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+bttv and sound mini howto
+=========================
+
+There are a lot of different bt848/849/878/879 based boards available.
+Making video work often is not a big deal, because this is handled
+completely by the bt8xx chip, which is common on all boards.  But
+sound is handled in slightly different ways on each board.
+
+To handle the grabber boards correctly, there is a array tvcards[] in
+bttv-cards.c, which holds the information required for each board.
+Sound will work only, if the correct entry is used (for video it often
+makes no difference).  The bttv driver prints a line to the kernel
+log, telling which card type is used.  Like this one:
+
+	bttv0: model: BT848(Hauppauge old) [autodetected]
+
+You should verify this is correct.  If it isn't, you have to pass the
+correct board type as insmod argument, "insmod bttv card=2" for
+example.  The file CARDLIST has a list of valid arguments for card.
+If your card isn't listed there, you might check the source code for
+new entries which are not listed yet.  If there isn't one for your
+card, you can check if one of the existing entries does work for you
+(just trial and error...).
+
+Some boards have an extra processor for sound to do stereo decoding
+and other nice features.  The msp34xx chips are used by Hauppauge for
+example.  If your board has one, you might have to load a helper
+module like msp3400.o to make sound work.  If there isn't one for the
+chip used on your board:  Bad luck.  Start writing a new one.  Well,
+you might want to check the video4linux mailing list archive first...
+
+Of course you need a correctly installed soundcard unless you have the
+speakers connected directly to the grabber board.  Hint: check the
+mixer settings too.  ALSA for example has everything muted by default.
+
+
+How sound works in detail
+=========================
+
+Still doesn't work?  Looks like some driver hacking is required.
+Below is a do-it-yourself description for you.
+
+The bt8xx chips have 32 general purpose pins, and registers to control
+these pins.  One register is the output enable register
+(BT848_GPIO_OUT_EN), it says which pins are actively driven by the
+bt848 chip.  Another one is the data register (BT848_GPIO_DATA), where
+you can get/set the status if these pins.  They can be used for input
+and output.
+
+Most grabber board vendors use these pins to control an external chip
+which does the sound routing.  But every board is a little different.
+These pins are also used by some companies to drive remote control
+receiver chips.  Some boards use the i2c bus instead of the gpio pins
+to connect the mux chip.
+
+As mentioned above, there is a array which holds the required
+informations for each known board.  You basically have to create a new
+line for your board.  The important fields are these two:
+
+struct tvcard
+{
+	[ ... ]
+	u32 gpiomask;
+	u32 audiomux[6]; /* Tuner, Radio, external, internal, mute, stereo */
+};
+
+gpiomask specifies which pins are used to control the audio mux chip.
+The corresponding bits in the output enable register
+(BT848_GPIO_OUT_EN) will be set as these pins must be driven by the
+bt848 chip.
+
+The audiomux[] array holds the data values for the different inputs
+(i.e. which pins must be high/low for tuner/mute/...).  This will be
+written to the data register (BT848_GPIO_DATA) to switch the audio
+mux.
+
+
+What you have to do is figure out the correct values for gpiomask and
+the audiomux array.  If you have Windows and the drivers four your
+card installed, you might to check out if you can read these registers
+values used by the windows driver.  A tool to do this is available
+from ftp://telepresence.dmem.strath.ac.uk/pub/bt848/winutil, but it
+does'nt work with bt878 boards according to some reports I received.
+Another one with bt878 support is available from
+http://btwincap.sourceforge.net/Files/btspy2.00.zip
+
+You might also dig around in the *.ini files of the Windows applications.
+You can have a look at the board to see which of the gpio pins are
+connected at all and then start trial-and-error ...
+
+
+Starting with release 0.7.41 bttv has a number of insmod options to
+make the gpio debugging easier:
+
+bttv_gpio=0/1		enable/disable gpio debug messages
+gpiomask=n		set the gpiomask value
+audiomux=i,j,...	set the values of the audiomux array
+audioall=a		set the values of the audiomux array (one
+			value for all array elements, useful to check
+			out which effect the particular value has).
+
+The messages printed with bttv_gpio=1 look like this:
+
+	bttv0: gpio: en=00000027, out=00000024 in=00ffffd8 [audio: off]
+
+en  =	output _en_able register (BT848_GPIO_OUT_EN)
+out =	_out_put bits of the data register (BT848_GPIO_DATA),
+	i.e. BT848_GPIO_DATA & BT848_GPIO_OUT_EN
+in  = 	_in_put bits of the data register,
+	i.e. BT848_GPIO_DATA & ~BT848_GPIO_OUT_EN
+
+
+
+Other elements of the tvcards array
+===================================
+
+If you are trying to make a new card work you might find it useful to
+know what the other elements in the tvcards array are good for:
+
+video_inputs    - # of video inputs the card has
+audio_inputs    - historical cruft, not used any more.
+tuner           - which input is the tuner
+svhs            - which input is svhs (all others are labeled composite)
+muxsel          - video mux, input->registervalue mapping
+pll             - same as pll= insmod option
+tuner_type      - same as tuner= insmod option
+*_modulename    - hint whenever some card needs this or that audio
+		  module loaded to work properly.
+has_radio	- whenever this TV card has a radio tuner.
+no_msp34xx	- "1" disables loading of msp3400.o module
+no_tda9875	- "1" disables loading of tda9875.o module
+needs_tvaudio	- set to "1" to load tvaudio.o module
+
+If some config item is specified both from the tvcards array and as
+insmod option, the insmod option takes precedence.
+
+
+
+Good luck,
+
+  Gerd
+
+
+PS: If you have a new working entry, mail it to me.
+
+--
+Gerd Knorr <kraxel@bytesex.org>