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Coding theory: data compression

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Coding theory: data compression

Data compression - Encoding

Data compression or encoding refers to procedures to reduce data storage requirements. Basically, a distinction should be made between 2 types of data compression, lossless data compression and lossy / lossless data compression.

In lossy data compression, an attempt is made to filter out irrelevant sound, image, or audio information that falls below the threshold of human perception. This includes, for example, certain color values and image spaces and tones and frequencies that are no longer perceptible in the audio data.

Encoding dTA COMPRESSION

In the audio sector, you can dispense with frequencies above 20 kHz with no qualms of conscience, which corresponds to a sample rate of 44,100 kHz, without noticing a noticeable audible loss (as an adult!). We speak here of irrelevance reduction or redundancy reduction. Even soft tones after loud sudden noises cannot be perceived by the human ear for a short period of time.

These themes can be summarized under the general term “psychoacoustic effects of perception”. Lossy / non-lossy data compression runs on the entire media area. Otherwise, the huge amounts of data would no longer be manageable.

If you go too far with data compression, a video will show typical compression artifacts or block artifacts. Video looks pixelated, often accompanied by blurry, blurry images, and streaks of color. Either the sound is muffled, squeaky, or has hearing compression artifacts as well. This video cannot be decompressed back into the original video in this way because essential video and audio information is simply missing.

The opposite of this procedure is lossless data compression. Data that has been losslessly compressed can be decompressed 1: 1 at any time without data loss. A known example of lossless compression is ZIP or RAR files. When you create a file from a text file, the file size is not reduced by omitting irrelevant data. With lossless compression, recurring information is routed and / or stored through counters. This can be explained with a simple example:

uncompressed text: Friday, Friday, always Friday

Method A
lossless compressed text: Friday, -1 over and over -4
Information “Friday” that is repeated 3 times is addressed in a space-saving way by specifying the position of the first definition.
Method b
Lossless compressed text: 3f, 3f over and over 3f
The information “Friday”, which is repeated 3 times, is addressed in a space saving way with the 2 characters “3f”.

The compromise: audio quality vs. Video quality

An audio and video analysis related to the content of the source video is essential before the encoding process. Get an accurate picture of the source video you have. A compromise must be found between good video quality / moderate audio quality and moderate video quality / good audio quality. H.264 is a very high compression video codec, but it should distribute whatever bit rates can be sensibly saved in advance.

What target bandwidth groups would I like to serve?
High / moderate video quality of source video?
Much / little movement in the source video?
High / moderate audio quality of source video?
For example, plan a higher bit rate for audio quality than video quality for a television interview. In contrast, for a complex documentary that requires a lot of movement, plan for a higher bit rate for video quality than for audio quality.

With this approach, you can distribute bitrates sensibly up front and save storage space if necessary. If you often have to do with web video encoding, we recommend that you use a checklist that you can work with on the corresponding video and audio prioritizations.

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What is a codec? – clearly explained

Codecs are crucial to the quality and efficiency of data encoding. In this practical tip we explain exactly what a codec is.

What is a codec? – 10 facts

The following information will help you understand what a codec is. The following example shows the differences between format, encoder, and decoder and shows how quality and efficiency depend on the codec.

-A codec is a combination of encoder and decoder.
-The objective of a codec is usually to reduce the amount of data for transmission or storage.
-Codecs are used in particular for audio, video and image data, as well as for text compression and encryption.
-In the audio sector there are special codecs for voice or music. -Video and graphics codecs are specially designed for comics / cartoons and real pictures / movies.
Lossy data reduction primarily uses psychophysical tricks to inaccurately encode those parts of image and sound that humans can barely or barely perceive.
-The most popular audio codecs are FLAC and MP3, where FLAC specifies both the format and the codec. MP3 is a codec for audio in MPEG I or MPEG II format.
-The MP3 decoder side is standardized by Fraunhofer and can be obtained without a license. However, the encoder is paid. Therefore, many use the free LAME encoder.
-G.722.2 and Full Rate (FR) voice codecs are used especially in telephony.
-The most popular video codecs are H.265 for UHD TV, DivX and XviD in MPEG-4 Part 2 and H.264 for QuickTime.
-Often various encoded formats like MP3 and JPG are combined into one container file, for example in AVI, MKV, MP4 and FLV.

