bhaagensen;547680 Wrote:
NewBuyer: thanks for the comments and in particular pointing out one
source of the 'long-cable'-argument. Another source that I see quoted
from time to time, is this:
http://www.positive-feedback.com/Issue14/spdif.htm
Along with your comment I also note, in this particular context, that
Naim's own (recommended) digital coax cable is 1.25m...
You're welcome. :) A source for the opposing short-length digital
coax cable argument is Dan Lavry - here's an extract of one comment
from him, on this subject:
Dan Lavry Wrote:
...In fact, the shorter the cable, the better you are. I am not
suggesting to use 3 inches cables, but a 3 foot is better then 10 foot,
and at over 30 feet you are certainly asking for trouble.
You said the reason for keeping the length at least 2 feet had to do
with reflections. Reflections have to do with MORE LENGTH, not with
less length! Reflection becomes an issue when the cable becomes LONG,
making the signal propagation delay longer (the signal travel time from
the driver end of the cable to the destination end). What does
longer time mean? Longer with respect to the digital signal rise (and
fall) time.
A typical cable delay is around 1.5 nano second (nsec) per foot. The
velocity is slower then the speed of light, in the range of 1/3 to 2/3
of the speed of light, and it depends almost entirely on one factor -
the cable inner material isolation (the dielectric).
The rise time for the digital signal is between 5nsec and 30 nsec.
30nsec is slow but still within the specifications. 5-15 nsec is nice,
and the reason that faster is not allowed has to do with setting a
limit on the electromagnetic radiation (transmission of interference).
At say 10 feet, the cable delay is around 15 nsec, and a 5nsec rise
time is 3 times faster then the delay, so one DOES NEED to terminate
the cable and do so properly.
But at say 8 inches length, the delay is around 1nsec and even a fast
5nsec rise is 5 times slower then the cable delay, and the signal will
have virtually no reflections at all. The shorter the cable, the better
it is from reflections stand point as well as from many other
standpoints.
I am not suggesting 8 inch cables. I am not suggesting not terminating.
In fact, as a rule the termination is built into the receiver side. The
issue here is cable length, and the notion that there is a minimal
cable length one should keep is just plain wrong.
Regards
Dan Lavry
Lavry Engineering
--
NewBuyer
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