Category: Audio and video

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Keyframes for moving images

In a moving image, things move or change significantly in a short time. Therefore, inserting keyframes at short intervals improves the reproducibility of small movements.

There is also the advantage that the search is smoother for images with many keyframes. The reason is that the search is based on keyframes, so the more keyframes you have, the easier it is to stop at the target scene. If the position where the search stops is not a keyframe but a difference information frame, the information is fetched to a nearby keyframe, but that time is short.
However, if you increase the number of keyframes too much, the keyframe bit rate will be taken over and the overall video quality will deteriorate, so be careful.
For moving images, the keyframe is approximately once every 3 seconds.

■ Keyframes for images with little movement

Since there is little difference information in a video with little motion, you won’t notice much difference in change even if there are few keyframes. However, if you insert keyframes for too long, search may not work properly and playback may take a long time to start. This is the opposite of the case where there are many keyframes, and if the rewind position is far from the keyframes, it will take time to read.
For those with little movement, the guideline for keyframes is approximately once every 6 to 8 seconds.

So far, “What is the encoding mechanism? Five points to consider for encoding HD video [Part 1]”, “Understanding the appropriate bit rate for the resolution you want to distribute”, motion oriented or image quality I explained three points how to change the “frame rate” and “how to insert keyframes” depending on whether it is important. In [Part 2], we will explain “the advantages and disadvantages of bitrate setting (CBR / VBR), proper usage” and “correct aspect ratio and interlaced processing”.

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Learn a suitable bit rate guideline for resolution

This is because the amount of data allocated per pixel is reduced, resulting in poor image quality.

The same phenomenon can be said of the videos. Bit rate is the amount of data allocated per second and affects the image quality of the video. If you want to display a video on a large screen, such as full screen display on a computer or TV monitor, you need a sufficient bit rate according to the resolution. On the other hand, when displayed at a small resolution, the roughness is not as noticeable even if the bit rate is reduced. On the contrary, even if you encode at a high bit rate, you will not notice the difference in image quality, and in many cases the file size will only increase.

Below is a list of the appropriate resolutions and bit rates commonly used for Internet video distribution.

<< Estimated resolution and suitable bitrate >>
* Figures are based on 30 fps assumption and based on our opinions as of June 2021.
* The appropriate bit rate may vary slightly depending on the video content.

resolution Video with little movement Video with a lot of movement
SD (720 x 480 px) 500 kps-1 Mbps 1 Mbps-2 Mbps
HD (1280 x 720 px) 2.4 Mbps-4.5 Mbps 4.5 Mbps-9 Mbps
Full HD (1920 x 1080px) 4.5 Mbps-9 Mbps 9 Mbps-18 Mbps
4K (4096 x 2160 px) 25 Mbps-35 Mbps 35 Mbps ~ 70 Mbps
However, in the case of moving images, the image quality at the time of encoding will differ depending on whether the video material has a lot of movement or the video material has little movement, even if the bit rate is the same. Therefore, it is necessary to thoroughly judge and encode not only the bit rate, but also the frame rate and keyframe settings, which will be explained later, according to the video material. I hope you understand that “there is a relationship between resolution and proper bit rate” as a determining factor for high definition.

Point 2: Increase the frame rate if motion is important and lower the frame rate if image quality is important.
The frame rate (number of frames) is set at 29.97 fps for televisions and 24 fps for movies, while the frame rate is freely configurable for Internet video encoding. A video is a collection of continuous images (frames) like a flip book. The more frames per second, the smoother the movement.

In video encoding, the bit rate per second is fixed, so if you increase the frame rate, the number of images in the flip book will increase and the movement will be smoother, but the amount of data allocated per frame will decrease, so the image quality will be better to fall.
On the other hand, if you reduce the number of frames, the number of images in the flip book will decrease and the smoothness of the movement will be a little slower, but the image quality will improve because a large amount of data will be allocated to each frame.

