Digital Audio Interfaces 571difference between the AES/EBU interface data format and the SPDIF data format is
at its most signifi cant. The channel status bits in both the AES/EBU format and SPDIF
format communicate to the receiving device such important parameters as sample
rate, whether frequency preemphasis was used on the recording, and so on. Channel-
status data are normally the most troublesome aspect of practical interfacing using the
SPDIF and AES/EBU interface, especially where users attempt to mix the two interface
standards. This is because the usage of channel status in consumer and professional
equipment is almost entirely different. It must be understood that the AES/EBU interface
and the SPDIF interface are thus strictly incompatible in data format terms and the only
correct way to transfer data from SPDIF to AES/EBU and AES/EBU to SPDIF is through
a properly designed format converter that will decode and recode digital audio data to the
appropriate standard.
Other features of the data format remain pretty constant across the two interface
standards. The validity bit, labeled V in Figure 18.3 , is set to 0 every subframes if the
signal over the link is suitable for conversion to an analogue signal. The user bit, labeled
U, has a multiplicity of uses defi ned by particular users and manufacturers. It is used
most often over the domestic SPDIF interface. The parity bit, labeled P, is set such that
the number of ones in a subframe is always even. It may be used to detect individual bit
errors but not conceal them.
It’s important to point out that both the AES/EBU interface and its SPDIF brother are
designed to be used in an error-free environment. Errors are not expected over digital
links and there is no way of correcting for them.
18.1.4 Practical Digital Audio Interface
There are many ways of constructing a digital audio interface, and variations abound
from different manufacturers. Probably the simplest consists of an HC family inverter
IC, biased at its midpoint with a feedback resistor and protected with diodes across the
input to prevent damage from static or overvoltage conditions. (About the only real merit
of this circuit is simplicity!) Transformer coupling is infi nitely preferred. Happily, while
analogue audio transformers are complex and expensive items, digital audio—containing
no DC component and very little low-frequency component—can be coupled via
transformers, which are tiny and inexpensive! So, it represents a false economy indeed
to omit them in the design of digital interfaces. Data-bus isolators manufactured by
Newport are very suitable. Two or four transformers are contained within one IC-style