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john dozier 2nd November 2012 05:07 PM

Bnc installation
I am replacing some rca's with bnc's on a cd transport. Guido Tent suggest that the bncs be mounted directly to the chassis with no insulating washers. What does the board think? Thanks

richie00boy 2nd November 2012 05:46 PM

Mount the same way that the RCA socket was. It depends on how the player is grounded.

mickeymoose 2nd November 2012 09:17 PM

Why BNCs? What is wrong with RCAs? E

Enzo 3rd November 2012 12:02 AM

Oh, BNCs are just lots more cooler.

When I was learning electronics and building stuff in the late 1950s post-war surplus era, I had tons of BNC cables and connectors and used them for everything. Clicking them on and off just seemed so darned industrial. NO electrical advantage, just style points.

john dozier 3rd November 2012 12:25 AM

These are replacing rca's used as digital outputs. I do like to use 75ohm connectors in these circuits. I do not even trust Canare's "75" ohm rca. Regards

marce 3rd November 2012 09:12 AM

RCA's are terrible (c**p) and add an impedance match, not what you want for a digital signal. The return and signal should also be kept together, from transmitter to reciever, for digital signals, do not seperate the return by staring it to the PSU or some other scheme.

DF96 3rd November 2012 11:52 AM

You meant to say that RCA add an impedance mismatch?

Fortunately frequencies are not all that high, so the impedance discontinuity of the RCA socket may be no worse than that of the circuit pigtails connected to it. In other words, good wiring of an RCA may be better than bad wiring of a BNC.

john dozier 3rd November 2012 02:02 PM

How long do you think pigtails can be without causing a significant impendence mismatch? Regards

DF96 3rd November 2012 04:35 PM

Half an inch should not do much harm, but the shorter the better.

Let's do a rough back-of-envelope calculation. Assume 16-bit data at 48kHz sampling rate. So each stereo sample has 32 bits of data, plus some overhead. I can't be bothered to look up the details, so let's just assume 40 bits per sample in total. This gives a basic data rate of 1920kbit/s. I think it uses Manchester encoding, but I can't even be bothered to look that up either. Let's assume a basic signal rate of 2MHz. You need a reasonable square wave, so let's assume at least the first 50 harmonics. This means the link has to be good to about 100MHz, which is not too difficult.

A disturbance in a transmission line will not do much harm if it is less than about wavelength/50 in size. 100MHz is 3m in free space, so we can have up to 6cm disturbance. Let's not use more than half of this allowance, so the other end can do it too. So we get 3cm.

This is a back-of-envelope calculation, so the result could easily be out by a factor of 2 either way. However, it gives a rough idea of what is needed. If you might want to use 24-bit audio at 192kHz then you may need to be more careful, and treat the whole thing as a UHF circuit.

john dozier 3rd November 2012 04:58 PM

Thanks DF96. My EE was almost 50 years ago and our computer had 64k of memory. Digital is not in my engineering vocabulary-give me tubes however.... Regards

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