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Formats and codecs

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As a result of the process of sampling an analog audio signal we obtain a sequence of binary numbers (numerical streams) that can be written to particular types of files (audio files or sound files) stored on various types of digital media (CD, DVD, HD or other).

Codecs and formats

These files can have three different encoding formats:

Uncompressed – All data derived from the sampling process is written to the stored file.
Lossy: the information contained in the stored file is less than that contained in the source data (information loss).
Lossless – The information contained in the stored file is identical to that contained in the source data, but the data is still compressed.

Formats & Codecs
In fact, these different possibilities arise from the need that when we want to store all the information related to a signal in a digital medium, we may need to reduce the storage space occupied in order to benefit from the portability or transmissivity of the encoded stream. We must resort to compression of the information itself in such a way that it also allows the reverse operation.

This operation is performed using codecs that are programs (or devices) that deal with both the digitization of signals (typically audio or video) and their digital encoding and / or decoding.

There are several types of codecs, different from each other by the type of signal on which they must operate and by the encoding / compression algorithm implemented in them. Each encoding format can be derived from several different codecs. In fact, these allow you to listen to proprietary formats opened by any file reader, keeping the physical layer separate from the logical format of its representation.

The advantages of compression are:

takes up less space on the target media.
spend less time during data transfer (bit rate).
The cost (disadvantages) is the increase in read / write times linked to decompression / compression times and, in the case of audio files, also in terms of audio quality.

Bitrate

Before delving into the different types of audio formats, let’s focus on concepts related to data transmission speed, since audio files are intrinsically linked to the time that passes: each second is associated with a certain information content and therefore both to a certain subsequence of figures. binary. The number of binary digits that make up these subsequences is called the bit rate.

Bit rate is the number of binary digits used to store one second of information.

CDs, for example, have a sampling frequency equal to 44,100Hz as standard, which therefore generates 44,100 values per second for each channel. In the case of a stereo file, they are multiplied by 2, and since sampling is done at 16 bits (exactly equal to 2 bytes), they must be multiplied by 2:

44,100 * 2 * 2 * 60 (seconds) = 10,584,000 bytes (~ 10MB) every minute

Bitrate is expressed in kilobits per second (kbps) and can range from 32 to 320 kbps. For example, if we wanted to calculate the bit rate of the previous file we would have to calculate:

44,100 * 2 * 2 * 8 (bytes to bit) = 1,411,200 bits / second (1,411 kbs)

The calculations we have just carried out refer to an uncompressed format, whereas in the case of compressed formats, as the total length of the file decreases, the average length of the subsequences also decreases, and consequently the bit rate. mean that will correspond to the compression factor.

In fact, if a file with a 1411 Kbps bit rate like the one in the previous example were compressed at an average bit rate of 320 Kbps, we would have reduced the original file size by a factor of approximately 4.5 (1411/320).

Currently in the most advanced codecs there are three types of bitrate implementation:

CBR (BitRate from Costant). The simplest, most used and now least effective method. The bit rate remains constant in each frame and this means that the encoder will always use the same number of bits to encode each musical passage. In practice, the more complex passages will have a lower quality than those with little dynamics or silence since they will be encoded with an always equal number of bits, while more would be needed for the former and less for the latter. A great advantage of this mode is that the size of the resulting file is always proportional to the length of the part and is easily evaluated.

ABR (Average Bit Rate). Average bit rate is a mode that outperforms CBR and consists of a kind of variable bit rate. The encoder encodes the regions that need it with more bits and the simpler ones with less.

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Video formats and compression codecs for video editing

To understand the basics of video editing, one must consider display frame theory. For this reason, we will often use a fitting comparison in the guide: that of motion pictures and the frames that make up the film.

Video Editing

What is a video format?

We all know that you need a screen, a projector, and a movie to get a movie projection. A sequence of images is printed on the film, translucent as the negatives of the photographs, which in the projection modify the light beam of the projector, allowing only certain parts to pass through, which will generate an image on the screen. Film flows, projected images change rapidly, and a motion effect is obtained.

Let’s go back to digital video.

Let’s say we have a series of images that reproduce a movement (we will see later how the capture phase, or video capture, allows this). These images alone are not enough to show a movie on our PC. In fact, we should be able to tell the machine where these images are, what type they are, how fast they should be viewed, and in what order. For this reason, the format of a clip is defined, that is, a kind of “container”, recognizable by the PC, in which the previous information is attached, in addition to the images.
To recognize a video file format of a clip, you can access the file properties panel or observe the file extension itself (for example, AVI, MPG, QTM, etc … all video extensions).

Digital video
Each editor chooses the best video format they consider appropriate, depending on how they work, the technology they have available and the end result they want to achieve.

In fact, try to think how much a color image of about 800×600 pixels takes up. So much … too much to see 25 per second (as the theory of optics says) and save them to our HD.

What is a video codec?

