers to the mono channel (lateral motion of the cutting head) and the 'Vert' or 'Vertical' refers to the stereo channel (vertical cutting motion). In modern terms these signals represent the 'Middle' and 'Sides' in a stereo recording.
  The Lat / Vert signals are created from a sum and difference matrix of the left and right channels, which are then passed through the two compressor / limiters and finally recombined to produce the left and right outputs. This allows the plug-in to affect the relative levels of 'Middle' and 'Side' information within the stereo image.

ACM70SA Technical Specification:
 
1. System Requirements (64bit Linux)

• Linux x86-64/AMD64 64bit e.g. Ubuntu 20.04 LTS or newer.
• A Linux VST, VST3 or CLAP compatible host application.

2. Product Specifications

• Linux VST, VST3, JACK and CLAP compatible.
• Mono or stereo operation.
• Interpolated controls for smooth parameter changes with no 'zipper' noise.
• -22dBFS reference level for 0dBu.
• Threshold: Adjustable via front panel control from Inf. to a factory set maximum at -20dBFS (+2dBu)
• Compression Ratio: Approx 2:1 at minimum threshold to approx 20:1 maximum.
• Time Constants:

  Time Constant	Attack Time	Release Time
  1.	0.2ms	0.3sec
  2.	0.2ms	0.8sec
  3.	0.4ms	2sec
  4.	0.4ms	5sec
  5.	0.4ms	Program dependent
  6.	0.2ms	Program dependent

• Maximum Input Level: +16dBu.
• Gain: +14dBu (no limiting).
• Onset of Clipping: +25dBu.
• Polarity invert switch.
• Output level trim (+/-6dB).

Please note: In the interest of continual product improvement, specifications may be subject to change without notice.

ACM70SA Download Free Demo
 
1. System Requirements (64bit Linux)

• Linux x86-64/AMD64 64bit e.g. Ubuntu 20.04 LTS or newer.
• A Linux VST, VST3 or CLAP compatible host application.

2. Free Demo Limitations

• Every 60 seconds the plug-in will fade to silence momentarily. To remove this limitation you will need to have purchased a product activation code.
• To remove the demo limitations, open the plug-in in your host application and click the lock button at the top left of the plug-in GUI to open the license entry dialogue. Type (or paste) your product activation code into the text box. You will then need to restart your host application to complete the process.

3. Product Support

• All downloads include their own installer, manual, and installation instructions. Easy to install with just a few clicks.
• If you need help downloading or installing the software please use our contact form to let us know.