bsnes-libretro/docs/faq.md

I see "tearing" when a game scrolls. How can I enable vsync?

higan supports synchronising video output to the display's vertical-synchronisation (or "vsync") signal, but the option is hidden because it often causes more problems than it solves (see the next question). To enable video synchronisation:

• Open higan's configuration file, settings.bml
  • On Windows, look in %LOCALAPPDATA%\higan or beside higan.exe
  • On Linux, look in ~/.local/share/higan
• Open it in your favourite text editor (Windows Notepad will corrupt the file, use WordPad if you don't have anything better)
• Find the line that says "Video"
• Somewhere below that, find an indented line that says "Synchronize:false".
• Change "false" to "true"
• Save your changes to settings.bml and restart higan

Why is video synchronisation a problem for higan?

The short version: Turning on video synchronisation cleans up video tearing, turning on audio synchronisation cleans up audio glitches, but turning on both makes audio glitches worse.

The long version: Enabling video synchronisation locks the frame-rate of the emulated console to the frame-rate of your computer's display. If your display's refresh rate exactly matches the emulated console's, games play at the correct speed and everything's fine.

However, modern 60Hz displays do not always match the emulated console's refresh rate:

• The Super Famicom usually runs a little faster than 60Hz
• the PAL variants of most consoles run at 50Hz
• the WonderSwan runs at 75Hz
• While the Game Boy does run its LCD at 60Hz, games can turn the LCD off and on at any time, requiring emulation to pause until it can get back in sync with the computer display.

Because of these frame-rate differences, enabling video synchronisation can force games to run faster or slower than intended.

The consoles that higan emulates produce video frames and audio samples at a particular rate. If video synchronisation causes the emulated console to run, say, 5% faster than intended, that means audio samples are also being produced 5% faster. You might not notice the changed game speed, but you'll almost certainly notice the game's audio glitching constantly as your sound card tries to keep up.

Enabling audio synchronisation normally fixes this kind of audio glitching, but with video synchronisation it makes things worse: audio is likely to glitch while higan waits for a video frame to be shown, and video is likely to stutter while higan waits for an audio buffer to complete.

Games run too fast

higan runs as fast as it can, but it will pause and wait for the audio and video drivers to catch up if audio synchronisation and video synchronisation are enabled, respectively. If games are running way too fast, here are some things to check:

• Make sure "Synchronize Audio" is ticked in the Settings menu
• Make sure the Audio driver is not set to "None" in the Advanced settings (remember to restart higan if you change driver settings)
• Make sure your computer has speakers or headphones connected (some computers disable all audio if no outputs are available)
  • If you want the game to be silent, tick "Mute Audio" in the Settings menu
• If you can't connect speakers or headphones to your computer, or you did but it didn't help, try enabling the secret video synchronisation option.

Games run too slow

Of all the consoles higan can emulate, higan's Super Famicom emulation is the most resource intensive. Full-speed emulation for the Super Famicom base unit requires an Intel Core 2 Duo (or AMD equivalent), full-speed for games with the SuperFX chip requires an Intel Ivy Bridge (or equivalent), full-speed for the wire-frame animations in Mega Man X2 requires an even faster computer. Low-power CPUs like ARM chips, or Intel Atom and Celeron CPUs generally aren't fast enough to emulate the Super Famicom with higan, although other emulated consoles may work.

If your computer meets the general speed requirements but games run too slowly, try choosing a different audio driver, since that's usually what drives higan's timing.

On some computers, the operating system's power-management system may be confused by higan's unusual pattern of CPU usage (it runs furiously to calculate the next video frame and the next few milliseconds of audio, then stops dead as it waits for output to complete). If holding down the fast forward hotkey runs too fast but the game normally runs too slow, try disabling "power saver" mode or enabling "performance" mode.

On Linux specifically, if your desktop environment doesn't provide a way to change the CPU usage governor, you can temporarily force your CPUs to run at full-speed by running the following command in a terminal:

echo "performance" | sudo tee /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor

To put things back to normal, reboot the computer, or run this command instead:

echo "ondemand" | sudo tee /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor

Why is higan so much slower than other emulators?

The consoles higan emulates generally contain different devices (say, a CPU, an audio chip and a video chip) that do their own thing, but can interrupt each other at any time. Games can and do depend on timing details like "when the audio chip is done playing this sound, it will interrupt the CPU at exactly the right time for the CPU to fiddle with the video chip". higan is therefore very cautious about timing: while it's emulating the CPU (for example), at every point the emulated audio chip might interrupt the emulated CPU, higan switches to emulating the audio chip up to the same point to find out whether it will interrupt the CPU.

In this way, higan is a little bit like an office-worker trying to do the jobs of three other people by running from desk to desk, sending the same emails that those three people would send to each other, leaving itself a note at each desk to remind it where it was up to when it comes back. Although this constant switching is slow and inefficient, higan does it to ensure the emulated console always matches the behaviour of the original.

Other emulators, especially ones that are popular on phones and tablets, typically work differently. Although emulated devices can interrupt each other at any time, usually they don't, and often the only exceptions are a handful of unpopular games. So it's pretty safe (and much, much more efficient) to let certain devices run "ahead" of the others. For example, letting the video chip render an entire scanline at a time instead of working pixel-by-pixel. The downside is that some games don't work properly in these emulators, with game-breaking bugs, minor visual glitches, or anything in between.

Some emulators try to recognise when the user loads a specific problematic game, and will adjust the shortcuts the emulator takes so that the game runs correctly. This is a good way to increase game compatibility without losing efficiency, but it means the emulator does not accurately reflect how the original console worked, and it requires the emulator authors to diagnose and work around each individual supported game, instead of having one emulator that works for everything.

Why can't higan use multiple CPU cores?

These days, most computers contain multiple CPU cores, allowing them to run different programs (or different parts of the same program) at the same time. Since higan requires high CPU performance, sometimes people suggest that it should split its work into multiple threads so it can run across multiple cores instead of stressing one core while the others lie idle. After all, the original consoles higan emulates had separate devices running independently, so why not run each emulated device in a separate thread?

The big problem with this approach is timing. The devices in a physical Super Famicom (for example) run at whatever speed they run at, and talk to each other whenever they feel like. Because every Super Famicom uses the exact same components, every Super Famicom runs compatible games at the exact same speed, and games can blindly assume that when operation X is complete on device A, device B will have finished operation Y and be ready to do something new. Meanwhile, higan's emulated components take an unpredictable amount of time to do their work, so without deliberate synchronisation things would break almost immediately.

It's not practical to make higan's emulated devices do their work in exactly the same amount of time as their hardware counterparts. The problem is forty years of technology designed to make programs run as fast as possible: optimising compilers and super-scalar, out-of-order CPU architectures change programs to make them faster, speeding up some programs more than others in ways that are very difficult to understand and predict. Even if higan's emulated devices ran at the exact, correct speed on one particular computer, they'd still run differently on a computer with a smarter CPU, or when compiled with a smarter compiler.

Since higan needs its emulated components to run at particular speeds, and it's not practical to make them run at those speeds naturally, it must force them manually. An emulated device runs for a little while, then all the others are run until they catch up. It's this careful management, regular stopping and starting, that makes higan slow, not the actual emulation of each device. Having multiple CPU cores waiting on each other would not help them wait any faster.