Why are MP3 bitrates often multiples of 32? (power of 2) part 2
MP3 Bitrate
Depending on the resource, VBR can be encoded by changing the bitrate between a fixed rate above each frame, or by sharing the available bits in adjacent frames (effectively producing a non-standard bitrate for the two frames combined).
MP3 Bitrate
the fixed frame depends on the sampling rate, 1152 samples per frame. There is no limit to the size of the frame itself, nor to the base 2 size of the frame (ie 417 bytes for a 128 kbit/s MP3 sampled at 44.1 kHz).
In the end, a file encoded at 126kbps will sound worse than a file encoded at 128kbps, and similarly a file encoded at 131kbps will sound better. However, MP3s are encoded according to the compression psychoacoustic model of a specific encoder. The amount by which a file sounds “better” or “worse” at a given bitrate largely depends on the algorithm used to implement the model, but in general higher bitrates allow for more data, presumably for rebuild a more accurate original transmission. audio signal
Why are MP3 bitrates often multiples of 32? (power of 2)
MP3 Bitrate
Some people say:
MP3 Bitrate
I understand why multiples of 2 often show up on computers since they are binary, but I can’t figure out how the most common mp3 bitrates (64kbps, 128kbps, 160kbps, 192kbps, 256kbps, 320kbps, etc.) also tend to follow this rule.
Since MP3 is just a sequential encoding of sound waves, why is it important to represent each second in kilobits divisible by 2?
Does a music player like iTunes continue to read the file and play the encoded sound regardless of the second limit, or does it read the file every second?
In the latter case, reading a 256kbps file requires reading slightly fewer memory pages than a 257kbps file, but the player can always read 256KB chunks, regardless of their bitrate, and just process them automatically. incremental, right, Bar?
Are 128kbps MP3 songs popular simply because it’s a generally accepted bitrate, or do they really have any advantages over 126kbps and 131kbps files, apart from a very slight difference in quality/file size?
For constant bit rate (CBR) encoding, the MPEG-1 Audio Layer III standard specifies standard bit rates of 32, 40, 48, 56, 64, 80, 96, 112, 128, 160, 192, 224, 256 and 320 kbit/second. There are a few others defined in the MPEG-2 standard, but they are also multiples of 2 (actually all multiples of 8 in the range 8 to 160 – see the table called “Bitrate Index” in the link above) .
Technically, there is nothing that limits the MP3 bitrate to a multiple of 2, since variable bitrate encoding can be used, or a custom bitrate can be achieved using some flags not used in the MPEG specification ( although this must be implemented manually). . In order for MP3 to be MPEG-compliant, and therefore compatible with most MP3 decoders, it must have a bitrate defined by the specification, so all CBR-encoded MP3 files have a bitrate of two.
Talking about some basic differences between VBR and CBR in mp3 files Part 3
CBR vs VBR
For VBR encoded mp3 files, since the bit rate of each frame is not fixed, the data size of each frame is arbitrary.
CBR vs VBR
Obviously, the size of the data reproduced per second is different. In this way, the duration of all the audio cannot be calculated with the above formula and other data fields are needed, which is one of the shortcomings of VBR technology: it is relatively difficult and complicated to calculate the duration of the audio.
There is another disadvantage of VBR technology. When playing an audio file, there will inevitably be an operation to jump to the position of the specified time to play (ie, the so-called seek operation). At this time, it is necessary to convert the time position of the target to the position of the file. Then jump to this file position offset to read and decode. If it is a download and play network playback mode, you must first calculate the position of the file during the search operation. Jump to this position and download a paragraph before continuing to play. . For CBR encoding, the conversion to file position offset is also very simple, using the following formula:
file position (byte) = target time position ( s ) * bitrate (kbps) * 1000/8 + id3v2 field size (if any)
But for VBR encoding, it is obviously impossible to use this formula to convert file position. The reason is also very simple: the bit rate of each frame is not fixed and the length of data per second is not average. Therefore, just like calculating duration, other data fields are needed.
The method to calculate the duration of the audio and implement the seek operation using VBR encoding
To solve the above two problems, VBR encoding adds some data fields. At present, there are mainly two types of VBR encoding technologies, one is the Xing specification proposed by the Xing Company, and the other is the VBRI specification of the Fraunhofer encoder. This article only presents how the Xing specification solves the audio duration computation and the implementation of the seek operation.
