What is bit rate? Knowledge of the MP3 audio format.

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What is bit rate? Knowledge of the MP3 audio format.

Digital audio formats are audio signals that are recorded, processed, and reproduced in digital form.

The emergence of digital audio formats is to meet the needs of high-fidelity playback, storage and transmission. Simply put, early analog audio formats had issues with playback distortion and glitches due to media wear. Since the advent of the CD, digital format audio files have become popular, but another problem has arisen: the limitation of the storage volume, and the CD still has the phenomenon of wear. Saving to hard drive (relatively longer storage time) is not a good solution when storage media (mainly hard drives) are still expensive at the time. The rise of the Internet has created a requirement for long-distance file transmission. Under the restriction of bandwidth, the demand to reduce file size has become more intense. All this has led to the generation of lossy compressed digital audio formats from external factors!

In terms of internal factors, with the improvement of computing and coding capabilities, the progress of various acoustic psychological models has promoted the emergence of various lossy compressed digital audio formats. Some of the most commonly used audio formats in MP3 players are briefly introduced below: MP3 (CBR, VBR, ABR), WMA, WAV, ADPCM, and the emerging audio formats AAC, ASF, and OGG.

Before introducing various digital audio formats, let’s clarify one concept: bitrate.

In the field of computing, all information is digitized. Bit is the smallest unit of data in a computer, it refers to a number of 0 or 1, which is a mathematical binary number, a “0” or “1” , is a bit. For example, when we say a 2-digit number, it means that it is a two-digit binary number, and there are 4 combinations of “00”, “01”, “10” and “11”, which represent 0, 1, 2 and 3 is four numbers.

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What does MP3 bitrate mean?

Bit rate

The rate at which a digital channel transmits digital signals is called the data transfer rate or bit rate.
The word bitrate has many translations, such as bitrate, etc., which indicates how many bits per second the encoded (compressed) audio data should be represented, and a bit is the smallest binary unit, either 0 or 0. 1. The relationship between bitrate and audio and video compression is simply that the higher the bitrate, the better the quality of the audio and video, but the larger the encoded file; if the bitrate is lower, the situation is reversed.

For example: encode audio and video at 500 Kbps.
where bps are bits 1K = 1010 = 1024
b is little
s is the second
p is for (for)
Therefore, encoding at 500 kbps means that the encoded audio and video data must be represented at 500 K bits per second.
In the baseband transmission system, the bit rate is used to represent the code rate of transmitted information.
The bit rate Rb refers to the unit of time
The number of binary bits transmitted within the unit, the unit is b/s. For example, the transmission speed of a computer serial port is up to 115200b/s.
The symbol rate or baud rate Rs refers to the number of modulation symbols transmitted per unit of time, that is, ternary and ternary
The information transmission rate of the multivariate digital code stream in the

In M-ary modulation, the relationship between the bit rate Rb and the baud rate Rs is:
Rb=Rslog2M
The sampling rate refers to the ratio of the sampling samples to the total number of samples, and the sampling rate refers to the number of samples per unit of time. If it is an instrument, the sampling rate is 40MSa/s, which indicates that the number of samples per second is 40M, but it cannot be represented by 40MHz.

The process of converting analog audio to digital audio is called sampling. In a nutshell, how much data is needed to record a 1 second duration of sound via waveform sampling. A sound with a sample rate of 44 KHz requires 44,000 data points to describe a 1-second sound waveform. In principle, the higher the sample rate, the better the sound quality.

What is bit rate? Knowledge of the MP3 audio format.

Digital audio formats are audio signals that are recorded, processed, and reproduced in digital form.

The emergence of digital audio formats is to meet the needs of high-fidelity playback, storage and transmission. Simply put, early analog audio formats had issues with playback distortion and glitches due to media wear. Since the advent of CD discs, audio files in digital format have become popular, but another problem has arisen: the limitation of storage volume and the phenomenon of CD disc wear is still present. Saving to a hard drive (in connection with longer storage time) is also not a good solution when storage media (mainly hard drives) are still expensive at the time. The rise of the Internet has created a requirement for long-distance file transmission. Under the restriction of bandwidth, the demand to reduce file size has become more intense. All this has led to the generation of lossy compressed digital audio formats from external factors!

Before introducing various digital audio formats, let’s clarify one concept: bitrate.

