[linux-audio-user] [OT] recording levels (mismatch)

[Mark Knecht]

On Tue, 12 Oct 2004 14:02:17 -0700, Erik Steffl <steffl at bigfoot.com> wrote:
> Andres Cabrera wrote:
> > Hi Erik,
> > Usually audio equipment is setup to output/receive at two possible
> > levels -10dBu or +4dBV, Semi-pro equipment and home equipment usually
> > work at -10, which comes up as very low when the receiver is setup for
> > +4. But I would guess the soundblaster card is setup to work at -10 as
> > well (I'd be surprised if it isn't).
> > Maybe there's some software fader somewhere else in the chain?
> 
>    I don't think so, at least I am not aware of anything. The volume is
> too low even when I just use the alsamixer to listen to it. When I
> connect other devices (ipod, line6 guitar pod, yamaha dd-55 percussion
> pad) the volume is OK.
> 
>    Actually maybe I could use headphone output, I didn't think of that
> (it should be pretty much the same as iPod or guitar pod, I use the same
> headphones with all of these).
> 
> > Maybe you need to turn the volume of the amp/tuner all the way up?
> 
>    nothing I do on amp changes the output in any way (volume,
> treble/bass, various effects, dolby settings etc.).
> 
>         erik
> 
> 
> 
> > You shouldn't need a preamp, because the amp/tuner thingy should do that
> > job.
> > Connecting to the speaker output wouldn't be good since speakers expect
> > a very high signal, so I would think it wouldn't work...
> >
> > Andres
> >
> > Erik Steffl wrote:
> >
> >>   I am trying to record from a receiver (not sure if that's the right
> >> name, it's the amp/tuner, the thingy that all other components like CD
> >> player, tape deck, video etc. connect to) and it seems like the levels
> >> are too low (the soundcard is soundblaster live platinum, using alsa
> >> drivers that come with kernel 2.6.5).
> >>
> >>   the receiver has two stereo audio outputs - one is marked as tape
> >> deck recording and the other one as VCR output but both seem to be
> >> about the same (and there doesn't seem to be a way to change them). I
> >> connect them to the RCA connectors on the front panel (live drive) but
> >> even if I set the mixer to 100% (on line in) the levels are too low.
> >> On vu-meter the levels only go to about 25-30% (max) while the
> >> 'normal' audio is somewhere between 50-75%.
> >>
> >>   I believe both the receiver and soundcard work OK (I can record from
> >> receiver to tape deck, I can record to soundcard from other sources)
> >> so I guess there's mismatch between the signal from receiver and what
> >> soundcard expects. Is there anything I can do? do I need some pre-amp
> >> or something? Seems crazy but does it make any sense to try to record
> >> from speaker outputs? Is there any way to change what soundblaster
> >> expects on input?
> >>
> >>   TIA
> >>
> >>     erik

Possibly it's an impedance mismatch? Possibly the input impedance on
the sound card is too low?

Much of the 'consumer' equipment out there uses high resistance output
drivers since they are less expensive to build. Pro equipment will
almost always use low impedance outputs.

Consider the following:

1V amp PP output  -> 1000 Ohm output <---cable----> 1000 Ohm input

In the above pirture to voltage the input receiver sees is 50% of the
voltage actually driven.

Vreceive = Vout * 1000/(1000+1000)

However, consider the following change for pro equipment

1V amp output  -> 100 Ohm output <---cable----> 1000 Ohm input

In this circuit the receiver sees 90% of the voltage driven

Vreceive - Vout * 1000/(100+1000)

On the other hand, if the output is high impedance and the input is
low impedance, then you are really messed up:

1V amp PP output  -> 1000 Ohm output <---cable----> 100 Ohm input

In this case you get Vreceive = 100/(100+100) or only about 9% of the
voltage transmitted.

This stuff comes up a lot when interfacing studio equipment. This, and
the levels provided as per the post from Andreas, are generall the
root cause of this sort of problem.

Good luck,
Mark