The main content of the Xing specification is the Xing header, which means that the first audio frame at the beginning of the VBR-encoded mp3 is not used to store specific audio data, but to store additional audio information. This information is marked with the four characters of “Xing” as the beginning of the field (some files also use the four characters of “Info” as the beginning of the Xing header).
The position of the Xing header in the first audio frame is after the standard 4-byte mp3 audio frame header. Between the table header and the Xing header, there will be a blank part with all 0 data content. This blank The length of the section is specified. After the decoder parses the frame header of the first audio frame, it skips the blank part of the specified length, and then judges whether the next content is the four characters of ‘Xing’ or ‘Info’ to judge the audio If the VBR encoding.
The length of the blank part is determined by the mpeg version and the number of channels, as shown in the following table (unit is byte):
non-mono infectious mononucleosis MPEG version
MPEG 1 18 32 (most common)
MPEG2 9 18
The following figure is an example of the field structure of the first VBR-encoded mp3 data frame:
The field structure and the content of the information stored in the Xing header are as follows:
Location (from marker ‘Xing’) longitude direction Example
0 4 VBR header tag, 4-byte ASCII characters, content is ‘Xing’ or ‘Info’ ‘Xing’
0 4 A flag indicating the specific content of the VBR header, the combination is logical OR. The area is
mandatory
. exists, excluding tags;
0x0004 – TOC index storage area set to exist;
0x0008 – Quality Indication Storage Area set to exist 0x0007 (meaning total number of frames, file length and TOC storage area are valid)
8 4 Stores the Big-Endian value of the total number of frames 7344
8 or 12 4 Stores the Big-Endian value of the file length, in Bytes 45000
8, 12, or 16 100 The TOC table, which is a byte array with a length of 100, is a positional index used for fast addressing in the file and is primarily used to resolve the implementation of the seek operation.
Talking about some basic differences between VBR and CBR in mp3 files Part 2
CBR vs VBR
The appearance of VBR encoding technology is to solve the problem of this waste of space.
CBR vs VBR
VBR technology selects the most suitable bit rate for each audio frame. For audio frames with a lower pitch, the bit rate will be lower and the data size will be smaller. If the pitch is higher, the bit rate will be higher. The size is larger. In this way, the storage space of the audio data can be saved and the size of the mp3 file can be further compressed without losing the audio quality.
The figure above briefly compares the differences in data content between CBR and VBR mp3 files. It can be seen that the bit rate of the VBR encoded mp3 is not necessarily the same due to the difference in data content between frames. Generally, VBR technology will compress and encode in the range of 8~320kbps, so the bit rate of the whole file is higher than that of the whole file. Constant CBR encoding, VBR encoding has a bit rate variable bit rate throughout the file, hence the name VBR (variable bit rate).
In addition to the two encodings CBR and VBR, there is also an ABR (Average Bit Rate, Average Bit Rate) type encoding, which is basically the same as CBR, most audio frames are encoded at the bit rate specified, but they will be The content is encoded with a higher bitrate than specified, but usually this content is short, so there is not much difference in file size compared to CBR, so this type is not common.
Disadvantages of VBR technology compared to CBR technology
Using VBR technology to encode and compress mp3 files can certainly optimize file size, but at the same time, it also brings some new problems in acquiring audio information and monitoring playback progress.
The first is the calculation of the duration of the audio. If it is CBR encoding, since the bit rate is constant, the data size of all audio frames is fixed, so the data size needed to decode for each second of playback is the same, so it is very simple to calculate the audio time length. Just use the following formula:
timelength ( s ) = (total file length (Byte) – total id3 field size (if present)) * 8 / (bitrate (kbps) * 1000 )
In the formula, the id3 field refers to the basic information field that is placed at the beginning or end of the mp3 file, and is generally used to record the audio file name, singer name, and album name. The id3 is divided into two versions, v1 and v2, and v1 only registers. The above three types of information, and the size is fixed, are usually placed at the end of the file; v2 is more flexible than v1, the type of the recorded information is not limited to the above three, and the size is not fixed, it is usually placed at the beginning of the file. The id3 field is an optional field, and the mp3 file doesn’t necessarily have it, so to calculate the audio time of the mp3, you must first read it to see if the id3 exists.
