A frame (フレーム) is a single still image (静止画) that makes up a video.
A video switches through frames that differ only slightly, at short intervals, so the eye sees motion. Each picture lined up on a strip of movie film is also one frame. Counting pictures per unit of time, or compressing by using the differences between neighboring pictures, both treat the frame as the basic unit.
AnalogyEach drawing on a page of a flip book is a frame, and flipping the pages quickly is like playing the video.
How to rememberThink of a picture frame: one frame holds exactly one picture.
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Mix-upA frame (フレーム) shows motion only when it is displayed in sequence with the pictures before and after it, which is a different use from a still image file viewed on its own.
The frame rate (フレームレート) is the number of frames a video shows per second, measured in fps (frames per second).
Frame rate
Time per frame
Frames in 10 seconds
15 fps
About 0.067 s
150
30 fps
About 0.033 s
300
60 fps
About 0.017 s
600
At 30 fps, for example, 30 frames go by each second, one about every 0.033 seconds. A higher value gives smoother motion, but the data size before compression grows as "data per frame × frame rate × seconds."
AnalogyIt is how many pages of a flip book you turn in one second. Turn more pages and the motion gets smoother, but you also have more pages to draw.
How to rememberIn fps, the f is frame and ps is per second. The unit itself spells out "frames per second."
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Mix-upA frame (フレーム) is the still image itself, while the frame rate (フレームレート) is how many of them are counted per second.
Mix-upResolution (解像度) is the number of pixels in one frame and describes how fine the picture is, while the frame rate describes how fine the video is over time. Raising either one increases the data size before compression.
MPEG is the umbrella name for a family of standards centered on methods for compressing video and audio data.
Compress and store a reference frame on its own
Find the differences from nearby frames
Store only the parts that differ
Rebuild from the reference and differences on playback
The player adds the differences to the reference picture to rebuild each frame.
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Neighboring frames tend to look very much alike, so MPEG video compression stores only the differences from nearby frames for everything except reference frames, which cuts the data size sharply. The family has numbered standards for different uses, such as MPEG-2 for DVDs and digital broadcasting, and some, like MPEG-4, even define the file format that holds the compressed data. The family also includes audio-only standards; MP3, for example, comes from the audio part of MPEG-1.
AnalogyIt is like describing a series of photos with the same background: instead of redrawing everything each time, you just say "the person moved a little to the right since the last one."
How to rememberMPEG comes from the Moving Picture Experts Group, the body of experts that wrote the standards. A "moving picture" is simply video.
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ExamMPEG is the name of a family of standards that covers both video compression and audio compression methods.
H.264 is a video coding standard developed jointly by ITU-T, the international standards body for telecommunications, and MPEG, and is also called MPEG-4 AVC.
Compared with the earlier MPEG-2, it is said to reach the same picture quality with roughly half the data. It has long been used for Blu-ray discs, online video streaming, and Japan's One Seg mobile TV broadcasts. The "H." name is an ITU-T recommendation number, while on the MPEG side the same standard is called AVC as part of MPEG-4.
AnalogyIt is like a well-known, standard way of folding luggage small. Many devices and services know this folding method, so they can unfold what was packed.
How to rememberAVC stands for Advanced Video Coding.
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Mix-upH.264 is its number as an ITU-T recommendation and MPEG-4 AVC is its name within the MPEG standards; both refer to the same standard.
H.265 is the video coding standard that succeeds H.264, also called HEVC (High Efficiency Video Coding).
At the same picture quality it is said to need only about half the data of H.264, at the cost of heavier computation for compression and playback (decompression). It is used to broadcast and stream 4K and 8K high-resolution video, which carries a lot of data.
AnalogyIt is a newer folding method that takes more effort but packs the same luggage even smaller.
How to rememberThe HE in HEVC stands for High Efficiency.
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Mix-upH.265 and HEVC are two names for the same standard.
AVI is a container format (コンテナ形式) that stores video and audio together in one file, with the extension .avi.
A container format is a set of rules for a "container" that holds already compressed video and audio data as a single video file. AVI is a long-standing format that Microsoft created for Windows, and it does not fix which coding method the video inside must use.
AnalogyIt is like a moving box. You can choose the box and the way you fold the things inside separately.
How to rememberIn Audio Video Interleave, "interleave" means to slot things in alternately. The way it stores video and audio in small alternating chunks gave the format its name.
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ExamA file with the .avi extension is a video file; it centers on video and can hold audio along with it.
MP4 is a container format (コンテナ形式) defined as part of the MPEG-4 standard (Part 14), with the extension .mp4.
Encode and compress the video
Encode and compress the audio too
Store both in one file
Extract and decompress on playback
The player takes the video and audio out of the file, decompresses each, and plays them together.
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Besides video and audio, it can also carry data such as subtitles in the same file, and the video inside is usually compressed with H.264 or H.265. It works not only on PCs but also on smartphones and video streaming services, making it one of the most widespread video formats today.
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Mix-upMPEG-4 is the name of the whole family of standards, including its compression methods, while MP4 is the name of the file container format defined within it.
AVI vs. MP4
Aspect
AVI
MP4
Key differenceDefined by
Microsoft (for Windows)
MPEG (Part 14 of MPEG-4)
Extension
.avi
.mp4
Main uses
Windows PCs from earlier years
Smartphones, video streaming, and many other devices
Key points
Video is a series of slightly different still images (frames) shown one after another, and the number per second is the frame rate (fps). A higher value makes motion smoother, but the data size before compression grows in proportion.
MPEG is a family of standards centered on video and audio compression, and its video compression uses the differences between neighboring frames to reduce data size.
H.265 (HEVC) is the coding standard that succeeds H.264 (MPEG-4 AVC), and it suits data-heavy video such as 4K and 8K.
A video file holds compressed video and audio together under the rules of a container format, and AVI and MP4 are names of such container formats.