On Sat, May 26, 2007 at 08:35:01PM +0200, Arnold Krille wrote: > Okay, your laptop introduces a hum and you are using balanced cables? I don't know of any laptop having balanced audio inputs/outputs. Using balanced cables if the interface isn't balanced will help sometimes - but only if you're lucky. The whole balanced / unbalanced issue is much more complicated than it is usually presented. Regarding inputs, the so-called balanced ones can be either: 1. Balanced and floating. Floating means that the common mode voltage doesn't matter at all - the signal pins are completely isolated from ground and could take any common voltage as long as it doesn't produce a flashover. One pin could be 1000V + x and the other 1000V - x, the only signal that gets through is the difference, 2 * x. The only way to do this is for analog audio is by using a transformer, big and expensive. You want this if you connect to anything outside your studio. 2. Balanced but not floating. As above, the useful signal is the difference between the two signal pins, but there is an extra requirement: the common signal has to be within limits, usually a few volts, at most of few tens of volts. If not it will leak through or even destroy the input circuit. The vast majority of so called balanced inputs are of this type. It's OK for connections within a studio or within a system that has a single ground. 3, Pseudo balanced. As above, but he two signal pins don't have the same impedance to ground. Seen very often on cheap equipment that pretends to be 'balanced'. Works for short connections. For outputs the situation is similar: 1. Balanced and floating. As for the inputs this requires a transformer. For such an output, the difference between the two signal pins does not change if you connect one of them to ground, or to any other voltage. 2. Symmetrical. The two signal pins provide antiphase signals, +x and -x, but they are independent - each one produces a voltage w.r.t. gorund, not w.r.t. the other pin. Connecting one of them to ground, or introducing some interference on one of them doesn't make the other compensate for it by maintaining a constant difference, as would happen with a truly balanced output. Can't be connected using a balanced cable to an unbalanced input, as this would short-circuit one the outputs. 3. Ground loop compensated. In this case, one of the output pins is actually an low-impedance input. Any voltage sensed on this input is added to the signal on the other pin, so a balanced input will only see the signal. Can be connected to an unbalanced input. A cheao and good solution for local connections. And even all this is a simplification.... -- FA Follie! Follie! Delirio vano è questo ! _______________________________________________ Linux-audio-user mailing list Linux-audio-user@xxxxxxxxxxxxxxxxxxxx http://lists.linuxaudio.org/mailman/listinfo.cgi/linux-audio-user
