Hi, Nikos and Florian. Thanks for the helpful elucidation.
-- Alan Mackenzie (Nuremberg, Germany). On Sat, May 28, 2011 at 04:13:18PM +0200, Florian Philipp wrote: > Am 28.05.2011 12:19, schrieb Nikos Chantziaras: > > On 05/28/2011 12:50 PM, Alan Mackenzie wrote: > >> Hi, Gentoo. > >> It occurred to me the other day that I am clueless about how a sound > >> card works. How do the data get into it? Does the sound card use an > >> interrupt to ask for more data? > > The data is placed in RAM. The card reads it from there using a DMA > > operation. You can read about it here: > > http://en.wikipedia.org/wiki/Direct_memory_access > >> What form do the data take? > > It's raw data, and its form depends on what the card is expecting. What > > the card is expecting is programmable by the card's driver. > Most likely it is some PCM format (pulse code modulation) not very > different from WAV, CDDA, etc. (just without headers, of course). In the > easiest case, the sound card then just feeds this into a digital-analog > converter connected to the output (together with a analog-digital > converter this is called an audio codec, for example AC'97). > AC3 or DTS, the compressed formats found on DVD, can also be "passed > through" the sound card to reach a home theater system over a digital > output without being converted into an analog signal. > >> Say I feed an mp3 through the card. Does > >> the Athlon do the decompression, or does the sound card do it? > > The MP3 is decoded by your CPU (by software like libmad, xine, > > gstreamer, etc.) The decoded data is send to the driver, the driver > > applies any needed conversions to it (according to what the card > > expects), and then places it in RAM so the card can get it by means of DMA. > This can be observed in some cases when the system crashes during > playback. Then sometimes the card just seems to loop over the last data > packet placed in RAM. > >> Last of all, is there a command line program which can play a CD by > >> feeding its data into the sound card? > > Today this works the same playing any other audio. The fact that audio > > in this case comes from a CD doesn't matter. An application reads the > > audio from the CD, sends it to the driver, and from there it gets to the > > sound card. > The cdparanoia FAQ provides a lot of insight into the special problems > of reading CD audio: > http://www.xiph.org/paranoia/faq.html > Regards, > Florian Philipp
