Stellerator is an emulator for the Atari 2600 VCS (Stella) developed as part of the 6502.ts project. Stellerator runs the vast majority of games for the VCS flawlessly, including both original games, demos and homebrews. Most cartridge types and bankswitching schemes are supported, including ARM-based DPC+ cartridges.
Among other things, it supports hardware accelerated graphics with phosphor simulation, scanlines and TV effects. and jitter-free audio emulation. Cartridges are stored locally in the browser, no data is transferred over the web.
The emulator UI is responsive and works well on mobile devices, too. The emulator uses a service worker and is able to load from the cache even if no internet access is available. On mobile devices, you can add the page to the homescreen to make it behave like a mobile application.
The source is available under the MIT license on github. Feel free to fork, explore and contribute.
The "Cartridges" page allows you to import, configure and run ROM image files. Image files can be loaded directly from ZIP files. If you load a ZIP file, all ROMs in that file will be imported.
- ROM Name: The cartridge name as displayed in the cartridge list. Defaults to the name of the image file.
- ROM type: Size and bankswitching type of the catridge. This is autodetected and should not be changed under normal circumstances.
- TV mode: Use this setting to change the emulated TV type. The default is guessed from the filename.
- Emulate paddles: Normally, Stellerator emulates four paddles. Some games (notably Thrust) have their controls messed up by the presence of the paddles unless they are turned off by this option.
- RNG seed: Upon startup, the emulated hardware is initialized to a random state. Fixing the seed will remove the randomness and will fix the initial state. This allows to reliably run buggy games that otherwise fail to start depending on the initial state (e.g. Dark Chambers).
- First visible frame line: Determine the first visible line of the image. The default is auto detection ("auto") and should work fine for the vast majority of ROMs. For problematic cases, this can be set to a fixed value. Typical values of real CRTs lie somewhere around 30 scanlines.
- CPU emulation: Override the accuracy with which the CPU is emulated. See below for the different options.
- Audio emulation: Override the method of emulating audio configured in the settings. See below for a detailed description of the available choices.
- Volume: Allows to control audio on a per-cartridge basis.
- Default phosphor: Use the global phosphor settings. If disabled, a per-ROM setting can be configured.
- Phosphor level: Phosphor blend level for this ROM (if default phhosphor is disabled).
The "Settings" page allows to configure global emulation settings.
- CPU emulation accuracy: Change the default accuracy with which the CPU is emulated. See below for the different options.
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Volume: Overall volume.
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Audio Driver: Switch the default audio driver. See below for a detailed description of the various options.
- Gamma correction: Adjust gamma correction.
- TV emulation: TV emulation mode. Note that this uses hardware acceleration
and may cause visual artifacts on some GPUs. In particular,
compositedoes not display cleanly on some iOS devices due to precision issues. - Scaling: Scaling mode. See below for a detailed description of the options.
- Phosphor level: Phosphor blend level. A value of 0 disables phosphor simulation.
- Scanline intensity: Intensity of the scanline overlay.
- Touch controls: Enable / disable touch controls. The default is "auto" and there usually is no need to change it.
- Left handed mode: Swap touch controls for left handed users.
- Touch joystick sensitivity: Sensitivity of the virtual joystick used by the touch controls.
- Display mode: Override the display size.
- Size: Choose a scling factor for the UI.
On the emulation page, the emulator can be controlled with the control panel on the right.
- Difficulty left/right: Controls the corresponding switches on the VCS.
- TV mode: Controls the corresponding switch (color / BW) on the VCS.
- Limit framerate: Toggle frame rate limiting. Without limiting, emulation will run at the maximum speed the browser can deliver.
- Hard Reset: Hard reset the VCS. Note that RAM contents are preserved, which causes some buggy games to restart in funky states.
- Pause / Resume: Stop and resume the emulation.
The keyboard controls are explained on the emulation page. Both joysticks and the select / reset buttons on the VCS are controlled via keyboard. In addition, there are keys for resetting the emulator, pause / resume and toggling fullscreen mode.
Stellerator supports controlling the two joysticks and select / reset with gamepads. As the emulator currently does not provide a way to change the button mapping, your mileage may vary. If one or two supported gamepads (for which the browser reports a well-defined mapping) are plugged in, the top bar will display "A" or "AB" in the top right corner.
