Hello Colin,

Terry Zagar wrote in message ...

d) AES recommends limiting TOSlink cables to a maximum length of
around 15 feet (5 meters). A particular TOSlink transmitter in an
audio component may not be able to drive that length of plastic cable.
For example, some TOSlink capable transmitters will guarantee only
0.2 meters maximum transmission distance. If you need to go to
distances longer than 5 meters, a repeater is required. A good CAT-5
cable, on the other hand is good for up to around 320 feet (100
meters). Interestingly, a glass (not plastic) fiber optic cable using
100baseFX is good for up to around 6,400 feet (2,000 meters).


Colin wrote:

Well this sounds like a limitation. 0.2 meters design length. If
this is a common limitation then fibre optic cable quality could make
a difference. Going back to college theory, there would be
reflections at both ends of the cable too(which if large enough could
interfere with the original signal) and the type of plastic/glass and
finish at the end of the fibre would affect the amount of internal
reflections. For the length limitation, the opacity would have an
effect too.

Maybe this all could make a difference to signal transmission?

You certainly seem to know your digital communications theory.

What do you think?


Rereading my earlier post, I noticed that I should have said that "0.2
meters" was the "minimum guaranteed transmisson distance", and not the
"maximum". So it's not quite as bad as it might seem. My apologies
for any confusion this may have caused.

Despite this, I would still be more concerned about TOSlink
transmitter performance than I would the performance of the TOSlink
cable itself. Consider the specifications for the Toshiba TOTX176
TOSlink transmitter:

http://www.sc-elec.demon.co.uk/totx176.pdf

As I read this spec, this particular part can "potentially" drive an
optical signal to a maximum transmission distance of 5 to 10 meters
(15 to 30 feet) depending on its implementation. But note that the
minimum guaranteed spec for these parts is 0.2 meters (about 9
inches!). Assuming a given audio component uses this particular part,
how does one know if:

1) It has a minimum or maximum performance TOSlink transmitter inside?

2) The manufacturer of the component actually tested the performance
of individual TOSlink transmitters as opposed to just plugging in
whatever the distributor sent them?

3) In the case that the manufacturer actually tests each TOSlink
transmitter part, what is their part acceptance criteria (1 meter, 2
meters, ...)?

I don't believe that the typical consumer will have the answer to any
of these questions available to them. So the safest course of action
is to err on the side of using as short a length of TOSlink cable as
an interconnect as possible.

But if you happen to have a well performing TOSlink transmitter inside
the component, I don't see the TOSlink cable as an issue within the
AES length recommendation (5 meters or less). And I wouldn't worry
about reflections/opacity adversely impacting transmission performance
if one follows this AES recommendation.

Cheers,

Terry