What's Your Frame Rate? - Part I 

Microsoft Flight Simulator has been stretching the capabilities of personal computers for more than 25 years. The earliest versions, in fact, were used as test applications on PCs to verify that they were truly compatible. Back then, PCs had Intel processors that ran at 4.77 MHz, and memory was measured in kilobytes.  Flight Simulator supported only two colors – black and white (or green or amber, depending on your monitor), and the entire product fit on a 5.25" 360 kilobyte floppy disk. If the PC wasn't up to the task, Flight Simulator could make 4.77 MHz seem even slower.

Fast forward two and a half decades, and have a look at Flight Simulator X. If we tried to put it on those original 5.25" floppy disks, we'd need something like 27,000 of them, stacked more than 80 feet high. Personal computer processor speeds are measured in gigahertz now, with clock speeds that are nearly a thousand times faster than the original PC, and memory is measured in gigabytes.

And Flight Simulator can still make them seem slow.

Importance of Performance
Just like a movie or a television picture, an application like Flight Simulator creates the illusion of motion by redrawing the screen multiple times per second. Each time the screen is redrawn, it is referred to as a "frame," borrowing a term from the movies. The frame rate is measured in frames per second, or fps, and generally speaking, the higher the rate, the smoother the motion, up to a point.

As a reference, a movie you watch in a theater runs at 24 fps, PAL video/television at 25 fps, and NTSC television in the US runs at 30 fps. Most people consider 15-20 fps a bare minimum for creating an enjoyable and believable experience in Flight Simulator. At anything below 15 fps, you'll see a noticeable jerkiness to the motion. Flight Simulator 1.0, for instance, ran at about 6 fps at its best, and the experience was not overly smooth.

There is a lot of debate about how many frames per second are enough. Some people maintain that anything over 30 fps is undetectable while others insist that the human eye can detect differences at 120 fps or higher. Anything above the minimum 15-20 fps is simply a matter of personal preference.

To badly paraphrase Yogi Berra, frame rate is 90% of the problem, and the other half is what we call volatility, or stutters. Not only is a reasonably high frame rate important, but that rate must be consistent, with volatility kept to a minimum. In other words, a solid and consistent 25 fps can look much better than 40 fps that periodically drops to 10 fps.

But How Does it Look?
Flight Simulator has always been a visually-oriented experience. Even in the early days, users could imagine that they were flying by watching the horizon line tilt and move, never mind the fact that the ground and the sky were the same color. Now, after a quarter century of hardware and software improvements, the view is dramatically better, with high-resolution terrain, three-dimensional clouds, and a palette of more than 16 million colors to choose from.

It's probably not surprising to learn that this visual fidelity comes at a price: the more detail that Flight Simulator draws, or renders, in each frame, the more taxing it is on your PC, and thus, the slower it runs. This means that there is a constant balance or series of compromises between visual quality, frame rate, and volatility.

Striking a Balance
So, how do we strike that balance? First, we throw your computer into a bucket, figuratively anyway. After you run Setup and launch Flight Simulator for the first time, it runs an algorithm that measures the following:
  • Processor (CPU) Speed (2GHz, 3GHz, etc.)
  • System Memory, or RAM (512MB, 1GB, etc.)
  • Video Memory (the amount of RAM installed on and dedicated to the video card - 128MB, 256MB, etc.)

As the algorithm inventories the PC's hardware, it defines which performance "bucket" the PC falls into. The buckets range from "Minimum" up through several steps to "Ultra High," and a PC must meet all the criteria for a given bucket in order to be placed in that category.
For example, the "Medium High" bucket requires the following values:

  • CPU Speed: 3.0Ghz 
  • RAM: 1GB 
  • Video Memory: 128MB

If your PC has a 3 GHz processor and 1GB of RAM, but just 64MB on the video card, it is demoted from the "Medium High" bucket to the "Medium" bucket.

If you don't know the details of all of your PC's specifications, click this link to run the Windows Game Advisor.
  An Important Note About CPU Speeds

When FSX evaluates your PC to determine the default graphics settings, it looks at the measurable "clock speed" in GHz. Some extremely fast and powerful processors run at clock speeds that are actually lower than those of less powerful processors. As a result, the default FSX graphics settings on some high-end machines (with lower clock speeds) may be set to a lower visual quality than is necessary. For example, both an AMD Athlon 64 4000 (with a clock speed of about 2.4 GHz) and an Intel Core 2 Duo (with a clock speed of 2.66 GHz) are high end processors with low clock speeds that FSX may underrate.

If you have a high-end machine but find the default graphics settings are lower than you'd expect, this may be why. While future versions of Flight Simulator will possibly use other methods to evaluate your PC, this issue regarding FSX reinforces the most important message when it comes to tuning your display settings: Don't be afraid to experiment. 

Once we've established which bucket your PC belongs in, we adjust Flight Simulator's settings to predetermined values found in our tests to provide the best balance between good visual quality and performance. For example, we can adjust broad settings, like the overall detail of the whole world's terrain, or specify the maximum number of auto-generated trees to be drawn per square kilometer. Or we can get more specific, turning off sun glare and reducing the overall complexity of aircraft paint jobs or textures.

Another important value that we set is what we call the Target Frame Rate. This is a slider with a range from 10 to Unlimited used to set the maximum frame rate. Why set a maximum? Shouldn't it always be set to "Unlimited"? No, because of issues with both volatility and visual quality. Setting a maximum and capping the frame rate at, say, 25 fps, allows the computer's resources (CPU / Video Card cycles, system and video memory, etc) to be redirected to provide a smoother, better looking experience. In other words, if 25 fps is smooth to your eyes, anything faster than that is wasted, so we set a cap and use your computer's resources in other ways. The important thing to remember is that we set a baseline, and give you, the user, as many choices as we can to customize your experience from there.

This general discussion of performance issues is meant to acquaint you with the tradeoffs involved in getting the best Flight Simulator experience your computer can provide. For specifics on how to make the right choices and tweak your Flight Simulator settings, read the Flight Simulator X Learning Center article on Optimizing Visuals and Performance.

For more discussion on performance, see What's Your Frame Rate, Part II.