How to distinguish the sound quality of Mp3 songs?

Mp3 quality

Factors that affect audio quality are the number of channels, the sampling rate, and the number of quantization bits.

Mp3 Quality

It’s not directly related to file size, I think friends who have used Audition or play more music will be more familiar with it.

-Number of channels

Channel count is easy to understand and is often referred to as channel count. Usually we talk about left and right channels, single and double channels, which refers to the number of channels.

The music that we listen to often in life is basically two-channel, that is, the left and right channels. Generally speaking, the higher the number of channels, the better the audio quality. Then the stereoscopic feeling of the sound will be stronger. It will feel more real. When a person speaks or an object makes a sound, the sound also spreads in all directions, and of course there are more than two channels. So, in fact, it is difficult for digital audio to achieve real sound realism.

-Sampling frequency

For example, when Audition exports audio files, there is a sample rate option. What exactly is this sample rate?

Sampling rate is explained in official words: the number of samples per unit of time (within 1S). The higher the sample rate, the more data it collects and the better the sound quality.

But you will find that music in real life is generally 44100HZ sampling rate, like the lossless music in the picture above. So there are so many miscellaneous sample rate options in the image below. What does this mean? The reason is that the audible sound range of the human ear is between 20 and 20,000 Hz. Even if you increase the sample rate, it will still sound the same to ordinary people, so there is no need.

-Quantization bits

This is also very understandable. It’s like the number of bits that people often say about the computer. Audio also has the concept of bits. A common number of bits for audio is 16 bits. Generally speaking, the higher the number of bits, the better the sound quality. The popular understanding of quantization is to digitize the sampled value, that is, in the binary form recognized by the computer.

The property display in Windows may not display these parameters intuitively, but you can see them with the help of tools. Sound quality is determined by the above three aspects. Instead of looking at the size of the file. Of course, the audio is basically compressed and transcoded when it is broadcast to the audience. After all, high volume digital audio is not conducive to broadcasting.

FAQ

How to distinguish the sound quality of an mp3?

It is important to look at several elements to distinguish its sound quality. Of course, first is the quality of the recording, then the bitrate and samplerate.

Your can improve the sound quality of an mp3?

It is possible, using Mp4Gain, to improve the perception of the quality of an mp3 or any other audio or video format. In addition to modifying the bitrate and sample rate, we can modify the “color” with an equalizer and even slightly modify the pitch and of course normalize the audio.

320 kbps audio bit rate, ideal for streaming.

320 kbps audio bit rate, ideal for streaming.

The triumphant march of the MPEG-1 Layer 3 audio recording format (colloquially called MP3) is explained by the fact that a simple and effective method of compressing an audio file was proposed, allowing to store up to 12 hours of quality music acceptable on a standard CD-ROM disc.

320 KBPS Bit Rate

Simply put, the MPEG-1 Layer 3 algorithm is based on the so-called “psychoacoustic” compression method, when frequencies and loudness levels that are not perceptible to the ear are excluded from the sounds in the spectrum. The spectrum “cleaned” in this way is divided into separate blocks (frames) of the same duration and compressed according to specified requirements. When played back, the signal is formed from a sequence of decoded frames.

The compression ratio depends on the parameters of the audio stream, which must be output after decoding the file.

The main parameter that determines the sound quality and the degree of compression is the so-called (what is) bit rate, the bandwidth measured in bits per second.

The higher the number, the better the sound quality and the lower the compression ratio. Since almost all MP3 files are recorded in stereo with a 44 kHz encoding rate and 16-bit depth, the determining factors for clear sound are: the source of the recording, the codec used, and the selected bit rate. .

The word codec is formed by combining the words encoder + decoder. This is a software module that allows you to encode or decode audio or video files according to your own algorithm.

The average value of the stream of 256 kbps provides a compression ratio of approximately 6: 1, for other values, the compression ratio changes proportionally. So, with 256Kbps stream, you can record music from six regular audio CDs to one CD, and with 128Kbps stream, you can record from twelve regular music discs.

There is a never-ending controversy between amateurs and professionals about the bit rate that provides good sound quality that matches the playback quality of an audio CD.

Some consider the level of 128 Kbps sufficient, others are satisfied only with the maximum value of the transmission: 320 Kbps. In all probability, both are right: the only difference is what is recorded and under what conditions it is played.

