Bus Processor 670

Introduction

This is not an emulation, it's an amalgamation. It's a "best of" and a "what if" made into a tone machine. It's a science project aimed at answering the question, "Where, exactly, does the magic of the 670 lie?" — turned into a seemingly complicated but surprisingly easy-to-use bus processing plug-in.

With Bus Processor 670 you can discover this for yourself. You can turn parts on and off to see where the magic is. You can exaggerate effects (the Calibration knob is fun!) and you can reduce them. But most important of all, you get that magic sound that's been a staple in well-equipped studios since the 1950s.

User Interface

Compressor Section

Bus Processor 670 is a stereo compressor at heart, with separate controls for the two channels. By default, both channels' controls are linked, meaning you always have the same settings on both channels. Turn off Link Channels to unlink them.

Input Gain

Sets the gain feeding into the compressor.

Compression Amount/Threshold

Sets the amount of compression, clockwise means more compression. Keep an eye on the VU meter while setting compression.

Time Constant

Sets the attack and release times of the compressor. The actual attack and release times depend on the amount of compression.

Of its 6 pre-set attack and release configurations, the ones that have stood the test of time and are most commonly used today are 1, 2 and 5.

1. Fast attack and release.
2. Fast attack, slower release.
3. Medium attack, slow release.
4. Medium attack, very slow release.
5. Medium attack, program-dependent release that is fast for transients and slow for sustained audio.
6. Fast attack, program-dependent release.

Output Gain

Sets the make-up gain of the compressor, before the Saturator.

Knee

Sets the sharpness of the compressor knee. This control was called "DC Threshold" on the original hardware.

Calibration

Adjusts internal calibration parameters. A typical hardware unit would have a coloration that corresponds to the Nominal setting. Clean has less distortion and coloration, while Worn adds substantial distortion.

Compressor Mode (Modern/Classic)

Switches between the classic feedback design and a super-clean modern feed-forward version.

Link Controls

Links left/right controls.

Mid/Side Processing

Sets the Bus Processor to work on Mid (sum) and Side (difference) channels instead of Left and Right.

In the original hardware, the mid/side functionality was called "lateral-vertical," or "lat-vert." This term comes from vinyl disc cutting, where stereo was encoded as lateral and vertical groove motion.

If left and right channels were used directly to cut a two-channel disc, a mono track would move the stylus both up/down and left/right at the same time, which could cause problems. Instead, the disc was cut using sum and difference signals—what we now call mid and side.

The stylus moves side to side (laterally) for mono information (mid), and vertically for side information. Hence "lat-vert".

Compressor Sidechain Section

Compressor Wet/Dry

Sets the blend between dry and compressed audio.

S/C Stereo Link

Sets how much the two channels' gain reduction should depend on each other.

For heavy compression, you typically want the two channels to be "Linked", so that both left and right channels get the same amount of compression and you don't risk the stereo balance "tilting" to one side when compression hits.

The drawback is that the stereo image might sound less wide. That's why many mastering engineers prefer unlinked sidechains. They typically use light compression, and the risk of image tilt is smaller.

In general, for heavy compression: more linked than dual mono. For light compression: more dual mono than linked.

When doing Mid/Side compression, S/C Stereo Link should not be set to "Linked".

S/C Low Cut

Removes low frequencies from the compressor detector.

S/C Tone Shift

Tilts the frequency balance before the compressor detector.

Saturator

Saturator Wet/Dry

Sets the total amount of saturation.

Transformer

Sets the level of transformer distortion. This distortion can typically only be heard with low-frequency audio, such as kick drum or synth bass.

The transformer distortion happens mainly before the compression.

Tone

Sets the character of the "Tube Stages" distortion.

Tube Stages

Sets the amount of tube saturation, mainly after the compression.

A Note On Distortion

In this type of compressor, there are a lot of things that can (and will) distort. We have tried to separate the main types of distortion into three different types:

1. Cross-over distortion from the push-pull stages in the variable-mu design. This distortion depends on the amount of gain reduction and can be controlled with the Calibration control. It can be eliminated by switching Comp Mode to Modern.

2. Transformer distortion. This distortion happens mainly at bass frequencies and is usually very subtle, but we added the possibility to exaggerate it using the Transformer knob.

3. Tube saturation. There are tubes throughout this design, and not all tube saturation leads to a "distorted sound"; some saturation can, for example, simply limit the maximum amount of gain reduction the compressor has. The Tube Stages knob controls the audible saturation—the distortion. From subtle to... a lot.

Spatializer

Air

Increases high frequencies using mid/side processing. This can be particularly useful when a slight tonal rebalance is needed.

Width

Turn right to add gain to the side signal while keeping the mid signal unchanged. Turn left to lower the gain of side information.

Mono Bass

Turns low frequencies mono below the cut-off frequency.

Spatializer Engage

Turns the Spatializer section on or off.

Compressor Engage

Turns the Compressor section on or off.

Saturator Engage

Turns the Saturator section on or off.

Credits

Cameron Clark – DSP programming
Fredrik Jansson – DSP programming
Niklas Odelholm – DSP programming, product design
Johan Bremin – Quality assurance
Alex Venguer – Sound quality assurance, presets
Stefan Boman – Testing, presets Talio Sanchez – Testing, presets Ulf Ekelöf – 3D rendering