C64 Non-Standard Hues: PAL & NTSC Palette

This guide examines techniques for producing colors beyond the Commodore 64’s native 16-color palette, covering implementation differences between PAL and NTSC hardware variants. Two primary approaches are documented: the Alternate Line Method (ALM) and Dynamic Chequerboard Method (DCM).

The VIC-II graphics chip generates 16 fixed colors through analog circuitry—a palette constraint often criticized compared to systems offering larger color selections. However, creative techniques can synthesize additional perceived colors through optical mixing, exploiting display characteristics and human visual perception to expand the effective palette substantially.

These techniques occupy interesting territory between hardware capabilities and perceptual phenomena. They produce colors the VIC-II cannot generate directly, yet the results appear on authentic hardware without modification. Understanding both the technical mechanisms and display-dependent factors enables effective application across different viewing environments.

Primary Approaches

Alternate Line Method (ALM)

ALM exploits CRT color bleeding by alternating pixel colors between adjacent scanlines. The phosphor persistence creates perceived intermediate hues. This technique requires PAL hardware with analog CRT output—digital connections and HDMI adapters bypass the color bleeding necessary for the effect.

The mechanism relies on CRT display characteristics. When a cathode ray tube renders adjacent scanlines in different colors, the phosphor glow from each line overlaps slightly with neighboring lines. Human eyes integrate this overlapping glow into a perceived intermediate color—placing cyan on even lines and yellow on odd lines produces a green appearance unavailable in the native palette.

ALM implementation uses character graphics or bitmap modes where individual scanlines can display different colors. In character mode, carefully designed characters alternate colors between consecutive pixel rows. In bitmap mode, direct control over each scanline enables arbitrary ALM patterns.

The CRT dependency limits ALM applicability. Modern flat-panel displays and digital capture devices render each scanline discretely without the analog blending that produces the effect. HDMI scalers, FPGA solutions, and emulators with “correct” rendering show visible striping rather than blended colors. For authentic ALM results, original CRT displays with composite or S-Video connections remain necessary.

Dynamic Chequerboard Method (DCM)

DCM implements dithering patterns that invert each frame, creating perceptual color blending through temporal mixing. Unlike ALM, DCM functions across both video standards. Combining red (#$02) with blue (#$06) produces deep purple tones suitable for atmospheric graphics.

DCM exploits persistence of vision rather than display artifacts. The technique displays a checkerboard pattern of two colors, then inverts the pattern on the next frame. Each pixel alternates between colors at 50Hz (PAL) or 60Hz (NTSC). Human visual processing integrates these rapid alternations into a perceived steady color representing the mixture of both source colors.

The frame-synchronized inversion requires interrupt-driven pattern updates. A raster interrupt at the top of each frame toggles a flag indicating current phase. All DCM-affected screen memory adjusts accordingly—XOR operations efficiently invert existing patterns. The CPU cost is modest given that pattern data remains unchanged; only the display relationship inverts.

DCM produces results across all display types because the blending occurs perceptually rather than through display artifacts. CRTs, LCDs, and capture devices all show the rapid alternation that eyes integrate into mixed colors. Some viewers perceive subtle flicker, particularly in peripheral vision; center-gaze viewing typically produces clean blends.

Verification Approach

Testing configurations included:

• VICE emulator configured for PAL and NTSC modes
• Original PAL hardware connected to CRT monitors
• NTSC-region hardware (C128) connected to 1084S display

Cross-platform verification ensures documented techniques produce consistent results across the C64 ecosystem. Emulator testing enables rapid iteration; hardware verification confirms real-world performance matches theoretical expectations. The combination identifies emulator-specific artifacts and hardware-specific behaviors.

Display selection matters significantly for these techniques. CRT monitors introduce analog characteristics (phosphor glow, shadow mask patterns) that affect perceived results. Modern displays eliminate these characteristics but introduce their own artifacts (pixel-perfect rendering, sample-and-hold response). Testing across display types characterizes the full range of viewer experiences.

Observed Results

PAL Performance

Hardware verification confirmed DCM generates visually smooth color blends. Red/blue combinations produced consistent dark purple without visible pattern artifacts, though camera capture introduced some banding not present during direct viewing.

PAL timing relationships favor DCM implementation. The 50Hz refresh rate and pixel clock characteristics create clean temporal mixing on most display configurations. The slightly longer frame time (20ms versus 16.67ms for NTSC) allows marginally more phosphor decay between frames, but the difference rarely produces visible effects.

