CD audio is samples per second. The number of bits per sample also depends on the number of audio channels. CD is stereo and 16 bits per channel. So, multiplying by 32 gives —the bitrate of uncompressed CD digital audio.
As less complex passages are detected by MP3 algorithms then lower bitrates may be employed. By lowering the sampling rate, MPEG-2 layer III removes all frequencies above half the new sampling rate that may have been present in the source audio.
This is easily accomplished using LAME version 3. For more info see Nyquist — Shannon. A sample rate of A great variety of bit rates are used on the Internet. Uncompressed audio as stored on an audio-CD has a bit rate of 1, The software was only able to use a uniform bitrate on all frames in an MP3 file. Later more sophisticated MP3 encoders were able to use the bit reservoir to target an average bit rate selecting the encoding rate for each frame based on the complexity of the sound in that portion of the recording.
A more sophisticated MP3 encoder can produce variable bitrate audio. The final file size of a VBR encoding is less predictable than with constant bitrate. Average bitrate is a type of VBR implemented as a compromise between the two: Layer III audio can also use a "bit reservoir", a partially full frame's ability to hold part of the next frame's audio data, allowing temporary changes in effective bitrate, even in a constant bitrate stream.
The ancillary data is optional and the number of bits available is not explicitly given. ID3 and APEv2 tag A "tag" in an audio file is a section of the file that contains metadata such as the title, artist, album, track number or other information about the file's contents.
The MP3 standards do not define tag formats for MP3 files, nor is there a standard container format that would support metadata and obviate the need for tags.
However, several de facto standards for tag formats exist. These tags are normally embedded at the beginning or end of MP3 files, separate from the actual MP3 frame data.
MP3 decoders either extract information from the tags, or just treat them as ignorable, non-MP3 junk data. Aside from metadata pertaining to the audio content, tags may also be used for DRM. MP3Gain may be used to reversibly modify files based on ReplayGain measurements so that adjusted playback can be achieved on players without ReplayGain capability.
Licensing, ownership and legislation[ edit ] The basic MP3 decoding and encoding technology is patent-free in the European Union, all patents having expired there by at the latest. In the United States, the technology became substantially patent-free on 16 April see below. The majority of MP3 patents expired in the US between and In the past, many organizations have claimed ownership of patents related to MP3 decoding or encoding.
These claims led to a number of legal threats and actions from a variety of sources. As a result, uncertainty about which patents must be licensed in order to create MP3 products without committing patent infringement in countries that allow software patents was a common feature of the early stages of adoption of the technology. An exception is the United States, where patents in force but filed prior to 8 June expire after the later of 17 years from the issue date or 20 years from the priority date.
A lengthy patent prosecution process may result in a patent issuing much later than normally expected see submarine patents. The various MP3-related patents expired on dates ranging from to in the United States. Patent 5,, , which had a PCT filing in October , expired. Patent 6,, , held  and administered by Technicolor ,  expired.
As a result, many free and open-source software projects, such as the Fedora operating system , have decided to start shipping MP3 support by default, and users will no longer have to resort to installing unofficial packages maintained by third party software repositories for MP3 playback or encoding.
The letter claimed that unlicensed products "infringe the patent rights of Fraunhofer and Thomson. To make, sell or distribute products using the [MPEG Layer-3] standard and thus our patents, you need to obtain a license under these patents from us. The project's position was that as source code, LAME was simply a description of how an MP3 encoder could be implemented.
Unofficially, compiled binaries were available from other sources. Motorola followed soon after, and signed with Sisvel to license MP3-related patents in December The three exceptions are: Patent 5,, , expired February ; U. Patent 5,, , expired February ; and U. Patent 5,, , expired 9 April The injunction was later reversed by a Berlin judge,  but that reversal was in turn blocked the same day by another judge from the same court, "bringing the Patent Wild West to Germany" in the words of one commentator.
Apple, Samsung, and Sandisk all settled the claims against them in January In November , before the companies' merger, Alcatel sued Microsoft for allegedly infringing seven patents. Microsoft for more information.