Originally Posted by billybob_jcv
What does this mean?
From usb_20.pdf found here: http://www.usb.org/developers/docs/usb_20_071012.zip
"Use of ferrite beads on the D+ or D- lines of full-speed devices is discouraged."
"The use of ferrites on high-speed data lines is strongly discouraged."
I was always under the impression that ferrite beads were used on substandard cables, but that on cables that are fully compliant with the USB 2.0 spec, they are not needed (and perhaps discouraged?)
I was also under the impression that a ferrite bead is a choke - a passive low-pass filter. They are used to eliminate EMI/RF noise. I was also under the impression this noise is well beyond the audio frequency range. How that would affect the audio encoded in a digital signal without also affecting the digital signal itself is beyond me.
A ferrite bead is like an inductor, except at high frequencies (also depending on model type, construction) the impedance is both high and also mostly resistive. i.e. most of the energy in the high frequencies gets dissipated. As you say, if it's in series with the load, you're thus lowpass filtering the signal so the load doesn't get as much of the high frequencies. The data lines in USB are carrying high-frequency signals. You don't want to lowpass filter them (out) if you want the other side to actually get the signal.
The ferrite bead is for filtering the +5V (supposed to be DC) line. The filtering is both for keeping the crud from the source out of the destination and vice versa. Or from other RF sources, though of course computers have plenty of high-speed clocks and switching devices already, even before you consider the actual radios.
The concern is with the RF noise getting into the DAC and affecting the D/A. These devices perform better with cleaner power. Noise at RF frequencies matters because... wait, somebody else better explain this to me too. That said, most DACs are oversampling or otherwise performing actions at frequencies much higher than audio, prior to the output reconstruction filter.
To be honest, electronics and RF/EMI issues are way out of my specialty...