Talking about some basic differences between VBR and CBR in mp3 files
CBR vs VBR
From the perspective of bitrate encoding, one of the most common audio file formats, MP3, can be divided into two types: one is constant bitrate CBR (constant bitrate).
CBR vs VBR
The bit rate of a frame is constant and unique. ; the other is Variable Bit-Rate VBR, which is the opposite of CBR. The bit rate of each frame is not fixed. The bitrate may or may not be the same. Due to the existence of these two types, some jobs that need to be done when playing mp3 files, such as getting audio information and controlling playback progress, need to be handled separately.
Introduction to some basic concepts.
To clearly understand the specific differences between CBR and VBR, you need to understand an important attribute of audio files: bit rate, also known as bitrate or bit rate, refers to the number of bits transmitted per second. The unit is bps (bits per second). The higher the bit rate, the higher the data transmission speed. Bitrate in audio refers to the amount of binary data per unit of time after converting an analog sound signal to a digital sound signal, which is an indirect measure of audio quality.
The bitrate unit of audio files is generally kbps, 1 kbps = 1000 bps. The default bitrate of mp3 is 128kbps, but the mp3 downloaded from the net is more common at 192kbps, and if you want to get high definition mp3 with better sound quality, the bitrate usually reaches 320kbps. The higher the bitrate, the better the sound quality, but the more disk space it will take up.
In general, the higher the pitch of the sound clip, the more space it needs to store and the higher the bitrate. The traditional mp3 file is encoded with CBR, that is, the bit rate of each frame is the same, which brings a problem: if the bit rate of each frame is the same, then the data size of each frame it’s the same way, no matter the pitch of this frame is high or low, the storage space of the audio frame with the highest pitch in all audio is used to store this frame, but for the audio frame with low pitch, not much storage space is needed. This will result in a loss of storage space and will virtually increase the size of the mp3 file.
Generally, there are three mp3 bitrates namely VBR, ABR and CBR.
VBR, ABR and CBR.
1.1 RBC
CBR is short for Constant Bit Rate, which means Fixed Bit Rate in Chinese.
For a CBR MP3 song with a bitrate of 128kbps, the first 128kb of the song describes the sound of the first second, and the second 128kb describes the sound of the second second…if the song is finished, it will take 640 seconds , then the size of the song is 128kb × 640 = 80Mb = 10MB. The so-called 128kbps means 128kb per second.
If you are careful, you will find that the volume compressed by this encoding method will be very large, because the bit rate is fixed. Of course, the sound quality has some advantages over the other two, although this advantage may be minimal.
1.2VBR
Dynamic bit rate VBR (Variable Bitrate). That is, there is no fixed bitrate and the compression software determines on the fly which bitrate to use based on the audio data being compressed.
A simple understanding is that the bitrate will be relatively high at the time the song is rich in detail, and relatively low at other times, so sound quality and size are taken into account. For example: at the beginning of the song, a person sings alone, the sound is relatively simple, we use 64kb to describe the sound within one second; at the climax of the song, everyone sings, the sound is more complicated, we use 256kb to describe a second voice within the species.
1.3 APR
ABR (Average Bit Rate) Average Bit Rate is an interpolation parameter of VBR.
For example, when you specify 192kbps ABR to encode a wav file, Lame will use a fixed 192kbps encoding for 85% of the file, then dynamically optimize the remaining 15%: complex parts are encoded with more than 192kbps, simple parts are encoded with less than 192 kbps. Compared to CBR 192kbps, ABR 192kbps has a similar file size, but the sound quality is much better. ABR encoding is 2 to 3 times faster than VBR encoding and has better quality than CBR in the range of 128 to 256 kbps.
It can be used as a compromise between VBR and CBR. Under normal circumstances, files with this encoding method are rarely found.
