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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
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, but it
does'nt work with bt878 boards according to some reports I received.
Another one with bt878 suport is available from
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),
in = _in_put bits of the data register,
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,
PS: If you have a new working entry, mail it to me.
Gerd Knorr <>