The bit rate at which digitized audio was encoded is usually indicated on the CD cover. For example, the entire Beatles collection can be purchased on three 128Kbps discs or six 256Kbps discs.

It is clear that in the second case the purchase price will be twice as expensive, but the quality is also better.

If the music is played in a home car, the 192Kbps transmission will provide sufficient sound quality, the best that you still won’t hear due to the strange noise. For listening on a computer or standalone player (MP3 player), a 256 Kbps stream is accepted.

But if the signal is sent unchanged to an external device and output to high-quality speakers, the maximum possible stream is desirable: 320 Kbps. Based on the above considerations, a stream of 256 Kbps can be considered universal: with a good recording quality, it will provide adequate playback in most cases.

INFLUENCE OF THE BIT RATE ON THE SOUND QUALITY OF AN MP3 FILE

INFLUENCE OF THE BIT RATE ON THE SOUND QUALITY OF AN MP3 FILE

Bit Rate

The term bit rate means how much information was transmitted per unit of time. Encoding in MP3 files can be done with constant and variable bitrate. Usually it determines the sound quality and consequently the file size.

BIT RATE

In normal practice, the bit rate for high-quality recording starts at 128 kbps. Although this is a purely subjective opinion and for everyone it can have its own value. It is undeniable that the higher the bit rate, the better the sound quality.

At a value of 320 kbps, the sound of an MP3 file is already difficult to distinguish from the original by the human ear.
The following table shows the comparative characteristics of the sound quality of a certain bitrate value with equivalents that are understandable to everyone. Although, we repeat, this is a subjective opinion.

Bitrate value comparison:

Bitrate (kbps) —– Mono / Stereo Equivalent
8 mono —– Telephone conversation
16 mono —– shortwave radio
32 monkey
—– Medium Wave Radio 64 Stereo FM Radio
96 stereo —– Nearly CD quality
128 stereo —– CD quality

Now let’s try to do a little quality comparative analysis of MP3 files of a certain bit rate with originals:
• 112 kbps: the sound is unnatural, the sound is far from the original. Almost any listener will notice the difference. Encoding with these parameters is ideal for listening to melodies through mobile phones (although progress has come a long way and 112 kbps is no longer sufficient on some devices). It is also very attractive for broadcasting live broadcasts over long distances.
• 128 kbit / s: many manufacturers consider this value as the standard for encoding (this is highly debatable). The quality is more or less acceptable here. Many music lovers will note that the recording is not original. Although, thanks to the many “manufacturers” of CDs, users will soon accept this quality as a source. The records can be comfortably listened to on jarring plastic office speakers, cheap headphones and iPods, and of course cell phones. If you need to recode this recording to WAV format, you will lose your sense of taste forever.
• 160 kbps – here the situation is much better than before. Many will no longer understand the difference, and those who notice slight discrepancies with the original, put 192 kbps. This bit rate suits you 100%.
• 256 or 360 kbps: For many people it will be impossible to tell the difference in sound. Recommended for fans of high quality sound, owners of hi-fi equipment, large flash players, etc.

In the following table, we will consider the effect of bitrate size on the amount of disk space occupied and the number of files that can be placed on a certain amount of media. As a comparative value, we take a sound file with a duration of four minutes, which corresponds to the average composition.
This table does not include heavier discs like DVD or the latest standards like HD DVD and Blu-Ray. But even from the last column, you can imagine how much information, even with the best quality, can be put into the new media formats.
File and disk size dependency on bit rate
Bit rate (kbit / s) 4 minute files in 64MB RAM 4 minute files on CD (650MB) Hours on CD (650MB) 4 minute files on 1GB
64 34355 24543
80 28 284 19 437
96 22237 16364128
18177
12273160
14142 9218192 12118
8182256 8 89 6137

Perhaps the greatest amount of software has been written for the MP3 format.
Not a single self-respecting software player developer, and even home gamers, will dare today not to provide support for the famous format for their creation. He’s at the top of his form, experiencing a real boom. All computers likely contain files of this type, while others have thousands of them (downloaded from the Internet or copied from discs). People are buying turntables that have become outrageously cheaper. Stores are packed with MP3-tagged CDs. People already know this word as “CD” before.
However, despite all this apparent lack of clouds, the format also has disadvantages. This is imperfect stereo encoding, problems with gapped files, multi-channel sound, high frequencies and others. Many formats are already on their heels. This is the ubiquitous Microsoft with its WM A, OGG Vorbis and others.
But the most competitive now is probably the AAC format, which Apple actively and successfully supports and promotes on its iPods and the iTunes Internet service. There is also support from Dolby, Nokia (on new phones) and many other companies.

