24-09-2016, 10:22 AM
600 ohms is a historical standard that isn't really used today. Instead, audio gear has a low output impedance (undefined, but should be <50 ohms), and a high input impedance (also undefined, but >10k). Much easier. Much better.
There are 2 sides to this coin:
1. Signal level.
2. Transmission line effects.
On the face of it, level is simple. But take it from me, level is a massive problem that is not fully understood by many in professional circles. Which is staggering in a way. Still...
When you match impedances, you get a 6dB loss in level. When impedances aren't matched, you need to calculate the loss (which is easy enough). What if you wish to connect the signal to more than one device? You might get double-termination, meaning the level is only 66% of what you were expecting. Working in a matched-impedance world is essential for things like video and RF and general test gear, but in audio, there is no need to do so, and happily, we don't. By adopting the "low Z-out, high Z-in", the loading effects are minimal, and you can safely join multiple devices together without significant changes in level.
So if matched impedance working is such a pain, why do it?
That gets us on to transmission line effects. That's a book in itself, but the short version is that for higher frequency work, transmission line effects start to become significant, so we need to terminate correctly at each end of the line. So a 50 ohm coax cable needs to "see" 50 ohms at each end - so the source impedance is 50 ohms, and the input impedance is set to 50 ohms. When done, the cable is working as well as it can be.
This is my favourite video explaining transmission line effects: https://www.youtube.com/watch?v=I9m2w4DgeVk
It's from Tek, made about 300 years ago! Enjoy
In short, for audio, BNCs are fine, and don't need termination unless they are longer than about a kilometre
There are 2 sides to this coin:
1. Signal level.
2. Transmission line effects.
On the face of it, level is simple. But take it from me, level is a massive problem that is not fully understood by many in professional circles. Which is staggering in a way. Still...
When you match impedances, you get a 6dB loss in level. When impedances aren't matched, you need to calculate the loss (which is easy enough). What if you wish to connect the signal to more than one device? You might get double-termination, meaning the level is only 66% of what you were expecting. Working in a matched-impedance world is essential for things like video and RF and general test gear, but in audio, there is no need to do so, and happily, we don't. By adopting the "low Z-out, high Z-in", the loading effects are minimal, and you can safely join multiple devices together without significant changes in level.
So if matched impedance working is such a pain, why do it?
That gets us on to transmission line effects. That's a book in itself, but the short version is that for higher frequency work, transmission line effects start to become significant, so we need to terminate correctly at each end of the line. So a 50 ohm coax cable needs to "see" 50 ohms at each end - so the source impedance is 50 ohms, and the input impedance is set to 50 ohms. When done, the cable is working as well as it can be.
This is my favourite video explaining transmission line effects: https://www.youtube.com/watch?v=I9m2w4DgeVk
It's from Tek, made about 300 years ago! Enjoy
In short, for audio, BNCs are fine, and don't need termination unless they are longer than about a kilometre
