Why aren't 75 ohm BNC connectors used for SPIDF or even L or R outputs?

[goraman]

Seems to me a well made BNC 75 ohm connector would be far better than an RCA connection.
Was there a good reason to use RCA jacks, cost? politics ?
I had a DAC that used one before and it worked well, why dose hi end audio still use RCA?

[Bencat]

I would suggest that the main reason for this was the original joint mfg companies Philips and Sony . They wanted to ensure that the medium was universal and that it was something main stream users were used to . BNC has always been a niche socket .

On the other hand there is lots of things that point to the non adoption of this connector as a chance for quality and consistency missed . BNC are the only connector which has a plug and socket specified as 75 ohm (be careful when buying there is also a 210 ohm BNC ) so if you have a fully specified 75 ohm cable with BNC 75 Ohm plugs at each end and the equipment at both ends fitted with 75 Ohm sockets then you would be able to at least be sure that you have a fully Red Book specified signal between equipment .

I would also expect that when CD became mainstream and player prices were tumbling that all the OEM and mass producing companies who also made amplifiers etc would not have wanted the additional cost of adding another type of connector which would have meant additional cost and seperate storage ranther than jus use the already well accepted RCA standard .

[struth]

50 ohm and 75ohm connectors are the norm.

[Light Dependant Resistor]

Seems to me a well made BNC 75 ohm connector would be far better than an RCA connection.
Was there a good reason to use RCA jacks, cost? politics ?
I had a DAC that used one before and it worked well, why dose hi end audio still use RCA?

Well done ! You are quite correct, it is high time manufacturers of CD players and DAC's provided a BNC and are advised by the buying public a RCA is
not a valid connector for SPDIF What an opportunity to corner the market in not only CD players but also digital interconnects.
Can't wait for it, to begin happening :)

[struth]

not sure i agree tbh. the differences for general use is minimal imo

[Sherwood]

My Allo Digione board has both RCA and BNC connectors. To my ears the BNC output is superior.

[NRG]

AES3 the professional version of SPDIF uses 110ohm STP and XLR connectors. I read somewhere but for the life of me can’t find it now that the use of RCA connectors came about as a quick n dirty way to test SPDIF in the lab and somehow was deemed good enough :eek: and the use of RCA ‘stuck’....

[Barry]

50 ohm and 75ohm connectors are the norm.

Agreed. I have never heard of a "210 Ohm" connector (if one does exist then it would have a very small centre pin, or have a very large OD.

I suspect one reason why the superior BNC connector was not adopted was because, as you correctly point out, they come in two versions: 75 Ohm and 50 Ohm. If one tries to connects a 50 Ohm plug to a 75 Ohm socket, the socket will be damaged as the centre pin of the 50 Ohm version is larger.

[goraman]

I have also seen 90 ohm used by NASA.

[Barry]

I have also seen 90 ohm used by NASA.

There is a BNC variant having a characteristic impedance of 95 Ohm, and used for video connections in the aerospace industry.

https://en.wikipedia.org/wiki/BNC_connector

[goraman]

There is a BNC variant having a characteristic impedance of 95 Ohm, and used for video connections in the aerospace industry.

https://en.wikipedia.org/wiki/BNC_connector

I was probably wrong 95 ohm sounds right, It has been awhile so my memory is from 20 years ago.
I was at a flea market and bought a coffee can full of Amphenol PL259 silver plated connectors new, for $10.00!
Same guy had boxes of 50,75 and 95 Amphenol BNC connectors and it was the first time I had seen any thing over 75 ohm so I asked what they where for and he said NASA video.

[Barry]

I was probably wrong 95 ohm sounds right, It has been awhile so my memory is from 20 years ago.
I was at a flea market and bought a coffee can full of Amphenol PL259 silver plated connectors new, for $10.00!
Same guy had boxes of 50,75 and 95 Amphenol BNC connectors and it was the first time I had seen any thing over 75 ohm so I asked what they where for and he said NASA video.

Until now I hadn't heard of 95 Ohm BNC connectors (and I worked in electronics R&D for nearly 30 years), which is why I clicked on the Wiki link cited to check.


There is a little confusion on the inter-matability between 50 Ohm and 75 Ohm connectors. If the dielectric support insulation is maintained, since all BNCs have the same outer diameter, 75 Ohm versions have a smaller centre conductor than the 50 Ohm types. This means that a 50 Ohm plug, having a larger diameter centre pin will, if mated with a 75 Ohm socket, damage the latter because the socket will have a smaller diameter centre recepticle.

However, there are some designs of 75 Ohm BNCs that forgo the centre support dielectric. This is possible because most BNC connectors use a support dielectric having a relative permittivity of ~ 2.3, and since the characteristic impedance varies inversely as the root of the permittivity (that is ~ 1.5), removing the support dielectric means a 75 Ohm version can have the same dimensions as that of a 50 Ohm version. So if these two are mated, no damage will occur. One has to look carefully to check if this is possible.

[goraman]

Until now I hadn't heard of 95 Ohm BNC connectors (and I worked in electronics R&D for nearly 30 years), which is why I clicked on the Wiki link cited.

I don't think they are in wide circulation.