A simple codec example

The principle of a codec is clarified with an example. An abbreviation codec reduces typed text to abbreviated text and uses it to generate typed text:

-Format: Your digital font format might dictate how all the letters and characters you use are encoded in binary.
-You define the type of encoding in a specification, for example, the rule that each character consists of 8 bits. To do this, write a table showing which bit sequence is assigned to which letter or character.
-Codec: You can now create an abbreviation codec to convert typed text into font formatted abbreviated text.
-To do this, your codec must specify which words will be abbreviated and how, for example, through a set of rules, a large list of abbreviations or a combination.
Encoder – An encoder can now convert typed text to abbreviation format using the rules and directory.
Decoder: a decoder can convert it to written text.
-The efficiency of data reduction is highly dependent on your codec.
-If you only created a directory with three abbreviations, most of the texts will not be noticeably shorter. Compression is low.
-If your list of abbreviations is huge, the device that is going to encode and / or decode the text must have a large memory and a high reading speed.
-However, if you set up a smart set of rules for encoding, your codec is ideally small and the end device doesn’t have to do a lot of computing or reading work.
-If a word can be abbreviated in more than one way, that’s fine. A good encoder would use the most efficient abbreviation.
-However, if an abbreviation can have multiple meanings, you have lost information. This will probably render your codec unusable.
-However, if only the words that you know you are omitting were abbreviated ambiguously, you would have invented a lossy psychophysical encoding. This takes advantage of the fact that you do not visually perceive every existing word optically.
-This idea is not entirely false. Because, as this example shows, we don’t necessarily read exactly what is written.

What is a Codec? Clarifying concepts

Speaking of multimedia terminology, there is a fairly widespread confusion between the concept of image and / or sound format and that of audio and / or video container format (also called multimedia container format) To finish bundling it, it is very common to refer to both with the term codec that does not express any of the two previous concepts.

The image, video, audio format… is the way in which this type of information is encoded so that it can be treated by computer, which includes both being able to store it and transmit it. That format, therefore, would be a series of rules to describe images and sound in a computer way; a mathematical model, if you want to see it that way.

A codec (or codec, with tilde, in its Spanish version) is a program that encodes or decodes multimedia information according to a certain format. Once the data is encoded, it can be processed, for example stored or transmitted. As you may have guessed, the word codec is an acronym formed by the English words coder and decoder: encoder and decoder in Spanish.

An audio / video container format (or multimedia container format) is a computer file structure according to which audio and / or video is stored or transmitted as well as information associated with them (such as subtitles or bookmarks) The same container You can include together (multiplexed) the different components (tracks) in a single document. It is very common to add an extension to the name of the documents (those few letters that are at the end of the name separated by a period) that usually refer to the type of document. If you are rigorous in establishing that extension, it can serve as a clue to know which container is the document.

Considering the above, we could say that the document «feature film.avi» is a container in AVI format that includes video in MPEG4 format encoded using the Xvid codec. As the orthodox version is a very long phrase, it is common, for example, to say colloquially that the document “short.avi” is an MPEG4: it is obvious that it is an AVI container and everyone will know what codec has installed in their system for what formats

Most of the time, when packaging the information according to a certain format, it is compressed, that is, it is transformed so that the resulting information requires less memory to be stored or less bandwidth when transmitted than the original information.

There are two ways to compress information, with or without losses. Lossless (or lossless) formats allow you to reconstruct the information as it was originally, before compressing, without any difference from the original. In most cases the lossless coding usually allows (much) less compression (takes up much more) and is reserved for the documents that are going to be edited, that is, that they are not final documents.

The most frequent image and sound coding is with losses (lossy) Of course, the coding system seeks to make these losses as less noticeable as possible, considering for this, for example, the movement (What changes the image between a frame and another) The level of quality-compression is usually configurable so you can choose the degree you want between occupying more and having more quality. In line with what has been said about lossless formats, it is not a good idea to code according to a material loss format that is going to be edited since the successive recompressions cause more and more quality information to be lost to images and sound. Do not compress again and again using a lossy format!

One moment, you said that it is better to encode the original material using a lossless format but also that it will take up more. Then I must give the button on my camera that records video without compression? For starters, unfortunately, it is quite rare for your camcorder to have that button (in those of taking pictures and in audio systems it is something that is usually possible) Only in some cameras you can do something like that, they will usually be professionals and of the highest range. On the other hand, be prepared to spend a lot of memory and quickly, since you will need to store a lot and at full speed. So, what should I do? Our advice is that you choose, if you can, the best possible quality, even with losses, for the images you are going to edit and suitable for those that do not require more editing than the “cut” assembly since, properly processed , does not have to be compressed again.