LPCM
Encyclopedia
Linear pulse-code modulation (LPCM) is a method of encoding audio information digitally. The term also refers collectively to formats using this method of encoding. The term pulse-code modulation
(PCM), though strictly more general, is often used to describe data encoded as LPCM.
as a sequence of amplitude values recorded at a sequence of times. LPCM is PCM with linear quantization
.
LPCM represents sample amplitudes on a linear scale. LPCM specifies that the values stored are proportional to the amplitudes, rather than representing say the logarithm of the amplitude (e.g., companding
– A-law/u-law), or being related in some other manner (e.g., DPCM
or ADPCM). In practice these values will be quantized.
LPCM audio is coded using a combination of various parameters – such as resolution/sample size (e.g., 8, 16, 20, 24 bit, etc), frequency/sample rate
(e.g., 8,000, 11,025, 16,000, 22,050, 24,000, 32,000, 44,100, and 48,000 Hz / "samples per second", etc), sign
(signed or unsigned), number of channels (monaural, stereo
, quadrophonic, etc) and interleaving of channels, byte order (little endian, big endian). If the sample is 16-bit signed, the sample range is from −32768 to 32767, with a centerpoint of 0 (for example, signed LPCM data is used on Audio CD, DVD Video, 16-bit LPCM in WAV, audio/L16, etc.) If the sample is 16-bit unsigned, the sample range is from 0 to 65,535, with a centerpoint of 32,768.
(used also on Super Audio CD
).
LPCM encodes a single sound channel. Support for multichannel audio depends on file format and relies on interweaving or synchronization of LPCM streams. While two channels (stereo) is the most common format, some can support up to 8 audio channels (7.1 surround).
Common sampling frequencies are 48 kHz
as used with DVD
format videos, or 44.1 kHz as used in Compact disc
s. Sampling frequencies of 96 kHz or 192 kHz can be used on some newer equipment, with the higher value equating to 6.144 megabit per second for two channels at 16-bit per sample value. The bitrate limit for LPCM audio on DVD-Video is also 6.144 Mbit/s, allowing 8 channels (7.1 surround) × 48 kHz × 16-bit per sample = 6,144 kbit/s.
standard supports 192 kHz/24-bit playback.
Pulse-code modulation
Pulse-code modulation is a method used to digitally represent sampled analog signals. It is the standard form for digital audio in computers and various Blu-ray, Compact Disc and DVD formats, as well as other uses such as digital telephone systems...
(PCM), though strictly more general, is often used to describe data encoded as LPCM.
Description
LPCM is a particular method of pulse-code modulation which represents an audio waveformWaveform
Waveform means the shape and form of a signal such as a wave moving in a physical medium or an abstract representation.In many cases the medium in which the wave is being propagated does not permit a direct visual image of the form. In these cases, the term 'waveform' refers to the shape of a graph...
as a sequence of amplitude values recorded at a sequence of times. LPCM is PCM with linear quantization
Quantization (sound processing)
In signal processing and digital audio, quantization is the process of approximating a continuous range of values by a relatively small set of discrete symbols or integer values...
.
LPCM represents sample amplitudes on a linear scale. LPCM specifies that the values stored are proportional to the amplitudes, rather than representing say the logarithm of the amplitude (e.g., companding
Companding
In telecommunication, signal processing, and thermodynamics, companding is a method of mitigating the detrimental effects of a channel with limited dynamic range...
– A-law/u-law), or being related in some other manner (e.g., DPCM
DPCM
Differential pulse-code modulation is a signal encoder that uses the baseline of pulse-code modulation but adds some functionalities based on the prediction of the samples of the signal...
or ADPCM). In practice these values will be quantized.
LPCM audio is coded using a combination of various parameters – such as resolution/sample size (e.g., 8, 16, 20, 24 bit, etc), frequency/sample rate
Sampling rate
The sampling rate, sample rate, or sampling frequency defines the number of samples per unit of time taken from a continuous signal to make a discrete signal. For time-domain signals, the unit for sampling rate is hertz , sometimes noted as Sa/s...
(e.g., 8,000, 11,025, 16,000, 22,050, 24,000, 32,000, 44,100, and 48,000 Hz / "samples per second", etc), sign
Signedness
In computing, signedness is a property of data types representing numbers in computer programs. A numeric variable is signed if it can represent both positive and negative numbers, and unsigned if it can only represent non-negative numbers .As signed numbers can represent negative numbers, they...
(signed or unsigned), number of channels (monaural, stereo
STEREO
STEREO is a solar observation mission. Two nearly identical spacecraft were launched into orbits that cause them to respectively pull farther ahead of and fall gradually behind the Earth...
, quadrophonic, etc) and interleaving of channels, byte order (little endian, big endian). If the sample is 16-bit signed, the sample range is from −32768 to 32767, with a centerpoint of 0 (for example, signed LPCM data is used on Audio CD, DVD Video, 16-bit LPCM in WAV, audio/L16, etc.) If the sample is 16-bit unsigned, the sample range is from 0 to 65,535, with a centerpoint of 32,768.
Implementations
LPCM is the method of encoding generally used for uncompressed audio, although there are other methods such as pulse-density modulationPulse-density modulation
Pulse-density modulation, or PDM, is a form of modulation used to represent an analog signal with digital data. In a PDM signal, specific amplitude values are not encoded into pulses of different size as they would be in PCM. Instead, it is the relative density of the pulses that corresponds to...
(used also on Super Audio CD
Super Audio CD
Super Audio CD is a high-resolution, read-only optical disc for audio storage. Sony and Philips Electronics jointly developed the technology, and publicized it in 1999. It is designated as the Scarlet Book standard. Sony and Philips previously collaborated to define the Compact Disc standard...
