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Explanation of the sample rate, bit depth and bit rate

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Explanation of the sample rate, bit depth and bit rate

Bit Depth

This is what happens when you compress audio files
High-resolution streaming and audio promise us particularly good lossless sound when playing digital music. But what exactly does that mean? Sample rate, bit depth, and bit rate are explained.

16-bit 44.1 kHz – CD Quality: This slogan is often used when marketing high-resolution and lossless audio formats and equipment. But what exactly is it about?

israelburrows | Israelsaepblog

The quality of digital audio signals is primarily determined by three things: sample rate, bit depth, and bit rate.

In principle, you have to imagine that a digital audio signal is not a constant flow of information. Rather, music is divided into very, many small “slices” (samples), each of which carries the information for the digital-to-analog converter in a short section.

The more samples a signal has per unit time, the more precisely the original analog signal can be mapped.

The sample rate reveals how tight the samples are: the 44.1 kHz of a CD represents 44,100 samples per second.

Bit depth

Bit depth (resolution) is specified in “bits” and indicates how many different values each sample can assume. With 16 bits (CD quality), this is 2 ^ 16, that is, 65,536. The recording studio usually works with 24 or even 32 bits.

This is where many high-resolution formats come into play that also promise studio quality at 32-bit resolution.

▶ ︎ MP3, FLAC, ALAC, WAV – the most important music formats in comparison

Bit rate

The bit rate (bandwidth) of an audio file in uncompressed formats is initially nothing more than the summary of the sample rate, the bit depth and the number of channels. It is specified in kBit / s and therefore provides an initial characteristic value for evaluating the quality of a file.

Example: A stereo CD stores two tracks at 44.1 kHz with a bit depth of 16 bits. This means that its bit rate is 44.1 kHz x 16 bits x 2 channels = 1411.2 kBit / s

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But what role does the bit rate play?

This term is known primarily for describing the quality of lossy compressed audio (eg MP3). Unfortunately, this makes it even more difficult than with channel-separated compression, the bit rate is split between the two channels: stereo MP3 (not to be confused with dual channel) with a 320 kBit / sec bit rate. uses only 160 kBit / sec per channel.

Bit Rate

Or different bit rates per channel: set stereo (mono signal calculated with additional stereo information), on the other hand, it works with volume differences between the two channels and therefore can use the bit rate much more efficiently .

Bitrate

DAC, bit rate

What exactly is the obviously so essential bit rate? For a CD based on the Red Book standard, the bit rate is calculated as follows: 2 channels * 44,100 Hz sample rate * 16 bit depth per sample results in 1,411,200 bits / sec., That is, 1.4112 Mbit / sec. Bit rate. Obviously, this is considerably more than what compressed formats (should) provide. You can see how the compression processes work: last but not least, they more or less cleverly reduce the bit depth per sample at a given sample rate of 44.1 kHz, for example. The amount of data decreases, and that is exactly the goal of every data compression: halving the bit rate means exactly halving the amount of data.

As an example, I have compared some common file formats for digital audio:

Format

Codec (s)

Multichannel

Sampling rate

Bit depth or resulting bit depth from bit rate

Compression
/
subject to acoustic losses

Wav

PCM et al.

yes

any

Optionally, depending on the codec, also lossless

AIFF

PCM et al.

Not

any

Not

FLAC

FLAC (Free Lossless Audio Codec)

yes

0.001 kHz-655.350 kHz

4, 8, 16, 20, 24, 32

without losses

Apple loses
less
MP4

ALAC (Apple Lossless Audio Codec)

yes

0.001 kHz-384,000 kHz

16, 20, 24, 32

without losses

MP3

MPEG I Layer 3 in various incarnations
as B. LAME

Not

8-48 kHz

8-320 kBit / sec. CBR / VBR or 640 CBR for free MP3 format

with mandatory loss

Most readers will know that lossy compression is often based on psychoacoustic models or natural limitations of human hearing: what humans cannot (should) hear is not stored in the musical signal and is irretrievably lost, apparently not. you need it. The most popular example of this is the old MP3 format already mentioned. For some audiophiles, lossy compression is by definition useless for serious music listening, regardless of whether or not they would notice the loss of compression. For others, the sound is 320 kBit / sec. MP3 encoded for pop music are identical to CDs, they are satisfied.

