Hector Martin

Plenty of cheap cards have gone to doing this, assuming later stages can either handle the ultrasonics, or they'll filter them out. "Can't hear it anyway, lol" Though that's usually high-oversampling sigma-delta cards that have the hash moved way out into RF.

Oh ick, I was just talking about not filtering, not constantly putting out a squarewave. Maybe it considers it some kind of ultracheap dither.

It's for DC bias. They put out a square wave on the 3.5mm sleeve pin (which isn't connected to USB ground) and then the same 50% square wave on the line channel is silence, lower duty cycle goes negative, higher duty cycle goes positive. No AC coupling caps.

OH, one of those! Yeah, it's trying to be a cheap-ass class D, and those are generally designed to assume they're driving a coil. The 51nb people use them in their audio daughtercards for the X62 and T70.

People get into trouble because the X62 has a single mono speaker, but the amp is being driven in stereo, and so most builders connect the speaker to left + and right -. Except the amp is not always driving the same squarewave to both channels.....

"why is my computer speaker squealing all the time?"