Stellerator can be controlled by touch gestures on touch capable devices. The touch controls divide the VCS video image into the four quadrants NW (top left), NE (top right), SW (bottom left) and SE (bottom right).
In "normal mode", the controls are:
- NW: Double tap to toggle fullscreen, hold to switch to "alt" mode (see below)
- SW: Fire
- NE / SE: Hold and drag to control a virtual joystick that is centered at your intial touch position.
While you are touching NW, the controls enter "alt" mode:
- SW: Tap to pause / unpause
- NE: Select
- SE: Reset
The touch controls can be changed to "left handed" moded in the settings. This basically mirrors quadrant assignment horizontally:
While Stellerator emulates all four paddles, currently only one paddle one can be controlled. The paddle is mapped to the horizontal mouse position; fire corresponds to joystick 0 / right.
Pressing enter toggles fullscreen mode.
Stellerator supports two different ways to emulate audio in a browser environment, which both have their issues and merits. The default is PCM.
The default ("PCM") should work fine. Consider switching to "Waveform" if you have performance issues, crackling artifacts or find the small audio lag unbearable.
This will produce a continuous stream of PCM audio data that is played back in realtime. The stream is cycle exact and fully supports both "normal" TIA audio and advanced effects like digitized samples (as used in Quadrun) or synthesized music (e.g. Pitfall II, Stay Frosty 2).
The downside is that a browser is a very bad environment for anything realtime, and even small lags will lead to popping noises. Stellerator attempts to mitigate this by buffering audio data, but the price is a small lag of ~3 frames between audio and video. Still, PCM provides highly accurate emulation and should work fine for most people, although the mileage might vary on slower systems and with certain OS / browser combinations.
In this mode, the emulator determines the current pattern (AUDC und AUDF) played by the TIA and instructs the browser to loop it until changed. Volume changes (AUDV) are handled natively by the browser.
This approach is much more tolerant to timing glitches than PCM. Lag will prolong the individual notes played by the TIA, but no popping occurs, and small speed variations are virtually unnoticable. There is no noticable lag between audio and video.
The price for this is emulation accuracy. Most games will sound just fine, but keen ears might spot small differences, and PCM playback via bitbanging will not work (e.g. affecting Pitfall II, Quadrun, Stay Frosty 2 and others).
There are two different accuracy levels for CPU emulation.
Use "Cycle" unless you encounter performance problems.
This mode provides a cycle exact model of every bus access. However, it is about 20% slower than the faster "Instruction" mode.
This mode provides cycle exact CPU emulation as well, but multiple bus accesses may be batched in a single cycle. This is faster than "Cycle" mode, but compatibility is slightly worse.
In practice, the only game known to habe issues with "Instruction" mode is Pole Position.
Stellerator offers three different algorithms for scaling the console image to the screen. Scaling is done on the GPU.
Scales to the nearest integer multiple and then does bilinear interpolation to the final size. This results in a crisp picture while still avoiding Moirée patterns.
Avoids Moirée patterns, but blurs the image.
Nearest Neighbour interploation. Generates a crisp image but is prone to cause Moirée patterns.
Stellerator aggressively uses many modern web technologies, some of which are part of the ES6 standard. Therefore, a recent browser is required to run the emulator.
Stellerator is tested and works fine on current versions of Chrome, Firefox and Safari. Performance is best in Chrome and Safari, Firefox may lag on DPC+ cartridges.
The emulator runs at full speed (approximately 3.5 MHz / 60 FPS in NTSC mode) on any modern x86-based machine. On ARM based mobile devices, speed varies greatly. Android devices usually are incapable of running the emulator at full speed. Modern 64bit iOS devices run the emulator well. During emulation, the current clock of the emulated system is displayed in the upper right corner. Chrome / V8 and JavascriptCore / Safari tend to be faster than Firefox.
If you run into performance issues, switch CPU accuracy to "instruction" and set the audio driver to waveform. Turning on frame merging and disabling the web worker can can help as well.
- Keyboard and gamepad mappings cannot be configured.
- Only the first paddle can be controlled.
- Audio on Safari on both iOS and MacOS may require a touch or click to start. This is a restriction imposed by Apple, and nothing can be done about it.