ALM results on PAL hardware showed excellent blending on properly adjusted CRT displays. The 312 scanlines per frame and specific luma/chroma phase relationships create favorable conditions for inter-line color bleeding. Displays with strong vertical convergence produce particularly smooth results.

NTSC Performance

NTSC systems exhibited vertical striping when using 1×1 pixel DCM patterns. The underlying cause relates to pixel-width versus color-cycle timing differences. Historical Commodore documentation describes pixels measuring 7/16ths of color clock width, producing cyclical color shifting horizontally.

The NTSC color encoding system creates phase relationships between adjacent pixels. When checkerboard patterns alternate colors at 1-pixel intervals, these phase relationships produce constructive and destructive interference patterns—manifesting as visible vertical stripes of varying intensity. The effect resembles interlace combing but stems from different mechanisms.

Expanding to 2×1 pixel patterns significantly reduced striping artifacts by better aligning pixel boundaries with color clock transitions.

The 2×1 pattern solution trades horizontal resolution for color quality. Each “color pixel” now spans two VIC-II pixels horizontally, reducing effective horizontal color resolution by half. For many applications—particularly background graphics where fine horizontal detail matters less than color quality—this trade-off proves acceptable.

System Specifications

Measurement	Ratio
PAL pixel-to-clock	approximately 14:16
NTSC pixel-to-clock	7:16

These ratios explain the artifact differences between video standards. PAL pixels span nearly a full color clock cycle (14/16 = 87.5%), reducing the phase variance between adjacent pixels. NTSC pixels span less than half a color clock cycle (7/16 = 43.75%), creating pronounced phase differences that manifest as the visible striping artifacts.

The underlying physics involves analog color encoding. NTSC and PAL both encode color information as phase modulation of a carrier signal. Pixel boundaries that align poorly with color clock transitions create visible interference patterns. The more closely pixel timing matches color clock timing, the cleaner the resulting image.

Testing Utility

An executable program (.prg format) enables experimentation with all 16 native colors. Controls allow selection of component colors A and B, with comparison color U for reference evaluation.

The utility implements both ALM and DCM techniques, allowing direct comparison on any given hardware configuration. Users can cycle through all color pair combinations, observing blending quality and artifact characteristics for each. This empirical approach reveals hardware-specific behaviors that theoretical analysis cannot predict.

C128 Extended Capabilities

The Commodore 128 supports additional color generation through VIC-II desynchronization, as demonstrated in Crest’s Risen From Oblivion production. Phase manipulation on compatible monitors produces further non-standard hues.

The C128 technique exploits timing manipulation unavailable on standard C64 hardware. By briefly desynchronizing the VIC-II’s color burst signal, the displayed image temporarily loses color reference, causing monitors to interpret pixel colors differently. Controlled application of this effect produces specific non-palette colors with consistent results on supporting displays.

This approach requires compatible monitor hardware—not all displays respond predictably to desynchronization. Additionally, the technique works only on C128 hardware due to specific timing relationships between the C128’s VIC-II and surrounding circuitry. Despite these limitations, the capability demonstrates that even thoroughly-documented hardware contains undiscovered possibilities.

Implementation Applications

DCM proves particularly viable for interactive software development where expanded color variety enhances visual presentation while maintaining hardware compatibility. The technique enables complex atmospheric effects within standard memory and processing constraints.

Practical applications include: gradient sky rendering with smooth color transitions, water effects requiring blue-green intermediate tones, atmospheric perspective where distant objects fade toward horizon colors, and UI elements requiring colors that stand apart from the standard palette.

Performance considerations remain modest. DCM requires frame-synchronized screen updates—typically one raster interrupt per frame—plus the memory operations for pattern toggling. The CPU overhead scales with affected screen area but remains manageable for typical applications. Full-screen DCM might consume 500-1000 cycles per frame; partial application proportionally less.

Limitations and Considerations

Extended color techniques involve inherent trade-offs worth understanding before adoption:

Display dependency: Results vary across display types. CRT-dependent techniques fail on modern displays; all techniques show some variation across display calibrations. Testing on target hardware remains essential.

Flicker sensitivity: Some viewers perceive DCM flicker, particularly those with above-average flicker detection sensitivity. Providing non-DCM graphics options accommodates affected users.

Capture challenges: Video capture rarely reproduces extended color effects accurately. Frame blending settings, capture timing, and post-processing all affect results. Documentation materials may show different colors than live viewing produces.

Despite these considerations, extended color techniques meaningfully expand C64 visual capabilities when applied appropriately. Understanding both capabilities and limitations enables effective application.

Related: Luma-Driven Graphics

See also: Extended Color Mode applications · advanced visual standards for C64 · font grid proportions