8.7: External vs. Internal Signal Processors

Plugins (Software-Based Processors)

In modern digital audio workstations (DAWs), most spectrum processing is done using plugins. A plugin is a piece of software that integrates into a host audio application to provide signal processing or effects. Plugins emulate the function of hardware devices but operate inside the computer environment. Common plugin formats include VST (Virtual Studio Technology), AU (Audio Unit), and AAX (Avid Audio eXtension).

Plugins can perform equalization, compression, reverb, noise reduction, and many other functions. They often replicate classic analog gear and provide additional digital precision such as spectrum analyzers and recallable presets.

Signal processors can be implemented either externally or internally, depending on the type of audio system and the workflow. Both approaches perform the same basic function — altering an audio signal for enhancement, correction, or creative effect — but they differ in how they integrate with the signal path and how the engineer interacts with them.

External (Hardware) Processors

External processors are stand-alone analog or digital units connected through the physical signal path of a console, interface, or recording system. They can be outboard rack gear, such as compressors, equalizers, reverbs, or effects modules.

Common examples include:

• Analog compressors like the Universal Audio 1176 or LA-2A
  

• Hardware EQs like the Pultec EQP-1A
  

• Classic digital reverbs such as the Lexicon PCM series or TC Electronic M-One
  

Advantages:

• Unique analog warmth and harmonic character.
  

• Hands-on, tactile control with real knobs and meters.
  

• Reliable real-time operation (no CPU latency).
  

• Often preferred for live sound and mastering.
  

Disadvantages:

• Limited flexibility compared to digital processing.
  

• Fixed parameter ranges.
  

• Requires additional cabling, routing, and maintenance.
  

Internal (Software) Processors

Internal processors exist as plugins or built-in effects within Digital Audio Workstations (DAWs) such as Pro Tools, Logic, Cubase, Ableton Live, and Reaper. These are virtual recreations of hardware processors or entirely new digital designs.

Advantages:

• Cost-effective and space-saving.
  

• Easily recallable and automatable within a project.
  

• Virtually limitless in parameter range and routing flexibility.
  

• Can emulate analog gear with remarkable accuracy.
  

• Allow for visual feedback through meters, graphs, and dynamic displays.
  

Disadvantages:

• Depend on computer processing power and can cause latency.
  

• May lack the subtle analog “color” or saturation of hardware units.
  

Signal Routing

The placement and routing of signal processors in an audio system determines how they affect the sound. There are two primary routing methods: insert processing and send/return (auxiliary) processing.

Insert Processing - Insert processors are placed directly in the signal path, so 100% of the audio passes through the device.
Common examples:

• Compressors
  

• Equalizers
  

• Gates
  

• De-essers
  

Usage:
Insert points are ideal for dynamic or corrective processing, where the entire signal needs to be controlled or shaped.

For example, inserting a compressor on a vocal channel ensures every word is evenly leveled before it continues to the mix bus.

Typical signal flow:

Mic/Line Input → Preamp → Insert Send → Processor → Insert Return → Fader → Output

Send/Return (Auxiliary) Processing

Send/Return processors operate in parallel with the original signal. Only a portion of the signal is sent to the effect processor, and the processed result is then combined with the dry (uneffected) signal.

Common examples:

• Reverbs
  

• Delays
  

• Chorus
  

• Flangers
  

Usage:
Send/Return routing is used for time-based or spatial effects, where blending the original and processed sound adds depth or ambiance. It also allows one processor to serve multiple channels simultaneously.

For example, multiple instruments can share a single reverb unit via an auxiliary send, maintaining a cohesive “room sound.”

Typical signal flow:

Channel Send → Effects Processor → Effects Return → Mix Bus

Parallel vs In-Series Processing

The way processors are connected — either in series (one after another) or parallel (side by side) — also affects the outcome.

In-Series Processing

With In-Series routing, each processor affects the full output of the previous one.
Example chain:

Microphone → EQ → Compressor → Limiter

Used for signal correction and gain control, series chains are common for vocals, drums, and mastering chains. The order matters greatly; for instance, compressing after EQ can yield very different results than compressing before.

Parallel Processing

In parallel routing, the dry (unprocessed) and wet (processed) signals are combined.
Parallel compression (also known as New York compression) is a classic example:

• The dry track retains dynamics and transients.
  

• The compressed track adds body and density.
  

• When mixed together, the result sounds punchy yet natural.
  

Parallel setups are also common for reverb, saturation, or harmonic enhancement effects.

Hybrid Systems

Modern studios often combine external hardware and internal software in hybrid systems.
Examples include:

• Using analog preamps and compressors during recording for warmth.
  

• Applying digital plugins for precision EQ, noise reduction, and automation during mixing.
  

• Routing audio through external gear using DAW insert points via an audio interface.
  

Hybrid setups offer the best of both worlds — analog tone and digital control.

Latency and Synchronization

When mixing hardware and software processors, latency (signal delay) can become an issue, especially in parallel routing. Modern DAWs use automatic delay compensation to align all tracks and maintain phase coherence. For precise timing effects (like delay or reverb pre-delay), maintaining synchronization between the hardware and software environment is essential.

Practical Applications

• Recording: Hardware preamps and compressors can shape tone before the signal reaches the DAW.
  

• Mixing: Software plugins offer recallable processing chains for fast revisions.
  

• Mastering: Hybrid processing often combines analog EQs and compressors with digital limiters and spectral analyzers.
  

• Live Sound: Hardware outboard units are valued for their reliability and zero latency.