Appendix 4: Digital audio interface format (S/PDIF-IEC958)

An interface format is designed for use between digital audio sets. This format was originally defined jointly by Sony and Philips (Sony/Philips Digital Interface Format – S/PDIF), but has become an international standard.

This format allows digital connection of digital audio sets, such as CD players, DAT players and others, thus excluding any quality loss related to multiple D/A–A/D conversion.

The transmission method can be either standard (coax cable) or optical.

In the S/PDIF format, both left and right channels are transmitted on one line, they are multiplexed.

Note that the SCMS is included in this S/PDIF format in order to avoid illegal copying. Figure A4.1 shows the S/PDIF data format.

Each subframe will give full information on one audio sample. If the audio sample is less than 20 bits, the remaining bits are meaningless. Two subframes make one frame, giving a left and right channel sample.

192 frames make one block, the channel status bits of one block, i.e., 192 bits taken from each channel bit in each subframe give more information about type of audio, sampling rate, etc.

Figure A4.1 S/PDIF data format.

Channel coding

For transmission of the S/PDIF format, a channel coding/ bi-phase modulation type was chosen to ensure proper clocking. If the input data is ‘1’, there is a transient in the middle of the bit; if the input data is ‘0’, there is no transient in the middle of the bit; at the end of each bit there will also be a transient. Figure A4.2 shows the channel coding.

Figure A4.2 Channel coding.

The preambles of each subframe are encoded in a unique way, as shown in Figure A4.3.

Figure A4.3 Preamble coding.