).
- LPCM is used for the lossless encoding of audio data in the Compact disc Red Book standardRed Book (audio CD standard)Red Book is the standard for audio CDs . It is named after one of the Rainbow Books, a series of books that contain the technical specifications for all CD and CD-ROM formats.The first edition of the Red Book was released in 1980 by Philips and Sony; it was adopted by the Digital Audio Disc...
(informally also known as Audio CD), introduced in 1982. - AES3 (specified in 1985) is a particular format using LPCM.
- On PCs, the term PCM and LPCM often refer to the format used in WAV (defined in 1991) and AIFFAIFFAudio Interchange File Format is an audio file format standard used for storing sound data for personal computers and other electronic audio devices...
audio container formats (defined in 1988). LPCM data may also be stored in other formats such as AUAu file formatThe Au file format is a simple audio file format introduced by Sun Microsystems. The format was common on NeXT systems and on early Web pages. Originally it was headerless, being simply 8-bit µ-law-encoded data at an 8000 Hz sample rate. Hardware from other vendors often used sample rates as...
, raw audio formatRaw audio formatRAW Audio format or just RAW Audio is a computer format for storing uncompressed audio in raw form. Comparable to WAV or AIFF in size, RAW Audio file does not include any header information . Data can be written in PCM, IEEE 754 or ASCII.-Extensions:The most typical extension is .raw or .pcm...
(header-less file) and various multimedia container formats. - LPCM has been defined as a part of the DVDDVDA DVD is an optical disc storage media format, invented and developed by Philips, Sony, Toshiba, and Panasonic in 1995. DVDs offer higher storage capacity than Compact Discs while having the same dimensions....
(since 1995) and Blu-rayBlu-ray DiscBlu-ray Disc is an optical disc storage medium designed to supersede the DVD format. The plastic disc is 120 mm in diameter and 1.2 mm thick, the same size as DVDs and CDs. Blu-ray Discs contain 25 GB per layer, with dual layer discs being the norm for feature-length video discs...
(since 2006) standards. It is also defined as a part of various digital video and audio storage formats (e.g. DVDVDV is a format for the digital recording and playing back of digital video. The DV codec was launched in 1995 with joint efforts of leading producers of video camcorders....
since 1995, AVCHDAVCHDAVCHD is a file-based format for the digital recording and playback of high-definition video....
since 2006). - Linear pulse-code modulation is used by HDMIHDMIHDMI is a compact audio/video interface for transmitting uncompressed digital data. It is a digital alternative to consumer analog standards, such as radio frequency coaxial cable, composite video, S-Video, SCART, component video, D-Terminal, or VGA...
(defined in 2002), a single-cable digital audio/video connector interface for transmitting uncompressed digital data. - RF64RF64RF64 is a BWF-compatible multichannel file format enabling file sizes to exceed 4 GB. It has been specified by the European Broadcasting Union.The file format is designed to meet the requirements for multichannel sound in broadcasting and audio...
container format (defined in 2007) uses LPCM and also allows non-PCM bitstream storage: various compression formats contained in the RF64 file as data bursts (Dolby E, Dolby AC3, DTS, MPEG-1/MPEG-2 Audio) can be "disguised" as PCM linear.
Standard sampling resolutions and rates
Common sample resolutions for LPCM are 8, 16, 20 or 24 bits per sample.LPCM encodes a single sound channel. Support for multichannel audio depends on file format and relies on interweaving or synchronization of LPCM streams. While two channels (stereo) is the most common format, some can support up to 8 audio channels (7.1 surround).
Common sampling frequencies are 48 kHz
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....
as used with DVD
DVD
A DVD is an optical disc storage media format, invented and developed by Philips, Sony, Toshiba, and Panasonic in 1995. DVDs offer higher storage capacity than Compact Discs while having the same dimensions....
format videos, or 44.1 kHz as used in Compact disc
Compact Disc
The Compact Disc is an optical disc used to store digital data. It was originally developed to store and playback sound recordings exclusively, but later expanded to encompass data storage , write-once audio and data storage , rewritable media , Video Compact Discs , Super Video Compact Discs ,...
s. Sampling frequencies of 96 kHz or 192 kHz can be used on some newer equipment, with the higher value equating to 6.144 megabit per second for two channels at 16-bit per sample value. The bitrate limit for LPCM audio on DVD-Video is also 6.144 Mbit/s, allowing 8 channels (7.1 surround) × 48 kHz × 16-bit per sample = 6,144 kbit/s.
DVD standards
Older DVD players only support 48 kHz/16-bit capability. Recent players have built-in 96 kHz/24-bit capabilities. The DVD-AudioDVD-Audio
DVD-Audio is a digital format for delivering high-fidelity audio content on a DVD. DVD-Audio is not intended to be a video delivery format and is not the same as video DVDs containing concert films or music videos....
standard supports 192 kHz/24-bit playback.
External links
- Summary of LPCM – Contains links to information about implementations and their specifications.
- How to control internal/external hardware using Microsoft's Media Control Interface – Contains information about, and specifications for the implementation of LPCM used in WAV files.
- RFC 4856 – Media Type Registration of Payload Formats in the RTP Profile for Audio and Video Conferences – audio/L8 and audio/L16 (March 2007)
- RFC 3190 – RTP Payload Format for 12-bit DAT Audio and 20- and 24-bit Linear Sampled Audio (January 2002)
- RFC 3551 – RTP Profile for Audio and Video Conferences with Minimal Control – L8 and L16 (July 2003)