Sustainability of Digital Formats
Planning for Library of Congress Collections
Sustainability of Digital Formats
|
|
|
|
| Introduction | Sustainability Factors | Content Categories | Format Descriptions | Contact | |
| Full name | Linear Pulse Code Modulated Audio (LPCM) |
|---|---|
| Description | Pulse code modulation (PCM) with linear quantization. PCM is a digital representation of an analog signal where the magnitude of the signal is sampled regularly at uniform intervals, then quantized to a series of symbols in a digital (usually binary) code. PCM has been used in digital telephone systems and is also the standard form for digital audio on audio compact disks (Red Book format; see Notes below. |
| Production phase | Used at all stages in production and archiving workflows. |
| Relationship to other formats | |
| Subtype of | PCM, Pulse Code Modulated Audio |
| Used by | WAVE_LPCM, WAVE Audio File with LPCM Audio |
| Used by | WAVE_LPCM_BWF, Broadcast WAVE Audio File Format |
| Used by | AIFF_LPCM, AIFF File Format with LPCM Audio |
| Used by | AES3, AES3, Digital Audio Interface Format |
| LC experience or existing holdings | WAVE_LPCM is used as the best digital format for audio converted for American Memory and in preservation reformatting by the Motion Picture, Broadcasting, and Recorded Sound Division and the American Folklife Center. |
|---|---|
| LC preference | WAVE_LPCM_BWF or WAVE_LPCM. |
| Disclosure | Widely described and documented. |
|---|---|
| Documentation |
The history of PCM is varied and extends back to at least the 1930s. Documentation that is relevant to digital sound recording includes the following:
|
| Adoption | Very widely used for encoding bitstreams. Used on audio CDs, Digital Audio Tape (DAT). Default bitstream encoding for WAVE and AIFF. One of the bitstream encodings supported for sound on DVD-Video. Coders and decoders available as chips. |
| Licensing and patents | None. |
| Transparency |
Linear PCM is a simple, uncompressed representation comparable in transparency to uncompressed bit-mapped images. From Appendix 1.1 of the report of the EU-US Working Group on Spoken-Word Audio Collections [SWAG]: "Linear PCM data, irrespective of sampling rate, word length, method of packing data into bytes and left-to-right or right-to-left arrangement of bits and bytes, can be decoded by relatively simple trial-and-error, and we can expect this to be the case indefinitely. PCM is in this sense a 'natural' representation for audio, and has very good long-term prospects regardless of the remaining problems of format migration." |
| Self-documentation | Not applicable. |
| External dependencies | None. |
| Technical protection considerations | None. |
| Sound | |
|---|---|
| Normal rendering | Good support. |
| Fidelity (high audio resolution) | Linear PCM captures and encodes audio without lossy compression. High fidelity is supported by high sampling rates and high bit-depth (word length); some LPCM coders and decoders on the market today support sampling rates up to 192kHz and/or sample sizes up to 32 bits. File or media formats that use LPCM for bitstream encoding may constrain sampling rate or bit-depth. Audio CDs use 44.1 kHz sampling rate with 16-bit samples; DAT tape uses 48 kHz sampling and 16 bits. The Audio Engineering Society standard AES5 recommends sampling frequencies, as described in the notes for AES3. |
| Multiple channels | Not applicable; LPCM encodes a single channel of sound. Stereo and two-channel audio is supported by interleaving two LPCM streams; see AES3. Formats like AES31 and MXF (and others) provide a means for bundling and synchronizing multiple LPCM bitstreams. |
| Support for user-defined sounds, samples, and patches | Not applicable. |
| Functionality beyond normal rendering | Not applicable. |
| Tag | Value | Note |
|---|---|---|
| Filename extension | raw |
The extension normally depends upon file wrapper used. However, LPCM data may be stored in "raw" form, with ".raw" as a file extension. A variant of this encoding is captured by devices manufactured by Snell & Wilcox, reportedly carring an ".aes" extension; see also AES3. |
| General | The Wikipedia article on the Red Book standard (consulted September 25, 2007) reports: "Red Book is the standard for audio CDs (Compact Disc Digital Audio system, or CDDA). It is named after one of a set of color-bound books that contain the technical specifications for all CD and CD-ROM formats. The first edition of the Red Book was released in June 1980 by Philips and Sony; it was adopted by the Digital Audio Disc Committee and ratified as IEC 908. The standard is not freely available and must be licensed from Philips. At the time of writing, the cost [from] Philips is US$5,000. As of 2006, the IEC 908 document is now known as IEC 60908 and is also available as a PDF download for $210." |
|---|---|
| History |
|
|