
Intro
=====

people start bugging me about this with questions, looks like I
should write up some documentation for this beast.  That way I
don't have to answer that much mails I hope.  Yes, I'm lazy...


You might have noticed that the bt878 grabber cards have actually
_two_ PCI functions:

$ lspci
[ ... ]
00:0a.0 Multimedia video controller: Brooktree Corporation Bt878 (rev 02)
00:0a.1 Multimedia controller: Brooktree Corporation Bt878 (rev 02)
[ ... ]

The first does video, it is backward compatible to the bt848.  The second
does audio.  btaudio is a driver for the second function.  It's a sound
driver which can be used for recording sound (and _only_ recording, no
playback).  As most TV cards come with a short cable which can be plugged
into your sound card's line-in you probably don't need this driver if all
you want to do is just watching TV...


Driver Status
=============

Still somewhat experimental.  The driver should work stable, i.e. it
should'nt crash your box.  It might not work as expected, have bugs,
not being fully OSS API compilant, ...


Digital audio mode
==================

The chip knows different modes.  Right now you have to pick the one you
want to use at insmod time.  Digital audio mode is the default.  The
chip gives you 16 bit stereo sound with ~32 kHz sample rate.  According
to the specs it should be possible to get up to 48 kHz, but I havn't
figured out yet how this works.  The specs are not very verbose :-(

Lower sample rates are possible too, but the code isn't written yet.
For now you are limited to the fixed 32 kHz.  Mono works throuth, the
driver will do that in software for you.

With my Hauppauge I get clear sound, but I got also reports saying that
digital audio mode does'nt work.  Guess Hauppauge connected the msp34xx
output to the bt878's I2S digital audio input port.


analog mode (A/D)
=================

You can tell the driver to use this mode with the insmod option "analog=1".
The chip has three analog inputs.  Consequently you'll get a mixer device
to control these.

The analog mode supports mono only.  Both 8 + 16 bit.  Both are _signed_
int, which is uncommon for the 8 bit case.  Sample rate range is 119 kHz
to 448 kHz.  Yes, the number of digits is correct.  The driver supports
downsampling by powers of two, so you can ask for more usual sample rates
like 44 kHz too.

With my Hauppauge I get noisy sound on the second input (mapped to line2
by the mixer device).  Others get a useable signal on line1.


more hints
==========

/me uses "sox -w -r 32000 -t ossdsp /dev/dsp2 -t ossdsp /dev/dsp"
to dump audio data from btaudio (dsp2) to es1730 (dsp,dsp1).

Have fun,

  Gerd

-- 
Gerd Knorr <kraxel@bytesex.org>
