Very good explanation. Thank you.

-Bill

"Midlant" wrote in message
news:v_fsb.20629$oB3.15068@lakeread03...
Mr Lampen of Belden Electronics Division answered this posts on
"rec.audio.high-end" some time back. I hope this answers your questions
in regards to cable quality and length of runs whether interconnect or
speaker.
John


In article , Lou Anschuetz

writes:

OTOH, I can understand how he comes to the conclusions he
comes to. As someone with 25 years in IT, I have thrown away dozens
of supposedly competently designed SCSI/serial
cables that don't work. SCSI cables in particular are darn
hard to to get right for the newer higher speeds - and it
all seems to come down to how the cable(s) is insulated.

I also agree that this doesn't necessarily have anything to do
with audio transmission. However, IMHO, this business about
insulation continues to bedevil me. Since it does play havoc
with digital signal transmission, it seems to me hard to leap
to an absolute conclusion that it has no effect in the analog
audio world. No hard science on this yet, but it is a very active
field of research...

Well, I'm on a plane flying from Beijing to Hong Kong (and on to
Melbourne,
Australia) listening to Chinese Pop music in my headphones, and nothing
to do,
so I thought I would just jump in here!

Why do some cables work on signal X but not with signal Y? The answer
is
simple: wavelength. (The answer is simple, the explanation is a little
more
complicated.) Every signal, data, audio, video, etc, etc. occupies one
or more
frequencies. Each frequency has a wavelength, that is, there is an
electromagnetic wave moving down the cable that has a specific length.
This
can be calculated by dividing 300,000,000 by that frequency. (The
answer comes
out in meters, so us backwards USA types have to multiply by 3.28 to get
to
feet.)

If you look at 20 kHz (or you can pick any frequency you want), that
gives you
a wavelength of 15,000 meters (about 9 miles). What that means is that
the
impedance of the cable (how it reacts to frequencies) is of no
consequence.
Most engineers agree that the ACTUAL critical distance is 1/4 wavelength
which
is, in this case 2.25 miles, still too far to have any effect. This
distance is
also affect by the plastic around the conductors. This affects the
speed
"velocity" of the signal. (Signals only move at the speed of light in a
vaccum,
i.e. outer space. If you ever have a chance to check out your speakers
there,
be sure and give me a complete test report!)

Let's say your interconnect cables were the worst ever made (a "velocity
of
propagation" of 50%). You would multiply the wavelength by 50% so we're
down
to 1.12 miles. Anybody with one mile speaker cables? data cable? mic
cables?
video cables? audio cable? Well, yes, there are people with AUDIO
cables of
that length or more. They call themselves the TELEPHONE COMPANY. And
the
maximum distance between your home and your central office is 13,000 ft.
(over
2 miles).

But the twisted pairs they use (even fancy new ones, Category 3) are not
the
correct impedance (too expensive). Luckily, the audio on your phone
ends at
3500 Hz. You can calculate the bandwidth and see you can go VERY far
before
you need to have impedance-specific twisted pairs. I'll be honest and
tell you
that I don't know the actual occupied bandwidth of a 56k data signal. (I
think
it's NRZI Manchester Coded, that would make it 28kHz bandwidth.) I'm
sure you
note that this puts it on the edge for performance if you are far from
the
central office, and you can't always get dialup that fast. And you'll
understand why in the USA, the FCC now mandates that all telephone
wiring must
be Category 3 or better. Category 3 is impedance-specific (100 ohms)
data
cable. But there are other reasons the FCC made this a standard besides
distance. The signals either go back to the source or simply stop in the
cable
(called "standing waves") and then radiate all that reflected energegy
into
cables around them. That's why the phone company is trying to use data
cable,
to avoid "alien crosstalk" (between cables).