If you want to emphasize the smoothness of motion in a video with a lot of motion, increase the frame rate (video demo 1). On the other hand, if the video has little movement, it is not necessary to increase the number of frames as much (depending on the degree), so it is effective to lower the frame rate and give priority to improving the image quality (Video Demo 2 ).

Point 3: insert “keyframes” at short intervals for moving images
A keyframe is a frame that exists as a single still image (an image that is not compressed between frames) and is the starting point for difference information. Depending on the encoder settings, the keyframes are inserted when there is a scene change and the difference information is inserted at regular intervals, such as XX frames and once every XX seconds.

What is the encryption mechanism? 5 Points to Consider for HD Video Encoding [Part 1]

Encode

The image quality of Internet videos is almost proportional to the bit rate. However, if the bit rate increases unnecessarily, the file size will increase. “Keep bit rate low”, “Reduce file size” and “Reduce load time” are linked, and there is nothing to say if you can encode in high definition while keeping the bit rate low.
Also, at the beginning, I wrote that “image quality is almost proportional to bit rate”, but I think some of you may have experienced that “I increased the bit rate and encoded, but I am not satisfied with the quality of the image. “So, this time, I will explain five points that are often used to do high definition video encoding.

” Table of Contents ”

Encoding Mechanism
Point 1: learn a suitable bit rate guideline for resolution
Point 2: Increase the frame rate if motion is important and lower the frame rate if image quality is important.
Point 3: insert “keyframes” at short intervals for moving images
* You can read the second part (Point 4, Point 5) here.

Encoding Mechanism
First, I will briefly explain the encoding mechanism.
An image is a collection of continuous images (frames), and by changing this in a short time like a flip book, it appears that you are visually moving. Japanese television images are 29.97 frames per second (short for 29.97 fps / frame per second) and most movies and animations have a standard of 24 fps. Since a large number of frames are required for video, the amount of data is also huge. Therefore, data compression is indispensable for distributing videos on the Internet.

When coding

“Prediction in frame” that compresses data within a frame
“Prediction between frames” that compresses data into consecutive frames
Information is reduced and data is compressed within the range that does not affect the visual sense.

■ What is in-frame prediction?
There are various methodologies for data compression, so I will skip the details here, but the basic idea of ​​within-frame prediction is to divide a frame into small blocks called cells and the colors adjacent to each other in the block. they are the same or similar, they are compressed together.
For example, if there is information “blue blue blue blue blue blue blue blue blue blue red red yellow yellow yellow” in the divided block, the amount of data can be reduced by combining this with “blue 11, red 2, yellow 3”. It’s an image.

An example of data compression in in-frame prediction (image)

■ What is cross-frame prediction?
However, in the case of video with a time axis, the number of frames is large, so there is a limit to the overall weight reduction based solely on the prediction within the frame. On the other hand, in the prediction between frames, based on the idea that “the contents are similar before and after the consecutive frames”, the cells that do not change from the previous frame reduce the amount of data by reusing information and the cells that change It becomes data as difference information.

Prediction between frames (image)

From here, I’ll explain five specific code points.

What format do you choose when copying? AIFF, ALAC, AAC … Check the sound quality of each one by “appearance” Part 2

The “lossless compression” method, to which Apple Lossless and FLAC belong, is a method that can completely restore the original audio data during playback, at the cost of a low compression rate.

The file size is large, but the sound quality is equivalent to that of a CD. The bit rate fluctuates automatically according to the content of the audio data, and the compression rate is not constant accordingly.

AIFF and WAV are “uncompressed” methods. Extract the original audio data and create a file as is. It does not compress, so it has a lot of capacity, but the sound quality is perfect.

Let’s take a closer look at the table.

■ Don’t say it again … Considering the playback environment, the format selection criteria for copying are as follows: As

You can see from the “Compression rate” and “Sound quality” items in the table, both are highly rated. It has around “AAC / 256kbps” and “MP3 / 192kbps”, which is a Rossy format with a higher bit rate. While ensuring good sound quality, the file size can also be reduced. It also has an excellent balance with the capacity of the integrated SSD / HDD of PC, iOS devices, smartphones, etc.

