Audio, hi-fi and car audio - 10 metres audio cable going into PC = too long?

In article , Floyd L. Davidson
writes
tony sayer wrote:
In article , Floyd L. Davidson
writes
Have you ever verified what the CMRR actually is on such a
circuit? The perception that CMR just cancels out everything is
naive. Typically common mode rejection is *not* sufficient to
provide a functional dial loop on a line with 40+ volts of AC
voltage.

Yes but quite some time ago now. FWIW we don't or very rarely have long
lumps of overhead line anymore that carry baseband audio. For voiceband
circuits these days its digital end to end with a A/D and D/A convertor
at each end.

Most residential POTS service in the UK is fully digital from
end to end?

That certainly is not true in the US, and I've never heard anyone
in the UK say that it was there before either.

To clarify that..

BT the main operator in the UK has its exchanges linked together these
days by Fibre and has done so for many years now. They break the fibre
down to copper at the exchange, add in loop volts and line break etc for
older phones, and then route that to the subs premises. However the
distances aren't that great, around 3 to 4 miles or so in rural areas
and less in urban ones. Using a multipair but what also might make some
difference between UK and US practice is that this will invariably be
underground plant in their own ducts. An overhead multipair cable is
quite rare, and is likely to be of a lowish number of pairs and not that
long probably only from a distribution pole to a group of subs not that
far away, i.e. a few hundred meters.

The Cable co's in the UK, firms like Comcast and Bellcable etc are now
all rolled up into the ntl and Telewest outfit, soon to be renamed
again!. They use a similar practice but the distance issue is changed
again. They will have roadside cabinets instead of exchange office type
buildings, (though they will have a central switch building) that breaks
the fibre down using Nokia equipment's to copper to the subs premises.
However once again these all are underground ducted cables but the
distances there are more likely to be of a few hundreds of yards!.

We don't have this sort of arrangement over here;!...

http://www.annsgarden.com/poles/poles.htm

In fact I've never seen a phone cable tacked to a power pole carrying
more than 240 volts!. And for that the cables are spece'd differently.
To have phone and comms cables with power lines at the sort of voltages
described on those poles just isn't done here!. The health and safety
police just don't allow!! apart from that there is an aversion to
overhead plant in the UK most all telecom power services is
underground.....

And for phones its going much the same way, well over here at least.
BT have the 21CN nets which are data circuits which you run data or
audio or whatever you like over them..

I'm not familiar with the terminology. However there are of
course such circuits here too (ISDN, for example), but by far
the majority of POTS service is delivered as an analog line,
after being trunked to a remote unit with digital services.

Yes as above. However the market for bizz and residential telephony
services is rapidly changing in the UK . Mobile use is now very high and
climbing. In fact a lot of young people won't have a landline phone.
They see it that a mobile is a "must have" and for around 30 or so
pounds a month you can get around 500 minutes a month inclusive calls.
Very cheap rates are applied at evenings and weekends.

The extra monthly cost of a phone line can buy quite a bit more call
capacity, and the mobility issue a cheap and easy to use text service
landlines are finding it hard to compete!.

BT 's 21st century programme is to do away with their circuit switched
networks and make it all IP based packet switched. A lot of people use
such services as Skype and other phone over the net services. The only
thing some people have a phone line in for is for broadband provision!..
However these can and are supplied over radio based nets as well as via
UMTS services.

However, none of that is relevant! Power line influence is, if
anything, *more* of a problem for digital services than it is
for old fashion POTS via an analog line.

Well it might be over there with all that overhead distribution but it
'taint here!. ADSL is very robust and I've never known a problem with
it. And as they use fibre a lot for phone lines between exchanges, not a
problem..

Virtually *all* "multicore" telecom cable is shielded. (Some
customer premise cable is not. But you won't find anything
within a telephone central office that isn't, and you won't find
any outside plant distribution cable that isn't.)

