Capture Signal Chain
Capture Signal Chain
A capture signal chain is the complete path a test or training signal follows from the capture system, through the equipment being modeled, and back into the recording or capture system.
Understanding this path is fundamental to creating reliable captures. Many apparent problems with a capture or model are actually problems somewhere in the signal chain.
Basic signal flow
A typical direct capture chain is:
Capture or training signal → audio interface output → reamp/capture device → device under test → audio interface input → capture or recording software
The exact chain varies according to what is being captured and which capture technology is being used.
The device under test (DUT) may be an amplifier, pedal, preamp, processor, or a combination of devices.
The outgoing signal
Most capture systems begin with a known test or training signal.
Depending on the platform, this may consist of sweeps, noise, impulses, specially generated audio, or a prerecorded training file.
The purpose is to present the device under test with a known input so that the system can compare the original signal with the returned signal and determine how the equipment changed it.
The test signal should normally be used exactly as specified by the capture platform. Changing its level, duration, sample rate, timing, or content without understanding the consequences may invalidate the capture.
Audio interface output
The test signal normally leaves the computer through an audio interface output.
This creates an important electrical issue: an interface line output is not necessarily equivalent to the signal produced by a guitar or other instrument.
Differences may include:
- Signal level
- Source impedance
- Balanced versus unbalanced operation
- Grounding
- Connector type
Simply connecting an interface output directly to an amplifier or pedal may work in some circumstances, but physical compatibility does not necessarily mean electrical equivalence.
See Gain Staging and Calibration and Reamping for Capture.
Reamp and capture devices
A reamp or capture device may be placed between the audio interface and the device under test.
Depending on its design, it may provide one or more of the following:
- Level adjustment
- Balanced-to-unbalanced conversion
- Galvanic isolation
- Ground-lift capability
- Appropriate connection to instrument-level equipment
- Convenient routing
A reamp box should not automatically be assumed to be necessary for every capture setup. Likewise, the presence of a reamp box does not by itself guarantee correct calibration.
The requirements depend on the equipment, interface, capture platform, and desired accuracy.
Device under test
The device being captured should be placed in a stable, repeatable state before beginning the capture.
Document settings that may affect the result, including:
- Gain
- Volume
- Tone controls
- Channel selection
- Switch positions
- Pedal settings
- Cabinet configuration
- Speaker or cabinet simulation
- Other devices in the signal chain
If the equipment changes state during the capture, the resulting model may be inaccurate or unusable.
Tube amplifiers should be allowed to reach normal operating condition before critical captures are made.
Capturing pedals and preamps
Pedals, preamps, and similar devices can often be returned directly to an audio interface input, provided that the electrical levels and connections are appropriate.
A simple chain may therefore be:
Interface output → reamp device → pedal or preamp → interface input
Multiple devices may also be captured as a chain:
Interface output → reamp device → pedal → preamp → processor → interface input
In that case, the resulting model represents the combined behavior of the chain rather than each device individually.
Capturing an amplifier without a cabinet
An amplifier output designed to drive a loudspeaker must not normally be connected directly to an audio interface input.
A suitable load and signal-level conversion method is required unless the amplifier provides a dedicated output designed for this purpose.
A direct amplifier capture might therefore use:
Interface output → reamp/capture device → amplifier → suitable load/direct capture device → interface input
This produces a model of the amplifier portion of the chain without necessarily including the acoustic behavior of a loudspeaker cabinet and microphone.
A cabinet simulation or impulse response may then be used during playback.
Capturing an amplifier and cabinet
To include a physical loudspeaker cabinet in the capture, the acoustic output of the cabinet must normally be converted back into an electrical signal with a microphone.
A typical chain becomes:
Interface output → reamp/capture device → amplifier → speaker cabinet → microphone → microphone preamp/interface input
The resulting capture may contain the combined characteristics of:
- Amplifier
- Loudspeaker
- Cabinet
- Microphone
- Microphone position
- Microphone preamp
- Other equipment in the return chain
Moving the microphone or changing any of these components can substantially change the result.
Return path
The returned signal must reach the capture system without unwanted clipping, processing, or alteration.
Check for:
- Correct interface input
- Appropriate input type
- Correct gain
- Phantom power where appropriate
- Unwanted high-pass filters
- Automatic gain control
- Software monitoring or processing
- EQ or dynamics processing
- Cabinet simulation
- Sample-rate conversion
Unless the capture procedure specifically requires processing, the safest general principle is to return the signal as directly and transparently as practical.
Gain staging
Every stage in the chain has the potential to change level.
A capture may involve several distinct level domains:
- Digital level inside the computer
- Interface output level
- Signal presented to the device under test
- Output level from the device under test
- Interface input level
- Recorded digital level
These should not be treated as though they are interchangeable.
For example, a recording that peaks safely below 0 dBFS tells you that the converter is not digitally clipping, but it does not tell you whether the amplifier or pedal received the intended input level.
See Gain Staging and Calibration.
Ground loops and noise
Capture systems often connect a computer, audio interface, amplifier, and other mains-powered equipment simultaneously. This can create ground loops.
Symptoms may include:
- Hum
- Buzz
- Computer-related noise
- High-frequency interference
Balanced connections and transformer isolation can help where appropriate. A ground-lift facility on a properly designed reamp or capture device may also be useful.
Do not defeat protective mains safety grounding as a solution to a ground-loop problem.
Monitoring
Monitoring should be arranged so that it does not accidentally become part of the capture path.
DAW routing can sometimes return the captured signal to an output, creating feedback or an unintended loop.
Before beginning a capture, verify independently:
- Which output carries the test signal
- Which input receives the return
- Where monitoring is routed
- Whether any plugins are active
- Whether direct monitoring is enabled
Document the chain
A useful capture should be reproducible.
When creating captures for yourself or others, record enough information to understand what the model represents.
Useful documentation may include:
- Device manufacturer and model
- Device settings
- Interface
- Output used
- Reamp or capture device
- Input used
- Calibration information
- Cabinet and speaker
- Microphone and position
- Sample rate
- Capture platform and version
- Relevant software settings
Good documentation can be almost as important as the model itself.
Troubleshooting the chain
When a capture fails or sounds wrong, troubleshoot the chain one stage at a time.
Do not immediately assume the modeling system failed.
Verify:
- The correct test signal is being played
- The correct interface output is active
- The signal reaches the device under test
- The device is responding normally
- The return signal reaches the intended interface input
- Nothing clips
- No unwanted processing is active
- Levels and calibration meet the platform's requirements
- The recorded or captured return has the expected duration and format
Breaking the system into individual stages usually reveals problems much faster than repeatedly attempting the entire capture process.