This is where the codec (COmpressor DECompressor or better DECoder CODER) comes in, or the software that contains the mathematical procedure through which the images are compressed (often with loss or loss of information) to allow agile management and reproduction correct clip.
In practice, compressed video formats are obtained.

The codec is used both to capture and compress the video from an external source, and to play and process the video once it is stored on the hard drive. It could be compared to a kind of very fast Winzip that, if necessary, compresses and decompresses the images of a film.

It is useless to dwell on how a codec manages to make a noticeable decrease in the space occupied by images, reducing the loss of quality to a minimum (sometimes surprisingly!) What little indication to say which is the best video codec or the best compression Of video . The answer is always the same: it depends on what you want to achieve (and, similarly, what is the best video format is a question that has multiple answers).
The important thing is that these codecs are available to us, there are many of them and each one has peculiar characteristics that suggest its working environment.

We suggest downloading the K-Lite Codeck Pack (often also abbreviated as Klite) which contains an important collection of useful and cross-cutting codecs, tools, formats and filters.

Structure of a video format

Hardware codec and analog capture

Until recently, analog capture cards (especially M-Jpeg) were almost all equipped with a proprietary chip that allowed smooth, lossless capture as it took the computer’s processor out of compression work (it’s this chip that kept the price of the cards high).
The hardware codec is still software, but it interacts with this chip by letting the system know that it exists and that it can do the job instead of the CPU.
Without the codec installed, the card chip is useless, whereas if only the software codec is installed, the PC processor may be able to do the compression job, but this in particular cases.

Entry-level PCs are still powerful, and often analog acquisition cards only have one analog-to-digital conversion chip, while the processor does the conversion. In some cases, it is even possible to capture with very complex and elaborate software codecs like DivX or Xvid.
It is clear that choosing the hardware codec is always recommended, as in the case of the MPEG2 capture which requires a lot of resources.

Video codecs

In photography, the choice of the recording codec is quite limited: we usually fluctuate between the JPG format and the Raw format. In video, the choice is slightly wider: MPEG-1, MPEG-2, MPG-4, H264, H265, RAW, ProRes, ProRes Raw, Motion jpeg, etc.

So what are these codecs for, what uses are they designed for, and which ones should you choose?

Códecs

CONTAINERS AND CODECS are NOT the same!

Digital videos are contained in files. These files are called “containers”. The extension of these files often makes it possible to determine their type. The most common containers are MP4, MOV, AVI and MXF. The MXF container is generally used in professional camera series from manufacturers such as Sony or Panasonic.

These containers are “boxes” or “folders” that consist of several elements: video track, sound tracks, time code or subtitles. The purpose of the container is to synchronize all the elements it contains.

Therefore, you cannot know the codec of a video from the file extension: for example, an .avi file and a .mov file could use the same codec to encode the video track.

WHAT IS A CODEC?

A codec is a mathematical procedure consisting of algorithms used to encode and decode the image (codec). It helps define how sensor information is stored and the strategy for compressing and decompressing images.

The size of a one minute 4K video file is about 50 GB. Therefore, it is easy to understand the main advantage of using a codec: compression. For the same minute of 4K recording, for example, using the H264 codec reduces the file size by a factor of about 27, or a final size of about 1.8 GB.

The more “codec” a codec is, the more complex the decoding will be and it will require a powerful machine to accomplish this task. At the same time, the higher the compression, the greater the loss of information that can make post-production difficult.

To achieve this optimization, most codecs use spatial compression: each image is compressed independently of the other images; This is known as an “intra” codec. For more compression, more complex codecs add temporary compression: we’re talking LONG GOP (GOP for “Group of Pictures” or “Long Group of Pictures”); Codecs of this type record only one full image per x images (often 12) and retain only what has changed between these images.

Codecs and bit rates, how much do they influence the quality of MP3s?

The importance of codecs

Everything that has been written about the bit rate is valid
“with the same codec”, ie with the same algorithm
Coding, but the quality of the algorithms is very variable,
since the perception model, fundamental control of the levels of
compression.

Those who are best able to mimic natural perception will get it
the best results since the cutoff frequencies will be
least audible and significant, while a codec with a perceptual model
Unrealistic creates flat and empty passages of important frequencies.

In addition, a bad codec easily introduces “artifacts”; or
Sounds that are not available in the original originate from poor quantization
of some frequencies, especially if the original is not perfect.

Like most things in the computer world, codecs are evolving
In addition to the codec type, it is important that it is current and updated
for increasingly powerful algorithms.

Codecs fall into two categories: “fast”
and slow “. The most important fast codecs include blade,
QDesign and Xing, fast compression, but low to medium quality,
among the slow but high quality Fraunhofer and Lame are these
latest free and especially valid for high bit rates.

