| For PCM encoded (uncompressed) audio data, there are a few
different cabling standards you'll see mentioned:
These actually send the same
information (stereo, uncompressed) through different kinds of cables --
optical, coaxial, or XLR/balanced. Assuming the cables are of good
quality and not too long, there will be no quality difference between
the different options, but you need to have the same input on the
receiving side as the output on the sending side.
Now, through this same digital cables, can be sent something
which is NOT linear, PCM, "uncompressed" audio. Most common is the
Dolby Digital 5.1 format, which compresses the bits of the sound to use
the same bandwidth as uncompressed audio, but fits six channels in the
same bandwidth (Front, Center, Right, RearLeft, RearRight and LFE).
Another format that can be sent through the same digital cables is
DTS. So, in the simplest of worlds, the digital formats
that come in through these cables is either PCM (which is uncompressed,
stereo), from a source such as a CD player, or DD (compressed, 5.1)
from a DVD. In either case, the receiver needs to decode
the signal. In the PCM case, the receiver forwards the bits from the
wire, to a stereo DAC (digital-to-analog converter), which puts out an
actual audio signal on your pre-outs and/or your speaker outs. In the
DD case, the receiver first has to do some computation on the
compressed bits to de-compress them, and then send the six separate
channels to a six-channel DAC for conversion to surround outs. There is
typically some other processing going on before converting to analog,
such as volume control, tone controls, etc. Now, HDMI comes
in and confuses things a bit. HDMI can support more than two
uncompressed channels in the cable. Thus, a HD-DVD or BR player can
decode a 7.1 signal, into separate PCM streams, and send across the
HDMI cable. The receiver then forwards those bits to 8 separate DACs.
However, HDMI can also forward a variety of compressed digital formats,
such as DD-Ex, DTS-ES, etc. Again, the receiver must do more processing
on those compressed signals to turn them into decoded (but still
digital signals), and then send the decoded signals to the DACs.
Different receivers have different amounts of decoding capability.
A basic home theater receiver may do only PCM (uncompressed) and DD
(compressed dolby digital). A fancy HDMI 1.3 receiver may do something
close to 20 different formats. In general, the media that's played
makes sure to include at least one of the two base formats (DD 5.1 or
PCM stereo) so that everyone can at least hear something, even if they
don't have the fanciest receiver. If your playback device
has settings for what to output, set it to output a compressed format
that your receiver knows how to decode, if you want multi-channel audio
over TOSlink or S/PDIF, else you'll only get stereo PCM. If you're
using HDMI, the format is usually smart enough to figure out what the
best option is to send data between the player and the receiver, so
that you get the highest quality with the most channels that your
system can deliver.
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