Digital Sound & Music: Concepts, Applications, & Science, Chapter 5, last updated 6/25/2013
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Figure 5.25 Signal path in digital audio recording with direct monitoring
5.2.4 Word Clock
When transmitting audio signals between digital audio hardware devices, you
need to decide whether to transmit in digital or analog format. Transmitting in
analog involves performing analog-to-digital conversions coming into the
devices and digital-to-analog conversions coming out of the devices. As
described in Section 5.2.1, you pay for this strategy with increased latency in
your audio system. You may also pay for this in a loss of quality in your audio
signal as a result of multiple quantization errors and a loss of frequency range if
each digital device is using a different sampling rate. If you practice good gain
structure (essentially, controlling amplitude changes from one device to the
next) and keep all your sampling rates consistent, the loss of quality is minimal, but it is still
something to consider when using analog interconnects. (See Chapter 8 for more on setting gain
structure.)
Interconnecting these devices digitally can remove the latency and potential signal loss of
analog interconnects, but digital transmission introduces a new set of problems, such as timing.
There are several different digital audio transmission protocols, but all involve essentially the
same basic process in handling data streams. The signal is transmitted as a stream of small
blocks or frames of data containing the audio sample along with timing, channel information,
and error correction bits. These data blocks are a constant stream of bits moving down a cable.
The stream of bits is only meaningful when it gets split back up into the blocks containing the
sample data in the same way it was sent out. If the stream is split up in the wrong place, the data
block is invalid. To solve this problem, each digital device has a clock that runs at the speed of
the sampling rate defined for the audio stream. This clock is called a word clock. Every time the
clock ticks, the digital device grabs a new block of data sometimes called an audio word
from the audio stream. If the device receiving the digital audio stream has a word clock that is
running in sync with the word clock of the device sending the digital audio stream, each block
that is transmitted is received and interpreted correctly. If the word clock of the receiving devices
falls out of sync, it starts chopping up the blocks in the wrong place and the audio data will be
invalid.
Even the most expensive word clock circuit is imperfect. This imperfection is measured
in parts per million (ppm), and can be up to 50 ppm even in good quality equipment. At a
Flash
Tutorial:
Word Clock
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