corrscope/docs/index.md

Corrscope Help

Homepage at https://github.com/corrscope/corrscope. Report bugs at https://github.com/corrscope/corrscope/issues/ or https://discord.gg/CCJZCjc.

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

Tutorial

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

Start by adding channels to be visualized: look at the bottom-right table and click the "Add..." button.

To add audio to play in the background, look at the top-right "FFmpeg Options" and click the Master Audio "Browse..." button.

To make the waves taller, go to the left panel's General tab and edit Amplification. Afterwards, click the Appearance tab and customize the appearance of the oscilloscope. (Note that colored lines will be discolored and blurred by Youtube's chroma subsampling.)

Click Preview to launch a live preview of the oscilloscope with audio.

Click Render to render the oscilloscope to video.

Click Save to save the current project configuration to a file. These project files can loaded in corrscope, previewed or rendered from the command line, or shared with corrscope's author when reporting issues.

Configuring the Trigger

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

Corrscope's triggering algorithm is configurable, allowing it to track many types of waves well. However it may be intimidating to newcomers. This will provide several types of waves, along with suggestions for how to tune trigger options.

Triggering options are found on the left panel. Trigger Width is located in the General tab. All other options are found on the Trigger tab. (Per-channel triggering options are found in the table.)

Sampled Trumpets and Trigger Direction (screenshot from Tales of Phantasia)

Sampled trumpets generally consist of a sharp falling edge, followed by gibberish with one or more rising edges.

• Set "Trigger Direction" to "Falling (-1)", which will track the falling edge well. (Using a rising-edge trigger will result in poor results, since the gibberish will vary between notes, especially for SNES SPC music using the echo functionality.)
• Slope trigger is useful, since the trumpet has a narrow tall positive section, followed by a narrow tall negative section.

Complex Waves and Trigger Direction (screenshot from Midori Mizuno - Sinkhole)

Corrscope's standard "edge trigger" does not look for "steep edges" but instead "sign changes". It operates by maximizing (signed area in right half) - (signed area in left half). This waveform has a clear falling edge from positive to negative, but no clear edge from negative to positive.

Either:

• Set "Trigger Direction" to "Falling (-1)".
• Alternatively set "Trigger Direction" to "Rising (+1)", set "Edge Strength" to 0, and increase "Slope Strength". This will latch onto the small rising edge.

NES Triangle Waves

NES triangle waves are stair-stepped. In theory, Area Trigger would work and properly locate the best zero-crossing on each frame. However, on every frame, corrscope looks at a different portion of the triangle wave, computes the average value (DC offset), and subtracts it from all samples. Unfortunately since the exact amount of DC (positive or negative) fluctuates between frames, corrscope will shift the wave vertically by different amounts, causing it to jump between different rising edges.

Try the following:

• Use any "Trigger Direction" you prefer. Rising and Falling both work equally well.
• Set "Sign Triggering" to 1 or so. This causes corrscope to preprocess the waveform before DC is removed, and add 0.5(peak-to-peak amplitude) to positive samples and subtract 0.5(peak-to-peak amplitude) from negative samples. The resulting steep edges will remain as zero crossings, even after DC is filtered out.
• Afterwards, set "Edge Strength" to nonzero (and optionally enable "Slope Strength"). Edge Strength will pick up the zero crossings (which match the zero crossings before DC removal), and Slope Strength will pick up the steep edges located at zero crossings.

NES triangle waves have 15 rising/falling edges. The NES high-pass removes DC and low frequencies, causing waveforms to decay towards y=0. As a result, "which edge crosses y=0" changes with pitch.

• Reduce "Buffer Strength" to 0 (or up to 0.5). Corrscope's buffer needs to be disabled, to prevent it from remembering "which edge used to cross y=0".

FDS FM Waves

FDS FM changes the width of waves, but not their height.

The NES high-pass removes DC and low frequencies, causing waveforms to decay towards y=0. If FDS waves contain anything other than pulse/saw, "which part of the wave crosses y=0" may change with FM and pitch.

• Experiment with "Trigger Direction".
• Use nonzero Slope Strength and low (or zero) Edge Strength, to reliably locate the sharpest edge in a waveform. This is because sharp edges are preserved by FM, whereas the width of waves is not.
  • If you have multiple steep rising/falling edges,

Yamaha FM and SNES/sampled Waves

• Experiment with "Trigger Direction".
• Try using Slope Strength, Edge Strength, or a combination of both.
• Reduce both relative to Buffer Strength to track evolving waves better (but center new/existing waves less strongly). To restore centering, you can enable Post Triggering and experiment with the radius.

Options

All tabs are located in the left pane.

• Global
  • Trigger Width (also controllable via per-channel "Trigger Width ×")
• Trigger, Wave Alignment
  • Buffer Strength
  • Buffer Responsiveness
  • Pitch Tracking
• Trigger, Edge Triggering
  • Edge Direction
  • Edge Strength
  • Slope Strength
• Trigger, Post Triggering
  • Post Trigger
  • Post Trigger Radius

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.
• edge_finder: computed once, never changes, reused for every frame.

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.

• If Edge Direction is "Falling (-1)", then both the main and post trigger will receive negated data from the wave, causing both to search for falling edges (instead of rising edges).

Sign Triggering

Some waves do not have clear edges. For example, triangle waves do not have clear rising edges (leading to suboptimal triggering), and NES triangles have 15 small rising edges, causing corrscope to jump between them.

