Editing S/PDIF (section)

=== Data framing ===

S/PDIF is meant to be used for transmitting 20-bit audio data streams plus other related information. S/PDIF can also transport 24-bit samples by way of four extra bits; however, not all equipment supports this, and these extra bits may be ignored.

To transmit sources with less than 20 bits of sample accuracy, the superfluous bits will be set to zero, and the 4:1&ndash;3 bits (sample length) are set accordingly.

==== IEC 61937 encapsulation ====
IEC 61937 defines a way to transmit compressed, multi-channel data over S/PDIF.<ref>Digitalton - Schnittstelle für nichtlinear-PCM-codierte Audio-Bitströme unter Verwendung von IEC 60958 - Teil 1: Allgemeines (IEC 61937-1:2007 + A1:2011); Deutsche Fassung EN 61937-1:2007 + A1:2011</ref>

* The control word bit 0:1 is set to indicate the presence of non-linear-PCM data.
* The sample rate is set to maintain the needed symbol (data) rate. The symbol rate is usually 64 times the sample rate.
* Data is packed into blocks. Each data block is given a IEC 61937 preamble, containing two 16-bit sync words and indicating the state and identity (type, validity, bitstream number, length) of encapsulated data present. Padding is added to match full block size as required by timing.

A number of encodings are available over IEC 61937, including Dolby [[Dolby Digital|AC-3]]/[[E-AC-3]], [[Dolby TrueHD]], MP3, AAC, [[ATRAC]], [[DTS Coherent Acoustics|DTS]], and [[WMA Pro]].<ref>{{cite web |title=FFmpeg: libavformat/spdif.h File Reference |url=https://ffmpeg.org/doxygen/trunk/spdif_8h.html |website=ffmpeg.org}}</ref><ref>{{cite web |title=Representing Formats for IEC 61937 Transmissions - Win32 apps |url=https://learn.microsoft.com/en-us/windows/win32/coreaudio/representing-formats-for-iec-61937-transmissions |website=learn.microsoft.com |language=en-us |date=15 May 2023}}</ref>