I asked a couple of cable jointers who were working beside the road the
other day re that one, and it seems that its the exception rather than
the rule these days. There is some cable which has a foil screen around
it, but as to woven braids seems they aren't used anymore..

I don't have a great deal of confidence in someone who is
getting their information from "cable jointers" alongside the
road.

Not quite so. One of these guys was quite old and very experienced, and
could recall the days of lead covered cable where they had to do wiped
joints etc. So don't despise the benefit of experience. Those guys have
probably seem more cable close up than you'll ever likely too!.

Course they don't specify it, but some old BT "Poles and holes" staff do
know a lot more that you'd give 'em credit for!..

Lets be blunt: you don't know what you are talking about.

If that makes you feel better, and in someway superior, so be it..
you're welcome;)

"Woven braid" has *never* been used for telephone cable. And
I'll repeat it just one more time: multipair cable for long runs
is virtually *all* wrapped with a shield, and additionally has
at least one single strand of bare wire running along with the
shield to provide greater conductivity.

I guess I need to tell you that I am *not* guessing.

Virtually *all* "multicore" cable is shielded. That is *not*
individual shields on each pair, but the entire cable is inside
a (foil) sheath.

Well it seems it isn't always the case here.

Well the ones ntl use here according to a friend of mine who works with
their plant day in and day out sez otherwise. Seems only some of the
cable they use has a foil screen but then again they use fibre and co-ax
for distances of any length, seems digital rules;)..

If you can't cite a valid source... please don't exaggerate what
you do know.

As above their use of multipair isn't that great. Its underground and
all in cable only ducts. They don't even run mains in there they source
that off street lightning cabs..

What do you define shielding as, just a wrap of aluminium foil with a
drain wire or a fully woven copper mesh?..

Shielding is shielding, whether it is aluminum foil or copper braid.

Yes except that if we're talking like we were about currents circulating
in the "screen" of a multicore cable, then there is going to be quite a
bit of difference in practice between a heavily woven copper braid and
the light foil wrap where the connection to that is by a fairly thin
drain wire...

But that doesn't change the way the shielding functions. All it
does is change the effectiveness of that functionality, and
clearly copper braid is much more expensive... to a degree that
the difference is not worth the cost.

Once again if its circulating currents in a cable screen as per the
original discussion, then for a given diameter of cable there will be a
difference between the current that can flow in that a braided multicore
will carry more current than one with a ally wrap and small drain wire..

I take it from your statements above and the lack of an answer
here that you have no experience with specifying or installing
telecommunications cabling.

I take your lack of a responsive answer as an affirmative one.

Nope. I don't work for a phone company like you do, but I do have to
have a very good working know how of telecoms, phone, appl, private
circuit, voice band and wider-band, and ISDN and data..

Yes we sometimes do, but very rarely these days, it s getting to be a
very digital world over here. Analogue circuits are quite rare nowadays
and BT have been known to have to get guys out of retirement to work on
the few remaining ones!.

I'm finding that to be a little difficult to believe, given the
other statements you've made.

Nope. Sorry but the "phone engineer" as we know is a vanishing breed:!.

Give you an example. We installed a PABX system for a radio station
recently. Its all based on a PC and uses SIP phones. Most all the
outgoing calls are over a GSM gateway to other mobiles 70% of calls, and
inter office calls are via ADSL. Probably 15 % are carried over the ISDN
circuits connecting it to BT. It is now no longer a phone provider
service contract issue its an IT one now!..

OK then, part 2 "On the other hand cable shields which are bonded at one
end etc". Read that thorough carefully, doesn't make sense.

"On the other hand, cable shields which are only bonded at
one end cease to provide shielding when their length exceeds
one-tenth of the wavelength of the frequencies to be
shielded against, so for example a cable 10m long only
provides any significant shielding for frequencies below
3MHz. When cable lengths exceed one-quarter of a wavelength,
shields which are bonded at one end only can become very
efficient RF antennas * radiating RF noise and picking up RF
from the environment more efficiently than if there was no
shield at all. Although the RF noise in pro-audio products
is usually caused by digital and switch-mode circuits, it
appears as common-mode (CM) noise on all the analogue inputs
and outputs too."

