Print

Please login or register.
1 Hour 1 Day 1 Week 1 Month Forever
Login with username, password and session length

[New Orleans Steve]

I have some questions about  rated cable impedance. We refer to digital cables as 75 ohm or 110 ohm. What exactly does that mean? I think the ratings are not for the cables themselves, but refer to the devices that they are meant to connect.
Really for the application I have at hand than question is: For a 4’ run can analogue audio be connected with a 110 ohm balanced Belden cable?
I am wiring a snake to connect mic pres to line in on a recorder.
Any links on the subject, or general discussion would be welcome.

[ssltech]

It does indeed refer to the characteristic impedance of the cable itself. There are several factors whic determine characteristic impedance, which becomes an issue at higher frequencies and moreso at longer lengths.

Steve Lampen at Belden is a wonderful expert on the matter and has published some readable stuff by way of explanation.

Try googling "steve Lampen impedance"... that should turn up something... but here's a quick redux:

Impedance (in ohms):

The total of resistance, capacitance and inductance. As frequencies get higher, resistance becomes less and less of a factor. At frequencies above 10 MHz or so, only capacitance and inductance are left, so the impedance settles to a "characteristic" value.

Keith

[Geoff Doane]

The characteristic impedance of a cable is determined by its physical construction, including what dielectric has been used for insulation.  For instance, with air dielectric coax, the impedance is the square root of the ratio of the squares of the outside (shield) and inside (conductor) diameters.  There's a similar formula for balanced (paired) lines, but I've long since forgotten it (Obviously, we use these bits of information frequently   ).

Characteristic impedance really only becomes important when the transmission line (wire) starts to become a significant part of a wavelength, or generally about 1/8 wavelength.  Unless you're the phone company, you probably aren't dealing with analog audio lines that long.  For digital audio, where the frequencies are much higher, we do (or should) care.

Typical "analog" twisted pair wire (Belden 8451) has a characteristic impedance of about 60 ohms, so it's not ideal for 110 ohm AES/EBU if you're running any length.  There is no downside to using 110 ohm cable for analog audio at any practical distance, except that the conductors tend to be smaller, and the insulation is thicker.

GTD

[Oh! My Sea Captain!]

The answers to all questions cable related lie here!

http://www.amazon.com/Wire-Cable-Fiber-Optics-Vid/dp/0070381 348/sr=8-2/qid=1159465618/ref=sr_1_2/102-5546025-5808122?ie= UTF8&s=books


BUY IT.

[Scott Helmke (Scodiddly)]

The deal with 70 ohm and 110 ohm cabling is that for coax cable, 70 ohms is about the most efficient (least loss) way to transfer high frequencies.  For balanced, 110 ohms.  So both the devices and the wiring use those impedances.  For short runs you can probably use non-ideal cabling without too much trouble.