That's why you can use crappy interconnect cables that come free with
your
receiver and it all sounds pretty good. They aren't long enough to make
a
difference. As long as you have continuity (electrical signal flow),
you'll be
just fine. But what if you use that cable for DIGITAL (i.e. S/PDIF).
Suddenly
we're sampling that audio at 44.1kHz (like a CD), and the bandwidth is
defined
as 128 times the sampling (5.6448 MHz, if memory serves). Let's just
use 6 MHz
to make it easy. (If you don't want easy, do it yourself.)

300,000,000 divided by 6,000,000 = 50 meters. 1/4-wavelength = 12.5
meters or
about 44 feet. How long is your cable? 3 ft? 6 ft? No problem. It's
still
not LONG enough to make a difference. Now, if you want to send S/PDIF
to the
other end of the house, that might be 40 ft. or even more. What would
you do?
Be sure and use cable which is the correct impedance. S/PDIF requires
75 ohm
cable (75 ohm cable has the lowest signal loss "attenuation" so it is
used for
many applications where low loss is required).

What if you get a really long piece of crappy cable? What impedance is
it?
Who knows!! And what will happen is that the signal, which will require
a 75
ohm "transmission line" and not see it. The farther it is away from 75
ohm the
more the signal will be REFLECTED back to the source and not get to the
other
end. This is called "return loss" in the cable world. You send me the
actual
impedance of your cable and I can calculate the mismatch and the return
loss.

But, if you get the right 75 ohm cable, how far can you go? It depends
on the
size of the wire (gage) in the middle. Let's assume it is a 20 AWG
center
(such as Belden 1505A, the world's most popular coax cable and a nice
generic
size).. Then the cable could go 716 ft. (This is based on a source
voltage of
0.5v and a minimum received voltage of 0.2v, at a bandwidth of 5.6448
MHz.)
Yup, 700 feet!!! (Snippy aside: try going 700 ft. on a piece of toslink
plastic fiber!)

And in the digital format, you can be real close to the maximum and the
signal
(i.e. data/audio) sounds just perfect. But add in one patch cord, or
one less
than perfect connector, or just a few more feet of cable, and you could
be down
the slippery slope of the "digital cliff". So, how many of you have to
run 700
ft.??? Not many, I assume.

This is not to say you shouldn't have cable with low capacitance.
Digital
signals need low capacitance to keep their sharp edges (i.e. the clock).
But
(amazing fact that no cable manufacturer ever told you) once you choose
the
construction (i.e. plastic/dielectric/velocity) and the impedance (i.e.
75
ohms) the capacitance is AUTOMATIC. High velocity foam coax 75 ohm = 15
pF/ft.
Old solid polyethylene 75 ohm coax = 20 pF/ft. And this is why you
should
avoid CATV coax, because it's not all copper in that center conductor
(it's
copper-clad steel for high-frequency-only applications where only the
skin of
the conductor is working.) This means the resistance is 5 to 7 times
higher
than an all-copper center and the distance you can go is 5 to 7 times
shorter.
(OK, we're down to 100 ft. and you're going 6 ft., but I wouldn't use
it.)

And all those SCSI cables (at the beginning of this thread) suffer from
the
same problem. Most of them use generic multiconductors (just pull one
apart if
you don't believe me). The impedance of a pair of wires is determined
by their
size (gage), the distance between them and the quality (dielectric
contact) of
the material inbetween. So, while the plastic is important, these
cables fail
at high frequencies because of the DISTANCE between conductors, which
varies
all over the place. Open up a SCSI 2 or 3 cable, what do you see?
Ahhhh,
twisted pairs!!! And the better the pairs (tighter impedance specs) the
less
the mismatch, the less the return loss, the greater the received signal
strength, the less the bit errors. Flat cable can also help maintain
impedance
specs, at least better than just a bunch of loose wires.

As frequencies go higher and higher, every dimension becomes more
critical.
Just take your car on a racetrack if you don't believe me. You need a
race car?
Buy a race car! You need a high-frequency cable? Buy one made,
tested,
verified, and guaranteed for whatever frequency band you wish.

OK, class, how did I do?

Steve Lampen
Belden Electronics Division