corrscope/docs/index.md

Corrscope Help

Corrscope is named because it cross-correlates the input wave and a history buffer, to maximize wave alignment between frames.

Program Layout

• Global options: Left pane
• Trigger options: Top-right pane
• Per-channel options: Bottom-right table

Correlation Trigger Explanation

Unlike other triggering algorithms (which only depend on the current frame's worth of data), Corrscope uses a correlation trigger, which remembers several forms of data between frames, to help align future frames.

Options

• Left pane, Global
  • Trigger Width (also controllable via per-channel "Trigger Width ×")
• Top pane, Wave Alignment
  • Buffer Responsiveness
  • Mean Responsiveness
  • Pitch Tracking
• Top pane, Edge Search
  • Edge Direction
  • Edge Strength

Variables Remembered

• buffer: array of samples, containing Trigger Width (around 40 milliseconds) recent "trigger outputs". Starts out as all zeros.
• mean: real number, estimated from recent "trigger inputs". Starts out at 0.

Obtaining Data (each frame)

On each frame, corrscope fetches [from the channel] a buffer of mono data, centered at the current time. The amount of data used is controlled by Trigger Width, which should be increased to keep low bass stable.

To remove DC offset from the wave, corrscope calculates the new mean of input data. Because each frame's mean may jitter depending on what window of the wave was selected, update saved mean = mean + Mean Responsiveness × (new mean - mean). Then corrscope subtracts this averaged mean from data.

Corrscope then estimates the fundamental period of the waveform, using autocorrelation.

(optional) Pitch Tracking

If Pitch Tracking is enabled:

If period changes significantly, corrscope computes the spectrums of data and data from 2 frames ago, and cross-correlates them to estimate the pitch change over the last 2 frames. It then resamples (horizontally scales) buffer to match this pitch change.

Correlation Triggering (uses buffer)

Corrscope multiplies data by data window of some sort.

Corrscope uses edge_finder, which is computed once and reused for every frame.

Corrscope correlates data with buffer + edge_finder to find the best part of data to pick, then sets position to be used for rendering.

(Optional) Post Triggering

If post triggering is enabled, the post trigger is called with position and returns a new position, which overwrites the original variable.

Updating Buffer

• Corrscope fetches [from the channel] a buffer of mono triggered data, centered at position
• triggered data is multiplied by a Gaussian window with width 0.5 × period
• Corrscope updates buffer = buffer + Buffer Responsiveness × (triggered data - buffer).