MP3 Bit Rate Guide – Quality and Differences Explained

Bit rate Quality

When compressing audio and video files, the MP3 bit rate indicates how many bits are available to the decoder to encode exactly one second of a track. The higher the bit rate of the MP3 file, the better the quality achieved. The bit rate can be constant (constant bit rate, CBR) or variable (variable bit rate, VBR). Our guide explains the differences.

Bitrate Quality

MP3 has established itself as a leading music format on the Internet in recent years and all popular MP3 players support this format. It was developed by the Fraunhofer Institute and is now considered the best known standard for Audiocodierun g. But where are the differences in the jungle of MP3 bit rates?

What MP3 bitrates are there anyway?

A distinction is made between the following common bit rates for MP3 files:

32, 40, 48, 56, 64, 80, 96, 112, 128, 160, 192, 224, 256 and 320 kBit / s.

This increases the quality of MP3s, but also the size of the files created. Compared to the original, an MP3 file only requires about 10 percent of the original storage space.

Starting at a bit rate of 192 kbit / s, you can hardly hear any difference from the quality of the original CD in many pieces of music.

Low bit rates: 32 to 128 kBit / s
Average bit rates: between 128 and 192 kBit / s
High bit rates: more than 192 kBit / s

What is the best bit rate for MP3 compression?

Again and again the question arises of what bit rate to select when converting songs to MP3 to achieve roughly CD quality. An MP3 compression with 192 kbit / s variable bit rate here is an ideal compromise between size and quality.

At just 128 kb / s, you can often hear a distinct difference from the original songs on CD. Music pieces with a lot of dynamics suffer more if the compression is too high (weak bass, lack of treble). So here it is better to use a higher bit rate.

How does the quality of bit rates differ depending on the compression method?

Constant Bit Rate (CBR)

With constant bit rate, each unit of time (for example, one second) is always allocated the same amount of storage space in the entire MP3 file. Therefore, the quality may vary depending on the piece of music. For this, the size of the resulting file can be calculated more precisely.

Variable Bit Rate (VBR)

Variable bit rate is usually the best compression method for normal use, as it can be used to produce consistent high quality. With Acapella parts, 320 kBit / s are not required, as only a few complex frequencies need to be encoded here. However, if you are playing a full orchestra, 128 kBit / s is usually not enough to cover the entire frequency spectrum of the various instruments. Depending on the piece of music, more bits are used when they are important, or those that are not can be omitted. In return, the file size varies more.

Average Bit Rate (ABR)

Some MP3 encoders also support average data rates. Technically, this variant is almost identical to Variable Bit Rate (VBR). Here, too, the encoder software always tries to achieve a uniform quality of the musical piece. However, the bit rate achieved often deviates slightly.

As an example: if you want a target bit rate of 128 kBit / s, then the bandwidth of the achieved bit rate is between 120 and 140 kBit / s. To achieve the desired average bit rate as accurately as possible, some codecs offer a two-pass compression process. The material is analyzed first and is only encoded in the second run. The ABR mode corresponds to a mix of CBR and VBR and is therefore qualitatively more in the middle.

What is the normal bit rate in an MP3?

A normal bit rate for an MP3 is between 128 kilobits per second, or kbps, and 320 kbps. Different bit rates in this range have different advantages; To extract music from a CD to MP3, choose the bit rate that best suits your purposes. In general, the higher the bit rate of an MP3, the better the audio quality, but the larger the file size. The most common bit rates in MP3s are 128 kbps, 192 kbps and 320 kbps.

Bit rate

The bit rate of an MP3 indicates the density of the audio information contained in the file. Therefore, every second in a 192 kbps MP3 contains 192 kilobits (24 bytes) of data. The higher the bit rate, the more information the MP3 contains; The more information it contains, the quality is closer to the original audio recording. The bit rate also determines the size of the MP3 file: a four-minute song encoded at 128 kbps occupies a little more than 3.5 MB, while the same song encoded at 320 kbps requires more than 9 MB of space.

