4.9 Data-Reduced Audio Over Standard Two-Channel Interfaces

4.9 Data-Reduced Audio Over Standard Two-Channel Interfaces

4.9.1 General Principles

The standard two-channel interface was originally designed for linear PCM audio samples but in recent years there has been increasing use of data-reduced audio coding systems such as Dolby Digital (AC-3), DTS and MPEG. Because consumer systems in particular needed the ability to transfer such signals digitally, the non-audio mode of the interface has been adapted to the purpose. This is described in a relatively new IEC standard numbered 61937 31 . In addition to a general specification detailing the principles it also has a number of parts that describe the handling of specific data-reduced audio formats, some of which are not yet finalized at the time of writing. A similar but not identical SMPTE standard (337M) 32 describes professional non-audio applications of the interface, including its use for carrying Dolby E data (see below). SMPTE 338M and 339M specify data types to be used with this standard. The SMPTE standard is also more generic than the IEC standard, designed to deal with a variety of data uses of the interface, not just low-bit rate audio. It can also carry time-stamp data in the form of SMPTE 12M timecode. The reader is referred to the standards for more precise details regarding implementation of specific formats.

In both SMPTE and IEC versions the low-bit rate audio data is carried in bursts in place of the normal linear PCM audio information, with bit 1 of channel status set to the 'other uses' or 'non-audio' state. (The SMPTE standard makes it clear, though, that professional devices should not rely on this. Some digital video tape recorders , for example, cannot control the non-audio bit yet it is desirable that they should receive/transmit and store low-bit rate audio such as Dolby E.) Sometimes the validity bit is also set to indicate 'invalid' or 'unsuitable for conversion to analog', as a further measure. In the IEC standard the data is carried in the 16 most significant bits of the audio data slot, from bits 12 to 27 in IEC nomenclature . In the SMPTE standard it is possible for the data to occupy 16, 20 or 24 bits. In the IEC standard the two interface subframes are treated as conveying a single data stream whereas in the SMPTE standard the subframes can be handled together or separately (for example, one subframe could carry PCM and the other data-reduced audio).

A data burst of low-bit rate audio (representing the encoded frame of a number of original PCM samples) typically occupies a number of consecutive subframes, the last subframe of the burst being packed with zeros if required. Each burst is preceded by a preamble of four 16-bit words, the first two of which are a synchronization pattern, the third of which indicates the mode of data being carried and the fourth of which indicates the length of the burst, as shown in Figure 4.23. In the IEC standard up to eight independent bitstreams can be carried in this way, each being identified in the third byte of the preamble. The SMPTE standard can carry up to 14 independent streams in the independent subframe mode and the data type preambles are not identical to IEC 61937. Because the total data rate may be lower than that required for linear PCM audio, packing zero bits can be used between bursts of low-bit rate audio data and there is a requirement for at least four subframes to have bits 1227 set to zero every 4096 frames .

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Figure 4.23: Format of the data burst in IEC 61937.

4.9.2 Data-Reduced Consumer Formats

A number of parts of IEC 61937 describe the transmission of audio signals encoded to different standards, some of which are manufactureror system-specific and others are internationally standardized. Data-reduced bitstreams of current relevance here are AC-3 (Dolby Digital), DTS (Digital Theatre Systems), MPEG 1 and 2-BC, MPEG 2-AAC and Sony ATRAC. The most commonly encountered applications in consumer systems at the moment are for the transfer of encoded multichannel surround sound data from DVD players to home cinema systems, using either Dolby Digital or DTS encoding. The digital output of DVD players is typically an IEC 60958 interface on a phono connector or optical interface that can be used to carry 5.1-channel surround sound data for decoding and D/A conversion in a separate surround sound processor and amplifier .

4.9.3 Data-Reduced Professional Formats

The SMPTE 337M standard allows a number of data-reduced audio formats to be transmitted over the interface. The most commonly used of these are Dolby AC-3 and Dolby E, but data types are also specified for MPEG 1 and 2.

Dolby E is a data reduction system designed for professional purposes, using mild data reduction in order to minimize generation losses. It was introduced to satisfy a need to transfer production multichannel surround sound signals over two-channel media such as digital interfaces and video tape recorder audio tracks, in order to ease the transition from two-channel operations to 5.1-channel operations in broadcasting and post-production environments. It packs the audio data into the two-channel frame in a similar way to that described in the previous section. The resolution can be adapted to fit 16-, 20- or 24-bit media, the most common implementation using 20-bit frame format mode (both sub-frames used together) at a data rate, including overheads, of about 1.92 Mbit/s. (The 16- and 24-bit modes run at data rates of 1.536 and 2.304 Mbit/s respectively.) Dolby E packets are aligned with video frames so that the audio can be switched or edited synchronously with video. For example, there are 25 Dolby E packets per second when synchronized with 25 fps video.



Digital Interface Handbook
Digital Interface Handbook, Third Edition
ISBN: 0240519094
EAN: 2147483647
Year: 2004
Pages: 120

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