What exactly is a 75ohm cable ?

[Liffy99]

Heard it mentioned often, e.g. 75ohm RCA but what does the 75 bit relate to ?
I assume it’s not the cable and its length as for most cable diameters i think the impedance would be negligible under 1m long. Is it something to do with the output device itself (eg 75ohm output impedance) ?

Explanation welcomed

[Yomanze]

Impedance is like resistance, except for AC instead of DC. It doesn't matter if the cable is a metre long, or 20 metres long, alternating currents are still going to 'see' a 75R impedance. Resistance & DC currents are different, because the longer the wire, the higher the resistance becomes.

However, RCA connectors are not 75R, more like 35R. To get a true 75R cable, you need to use something like BNC connectors. At RF frequencies (such as SPDIF), a change of impedance means signal reflections.

[Barry]

The 75 Ohm refers to the characteristic impedance of the cable. It is only relevant when the length of the cable is longer than about 1/6 of the signal wavelength. At audio frequencies it has no real meaning since even at the highest audio frequencies the wavelength will be ~ 2km, compared to a cable length of ~ 1m.

The situation is quite different for a digital signal passing from a CD transport to the DAC. Here the sampling waveform has a very fast rise time of order of a few tens of ns, so the cable connecting the two should be treated as a transmission line with source impedance of the CD transport and the load impedance of the DAC matching the characteristic impedance of the cable. For the Sony/Philips Digital InterFace (S/PDIF) the characteristic impedance chosen is 75Ohm.

Surprisingly the connector chosen is the humble RCA phono connector, which from its geometry cannot have an impedance as high as 75Ohm (depending on the insulation used, it will be ~ 30Ohm), but being electrically 'short' the mismatch is somewhat ameliorated.

[Firebottle]

A 75 ohm cable is essentially the description of the impedance of the cable at high frequencies, a lot higher than audio frequencies.

By impedance that automatically implies an AC signal, as confirmed by the 'high frequency' descriptor and usage.
It is not related to the resistance of the cable (or length) as the resistance is measured at DC.

A 75 ohm cable will typically be used with video and digital signal, with typical frequencies of up to around 10 MHz, that's 10 million 'cycles per second' in old money. At these high frequencies the distributed capacitance and inductance of the cable will present a constant impedance to the signal, along the complete length of the cable, so the cable can be any length and will still be a 75 ohm cable.

The actual impedance is given by the ratio of the size of inner conductor to outer conductor, usually a complete screen as in coaxial cable.
The other common cable impedance is 50 ohm, almost universally used for RF equipment and antenna matching, which has a slightly larger inner conductor diameter (for a given overall cable diameter) which makes it the lower 50 ohm impedance.

:)

[Firebottle]

I think between us we have pretty much covered it Barry :thumbsup:

[Liffy99]

So glad i asked . . . 😨

But thanks guys.

So, as i have no BNC inputs or outputs is there any point in trying a cable with BNC to RCA adapters at the ends ? Or even using a cable with BNC at one end and RCA at the other ,(can a cable even be built like that ?).

I am thinking of trying the Allo SPDIF card and want to get the best out of it I can.

[Barry]

If you don't have any BNC style connectors on your gear (and it would ideally have to be a 75Ohm BNC connector (BNCs come in two impedance versions: 50Ohm and 75Ohm)), you are 'stuck' with using RCA connectors. So whilst they are not perfect, Philips and Sony think them good enough (or cheap enough!), and as I say the electrical mismatch is sufficiently small to be tolerable.

All you have to do is to use an RF cable having a characteristic impedance of 75Ohm. There are many to choose from: RG59CU for example, so you shouldn't have any difficulty. I would recommend that you make the cable no shorter than 1 metre.

[Sherwood]

So glad i asked . . . ��

But thanks guys.

So, as i have no BNC inputs or outputs is there any point in trying a cable with BNC to RCA adapters at the ends ? Or even using a cable with BNC at one end and RCA at the other ,(can a cable even be built like that ?).

I am thinking of trying the Allo SPDIF card and want to get the best out of it I can.

https://www.ebay.co.uk/p/kenable-Pure-Copper-CCTV-BNC-to-Phono-Plug-Cable-Gold-Connectors-1m/14012054198?iid=190484309091