IEC 60958-1 :2021 pdf download – Digital audio interface – Part 1: General.
4 Interface format 4.1 Structure of format 4.1 .1 Sub-frame format Each sub-frame is divided into 32 time slots, numbered from 0 to 31 (see Figure 1 ). Time slots 0 to 3 (preambles) carry one of the three permitted preambles (see 4.1 .2 and 4.3; also see Figure 2). Time slots 4 to 27 (main data field) carry the audio sample word in linear 2’s complement representation. The most significant bit (MSB) is carried by time slot 27. When a 24-bit coding range is used, the LSB is in time slot 4 (see Figure 1 ). When a 20-bit coding range is used, time slots 8 to 27 carry the audio sample word with the LSB in time slot 8. Time slots 4 to 7 may be used for other applications. Under these circumstances, the bits in the time slots 4 to 7 are designated auxiliary sample bits (see Figure 1 ). If the source provides fewer bits than the interface allows (either 20 or 24), the unused LSBs are set to a logical “0”. For a non-linear PCM audio application or a data application the main data field may carry any other information. Time slot 28 (validity bit) carries the validity bit associated with the main data field (see 4.4). Time slot 29 (user data bit) carries 1 bit of the user data channel associated with the main data field channel transmitted in the same sub-frame. For the applications, refer to the other parts of IEC 60958. Time slot 30 (channel status bit) carries 1 bit of the channel status information associated with the main data field channel transmitted in the same sub-frame. For details refer to the other parts of IEC 60958.
4.1 .2 Frame format A frame is uniquely composed of two sub-frames (see Figure 2). For linear coded audio applications, the rate of transmission of frames normally corresponds exactly to the source sampling frequency. In 2-channel operation mode, the samples taken from both channels are transmitted by time multiplexing in consecutive sub-frames. The first sub-frame (left or “A” channel in stereophonic operation and primary channel in monophonic operation) normally starts with preamble “M”. However, the preamble changes to preamble “B” once every 1 92 frames to identify the start of the block structure used to organize the channel status information. The second sub-frame (right or “B” channel in stereophonic operation and secondary channel in monophonic operation) always starts with preamble “W”. In single channel operation mode in a professional application, the frame format is the same as in the 2-channel mode. Data is carried in the first sub-frame and may be duplicated in the second sub-frame. If the second sub-frame is not carrying duplicate data, then time slot 28, (validity flag) shall be set to logical “1 “. NOTE For historical reasons preambles “B”, “M” and “W” are, for use in professional applications, referred to as “Z”, “X” and “Y”, respectively. Annex C describes the relation of the IEC 60958 series families based on the frame format.
4.4 Validity bit The validity bit is logical “0” if the information in the main data field is reliable, and it is logical “1 ” if it is not. There is no default state for the validity bit. NOTE For transmissions not using a linear PCM coding, this bit can be set. This is intended to prevent accidental decoding of non-audio data to analogue before a complete channel status block is received. See Annex A. 5 Channel status 5.1 General For every sub-frame, the channel status provides information related to the data carried in the main data field of that same sub-frame. Channel status information is organised in a 1 92-bit block, subdivided into 24 bytes. The first bit of each block is carried in the frame with preamble “B”. The channel status data format is defined in Table 2. The specific organisation depends on the application. In the descriptions, the suffix “0” designates the first byte or bit. Where channel status bits are combined to form non-binary values, the least significant bit should be transmitted first, unless otherwise indicated. 5.2 Applications The primary application is indicated by the first channel status bit (bit 0) of a block as defined in 5.3. For professional applications, refer to IEC 60958-4. For consumer applications, refer to IEC 60958-3 and IEC 60958-5. Secondary applications may be defined within the framework of these primary applications. Application documents or specifications are listed in Annex B.