Btaudio (bt878)

The btaudio kernel module allows you to get the audio from your tuner card directly over the PCI bus without using a patch cable to your sound card. This is useful if you would like to reserve your sound card for other functions, if you want to have multiple tuner cards in one computer, or if your sound card does not handle full-duplex.

Instructions

• Gerd Knorr's instructions (copied from the 2.6.12-rc2 kernel and lightly updated)
• MythTV wiki: setting up btaudio on an ATI TV Wonder card
• Failed attempt at getting the sound directly off a LifeView FlyVideo3000 bttv card
• Note that saa7133 cards unproblematically let you get the audio directly

Supported cards

For btaudio to work, you need a bttv tuner device with an MSP34xx chip. The chip also needs to be connected to the board in a certain way to work.

These cards are known to work with btaudio (thanks in part to MythTV HowTo):

   * ATI TV Wonder
   * Hauppauge WinTV-radio with dbx-TV stereo, model 401
   * Hauppauge WinTV-Theater, model 495, 498 (Europe)
   * Hauppauge WinTV Express, models 751, 753 (but see note)

Note: Hauppauge model labels are often uninformative. For example, the box labeled model 753 may contain one of these:

* board #44804 rev #D183, auto card=10; tuner TCL2002MB-3 (hidden sticker), but has to be forced to tuner=41 (Phillips PAL-MK(FI1216MK)
* board #44804 rev #D108, auto card=10; tuner FI1215MK5 (hidden sticker), autodetected as tuner=5.

The following cards do not work:

   * Pinnacle Studio PCTV Pro (this has an MSP34xx, but it's not wired correctly to the BT878 chip)
   * ATI TV Wonder VE
   * Leadtek Winfast 2000 XP (PAL, UK and NTSC)
   * I/O Magic PC-PVR (no MSP34xx chip)
   * Avermedia AverTV Stereo (no MSP34xx chip)

The following cards have been reported to work, but have issues:

   * Avermedia AVerTV Studio (no digital DSP output, "whiney noise" on analog)

The list of supported and unsupported cards is still a work in progress; please edit this page if you have new information.

Technical background

This account is preliminary and may be incorrect.

The analog device created by the btaudio driver has three channels, called Line1, Line2, and Line3. The sound comes through on Line2 (cf. Instructions section above). The 878/879 audio A/D has two inputs - STV and LINE. One of the line inputs is an internal mux switch between STV and LINE. The other is a switch for a single stage 6dB preamp boost for the audio signal.

On cards that support the btaudio driver, the A/D audio stage runs a 1.792MHz clock with valid hardware decimation values between 4 and 15 (controlled by bits 8-11 of local register 0x10C). This would give the following valid hardware rates:

 4:   448    kHz
 5:   358.4  kHz
 6:   298.67 kHz
 7:   256    kHz
 8:   224    kHz
 9:   199.1  kHz
10:   179.2  kHz
11:   162.9  kHz
12:   149.3  kHz
13:   137.8  kHz
14:   128    kHz
15:   119.47 kHz

The input gain control in the audio A/D has 15 gain settings, from 0.5 times (-6db) to 3 times (10db) - this is in addition to the 6dB switchable audio preamp boost. What gain setting you need depends on the magnitude of the input signal from the tuner. The audio A/D is designed with a nominal input signal of 0.5V RMS, but the tuner input could be wildly different.