If Sign Strength (Sign Triggering on the GUI) is set to nonzero strength, corrscope computes peak = max(abs(data)). It adds peak * strength to positive parts of data, subtracts peak * strength from negative parts of data, and heavily amplifies parts of the wave near zero. This helps the correlation trigger locate zero-crossings exactly.

Mean/Period

To remove DC offset from the wave, corrscope calculates the mean of input data and subtracts this averaged mean from data.

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

Corrscope multiplies data by data window to taper off the edges towards zero, and avoid using data over 1 frame old.

(optional) Pitch Tracking

If Pitch Tracking is enabled:

If period changes significantly:

• Cross-correlate the log-frequency spectrum of data with buffer.
• Rescale buffer until its pitch matches data.

Pitch Tracking may get confused when data moves from 1 note to another over the course of multiple frames. If the right half of buffer changes to a new note while the left half is still latched onto the old note, the next frame will latch onto the mistriggered right half of the buffer. To prevent issues, you should consider reducing Buffer Responsiveness (so buffer will not "learn" the wrong pitch, and instead be rescaled to align with the new note).

Correlation Triggering (uses buffer)

• Buffer Strength controls the strength of buffer (previous on-screen content), which searches for waves which line up with previous on-screen content.
• Edge Strength controls the strength of edge_finder, which searches for waves which are negative on the left, but positive on the right.
• Slope Strength controls the strength of slope_finder, which searches for waves which steeply increase near the center of the screen.

Corrscope cross-correlates data with (Buffer Strength * buffer) + (Edge Strength * edge_finder) + (Slope Strength * slope_finder) to produce a score for each possible data triggering location. Locations which line up well with the complex expression (line up well with the previous frame, transition from negative to positive, or increase in value) have high scores. Corrscope then picks the location in data with the highest score as the position to be used for rendering.

(Optional) Post Triggering

If post triggering is enabled:

• We recalculate the post mean of data around our new position value. If position is a good trigger position (and there are no nearby discontinuities like note changes), then post mean should be stable and not jitter.
• The post trigger is called with position and returns a new position, which overwrites the original variable.

Zero Crossing Trigger

Setting Post Trigger to "Zero Crossing Trigger" causes corrscope to "slide" towards edges. The maximum distance per frame is determined by GUI Post Trigger Radius (in samples).

• Grab some new data around position. Subtract post mean from the new data.
• If the data is positive, search left for a rising edge. If the data is negative, search right for a rising edge.
• If no edge is found within Post Trigger Radius samples, return position ± Post Trigger Radius.

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).

Video Encoding

Corrscope uses FFmpeg to encode videos. All video encoding settings (both picking an encoder and options) are configured in Corrscope's "Video Template" textbox, which is passed to FFmpeg. By default, it tells FFmpeg to use the x264 video encoder (producing H.264 videos). Tuning video encoders like x264 is a complex task, but this is a brief summary of the default settings:

Videos are first converted from RGB pixel values to YUV (brightness and color).

• -pix_fmt yuv420p enables chroma subsampling of the YUV frames, which halves the horizontal and vertical resolution of the color channels (blurring color information) before compressing the video. For example, a 1280x720 video only has 640x360 of color information!
  • This improves compatibility with players like web browsers, smartphones, and Windows 10's Videos app, but degrades the quality of colored lines.
  • Removing this argument produces a better-looking video (with a relatively small file size increase), but the quality boost is lost when uploading to YouTube (which transcodes the video to yuv420p).

Afterwards, the video is sent to the video encoder, which has its own arguments:

• -c:v libx264 picks libx264 as the video encoder.
• -crf 18 determines the quality of the compressed video. Higher values discard more information, producing smaller but lower-quality files.
• -preset superfast speeds up the rendering process at a given quality level, at the cost of a larger file size.

Avoiding Color Artifacts

Video encoding/compression degrades color more than brightness, especially fine color detail. As a result, thin colored lines look desaturated, fuzzy, or discolored. (Thin lines generally arise when rendering at a low resolution, or when YouTube takes a high-resolution video with thick lines and reencodes lower-resolution streams with thinner lines.) To avoid this loss of quality, corrscope defaults to white lines on a black background.

Loss of color information is especially damaging with "Color Lines By Pitch" enabled. At the default settings (720p, 1.5 pixel thick lines), the vibrant line colors seen in the preview lose saturation when rendered, and turn into grayish messes when uploaded to YouTube (blues and purples lose the most color).

To render colored lines while minimizing quality loss, render at a higher resolution (slower) with thicker lines. This will improve color fidelity for people who watch the resulting videos above 720p.

I do not have experience with other encoders (like x265, VP8, VP9, or AV1), but the principle of losing fine color detail to chroma subsampling and lossy codecs should remain the same. AV1 should preserve colored lines better due to chroma-from-luma, but AV1 encoders are still impractically slow.

Audio Encoding

corrscope defaults to rendering to .mp4 files, which support a limited set of audio codecs. MP3 is a "good enough" audio codec. AAC is better in theory, but ffmpeg's AAC encoder (which corrscope uses by default, -c:a aac) is bad. However, corrscope defaults to 384 kilobits/sec (-b:a 384k), which should be sufficient to produce audio without obvious artifacts.

In the future, I may enable VBR encoding, or switch to an audio codec not supported in .mp4 files, like Vorbis or Opus (which has better quality at any bitrate than even good AAC encoders). This would requires switching to a different file format like .mkv.