You can achieve good results with Fraunhofer coding
Most listeners cannot do this even at 128 Kbit / s and 320 Kbit / s
Distinguish MP3 from the original while compressing with Xing
o Blade with 128 Kbit / s can detect obvious artifacts and only with 256 Kbit / s
Audio reaches the quality of an MP3 compressed with 128 Kbit / s
Fraunhofer.

The just released Xing algorithm had the advantage of lower costs
Rights compared to Fraunhofer and faster coding speed
(It was also the first to implement variable bit rate VBR coding), but
Today’s processor performance is like that. for this
Factor is less important.

The lame algorithm has the advantage that no fees have to be paid
and of remarkable quality, especially at high bit rates – an MP3 from
128 Kbit / s encoded with Lame are less than one compressed with Fraunhofer.
but significantly better than Xing, and 256 Kbit / s Lame has the same quality
Fraunhofer.

At the moment, Lame is the best option for those who
You can afford larger files, while those that need to be careful
about the size, maybe because they have to offer files on an amateur website
or because they have an MP3 player with limited memory, they can pay well
Get a Fraunhofer encoder.

Xing, Blade, and other faster algorithms are not recommended, though
Pay attention to quality.

General quality and bit rate considerations
So that most listeners are of acceptable quality,
using a quality codec like Fraunhofer the bit rate that is normally used
(128 kbps, 1 MB per minute) is usually sufficient, especially if
Songs have a limited frequency range.

For some traces or for more sensitive ears a
highest bit rate, e.g. B. 256 Kbit / s (2 MB per minute, 1/5 of
Corresponding WAV).

64 kbit / s bit rates are always insufficient and with low quality codecs
like Xing or Blade are practically unknown.

Finally, we have to dispel a false myth: it is not possible to point it out
Music genres that need a higher bit rate, classical music too
Sometimes it can be made acceptable at 128 kbps because
Use restricted frequencies, while some rock or pop songs can
suffer because synthesizer tones and human voices are
easily exposed to the “artifact” effect.

Only lyrical music that has always been the most difficult
play and jazz (with cymbals,
Artifacts easy to suffer) certainly not suitable for MP3 and require bit rates
higher.

However, the results vary from song to song and are not
possible to give a general rule. It must be remembered that the audiophile,
The hi-fi musician or hobbyist can almost always
Distinguish an MP3 file with 320 Kbit / s from the original on the audio CD.
So if you have storage space, WAV files or codecs without loss of information
like WMA9 Lossless or FLAC (Free Lossless Audio Codec) always have a quality
Consider MP3, even if they are two to four times larger than
an MP3 with 320 Kbit / s.

Codecs: How an audio or video codec works

To understand in a simple way what a codec is and how it works, the first thing we have to keep in mind is that the human voice is a continuous (analog) waveform, while the information that circulates through a data network is discrete ( digital). This means that if we want to send packets with data that contain the information needed to reproduce a voice fragment, we will have to digitize the voice beforehand.

The process of converting an analog signal to a digital signal is done through an element called an encoder. If the encoder is also capable of performing the opposite process, that is, moving from a digital signal to an analog signal even if only approximately, then we will have an encoder-decoder. An encoder-decoder is also called a ‘codec’.

Although it may seem that we would only be interested in selecting one or two codecs taking into account the voice quality (MOS, Mean Opinion Score) and bit-rate parameters that each codec needs, the truth is that we also have to face our network Look closely at the size of the packets that we are going to send since each voice packet is also made up of bits that are used for routing and error correction issues. As they add load, they saturate the network and make the entire system go slower. Do we reduce the number of packages we have to send making them larger? It is not always a good idea because the loss of a large package will have a greater impact on the output, even a cut in communication! What is recommended in these cases is to have a certain compromise between the number of packets that are sent to the network and the protection we want to have. The reference size should be about 20ms of voice per package.

How codecs work

The codecs reduce the information of the clips to facilitate and enable their publication and viewing through the Internet. There are two methods of compression, the so-called spatial and temporal compression.

In the first one, reduce the information by compressing the existing one inside each frame. Instead of describing the pixel-to-pixel image, pointing out for example the position and color of the pixels, the compression codec generalizes describing similar areas and their light and color characteristics. For example, instead of reproducing a blue sky pixel by pixel, it would be described as an area with similar light and color characteristics. In this sense, the less varied details an image presents, the codec can generalize and compress more easily. Creating videos with simple backgrounds facilitates compression and reduction, just as working with a tripod instead of a camera in hand means stabilizing the backgrounds and therefore facilitating subsequent compression.

The other compression method is temporary, where the information between consecutive frames is compared and only the details that vary are stored. The reference frames from which the differences are analyzed and the subsequent ones are supported are called keyframes and contain the complete image. On the contrary, the frames that reflect the differences are called “delta frames” and only contain the information of the areas that vary with respect to the previous images.

In general, videos that show few changes between frames are compressed better and this necessarily affects the realization. At present, both the television and the cinematographic realization tend to use the camera in motion. However, the compression of dynamic videos is more problematic than the cases of more static images.