> Subject: [Alsa-user] Noise with Ardour, jackd...via
8235
>
> Hi there,
>
> I'm trying to get Ardour running, to record some
original
> music. When I start jackd I get a continually
repeating
> message...
>
> **** alsa_pcm: xrun of at least 0.0xx msecs.
>
> ...which apparently is OK? I then start Ardour, and
open
> a session, plug my bass guitar into the mic-in port.
When
> I listen to the audio output it is a mish-mash of
digital
> noise that has a pattern that repeats as often as
the
> jackd message repeats, but my bass guitar sound is
still
> partly in there. It does not matter if I run jackd
and/or
> Ardour as root, I still get the same noise...
>
> I have a Via KM400 mobo with the 8235 southbridge.
Alsa uses
> the via82xx driver, and most things seem to work.
What can I
> do to get more information about my audio subsystem
in order
> to eliminate the cause of this noise?
>
> --
> Kind regards,
>
> Chris Wilkinson, Christchurch, New Zealand.
>
You may need to use an impedance matching transformer,
or a microphone preamplifier, between your guitar and
the microphone input on your sound card. Your sound
card probably has a "high impedance" input, and I
think all electric guitars have "low impedance"
outputs. Most good microphones also have "low
impedance" outputs.
The amount of electrical power feeding into your sound
card's microphone input, determines how high you will
need to set your microphone gain. If the power is low,
you will need to set the microphone gain high. If the
microphone gain is set high, you will pick up all
sorts of extraneous electrical noise, and the sound
will be terrible. The guitar will sound very weak and
the noise will sound very strong. The guitar may sound
so weak that you can not hear it at all, all you will
hear is noise.
The amount of electrical power feeding into your sound
card's microphone input, is equal to the voltage
supplied by the guitar pickup, multiplied by the
quantity of current that is made to flow in the sound
card's input circuitry by the application of that
voltage.
1) POWER (input to the sound card) = VOLTAGE (supplied
by the guitar) X CURRENT (flowing into the sound card
input)
2) CURRENT (flowing into the sound card input) =
VOLTAGE (supplied by the guitar) / RESISTANCE (of the
sound card input)
substituting #2 into #1 :
3) POWER (flowing into the sound card input) = VOLTAGE
X VOLTAGE (supplied by the guitar)
------------------
RESISTANCE
(of the sound card input)
Note: for the purpose of this discussion, resistance
is close to the same thing as impedance. When
alternating voltage and current are discussed, their
ratio is called impedance. When direct voltage and
current are discussed, their ratio is called
resistance.
So, to get enough power to flow into the sound card's
"high impedance" input, you will need to increase the
voltage coming out of the guitar to an acceptable
level.
The guitar has a "low impedance" output. That means
the ratio of voltage / current coming out of it is
low. Low voltage and high current.
The sound card microphone input is "high impedance".
That means it requires an input signal where the ratio
of voltage / current is high. High voltage and low
current.
A transformer, or microphone preamplifier, is capable
of taking the low voltage signal coming from the
guitar, and increasing it so that it is great enough
to supply enough power to the microphone input.
This is a little tricky to understand. A transformer
does not increase power (in fact, it consumes a small
amount). It is not taking the power from the guitar
and increasing it. The transformer increases voltage
so that it is large enough to cause enough current to
flow into the microphone input circuitry to get enough
power so that you don't have to turn the microphone
gain up too high.
If you have the correct transformer, you may not need
a preamplifier. If depends on how much amplification
the microphone amplifier inside the sound card
provides; how sensitive it is.
When I say the correct transformer, I mean one with
the correct impedance ratio. It must have a low
impedance at the input to match the guitar pickup, and
a high impedance at the output to match the sound card
microphone input. The impedance ratio of a transformer
is determined by the ratio of the number of primary to
secondary turns it has. This information is usually
printed on the body of the transformer.
There is also the issue of connectors. The input
connector on the transformer has to mate with the
guitar plug, and the output of the transformer has to
mate with the sound card's microphone input connector.
You may need adapters and cables to match up the
connectors. There is also the issue where the
microphone input is stereo and your guitar is output
is mono. You may need to consult with a local expert
that knows about connectors.
An electronic amplifier, such as a microphone
preamplifier, is capable of increasing power. This may
be your best solution. You will need a preamplifier
designed for low input impedance, so that it is a
match for the guitar pickup.
With a preamplifier, you can turn up the gain if the
guitar sounds too weak without having to turn up the
gain on the microphone channel of the sound card so
high that you get noise.
Your guitar amplifer has the correct low input
impedance preamplifier built into it. If your guitar
amplifier also has a "high impedance" output (not the
speaker output, this is low impedance) you can connect
a cable from that to the microphone input. It might be
the case that the signal level from the guitar
amplifier is to great and will overload the microphone
input of the sound card. In that case, you may be
better off connecting to the "line input" on the sound
card instead of to the microphone input.
You may wonder why engineers would design some things
with low output impedance and some things with high
input impedance. Wouldn't it make more sense to design
everything with the same impedance so that you could
plug anything into anything and always have it work.
The answer is mostly to save money. High impedance
microphones are cheap to make. (They also usually
sound cheap.) High impedance microphone amplifier
circuits are also cheap to make. So, you need a cheap
microphone to go with a cheap sound card. A
professional sound card will have low impedance input
circuits build into it.
Mark Rainess
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