For audio compression, such as MP3, the traditional CBR (Constant Bit Rate) is a static bit rate.
vbr
CBR accepts that the MP3 sampling rate is a fixed value. An MP3 is compressed with a fixed value like 128 KBit/s from start to finish. VBR adopts a new compression method with full dynamic adjustment technology to strike a balance between sound quality and file size. When VBR encodes an audio file, the program will try to maintain the selected quality of the entire file and will select different bit rates to encode different parts of the music file. When in the low range (simple part), VBR will automatically use a lower bit rate, such as 32 KBit/s, to compress the sound quality; when in the high range (complex part), it will use a higher bit rate, such as 224 KBit/s. s to compress sound quality Compression is performed; at the high end, use up to 320 KBit/s for compression. VBR MP3 maximizes MP3 sound quality while controlling file size. VBR automatically allocates fewer bits to the simple part of the content during encoding, leaving enough bits to generate high-quality complex parts. The output result of VBR encoding is of better quality than that of CBR encoding, and the encoded file is relatively small and suitable for storage. However, the disadvantage of VBR is that the amount of computation is large, the compression time is long, the size of the compressed file cannot be estimated during encoding, the requirements for the decoding algorithm are also much more complicated, and the Requirements for the hardware (such as the CPU) of the decoder are also high.
Differences between CBR and VBR:
(1) CBR: The size of the FRAME with a fixed bit rate is also fixed. As long as the total file length and frame length are known, the total mp3 playback time can be calculated from the 26 ms required to play each. frame, and can also be controlled by counting the number of frames. Fast forward, fast rewind, slow playback, etc.
(2) VBR: VBR is an algorithm released by XING company, so there will be the keyword “XING” in the FRAME of MP3 (many popular small software can also do VBR compression, whether they abide by this agreement, then it is impossible to Got it), it is stored in the first valid FRAME in the MP3 file, and identifies that the MP3 file is VBR. At the same time, the first FRAME stores the total number of FRAMEs in the MP3 file, making it easy to get the total playing time. At the same time, there are 100 bytes that store the INDEX of the 100 time-sliced FRAMEs. of the total playing time.
beginning
Edit
VBR is a variable encoding rate MP3 compression method. Its principle is to encode the complex part of a song with a high bit rate and the simple part with a low bit rate. Through this dynamic adjustment of the encoding rate, the sound quality can be improved. additionally obtained and the size of the file. Its main advantage is that the entire song can approximately meet our sound quality requirements, but the disadvantage is that the size of the compressed file cannot be estimated during encoding.
Most MP3 players released now support VBR, but although some machines can play songs in VBR format, they cannot display the playing time correctly (especially in car stereos). Many high-quality MP3s today are encoded with VBR. when purchasing, please pay attention to whether MP3 Walkman supports VBR effectively.
other acronyms
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VBR: Valid Business Reason
VBR: Vector Base Register
VBR: Vertebral Body Replacement
VBR: Virginia Blue Ridge Railroad
VBR – Variable Diameter Ram (Oil Drilling)
VBR: Basic Rule Violation (Oregon Traffic Division)
VBR: Kind regards
VBR: ventricle-brain ratio
VBR: Virgin Branson Richard
VBR: Visual Basic custom control file control file,
Visual Basic customization
VBR: VärnamoBygdens Ryttarförening VärnamoBygdens Ryttarförening
VBR: Visions come true
VBR: Vinnell, Brown and Root Vinnell, Brown, Root
VBR: Varsity Brands, Inc. Some brand companies
Bit rate refers to the number of bits (bits) transmitted per second. The unit is bps (Bit per second) The higher the bit rate, the faster the data transmission speed.
Mp3 bitrate
Bitrate in sound refers to the amount of binary data per unit of time after converting an analog sound signal to a digital sound signal, which is an indirect measure of audio quality.
Bitrate refers to the sampling rate at which digital sound is converted from analog to digital format. The higher the sampling rate, the better the quality of the restored sound. As a benchmark for the efficiency of digital music compression, bit rate indicates the rate of the number of bits bps (bit per second, bits per second) transmitted per unit of time (1 second). Kbps (in layman’s terms is 1000 bits per second) is usually used as the unit. The bit rate of digital music on CD is 1411.2 kbps (that is, to burn 1 second of CD music, 1411.2 × 1024 data bits are required), the high BIT RATE of the digital music file music means that it should be processed in a unit of time (1 second) The amount of data (BIT) is large, which means that the sound quality of the music file is good. However, when the BITRATE is high, the file size increases, which will take up a lot of memory capacity. The most commonly used bitrate for music files is 128 kbps, and MP3 files can generally use 8 to 320 kbps. In the same way, most of them are 32-256 Kbps. Of course, the wider the rate, the better, but 320 Kbps is the highest level at the moment.
