Audio Engineering

Digital Audio Interfaces 569

defi ned as 1 V pk-pk when unterminated. Transformer coupling is by no means always
used with this interface but it is recommended on at least one end. Figure 18.2 is a
drawing of a common implementation of the SPDIF interface.

18.1.3 Data

Despite the very considerable electrical differences between the AES/EBU interface and
the SPDIF interface, their data formats are very similar. Both interfaces have capacity
for the real-time communication of 20 bits of stereo audio information at sampling rates
between 32 and 48 kHz, as well as provision for extra information, which may indicate to
the receiving device various important parameters about the data being transferred (such
as whether preemphasis was used on the original analogue signal prior to digitization).
There is also a small overhead for limited error checking and for synchronization.

Some of the earlier digital–audio interfaces such as Sony’s SDIF and the Mitsubishi
interface sent digital audio data and synchronizing data clocks on separate wires. Such
standards obviously require multicore cable and multiway connectors, which looked
completely different from any analogue interface that had gone before. The intention
of the designers of the AES/EBU and SPDIF interfaces was to create standards that
created as little “ culture shock ” as possible in both the professional and the consumer
markets and therefore they chose connector styles that were both readily available and
operationally convenient. This obviously ruled out the use of multicore and multiway
connectors and resulted in the use of a digital coding scheme that buries the digital
synchronizing signals in with the data signal. Such a code is known as “ serial and self-
clocking. ” The type of code adopted for AES/EBU and SPDIF is biphase mark coding.
This scheme is sometimes known as Manchester code and it is the same type of self-
clocking, serial code used for SMPTE and EBU time code. Put at its simplest, such a
code represents the “ ones and noughts ” of a digital signal by two different frequencies
where frequency Fn represents a zero and 2 Fn represents a one. Such a signal eliminates
almost all DC content, enabling it to be transformer coupled, and also allows for phase

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Figure 18.2 : SPDIF interface.