DAC bit rate

Lossless compression, on the other hand, has become increasingly popular since Internet bandwidths and storage capacities have steadily increased. An example is the FLAC format, which fortunately is also “open source”, which means that it can be used freely and even changed in terms of the program. Meanwhile, most of the time it is directly compatible with proprietary audio hardware, so FLAC files can be played without the help of a computer, and in some cases even created (ripped CDs). As the table above shows, FLAC supports very high sample rates and bit depths, as well as multi-channel sound.

With FLAC, the audio signal is encoded based on computable fixed-point algorithms that conserve computational power, in which blocks are formed step by step and stereo separation is converted to mid-side separation and performed the remodeling of the signal with differential storage. No information is lost, it is stored more efficiently than, for example, on a CD; Depending on the complexity of the audio signal, compression rates of up to 30% can be achieved.

FLAC

It should be clear once again that FLAC or MP3 are file formats and therefore cannot be directly compared to the PCM of the Red Book encoding of a CD or DSD (see next section). Let’s leave it at that on the subject of data compression.

What does “bit rate” mean in a video?

When downloading videos from the Internet or copying videos from disc to your computer, you may have the option of choosing a different bitrate. Even DVDs have specific bit rates. As technology has advanced, video media are able to achieve higher quality, such as high definition DVD, and this is achieved by increasing the video bit rate.

Bitrate

Definition

Bit rate, sometimes referred to as “bit rate”, is the term used to define compression of audio, video, or other multimedia compression in terms of the amount of data, measured in bits, per unit of time in the printed material. Bit rate is the ratio of quality to time. Bit rate is also often measured in kilobits or megabits per second.

Bitrate

Sense

Videos with a higher bit rate will have a larger file size. For example, it will take longer to download or stream from the Internet and upload to a device or computer, and you can burn fewer videos to DVD. However, a higher bit rate usually indicates a higher quality. In this way, your video can be clearer and sharper. When browsing for video or importing it to computer from DVD or camera, choose the highest bit rate for higher quality. However, if space is your main concern, choose the lowest bitrate when downloading or compressing video files.

bit rate factors

Programs will have different bit rates depending on different factors. For example, the output material may have more depth and quality, thus producing a higher bit rate. High definition movies have a higher bit rate than standard definition, for example. Different types of video files have different bit rates and you can also choose different bit rates for the same file. For example, iTunes allows you to compress audio files with a bit rate greater than 128 kilobytes per second to save space on your iPod.

Standard bit rates

Most standard definition DVDs have a maximum bit rate of 9.8 megabits. High definition television has a bit rate range between eight and 15 megabits per second, while the standard for 720p high definition video is approximately 19 megabits per second. At the time of publication, Blu-Ray DVDs have the highest bit rate of all consumer videos with a maximum of 40 megabits per second. This makes the content of Blu-Ray DVDs more than four times that of standard DVDs.

Understanding the bit rate

When you start to make comparisons between video cameras you are faced with a series of aspects to take into account: numbers, characteristics, options … One of the lesser known but no less important issues is the bit rate. Let’s find out what it is, once and for all.

Bit rate

What is the bit rate?

Bit rate is the amount of digital information (bits) that is transferred or recorded in the unit of time.
In the case of video, the bitrate is generally expressed in bits per second, and the amounts being played to date are Mbps (Megabit, that is, millions of bits, per second) or kbps (kilobits, thousands of bits per second).

Bit rate

Because it is important

The bit rate is a parameter to take into account because it affects many aspects of digital production, both technical and more practical (ie economic).
First of all, it affects the quality of the video and the size of the single file: obviously, the more data is transmitted in the unit of time, the higher the quality of the recording. And at the same time, it will increase the size of the recorded file.
However, this should not lead us to think that a very high bit rate is always necessary: in fact, we must bear in mind that as the bit rate increases, the difference becomes more and more marginal.
Basically, bit rate variations made from already high values to reach higher values are less visible than variations made from low values.

Then there is the question of file accessibility on the network: if the bit rate is very high, it will take a long time to download a video and watch it (for example, YouTube recommends not very high bit rates).

Finally, the bit rate also influences our portfolio: if we want to record at very high bit rates, we must first have a higher performance equipment, and then we will have to have more capable and higher performance media. Hence, larger SD, with high write speeds and consequently higher costs.

