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