On Mon, Jun 19, 2017 at 10:33:28PM -0700, John Jason Jordan wrote: > laptop for getting VLC to play a Blu-ray movie, and it does so, but > there is a loud hum, loud enough to drown out the audio in the movie. I > would say it is a 60-Herz hum, except that it seems just a bit higher.
Given the strange behavior, this could be audio software written by folks who do not understand signal theory. Digital versions of audio signals are sampled into a fast series of mumbers. The sample rate might be 32,000 samples per second or 64,000 samples per second or 65,536 samples per second or something really weird like fer instance 63,262.3 samples per second. Different devices use different sample rates; a video device might sample at a different frequency than an audio device. I don't listen to much audio or watch movies on my computer, so I don't know the current standards. Adding insult to injury, computer CPU clocks are "dithered" - the clock rate varies a bit so that the radio noise that the computer broadcasts is spread out, not a single pure tone that the FCC can point at and say "you are interfering, add more shielding!" This is a cheat permitted under current rules, shielding is more expensive but more polite, but I digress ... Lastly, your computer responds to interrupts to do stuff, a device or a service periodically (60 Hz is common, so is 100 Hz) requesting CPU time to take care of things. Now ... with all those different things happening at different times, imagine an audio source that is sampled at 64,000 samples per second (perhaps the Blue Ray sound track) feeding an audio program that wants 65,536 samples per second. Not the same rate! Where do the extra 536 samples per second come from? How do you turn samples that are 15.625000 microseconds apart into samples that are 15.2487891 microseconds apart? And what do you do if your audio output system wants more samples, and the video display is refreshing at that time? An excellent media programmer will understand all that and deal with it. A not-quite-excellent programmer will fail to provide for some combinations of software, hardware, sources and outputs. Indeed, in some cases a "quiet" channel with the wrong kind of zeros (two's compliment versus integer) mixed up will make a hell of a racket. I hope I've flummoxed you with detail, so that you understand how a less-than-perfect programmer might be flummoxed as well. There's a good chance you are listening to the result of such a flummox. Keith -- Keith Lofstrom kei...@keithl.com _______________________________________________ PLUG mailing list PLUG@lists.pdxlinux.org http://lists.pdxlinux.org/mailman/listinfo/plug
