g.bon said:
Hello,
I'm wondering what kind of input I could expect on a soundcard.
Are they mono or stereo ?
My aim is to use it like a little scope but type of input is never
precised on datasheets.
I would prefer stereo for having 2 inputs.
Can someone tell me if it's usualy mono or stereo and on what cards ?
on realtek mobo chipsets, on soundblaster...
Thanks,
GB
At the audio inputs on the sound card, all inputs and outputs
are capacitively coupled. That means, if there is any
DC component to the signal, it should not appear when
measured by the ADC.
Line-In is definitely stereo.
The Microphone can be mono or stereo. In the past, on AC'97
codecs, you'd see some of both. (Cheapest codecs were mono.)
With HDAudio codecs, there is no longer an incentive to make
all the ports have different capabilities. The ports
tend to have the same capabilities. Some restrictions
are imposed by the driver, but it's hard to say how
important that is. (I don't think you can get 32 ohm
drive on all 5.1 outputs for example. The max seems to be
2+2 32 ohm drive. All the ports may have a widget with
amplified output.) Even the front panel microphone
and headphone jacks, can switch roles, such that the
headphone jack becomes a microphone input and the
microphone jack becomes a headphone output. The control
panel for the audio device, may ask you to verify what
you just plugged in (using jack sensing), and then you
can indicate the port needs to change roles.
The input resolution may vary between 16 bits and 24 bits.
But the noise floor of the card, may not make good use
of all 24 bits. Motherboard sound chips are the worst in
that regard, as they tend to pick up more background
noise. A sound card with a Faraday cage, is about as
good as you can do, for an internal installation. There
can be a 30dB difference in noise floor, between good
and bad sound designs.
On this page, the writer of the article uses a buffer
amplifier, in front of the sound card. This helps
remove the poor loading characteristics of the sound
card, from external probes. But the circuit in this
example, also continues to be AC coupled, and has
caps on both input and output.
http://www.sciencetronics.com/geocities/electronics/projects/soundcard_osci.html
This page does a bit better job.
http://www.qsl.net/om3cph/sb/dcwithsb.htm
( linked from
http://www.epanorama.net/phpBB3/viewtopic.php?f=4&t=31133 )
I've seen one web site years ago (but I appear to have lost the
link), with a circuit diagram for a proper input stage for an
oscilloscope. It would offer switchable gain selection on the
input, and allow usage with standard oscilloscope probes. So if
you continue your search, you may find an even better article
on how to form a nice input interface on the thing.
I would say, if the motherboard is HDAudio type, you should be
satisfied with the capabilities. If you have an older computer
(one with an AGP slot), then you could end up with a monophonic
AC'97 chip. Installing a PCI sound card, can fix that to some
extent. Some of the more expensive sound cards, have separate socketed
op amps for buffering, which could improve the characteristics
a bit.
You might also look to see, if there are any USB chips with
ADC inputs on them. Perhaps you can build something slightly
better than can be provided by the sound card.
Paul
