Before the advent of high fidelity , conventional wisdom held that matched impedances were the way to go when transfering audio signals. I don't know why or when, but 600 ohms developed as a standard and still holds today for some applications. I believe it has to do with the fact that earlier systems tended to be transformer coupled and input transformers tend to have lower impedances.
Noise rejection is one advantage of this lower input impedance. If noise currents are induced in the cables, they will develop lower voltages at the input because the impedance is lower. (Voltage = current x impedance)
I found this link which may help a bit.
Noise rejection is one advantage of this lower input impedance. If noise currents are induced in the cables, they will develop lower voltages at the input because the impedance is lower. (Voltage = current x impedance)
I found this link which may help a bit.