So be specific. It makes sense to me. What part would you like
explained?

Well they don't define what you are doing with that. Consider say 10
meters of Andrews LDF 4-50 cable connected to a transmitter with the
correct plug, what are they connecting that other end to?. Nothing or a
load partially connected?.

They detailed it precisely enough. The outer conductor is not
connected. It makes virtually *no* difference what you do with
the inner conductor. :-) The point is that depending on the
frequency and the length (not on what it is connected to) it
will (or not) act as a very good antenna.

Actually read that through again;) It isn't that wonderfully written for
what they want to convey. I've mailed that off to a few other people to
see what they "visualise" that cable to be doing in that description!..

Or do they mean the connection to the shield, referred to the point
where that would normally be connected, is greater than one tenth of
lambda?. If thats what they meant then they didn't describe that very
well.

They mean the length of the cable is longer than 1/10 of a
wavelength, and that there is no connection to the shield, but
there is (to virtually anything you'd like to connect, including
a box of "nothing") to the center conductor. Under some
circumstances, which depend on the length and the frequency, it
will act as an antenna.

It seems that they were thinking of say a braided cable like perhaps
RG214 or similar when you "could" take that out as a pigtail
perhaps......

That would be one example.

Then take a
lump of Andrews 4-50 Heliax and see what a good radiator that is even
greater number of wavelengths . They didn't even state if it were open
circuit or terminated on a load...

Please review this portion of what I wrote in my last message:

Heliax is, just as they state, a good radiator if it is not
bonded properly. It provides good shielding when properly
bonded, and can become a very effective antenna at lengths
approaching or exceeding 1/4 wavelength when not bonded.

That is true regardless of whether there is a resistive load, or
not.

Please review any book on antennas! The statement made
describes the physical construction of more than one popular
variation of an antenna.

Yes I design and install aerials thanks.

Antenna's are what insects have;)

Actually we've had a lot of EMC experience over the years in radio,
audio and automotive environments and what's made by far and away the
biggest effect is bypassing of transistor junctions at RF
frequencies....

I like chocolate chip cookies myself. But that has nothing to
do with the topic we are discussing either, so I haven't brought
it up. You probably should stay on topic too?

I think its relevant on the subject, but YMMD as they say..

But chocolate chip cookies are more relevant.

Yummie. Biccy's is what we call 'em here, dunk 'em in your tea
traditionally best stirred with a screwdriver:))

I think you have that wrong. Provided that the rejection is what it
should be then whatever is induced on the pairs will cancel out.

That is simply not true. Have you ever *measured* it? It does
*not* simply cancel *everything* out.

Do you know what "longitudinal balance" is? That is the
characteristic which most determines how much is canceled out by
common mode rejection.

It is *never* perfect.

Well how far do you want to go with that;?...

The intent is to go as far as is practical, in terms of cost.

OK on that then..

We could all agree that common mode rejection is not always
sufficient, and that reverse induction is virtually *always*
applied to outside plant communications cables because of that.

What do you do over there are you involved in a Telco?..

Just over four decades in telecommunications.

I've been in TV transmission, Radio broadcasting, studio design and
maintenance, data comms, and two way radio......among others...

Now if say you ground that to the local mains earth at one end, and say
10 meters away at the microphone case end earth that to a driven rod
earth, will it or wont it hummmmmmmmmmm?.....

Your circuit is using a single ended coaxial cable. The return
path for the circuit includes the shield. Hence you've just
connected the ground differential to the signal circuit. It
won't hum if you are 100 miles from the nearest power line...

Humm... What do you use out there in deepest Alaska, batteries;-?.....

Proper technology seems to work the best.

Indeed it does.

Now what was the original argument again;?.....
--
Tony Sayer