Re: [linux-audio-user] Recording Level using Audacity and a Multiface card

From: Fernando Lopez-Lezcano <nando@email-addr-hidden>
Date: Sat May 13 2006 - 01:32:55 EEST

On Sat, 2006-05-13 at 06:42 +1000, D. Sen wrote:
> Ok, so the gain option quoted by the device isnt the gain quoted by the
> conventional pre-amps. However, you are assuming a lot here. In essence,
> you are assuming that just because you dont see "60 dB gain", the device
> is not amplifying the signal to line-level when in-fact, even at x1
> gain, the device is producing line-level and the extra 20 dB gain using
> the x10 setting is beyond what is required for line-level inputs.

AFAIK I'm not assuming anything about the box[I _am_ assuming that the
microphones you use produce typical mic level output]. I can only read
what the spec you posted says. It clearly states that the box can act as
a unity gain signal conditioner (X1 gain) or as a X10 amplifier. That's
all it says - it does not talk about line levels or anything like that.
It then says that the linear output voltage is going to be 10V p-p or
greater. In my understanding that is the clipping voltage or max voltage
at the distortion levels they specify (ie: you can't get more than that
without introducing distortion). The actual output voltage you get will
depend on the input voltage you feed it.

> The gain does not tell you anything about the voltage at the output.
> The pre-amp from RME quotes an output of 21 dBu max. This converts to
> about 24 V p-p. The device I am using produces an output of 10 V p-p.
> This is a difference of slightly above 6 dB.

[I don't mean to patronize you]

Gain multiplied by the input voltage gives you the output voltage:
   vo = vi * gain

You have a fixed gain (X10) and you want a given output voltage (10V
p-p, just an example), to get that you need to supply a 1V p-p input
signal. What I don't know is what output signal you are getting from the
http://www.bksv.com/pdf/Bp1380.pdf (BTW, very very nice :-) at the sound
pressure you are measuring, but your results would indicate it is not
enough to get full scale output from the signal conditioner, as seen by
the RME card.

> If at the end, the gain was the root of the problem, I would surely see
> the same kind of low level recording on other digital recorders (namely
> a FOSTEX and a SONY line-level input recorders), and I dont.

I imagine that depends on what kind of input stage you have in those
machines. Usually they have a gain control which you can use to increase
the input stage gain (and thus maybe compensate for the gain of your
conditioner box).

It could also be as Sampo suggested that the RME multiface is set for
the less sensitive input gain (both gains are considered "line level" -
quite confusing).

-- Fernando

> Fernando Lopez-Lezcano wrote:
> > On Sat, 2006-05-13 at 06:00 +1000, D. Sen wrote:
> >> Look at the specs. The output is definitely line-level. The function of
> >> a pre-amp is to produce line-level output and this does it.
> >
> > Not really. Please compare to the specs of a real mic preamp, please,
> > for example:
> > http://www.rme-audio.com/english/micpreamps/quadmic.htm
> > (which would be an appropriate companion to the multiface)
> >
> > In particular: "Amplification can be set between 10 and 60 dB". 60dB of
> > gain expressed as a voltage ratio would be amplifying the signal by
> > 1000. I would imagine you would need to use 40 to 60dB to amplify a mic
> > to line level so you are at least a factor of 10 below what would be
> > needed.
> >
> >> It is sold by B&K and they swear by it.
> >
> > (they can swear all they want but that's not gonna make it amplify the
> > signal more :-)
> >
> > The key here (as outlined before in the thread) is gain. This signal
> > conditioner's gain is too low (probably by a factor of at least 10) to
> > act as a mic preamp _unless_ your line input has additional gain you can
> > dial in (which is probably why you can use it with other equipment).
> >
> > -- Fernando
> >
> >
> >> Paul Winkler wrote:
> >>> On Sat, May 13, 2006 at 04:16:28AM +1000, D. Sen wrote:
> >>>> This is what is being used between the microphone and the multiface:
> >>>>
> >>>> http://www.bksv.com/pdf/4416B.pdf
> >>>>
> >>>> This is providing line-level output to the multiface.
> >>> What you have there isn't really a microphone preamp. Max gain of 10
> >>> isn't very much at all, it's what, 20 dB?
> >>>
> >>> Commercial mic preamps typically have at least 60 dB of gain available.
Received on Sat May 13 04:15:05 2006

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