In this article, we would like to lift the veil of secrecy and explain what is the difference between hz vs fps or Frame Rate vs Refresh Rate, but essentially different terms.
It’s understandable why there’s confusion: both refresh rate and frame rate represent the number of frames per unit of time (frames per second is the common unit). For example, a signal with a refresh rate of 60 Hz and a signal with a frame rate of 60 fps consist of 60 images that change each other in 1 second. And yet they are not the same!
Frame Rate vs Refresh Rate (hz vs fps)
The term refresh rate (hz) applies to video signals such as HDMI™, VGA, DVI, etc., while the term frame rate (fps) applies to encoded videos such as an AVI file or YouTube streaming.
Refresh Rate (Hz)
To better understand the difference between these two terms, we must refer to the concept of “refresh rate”, which was originally inextricably linked to the cathode ray tube (CRT) of a television or computer monitor. The CRT monitor is a glass vacuum tube, i.e. a cavity in which air is completely removed. On the front side, the inner part of the tube glass is coated with a phosphor. A phosphor is a substance that emits light when bombarded with charged particles. An electron gun is used to create an image on a CRT monitor.
The electron beam passes sequentially through all points of the screen from left to right and from top to bottom, forming an image, or frame. The number of possible image changes per second is called the refresh rate and is measured in Hertz (Hz).
Having descended to the lower edge of the screen, the electron beam quickly returns to the upper left corner to begin drawing a new frame of the image. The pause between updates is called the blanking interval. In serviceable monitors, the reverse path of the beam is imperceptible. But sometimes you can see that the monitor “flickers”. The low refresh rate, typically less than 80Hz, is to blame.
A liquid crystal display (LCD) operates on a different principle from a CRT monitor. The LCD does not have a pause after each frame update, as it simply changes the structure of the liquid crystal substance to display each subsequent picture, and its brightness remains constant regardless of the refresh rate of the incoming signal.
The concept of “refresh rate” has played a significant role in shaping the modern digital signal encoding process. But before we get to this point, we would like to explain in more detail the difference between the frame rate of the encoded signal and the refresh rate of the raw signal.
Frame rate (fps)
In fact, the encoded video signal is quite different from the raw video signal, if only because it does not depend on such physical objects of the real world as electron guns.
Consider the process of encoding a signal without compression. For example, for a 1080p, 60fps raw signal, the total bitrate would be: Vertical resolution × horizontal resolution × frames per second × bits per pixel = 1920 × 1080 × 60 × 24 = 2,985,984,000 bits per second
… or about 356 MB/sec.
Then you can easily calculate that one minute of uncompressed video will weigh about 21 GB. It is obvious that it is not advisable to use such a volume of data for storage, and even more so it is problematic to broadcast it over the Internet. But after encoding and compression, the same video without loss of quality will be about 1.5 MB / s (~ 12000 Kbps).
To reduce the data size, the encoded video must include color space compression. For example, changing the color space from RGB24 to YUV420 reduces the number of effective bits per pixel from 24 to 12.
After the encoding process is completed, it becomes clear that the received data significantly reduces the amount of free space on the hard disk and allows you to use less network bandwidth when transmitting over the Internet.
More shots doesn’t mean better!
When capturing and encoding raw video, the output frame rate may not match the original refresh rate. Devices for capturing (as well as broadcasting and recording) video signals, such as AV.io HD, work with almost all signal standards, but due to bandwidth limitations of the USB 3.0 cable, the encoded signal will result in a lower frame rate as a result of the conversion. For example, our capture cards can accept HDMI at 1080p and 120Hz, but you won’t get 120fps as a result, but 60fps is much more likely.
And how many frames per second is considered sufficient? It depends on many factors! More fps is not always better. Just because a 60Hz setting doesn’t mean you need 60fps for recording or streaming – it really depends on the nature of the video content.
The easiest way to explain this is on the example of films. The standard frame rate is only 24 fps, and most Blu-ray movies (the current highest quality standard for home theater) also use this frame rate. Interestingly, the film industry is currently testing the possibilities of using higher frame rates. The film The Hobbit: An Unexpected Journey, directed by Peter Jackson, was the first film to use 48 fps, which caused very mixed reviews from critics and viewers.
Many movie gurus have argued that the move from the standard 24 fps to the higher 48 fps made the film too realistic, virtually blurring the line between fact and fiction. In the case of movies (or other video at a similar speed to a movie), the frame rate of 24 fps is the most preferred and is considered the gold standard for DVD and Blu-ray.
Another example of a typical use for low frame rates is when displaying relatively static video content, such as presentation slides. If nothing else is shown besides the presentation, then there will be no significant difference between 30 fps and, say, 10 fps, since the image changes completely only once in a few minutes or seconds, and remains static the rest of the time. In case this does not affect the result for the viewer in any way, lowering the frame rate can help to significantly reduce traffic, file size for storage on the hard drive, and also the load on the PC processor.
Higher rates, such as 60 fps, are used successfully in the field of visual media, where changes occur at high speed, and where you need to show every detail in great detail. In addition, do not forget about dynamic sports and action video games – in these cases, encoding with higher frame rates will not let the viewer miss a single moment.
Summing up
Now you know what it is – frame rate and refresh rate; two terms that refer to the number of images shown per second, but which are fundamentally different from each other. The refresh rate originates from the advent of CRT monitors and expresses the number of images per second for the raw video signal, while the frame rate refers directly to the encoded signal and can be set by the user depending on the nature of the video content.
We will be very glad if this article will be useful for you and will allow you to better navigate in technical terms.
