Newsgroups: rec.audio.misc,rec.audio.pro,rec.audio.tech
Subject: Re: Cable Length Limitations
References: 
"Toby"  writes:

> I was wondering if anyone knew of a source for cable length limitations?

Cable lengths for audio signals are dictated by
the equipment (output impedance, interface type)
and cable (cable capacitance). In practice cable usable audio
lengths are mostly dictated by the equipment connected to
the end of it, because the equipments vary more than
the cables. When carrying "normal" unbalanced audio signals
(thoe with RCA connectors) specifications of the signal
driver source determine how long the signal will travel
before it is too much affected by the cable capacitance.
The output impedances in normal HiFi equipment, computer sourcard
etc. can vary from 30 ohms up to 5 kohms. That's over 1:100
difference in the effect of the cable!

For video applications all equipment use standardized 
75 ohm impedance. The cable is also matched to 75 ohm load.
Here the cable attenuation factor affects how long the signal
will travel well. This is from tens of meter to over
100 meters depending on the cable.

> I was hoping to run RCA type cable for Audio/Video from one room to
> another... about 40'.  Bad idea?

Not bad with proper equipment.

Good 75 ohm coaxial cable for video.
For audio a distrbution amplifier + balanced connection
this definately works well.

But might work with some devices more cheaply...



Here is a repost of an earlier post I have done on thos
topic some time ago to this newsgroup also:


Newsgroups: rec.video,rec.audio.tech,comp.dcom.cabling,ieee.ces.broadcast-cable
Subject: Re: Standards for maximum cable length of analog audio and video
References: <[email protected]>
Thomas Sluschny  writes:

> Hi!
> 
> Is there any offical standard or norm or regulation or ...
> describing maximum cable length for analog audio/video
> transfer?

Thers is no official standards for or norms for maximum cable
lenght of analog audio/video signal transfer cable length.
Because of the nature of the signal, the lonfer the cable is
the more attenuated and distorted the signal will be. How
much attenuation you can take and how much other problems
(distortion like frequency response changes and noise)
you can accept in your picture and sound.

The length of the cable for audio transmission is very much
dependent on cable characteristics like shielding properties
(if you are in noisy environment) and cable capacitance.
More capacitance, the more the cable will attenuate high frequencies.
And one thing which has very much effect on signal transmission
in normal unblanced audio connections is the impedance of
the signal source. Some hifi equipments have output impedance of
few kilo-ohms. Some prefessional equipments have output impedance
of only tens of ohms. This means that you can run tens
of times longer cable with the same frequency response change
effects on professional equipment with output impedance of 60 ohms
than with an homehifi CD player with output impedance of
5 kilo-ohms. (both of the example impedances are taken from
specs of real-life equipments I have seen and used). So for
the first equipmet I would happily connect few tens of
meters of audio cable. And for second more than few meters
might be heard even more!

On professional audio connections with unbalanced equipments
distances up to few tens of meters is quite typical in
real-life applications without sound problems on
good cases. Using professional audio equipments with
balanced connections the links up to few hundred meters is
typical. In those applications I expect that good quality
audio cable ment for professional use is used for connections.

Home audio hifi equipments the distances more
than 3-15 meters on normal cables might be heard or not
(the distance depends mostly on the equipments and somwehat on
the cable characteristics).

> I'm particulary interested in Coaxcable for Videosignals
> (PAL/NTSC) and Twisted Pair for Audiosignals (NF).

Normal shielded audio connections were listed above.
For coaxial PAL/NTSC connections the cable is one thing.
If you use some whatever audio cable, less than few meters
will not show on picture. If you use the right cable type
which is 75 ohm coaxial cable (RG-59 being the most
common cable for this) then you can easily go up to
tens of meters without you seeing any problems in pictures.
I have used many times runs up to 20 meters to vide projectors.
And if you can take some "softening" in the picture, you
can eaily go to 100 meters with RG-59 or similar coaxial cable.
I have used few times cable length of up to nearly 100
meters for simple video cameras (security camera type application)
and picture quality was usable (propably not the most sharpest).

And I have been oprating video systems where have been 40-100
meters of RG-59 (or similar cable) between the video equipment
and large video projector. With some cable equalizatio and aplification
the picture has been practically perfect even on that long
runs. 

> The only things I found in the web or books are rules
> of thumbs, no offical papers to refer.

There are only general rules of thumb which you can use 
as general advice. For a particular situation if you have
technical information on equipments and the cable you
can make calculations or test what does it effect to the
signal. For some basic information take a look at the
wiring guides and other papers at
http://www.us-epanorama.net/audiopro.html#wiring
They should give you a general view on this.

I feel that this question might just be an another FAQ
which should have a good document on it with good
description of the problem. And with both sules
of thumb general information and also the technical
information needed to analyze the connections using
mathematics and electircal circuit models.


-- 
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at 
http://www.epanorama.net/