H
D
T
V
Audio
In the December ‘07 issue, we examined the various ways to hook
up pieces of your home entertainment system to your HDTV. We
specifically focused on the different video interfaces. We’ll continue
now with the choices for passing audio from one device to another.
by Jeff Mazur
Once again, the most common
connection by far is the standard
analog stereo pair using RCA
jacks and cables. With good quality
cable and connectors, this method can
provide excellent results. The most
common issue with analog audio connections is its susceptibility to picking
up hum and/or other extraneous
signals, especially from components
within your system (or perhaps from
the ham operator who lives next
door!). To solve this issue — as well as
complete the total conversion to binary
1s and 0s — there are three basic ways
to pass audio signals digitally between
devices: coax, optical, and HDMI.
S/PDIF (Sony/Philips
Digital Interconnect
Format)
Figures 2-4 are courtesy of Wikipedia, the
free encyclopedia (licensed to the public
under the GNU Free Documentation
License).
Named after the two companies
that developed this interface, S/PDIF
is a means to carry audio between
devices in a digital format. The signals
can be carried over
standard 75 ohm coaxial
cable using RCA jacks
(or BNC connectors in
professional equipment)
or via optical fiber
(glass or plastic, usually
terminated with F05
connectors). See Figure 1.
The optical connection — created by Toshiba
and also known as TOSLINK — uses 1
mm fiber terminated in a 5 mm
connector. While earlier cables were
restricted to less than 15 feet, you can
now buy high quality TOSLINK cables
up to 100 feet in length. TOSLINK can
carry data signals of up to 125 Mbits/s,
which allows for three audio channels.
However, it is usually used to carry a
single pair of stereo audio signals.
As an electrical signal, S/PDIF is
represented by a roughly 1V digital
pulse train using Biphase Mark Code
(BMC) to carry the audio data. While
no specific sampling rate or bit depth
is specified in the standard, audio is
usually carried as either 48 kHz (DAT)
or 44.1 kHz (CD) data with either 20
or 24 bit samples. We’ll describe the
actual data format in a moment.
HDMI
FIGURE 1. Digital audio
connections (top, coax
and bottom, optical).
We’ve already discussed the
HDMI interface that can carry digital
video between devices. HDMI also
includes support for up to eight
channels of uncompressed digital
audio at a 192 kHz sample rate with a
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February 2008