On Sat, 12 Feb 2005, colliepon wrote:
Researcing into Bt848 and Bt878 based cards and a bit confused on some issues. From what I understand BT848 cards have NO audio built in and require a sound card to rip, but can rip stereo. BT878 cards digitize audio on board, but are only mono. (please correct me if wrong) And since I want to run 2-3 dumb cards (alongside 1+ Hauppage cards eventually) this makes things a bit more complex too. :)You may wish to rethink this plan. BT8xx capture cards take quite a bit of PCI bandwidth (uncompressed data going over the PCI bus). One's OK, two's probably OK, but three's pushing it. MPEG2-encoding aside, The capture quality of the BT8{48,78,79} is quite a bit poorer than the SAA7115 or CX88 used on the PVR-[23]50 and PVR-150, respectively. It's pretty much VCR-quality, whereas the SAA7115 is broadcast quality.
1) First up, can someone explain BTaudio a bit better? I've read the FAQ, but I didn't see whether it obtains stereo or mono sound and the compatible card list isn't updated in nearly two years.
From the kernel documentation (Documentation/sound/oss/btaudio): <snip> The chip knows two different modes. btaudio registers two dsp devices, one for each mode. They can not be used at the same time.
Digital audio mode ==================
The chip gives you 16 bit stereo sound. The sample rate depends on the external source which feeds the bt878 with digital sound via I2S interface. There is a insmod option (rate) to tell the driver which sample rate the hardware uses (32000 is the default).
One possible source for digital sound is the msp34xx audio processor chip which provides digital sound via I2S with 32 kHz sample rate. My Hauppauge board works this way.
The Osprey-200 reportly gives you digital sound with 44100 Hz sample rate. It is also possible that you get no sound at all.
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 <snip>
2) is it possible to rip stereo with a BT878 card by routing it to the sound card anyway despite the built in mono? (or is there no provision or ability provided) Alternately do any BT878 cards have stereo supported in Myth? AKA trying to figure out if it's better to get an 848 or 878 card - i'd like to rip stereo for many of the programs I want to record.The original purpose of the audio A/D in the BT878 chip was to do software audio decoding. That's why the samplerate will go very high (448kHz stock, or 4x that rate with a mild hack). In order to do stereo demod in software, the whole baseband spectrum must be captured. I don't know of anyone besides the gnuradio folks that have played with it much. To get stereo from the cards, I believe you operate under mode #1 above... which requires the msp34xx chip. I think that the audio device you get with btaudio is then stereo. Someone who knows more about this can feel free to correct me... I've only played with the mono high-rate version.
I know that most people who use bt8xx cards use them in this way (sound routed to soundcard). The separate difference in latency between the video path and audio path does have the potential to generate a slight sync mismatch, but it won't be progressive like on a bad MPEG2->DIVX rip. It'll pretty much stay at it's (small) value and (imperceptibly) jitter around that.
3) If BTaudio doesn't work, and I get a BT848 needing a sound card, are there any non-easily-fixable (like a programmed offset or something) problems with sync? (having read of sync and speed mismatch problems when people try to rip movies with cards that have no on board audio digitizing) Wondering whether i'd likely have trouble with an 878 (mono) card, and two 848 cards routed to the onboard audio and an add in sound card with stereo.
-Cory
************************************************************************* * Cory Papenfuss * * Electrical Engineering candidate Ph.D. graduate student * * Virginia Polytechnic Institute and State University * *************************************************************************
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