This is why you need to know what bitrate is, especially if you are serious about creating videos.

Bitrate, framerate and resolution

To get an idea of the concept of framerate you can read this article that talks about 24 fps and the cinema effect. Instead, resolution is simply the number of pixels the camera uses to “describe” the scene – more pixels means more resolution and therefore more detail (up to a certain limit).

Why are we talking about these other two concepts? Because the bit rate is a bottleneck that can make you lose part of the information we want to record.

A little math

Let’s imagine that we have our machine and we set it to 4K and choose a frame rate of 60 frames per second.
Now let’s take the same machine and set it up in HD choosing a frame rate of 30 frames per second.

How will the bit rate change?

Since the bit rate is bit / second, let’s try to calculate the bits to transfer in each case.

-In the first case we will have 4096 × 2160 pixels, all multiplied by the 60 times in one second that the frame is recorded: a total of 530841600 pixels.
In the second case, however, 1920 × 1080 pixels multiplied by 30 frames: a total of 62,208,000 pixels.

That is why the bit rate can be a bottleneck: each pixel is represented by a certain number of bits, and if we record with resolution and framerate parameters that exceed the bit rate possible with a certain machine, this will have to apply compression to the file. And we know that the more compressed a file is, the lower its quality.

What is the bit rate and how does it influence the quality of the videos?

Amid the pandemic we are facing and authorities increasingly asking people to stay home, the use of the Internet, especially broadcast services, is skyrocketing. Amazon Prime Video, Netflix, YouTube, Twitch and others have already announced measures to try to ensure everyone accesses their videos without gagging, but there are differences between the measures on some platforms.

YouTube reduces standard video quality to SD worldwide

Netflix, for example, opted for a reduction in the data rate or bit rate without lowering the resolution of the video itself. YouTube is already one of the platforms that now forces you to load at a lower resolution while maintaining the data rate, that is, the final quality, and allows you to increase the image size manually by the user.

bitrate

Oops wait! So does reducing resolution not reduce the quality and quantity of data to transfer? Not quite. Resolution is not the only thing that defines the quality of a video image, or even a JPG image, audio and the like, there are a number of factors. Among them, in addition to the image size, the bit rate that will be transferred from a server to a device, for example.

Data rate versus bandwidth

Reducing the amount of data sent to each user can help guarantee the service for everyone (Photo: Pixabay)
To understand metering on Netflix, you first need to understand the difference between the data rate of a media file and the bandwidth of the connection. Both can be called bitrate and throughput, and whether those terms are correct or not matters little. What matters is that it confuses, so it is necessary to clarify it.

The data rate of a media file refers to the maximum size of a second of the video. A video, as you may already know, is divided into frames, the most common speed is 30 fps (30 frames per second). Each table has its own set of information, and the total of 30 (or 24, 60, 120, and the like) forms the bitrate.

The bandwidth of the connection is the speed of your internet. Brazilian 4G, for example, has an average of 20 Mbps. Some fixed broadband operators offer packages of 100 Mbps, 200 Mbps or even more. This rate is the maximum amount of data that your Internet can receive from a central server.

Data volume

YouTube chose to lower the standard resolution without decreasing the volume of data (Photo: Pixabay)
The bit rate of a media file, therefore, is the amount of data it has each second. Netflix reduced this rate to decrease the amount of information that its servers need to send to each user connected to the platform. This reduces the quality of the image because it is less data, which can slightly reduce sharpness and cause noise, which some call “pixelization”, especially in very dark areas of the frame.

The resolution is not the lowest: it is that this image has less information. When loaded onto a large screen such as a TV, details are missing to display. On smaller screens, you will hardly notice the difference.

Facebook and Instagram will reduce the quality of video in Brazil
Globoplay will reduce video quality to meet demand
In short, Netflix’s decision ensures resolution, keeping sharpness a little closer than the user is used to, but reducing the amount of data transferred, resulting in images with a lower level of detail and sometimes , even more noise, even.

By sending less information to each user, Netflix, as well as Facebook, Globoplay and other services with similar measures, the platform avoids what we call server bottlenecks. This could cause suffocation and connection failures. YouTube’s measure also alleviates traffic because not everyone will manually increase the resolution, which in itself already reduces the amount of information to be transferred.