Bitrate calculation formula
The basic algorithm is: [Bit rate] (kbps)=[file size] (bytes) X8/[time] (seconds)/1000
Special algorithm for audio files: [bit rate] (kbps) = [quantization sample point] (kHz) × [bit depth] (bit/sampling point) × [number of channels] (typically 2)
For example, the D5 drive has a capacity of 4.3G, which takes into account different audio formats, so it is calculated as 600M (so the remaining capacity is 4.3*1000-600=3700M), so the video file should not be larger than 3.7G, in this example, take The capacity of the video file is 3.446G, and the length of the video is 100 minutes (6000 seconds). The calculation result: the bit rate is approximately equal to 4933kbps.
What is a good bitrate guide for mp3 files? Part 2
Mp3 Bitrate
To produce high-quality MP3 files of classical and jazz music, the optimal bitrate depends on the characteristics of the song.
Mp3 Bitrate
Smooth jazz can usually be copied at 192kbps to create a good balance between file size and diminishing returns, although 256kbps may sound better in a home entertainment center. A classical orchestra should be 256kbps for a portable player, but if you want to burn a CD at home or in your car, a 320kbps file might be a better option.
For saturated music such as hard rock, metal, arena, pop, electronic and house music, 320 kbps will provide the best results. The higher the number of bits per second, the more complex acoustic envelope will be preserved.
If possible, it’s best to create MP3 files with variable bit rates. This allows the encoding program to determine if a particular frame of music requires the full bit rate. Otherwise, the program will reduce data retention for that frame, resulting in a smaller file without sacrificing quality. Forcing the program to “oversample” frames can produce artifacts.
While this article is intended as a general guide, he or she may be equally satisfied with a lower bitrate for a particular song or songs in general. Many factors affect our ability to judge the quality of music, not only the devices we use but also our activities while listening to it. For example, for those who listen to MP3 files while exercising or taking a walk, external noise can make it more difficult to tell the difference in quality. Conversely, audiophiles may prefer to sample at 320kbps, regardless of their equipment, type of music, or listening habits.
If you create your own MP3 files, there are other settings that affect quality. LAME is an excellent MP3 encoder that is free and has many graphical interfaces as the interface for this popular command line program. LAME allows users to adjust many settings to generate high-quality MP3 files in seconds. You can also experiment with various bitrates in your source file to find the best subjective balance between quality and file size.
MP3 files are compressed audio files created from audio formats such as wave (.wav). Wave files replicate analog recordings and digital sound files at the expense of large file size, while MP3 files sacrifice some quality for a smaller footprint. There are several factors that mitigate the quality sacrifice during the conversion process. With the correct bitrate and settings, MP3 files can provide very high quality results, making them very close to the original wave files when played on portable audio players.
An mp3 player.
The balance between file size and quality is somewhat subjective. For audiophiles, any difference is noticeable. Others may simply not be able to tell the difference between a high quality MP3 file and a raw wave source. In many cases, the nuances of the sound environment will only become clearer when played through a high-quality stereo system.
MP3s are compressed digital music files that sacrifice quality for file size.
MP3 files are primarily targeted at portable audio players. In this field, high-quality MP3 files are played with incredible sound due to their small file size. With the limited memory of portable players, it makes sense that one would want MP3 files to be as small as possible while maintaining the highest possible quality.
For this, one of the most important factors when creating MP3 files is the bit rate. In general, the more bits per second that are preserved from the original file, the higher the quality of the MP3 and the larger the file size. Lower bit rates reduce size and quality. The idea is to use the bitrate for maximum realism without saving unnecessary data, which just creates larger files with no noticeable difference to the ear.
For voice recordings such as lectures or language lessons saved to waveforms, a bit rate of 32 kilobits per second (kbps) is acceptable, although 64 kbps may offer better quality, depending on the source. At 32 kbps, the sound may sound “flat”, but that’s understandable. A 64 kbps MP3 file created from a voice recording should sound nearly identical to the original.
Desaturated acoustic music with simple arrangements should work fine at 192kbps bitrate. You can choose 256 kbps if the music will be played on a high quality device. Music that falls into this category includes folk, boy band songs, easy listening, and folk music. There are also works by many classic artists such as James Taylor, Linda Longstadt, Jonny Mitchell, and Simon Garfunkel.
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