FPS, Hertz, Sync, Input Lag
Let’s start this path that I hope will clarify the functioning of the monitor and the visualization of the images sent by the video card, extending the speech to the input lag.
Let’s try to understand one fundamental thing, the refresh of the monitor and the frames per second (fps) are two distinct and unrelated quantities.
Correlation occurs when they are synchronized via software or hardware, adapting the frames to the monitor refresh (vsync, fastsync, etc.) or the monitor frequency to the frames (Freesync or Gsync).
This article is complex and when you go into detail for verification you need specific measurement tools: you need a camera that can record video with a minimum of 240 fps and for this I purchased a
Casio Exilim which can film at 480 fps.
We will define the basic concepts that we will try to clarify with examples and photos passing then to the videos and the tests addressed to the input lag and the fluidity video.
These are the topics that we will see in detail:
Hertz, Fps, Sync, Fps Limit, Video Fluidity, Input Lag, Tearing, Stuttering
The Hertz (Hz) of the monitor indicates how many times per second the panel shows an image and is also called Video Refresh.
For example at 60 Hz the image generated by the gpu is updated 60 times per second.
The operating range of the monitors varies between 60 and 240 Hz for the latest generation of monitors.
Tearing occurs when, without synchronization between the video card and the monitor, the frame coming out of the front buffer and the frame coming in are displayed.
As you can see in the picture the frame is located in a memory area of the video card indicated with buffer and without synchronization is sent to the display,
so depending on the timing of the video card, two frames can be sent during the same video refresh causing tearing.
Different in case of sync where the refresh is waited to show the frame ready in the buffer.
The buffer is a memory area of the video card that contains the frames to be sent to the monitor and not being synchronized with the monitor may happen that two or more are sent during the refresh time of the monitor,
It all depends on how fast the video card can generate and send frames.
The Nvidia test gives an idea of the tearing and operation of Vsync and Gsync (proprietary technologies).
As you can see, the position of the “line” of interruption of the image is not fixed because the frames are not synchronized and therefore never overlap with the same timing.
Thank you Vanessaezekowitz – Own work for the simulation that shows three frames with double tearing, instead of a single line of interruption between two frames the refresh shows three frames and two interruptions.
We can see in the picture how there are two lines of interruption of the image, suppose that the image is that of a monitor at 60hz, if the gpu produces 180 frames per second is easy to see two lines that highlight the three frames sent by the video card.
Leaving aside the visual effect, we can understand how the most framerate affects a lot in shooter games with the updating of the data in the frames, thus reducing the input lag,
This happens because the constrain-free display shows all the frames that are produced by the gpu but causing the interruptions that we saw in the screenshots,
but also showing the input frame by frame, thus reducing the input time.
As we will see later on, in fact, the lag of the inputs is strictly related to the vision and the speed of the frames.
However, if we synchronize the frames with the Vsync signal, the times expand, increasing the response delay of the game events from 5 (with the 180 fps mentioned above) to 16.7 msec (with a monitor at 60hz).
Stuttering and microstuttering is a loss of fuidity, an irregular advancement of images, occurs when the time of frame generation is not constant,
this can happen because of the inadequate programming of the games, for the video drivers, for the gpu that does not generate frames with regularity
or the cpu that provides the data to the video card with inadequate speed.
You notice more when the frames generated by the video card are few compared to the refresh of the monitor and the same image is displayed multiple times causing a loss of visual fluidity returning a display that proceeds to tears.
The scarcity of rendered frames also leads to still images lasting a few tenths of a second.
From a video about S.T.A.L.K..E.R. Call of Pripyat you can see microstuttering and freeze.
I limited the video card (cap) to 60 frames with 144Hz refresh display, the game that has always had these problems that are highlighted by the cap and refresh rate of the monitor.