The Sega Mega Drive, known as the Genesis in North America, is remembered as a console that emphasized speed, action, and vibrant arcade-style games. But beyond the blast processing slogans and iconic titles like Sonic the Hedgehog, the Mega Drive had a surprisingly advanced graphics system capable of more than what most developers asked of it. One of the most fascinating and underutilized features of the system is Interlace Mode 2.
Understanding Interlacing on the Mega Drive
The Mega Drive’s VDP, the Yamaha YM7101, is capable of multiple display resolutions. Horizontally, it can run in either H32 mode (256 pixels wide) or H40 mode (320 pixels wide). Vertically, most games use a progressive resolution of 224 lines (or 240 lines on PAL region systems). This means each frame is drawn from top to bottom, one line after the other, in a single pass.
But the VDP can also output interlaced video, just like CRT televisions. In interlaced modes, each frame is split into two fields. The first field draws only the even-numbered lines, and the second field draws the odd-numbered lines. When these are alternated quickly, the viewer perceives a full-resolution image. This trick allows the Mega Drive to output a screen with 448 vertical lines, effectively doubling the visible resolution.
Interlace modes: what’s the difference?
The Mega Drive supports three interlace modes:
Mode 0, the non-interlaced mode. This mode is plain 240p. It uses a trick to prevent the odd lines from ever being drawn, meaning that two sets of 240 even lines are drawn instead.
Mode 1. Similar to Mode 0, but doesn’t prevent odd lines from being drawn. The odd lines will display the exact same graphics as the even lines. The official ‘Genesis Software Manual’ developer document warns that this mode will result in severe vertical blurring.
Mode 2. Like Mode 1, except the odd lines will not display the same graphics as the even lines. The Mega Drive’s vertical resolution will double, being 320×480 or 256×480. However, because only even or odd lines are displayed in a single frame, this means that the image will be downsampled back down to 320×240 / 256×240 when displayed.
Regardless of the interlace mode, it’s always 240 lines that are output in a frame.
How Sonic the Hedgehog 2 uses Interlace Mode 2
Sonic the Hedgehog 2 uses Interlace Mode 2 for its iconic split screen multiplayer gameplay, and takes advantage of the doubled vertical resolution. The interlacing does nothing but halve the game’s vertical resolution and introduce a very ugly jittery effect to the screen. This approach allowed Sega Technical Institute to display both players’ screens simultaneously and maintain separate camera systems. It’s a technically ambitious use of the VDP, and very few games attempted anything similar.
How Sonic 2’s 2P Mode is intended to look
How Sonic 2’s 2P Mode actually looks
Why Was Interlace Mode 2 So Rarely Used?
Despite the obvious benefits of higher vertical resolution, there were several reasons why developers avoided Interlace Mode 2.
- Visual Flicker on CRTs: Interlaced video can cause shimmer and flickering, especially for small, high-contrast elements like thin lines or text. For fast-moving games, this could make the image appear unstable.
- No Native Sprite Scaling: Because the resolution doubled vertically, all assets appeared shorter unless scaled manually. The Mega Drive didn’t support sprite scaling natively, so artists had to create additional sprite sets or adjust layout logic, adding development overhead. Sega didn’t introduce native sprite scaling till 1992’s Mega CD add-on.
- Increased VRAM Demands: Rendering double the lines requires more data to be sent to VRAM per frame. This taxed the VDP bandwidth and could affect performance, particularly in action games striving for smooth 30 or 60 fps.
Final Thoughts
Interlace Mode 2 is one of the Mega Drive’s most fascinating under-the-hood features. It offered developers a way to dramatically improve visual clarity and interface complexity, but came with trade-offs in terms of performance, compatibility, and complexity. Still, in the hands of skilled developers, it could deliver results that pushed the 16-bit system far beyond its usual limits.
If you’ve ever thought the Genesis was all about speed and not sophistication, Interlace Mode 2 proves there’s a lot more to explore beneath the surface.