Choice of a bit rate

When you convert a sound file or extract your CD collection to MP3, you must specify the bit rate you want the encoder to use. Choose a bit rate based on the type of audio you are encoding and on your desired purpose. If you are converting a conference or other piece of vocal audio, a bit rate of 128 kbps is more than enough. If you are encoding MP3 files for use with a portable media player, a bit rate of 192 kbps usually provides a good balance between audio quality and file size. If you are converting your CD collection to MP3 for archiving, encode the files with a bit rate of 320 kbps.

Variable bit rate

Coding with a variable bit rate changes the bit rate of the MP3 depending on the type of data present at a certain point in the audio. For example, a variable bit rate encoder could encode a portion of dense information from an MP3 at 320 kbps; however, when the audio contains a silent section the encoder lowers the bit rate to 32 kbps. If the bit rate of an MP3 has an unusual number (for example, 204 kbps), this indicates that the creator of the MP3 encoded it with a variable bit rate.

Bit Rate Conversion

Although most MP3 conversion and encoding programs can modify an MP3 at a different bit rate, this is not a good idea, generally speaking. Converting an MP3 to a higher bit rate does not add audio information to the file, so the sound quality does not increase. Converting an MP3 to a lower bit rate reduces the file size, but it could also introduce audio defects, since the encoder is compressing a file that is already compressed. If you need an MP3 file to have a different bit rate, re-encode an MP3 from the original audio CD or WAV file.

Everything you wanted to ask about the mp3

WHAT IS A CODEC AND WHAT IS IT FOR?

CODEC is an abbreviation of CODifier-DECodifier, which already indicates its functions. In the case of MP3, it deals with processing the sound, applying various compression methods to reduce the size necessary for its storage, and conversely, to reconstruct the sound from the encoded information. The codecs can be found in the form of an executable file, a DLL, or less frequently, incorporated into the code of another program. The DLL format allows the same codec to be easily used by different programs, while the executable format is usually more flexible in terms of configuration, although it is more complex to use.

WHAT DOES “PSYCHOCAUSTIC COMPRESSION” MEAN?

This very complicated word refers to the main compression technique that MP3 uses (although it is not the only one), which simply consists of eliminating the information (sound, in this case) that, in theory, is not able to perceive the ear human. I say “in theory” because, depending on the compression ratio that is applied, that loss may be more or less noticeable, hence some files do not sound as good as would be desired. The different codec also do not use psychoacoustic principles in the same way, so the quality they achieve will not be the same.

DOES THE SOURCE ORIGIN INFLUENCE THE MP3 QUALITY?

Evidently. The ideal is to start from an original CD. Analog sources, such as vinyl records or tapes, have lower quality in themselves, no matter how much we “digitize” or “improve” them with some programs. Recorded CDs may contain errors (which will end up being noises when played), and we usually do not know from which source they have been recorded (perhaps from previously encoded MP3 files). Re-encoding an already compressed music file, in the same or another format, is also a mistake, since quality is always lost.

DO THE PROGRAMS WE USE TO EXTRACT AND CODIFY THE QUALITY OF MP3?

It also seems obvious, so much so that this point should not be here. But many people do not take it into account, without knowing the reasons very well.

DOES THE QUALITY OF PLAYERS INCLUDE IN THE QUALITY OF MP3?

To hear it, you already know the answer, don’t you? J. To have it stored on the hard drive or on a CD, nope. Jokes aside, and in theory, all decoders should get the same sound from an MP3 file. But unfortunately this is not the case, usually because they do not strictly conform to the MP3 standard. You can find the best explanations on this topic here.

DOES THE QUALITY OF THE SOUND CARD AND SPEAKERS INCLUDE IN THE QUALITY OF THE MP3?

Well, I think you already know where the shots are going, we will not continue along this line. But nobody says “is that MP3 does not sound good” when it has a Sound Blaster 16 (what times those!) And some 800 W TurboPowerSound speakers that cost € 18 uros.

WHAT IS BITRATE?

It is the number of bits that are used to encode a second of sound, if we talk about MP3. It is measured in Kbps (kilobits per second).

As there is often confusion, we will give an example. If we have a 4-minute MP3 encoded at 128 Kbps:

-One second occupies: 128 Kbits x (1 Byte / 8 bits) = 16 KBytes.

-The complete song: 16 KBytes / second x (4 minutes x 60 seconds / 1 minute) = 3840 KBytes = 3.75 MBytes.

