Hi,
I've just bought a brand new HP pavalion which came bundled with the
Asus TV FM card, and a Realtek HD onboard audio card. I've got the
digital audio output hooked up to my surround receiver, and working
fine in 5.1 dolby, e.g when watching DVD's but any audio from the Asus
card is only output via the analog output on the PC...nothing goes
through the digital output.
I'm slightly confused....is this an issue with the sound card, the
capture card, or the software (InterVideo home theater) ? How is the
audio on the audio card connected through to the sound card ? Do I
need a new capture card that has a digital output, and route that to
the sound card's digital input ? Am I trying to do the impossible ?
Any help is much appreciated.
Andy
First question. How is sound supported. The first method
is what happens when you have a separate sound card, while the
second method has a sound chip on the motherboard, and that
sound chip is either AC97 or HDaudio standard. HDaudio is
just a doubling of the datarate of the output to the AC97
chip, and many of the functions in the hardware work the
same way.
PCI sound:
| |<--- Sound
Processor - NB - SB - PCI_Bus - PCI_Plugin_Soundcard |<--- Card
| |<--- Jacks
Onboard sound (your system):
|<--- Jacks
Processor - NB - SB - ACLink - Codec |<--- on
|<--- motherboard
Onboard sound uses a digital stream, that is decoded by a CODEC
chip (coder/ decoder). The CODEC is the glue between the digital
and the analog world.
A PCI plugin card, like a SoundBlaster or an Audigy, performs
many of the same functions. The PCI bus is used to transfer
blocks of data into the Soundcard. The hardware on the soundcard
makes a stream of data, and coder/decoder circuits on the PCI
card make the analog signals, just like with onboard sound.
One difference between the two sound solutions, is the AC97/HDaudio
type solutions are constrained by their architecture, to having
many standardized features. The PCI cards, on the other hand,
are completely under the control of the designers, like
Creative. They can put whatever they want on there.
Another difference between sound solutions, is they use varying
amounts of CPU. Gamers sometimes complain about these differences,
but from a hardware perspective, there is no excuse for a difference
between the two solutions. (And as I result, I cannot explain why
one solution is more efficient than the other.)
Now, the next issue is software. Obviously, a Microsoft OS is only
going to support doing certain things with the sound hardware.
There could be hardware capabilities in there that go unused,
for reasons of licensing or for DRM reasons (like preventing you
from making exact copies of media).
Not everything we need to know about your problem, is written
down somewhere.
Asus has a couple of TV FM cards. These look to be different
from one another, and not simple copies.
http://usa.asus.com/products/vga/tvfm_7135/overview.htm
http://usa.asus.com/products/vga/tvfm/overview.htm
The 7135 has an internal audio cable, and that will plug
into a header on the motherboard. The other card uses
a male-to-male 1/8" stereo cable, and that goes from
audio out on the TV FM card, to line in on the motherboard.
In both cases, it should be possible for aux_in or line_in
analog audio signals, to be looped to the lineout analog
output. That path is right in the hardware, and you've proved
it works already.
But, for an analog AC97 input to come out on a digital
output like S/PDIF output, requires cooperation from the OS.
The analog signal is converted to digital form, and
travels across the AClink to the processor. It is up
to the OS, to turn the data stream around, make it
go back across the AClink, and out on the SPDIF output.
Digital rights management prevents certain paths from
working through the OS. For example, SPDIF_in to SPDIF_out
will be forbidden, when the "pro" bit in the SPDIF
stream is set to consumer mode. That is a way for the OS
to enforce DRM and prevent an exact copy of the source
material.
There is less reason to be doing that, for what you want to
do. In your case, turning an analog input back around, for
output on the digital side, is a degrading step, so an
exact copy doesn't happen. There is no reason for DRM to
stop something like that, that I can think of.
As for how the SPDIF output works:
SPDIF is by nature a two channel (stereo) hardware interface.
The only way to get 5.1 channels across it, is via a compression
format called AC3. When you play a DVD, there is already a
digital stream encoded in AC3 format, and the computer does
not interact with the stream at all. The computer just
passes the AC3 stream out the SPDIF_out, and the external
audio device is responsible for decoding the AC3 and making
the 5.1 signals. You've tested that and it works.
Encoding AC3 (i.e. converting 5.1 analog signals into the
two channel digital stream) is done by a Dolby Audio encoder.
This is licensed from Dolby. The Realtek ALC880 (assuming that
is the chip on your motherboard) comes in two versions, and
I believe the 880D version comes with a bundled AC3 software
encoder.
What the software encoder would allow, is taking an arbitrary
5.1 audio source, and converting to AC3, so it can pass over
SPDIF_out. I've seen one posting, concerning a Cmedia
implementation that does the same thing, and they report
the latency for conversion is 1.5 seconds. That makes it hard
to line up audio and video (lipsink), for example, unless
the video stream can be tracked via a timestamp somehow.
HP may or may not have bought the 880D version, for use on
your computer. And, in any case, software solutions like that
have limited utility (i.e. Windows desktop sound, like an
alert sound, taking 1.5 seconds before it reaches your
SPDIF equipped audio solution, would suck).
I wish I could answer your "analog in to digital out loopback"
question, but I don't know if OSes prevent this path on
purpose, or the absence of the path is just a driver bug
or oversight. Certainly, your question has been asked before.
SPDIF is a "Swiss cheese" that needs quite a few holes filled.
It is not a very useful standard.
To learn more about your onboard sound chip, try:
http://www.realtek.com.tw/products/products1-1.aspx?lineid=2004052
HTH,
Paul
