What is the bit rate?
Bit rate refers to the data transfer rate (that is, how many bits are transmitted in a given time), generally expressed in bits per second.
Common units of bit rate are kilobits per second (Kbps) and megabits per second (Mbps). The term is also commonly used when talking about digital sampling and sample rates. For example, the MP3 audio compression algorithm is often configured to output files at a bit rate of 128 kbps. This means that the file contains an average of 128 kilobits for every second of audio (960 KB per minute). This is in contrast to CD audio, which is encoded as 44,100 16-bit stereo samples per second: 1411.2 kbps (16-bit x 44100 Hz x 2ch).
Often times, bytes are written in uppercase and are multipliers (for example, “KB” for kilobytes) and lowercase factors are bits (for example, “kb” for kilobytes). All modern computers use 8-bit bytes.
MP3 bit rate
The MP3 bit rate can be misleading. For example, an MP3 “constant bit rate” (CBR) of 128 kbps will use approximately 128 kilobits for every second of encoded audio (so the file size in bits divided by the length of the audio is approximately 128,000) , and headers, your frames will appear at regular intervals, but internally, from frame to frame, you can encode audio at bit rates higher or lower than 128 kbps by using a bit bucket (the ability of a frame to use bits of replacement of a previous block). However, the size of this bucket, and therefore the amount of variability, is limited, so 128 kbps will be very close to the effective bit rate throughout the file.
See also: 8D surround sound and how to do it
As another example, “128 kbps VBR MP3” is often incorrect, since the purpose of VBR is to allow each of the internal MP3 sectors to have its own bit rate. When people refer to the VBR MP3 bit rate, they are generally referring to the actual average bit rate of their frames. If the length of the encoded audio is known, then the “bit rate” can be the data size of the file divided by its duration, which will be fairly close to the same number. However, the length of an MP3 VBR cannot be accurately determined without scanning all the frames.
Let us remind you that when mixing and mastering, our engineers encode mp3s with various algorithms, and choose the best result so that the client gets the highest acceptable quality from their composition.
