> On this list, the word "gain" has been used in many various ways, two
> are more common and it gets very confusing.
Klas,
I don't think that there is anything wrong with how we use the term "gain", at
least from an engineering point of view.
In my option (I'm a trained engineer), there is actually no big difference
between the two things.
> 1/ Professional people mostly mean by "gain" that it is the amount of
> amplification that a amplifier is been set to, as 60, 50, 40, 30 etc
> db. That is one thing.
Yes, in this case the gain (expressed in dB) is a positive number.
> 2/ When talking about consumer products, we often mean by "gain" the
> maximum gain, like "high sensitivity" and "the wheel" at ten. The
> wheel, is then a open potentiometer.
The "high sensitivity" at this setting is also realized by an amplifier, just
in the same way as in your paragraph 1/
> When being closed, from 10 to 0,
> it shortens the output of the mic preamp to ground and we will have a
> linear relationship between "gain" and noise. When the potentiometer
> is shorting the signal to ground, also the preamp noise will be
> shortened to ground.
Right, but this can also be interpreted as a negative gain (attenuation).
Imagine the potentiometer was dividing the input voltage by a factor of 10.
This would be an attenuation of 20 dB or a gain of -20 dB.
Most recorders contain a preamplifier that has a fixed gain or sometimes two
different gain settings. On the Olympus recorders there are two gain settings:
SENSE HIGH and SENSE LOW.
In addition that, they usually have a recording level control wheel that is
basically a potentiometer that attenuates the signal somewhere within the
signal chain. The total "gain" of the recorder is the gain of the preamp (let's
say +60 dB) plus the negative "gain" of the recording level potentiometer
(let's say -20 dB). This just means that the total gain of the recorder is 60 -
20 = 40 dB.
In theory, there should be no difference if the gain of the preamp was only +40
dB and the potentiometer was "open" (-0 dB setting). In other words, it would
be valid to treat the preamp / potentiometer combination as a "black box"
amplifier that indeed has a certain "gain".
There is however a difference in the real world: A high preamp gain (+60 dB in
this case) can lead to clipping at the output of the preamp that is sometimes
difficult to detect (Dan talked about that in a recent post).
> This is vice versa from the professional
> concept, where noise increases, as you turn down the gain.
Hmmm, that's not always true. We have to distinguish here between the different
sources of noise:
The noise originating from the microphone or preamp input (EIN) will be
affected by both the preamp and the potentiometer in the same way (see the
above black box principle).
Nevertheless, it is true that a low gain can (depending on the specific design
of the recorder) can lead to a condition where other noise components than the
mic noise or EIN of the preamp dominate the total system noise. This can either
be the quantization noise of the A/D converter or just the thermal noise of
other components in the along signal chain. But note that this low gain
condition can also be caused by a low position of the recording level wheel!
In most recorders it can also happen that a low recording level wheel position
(such as level 2 on the Olympus LS10/11 at SENSE HIGH) reduces the signal level
within the preamp chain below a critical voltage. To understand this, you need
to know that any analog signal path does not have an unlimited dynamic range
towards the low end. The thermal noise even of simple resistors would be the
limit here. In addition to that there might also be tiny digital noise
components that enter the analog part of the recorder and which can also
degrade the performance of the recorder.
The general rule here is : Avoid low signal voltages anywhere in the analog
signal chain!
I hope that this will help a bit to better understand the "mysteries" of the
recorders ;-)
Regards,
Raimund
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