The formulas could be more correct, but this is not a mathematical thesis and I think they are enough to understand us.

WHAT DOES CBR, VBR and ABR MEANS, AND WHAT IS THE DIFFERENCE?

They are different coding methods. CBR means “Constant BitRate”, VBR “Variable BitRate” and ABR “Average BitRate”. It is easy to deduce that if CBR is used, the bitrate remains fixed, and if VBR is used, the bitrate changes. ABR can be considered a variant of VBR in which the objective is to achieve a certain final size, since we can set the average bitrate to be used (although it will never be as accurate as CBR).

The great advantage of VBR is that it uses more bits when the fragment to be encoded is more complex, and less when there are silences or it is simpler, so that we will achieve the highest quality using the minimum possible space. If the final size is not the deciding factor, it is the most recommended option.

WHAT IS THE BEST MP3 CODEC TO COMPRESS MUSIC? WHAT IS YOUR BEST CONFIGURATION?

The first question is easy to answer: LAME. In many different tests, done independently and with means that probably none of us will ever have, LAME is considered again and again as the best codec. And since there is no commercial motivation, since it is free distribution software, we can give full validity to these tests. The Fraunhofer codec and some of its “versions” could be valid in certain circumstances, but … do we need to use a commercial or pirate codec with a free and legal substitute? I think not (and from here, what each one does, is his own thing). For me, codec MP3 = LAME.

The second question is much harder to answer. There is no configuration that is perfect for everything (because if not, the modifiable options would be left over and everything would be much easier) J. But there are some recommended options in general, which we can establish with the assurance that the result will be of good quality. I have to admit that I barely change them, but as in this case (and in many others) there are no absolute truths, everyone has to try until they establish the one that best suits their preferences (or their ears).

The first thing that we must be clear about is that, in an MP3 file, size and quality are conflicting terms. We have to look for a practical middle ground. I put that term between 128 and 192 Kbps (some will say it is too low … I will try to justify my opinion). In principle, the purists established the CD quality at 256 kbps CBR. That is, using that bitrate, the sound of the CD and that of the MP3 file were absolutely indistinguishable. But, right now, LAME is already very developed. It uses the most advanced techniques, and manages to do perfectly things that other codecs are not able to solve properly (VBR, Joint-Stereo), which allows you to improve the quality and reduce the size.

In r3mix.net, after the corresponding tests, they have established an “ideal” configuration based on quality criteria. This configuration is so accepted, that in the latest versions of CDex, and other programs, it is already included as one of the preset configurations. But be careful: this does not mean that this is the only option, nor that it will always achieve the highest possible quality. Another thing is that using this option our ear can differentiate the results of the original audio CD, and this is where the “trick” of MP3 compression is.

After having used this configuration, I have observed that, although there are important variations, the average of the files is about 170-175 kbps, and that they rarely go beyond those 192 Kbps that I mentioned before, so I consider that overcoming that bitrate does not it will hardly improve the quality, but it will significantly increase the size. Although I repeat, there will always be someone to whom this bitrate seems insufficient, and will use even 320 Kbps, which is the maximum supported by LAME.

The second part is to take into account the equipment we have. If the MP3s are not going to sound through a card, an amplifier and very good quality speakers, the bitrate can be adjusted a little more without practically noticing variation in quality. The average 160 Kbps seems like a very reasonable amount to me, and that’s how I usually code my CDs. And I can say that they don’t sound bad at all. If someone is able to do a “blind” test and differentiate between several files encoded (correctly) at 160 and 192 Kbps, you really have to congratulate him on his ear.

Finally, if we consider size as a decisive factor, we could go down to 128 Kbps, but considering that the quality may already be too compromised. Summarizing:

Configuration that seems more appropriate for practical purposes, and looking for a high quality (about 192 Kbps):

-Joint-Stereo -> Get reduce the final size without losing quality. Sometimes, where there is a lot of variation between channels and with high bitrates, it could be configured as Stereo.

-VBR MTRH Quality 1 or R3Mix Preset -> Change the bitrrate according to the needs of each moment, improving the quality without increasing the size with respect to CBR (normally).

-Bitrate minimum 64 Kbps -> To compress to the maximum silences and very simple fragments.

-Bitrate maximum 256 Kbps -> So you have room to improve the quality in complicated fragments.