The default setting for iTunes is AAC / 256kbps, and the specifications for music files sold on the iTunes Store are the same. You can tell that the current standard is around here. In terms of playback compatibility, AAC and MP3 are widespread, and no matter which one you choose, you don’t have to worry about the playback environment.

On the other hand, it is the lossless format that Apple Lossless and FLAC belong to that can reduce the file size to some extent while maintaining the best sound quality equivalent to that of a CD. If you want to give the highest priority to sound quality from an audio point of view, I would like to select this. It will be a bit difficult to balance it with the capacity of SSD / HDD, smartphone, etc. from the PC, but if it can be erased, it is convenient to use this format.

For example, if you don’t have that many CDs, creating a library in a lossless format will not put too much pressure on your PC’s SSD / HDD and you will be able to sync all the songs on your iOS device. Alternatively, you can deal with this by coming up with sync settings for iOS devices, etc. (I’ll explain later). If so, it is better to have a lossless format that can maintain the best sound quality, and there are few errors in the long run.

Just keep in mind that Apple Lossless and FLAC are a bit difficult to choose in terms of the playback environment. Until now, iTunes and iOS devices do not support FLAC, and many other devices and software do not support Apple Lossless. As of June 2012, at the time of writing, many network players only support FLAC. However, with Apple Lossless opening font in October 2011, support for the same format is expanding, so I’d like to keep an eye out for this trend as well.

AIFF and WAV are uncompressed formats. Of course, the sound quality remains the same as that of a CD. However, the data capacity is not compressed at all. In other words, the uncompressed format “has the same sound quality as the lossless format and has a larger file size than the lossless format.” In that sense, there is no reason to choose it unless you are particular about it.

Well finally the highlight of this era. Let’s review the “appearance” of “what is the actual deterioration in sound quality for each compression format?”

What format do you choose when copying? AIFF, ALAC, AAC … Check the sound quality of each by “appearance”

Music files are the mainstream of Imadoki’s audio playback sources.

Except when purchased from an online distribution, the sound quality of the music files used here is largely related to the work of reading audio data from a CD to a PC, the so-called “ripping” setting. This time I would like to review that part in a little more detail.

The first half of this article describes the basics of the extraction format for those who want to know what sound quality to choose when extracting. Perhaps this first half is common information to many file and web readers.

However, the highlights are beyond that. “Really Terrifying Audio Compression” … So, in the second half of the article, “How much does the compressed file actually deteriorate the sound?” And “How much does the sound quality change depending on the bit rate value?” you will check with (→ Visually check the sound quality of AIFF, ALAC, AAC!). I would like you to stay with us until the end.

■ Don’t say it’s time to change … First, let’s review the basics

The scheme of the options for copying is “compression format (file format)” and “bit rate”. These two determine the sound quality and file size, which is another important factor.

“Compressed format (file format)” refers to formats such as AAC, MP3, Apple Lossless, and FLAC. This selection determines the sound quality, file size, and playback environment.

The “bit rate” is the amount of data allocated per second of audio. The higher the value (kbps), the higher the sound quality, but the larger the file size.

In the case of iTunes, call this screen “Load Settings” from the environment settings and configure the extraction.

Please refer to the following table based on that. We have summarized the characteristics of typical compression formats and bit rate settings.

The item “Sample Bitrate Settings” in the table is quoted from the default settings provided in iTunes (* iTunes does not support FLAC)

First of all, pay attention to the second item from the left of the table. Compression formats can be broadly classified into “lossy compression”, “lossless compression” and “uncompressed”.

The “lossy compression” method, to which AAC and MP3 belong, achieves a high compression rate = a significant reduction in file capacity by reducing some of the data when compressing audio data. In contrast, the original audio data cannot be fully restored during playback and deterioration in sound quality is inevitable. The degree of deterioration in sound quality changes depending on the bit rate setting. The lower the value of the bit rate, the greater the deterioration in sound quality and, conversely, the higher the value of the bit rate, the more mitigated.