Ted Spencer
Veteran
Please excuse the very long post, but hopefully the following might be useful to some...
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First a little background:
In a mixing console or mic preamp, a "trim pot" (pot = potentiometer, or gain control) is a level control at or near the earliest stage of a signal path. Its purpose is to adjust gain before it hits the later stages of the signal path (equalizer, output stages, A/D converter, etc.) in order to A: prevent distortion, and/or B: optimize signal-to-noise ratio.
A mixer or preamp's "pad" does the pretty much the same thing, but is a switch as a rule, not a pot, most often reducing gain by about -20db or so. It is 'ahead' of the trim pot in the preamp/console's signal path, and is thus the first line of defense against preamp overload/distortion. Thus the GAIN setting in a camcorder's menu would be somewhat more accurately described as a variable pad, not a trimmer, since the settings are fixed.
So much for semantics... : )
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The next stage of gain control is the main volume control - the channel volume fader or knob on a mixer, or the knob you see on the outside of a camcorder (mixers usually have yet another control for the output stage, but not cameras). The HVX200a's audio block diagram (presumably similar to the HPX300's) appears to indicate that the "gain" menu item is indeed an early, "pad" type control and that the outside volume knobs are much later in the circuit. So the setup drill for these types of camcorders would be:
-------------------
-Set the "gain" menu item with a given mic (mics vary hugely in nominal output) so that a proper record level (average meter indication of about -12 to -16db, and all peaks safely below clipping) is achieved with the physical volume knob position at 45-65% of its rotation.
-If that knob position doesn't produce enough gain, change the gain menu item to a *lower* number (from -40 to -50 for example). If the knob position produces too much level, switch the gain setting to a higher number.
-Then use the volume knob again to fine tune the level so it's ideal. The gain menu changes go in big jumps, so the knob is there to allow you to make the smaller adjustments between the steps.
-If you've got the level knobs at about 45-65% of their travel and the level indicated on the meters is as described above, you're done.
-Regarding the GAIN menu numbers: they indicate the *nominal output level of the microphone* (-30, -40, -50, etc.). A mic that puts out a nominal -30db is 10db *hotter* than a mic that puts out -40db (remember we're talking negative numbers here). So, changing the menu GAIN setting from a LOWER number (like -40) to a HIGHER number (like -30) DECREASES gain. You're telling the camera to expect a 10db hotter mic than at the -40 setting.
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Note: the noise in any device like a camera or mic always exists in a *fixed* amount.
No individual gain change will actually increase or decrease noise - it and the signal will stay in exact proportion wherever the gain is set. But *combinations of more than one gain control* can. So, for example, if you've got the gain menu set very low (a higher number, like -30), and you have to 'make up' the gain by turning up the volume knob way up, you will increase noise because you are amplifying noise-generating electronic components inbetween the two gain controls (low gain early, noisy bits in the middle, high gain later).
The solution is to raise early gain (menu) and lower late gain (knob). Now, the noise-generating stages inbetween, which are still making the same amount of noise they always do, are effectively made less so because the signal has been increased *before* the noisy bits get in the game, and then reduced *after* them. All the noise generated by them thus decreases with the lowering of the volume knob. The chain of events looks like this:
FIXED GAIN, normal or low level mic (or quiet source)--->VARIABLE gain menu setting--->FIXED noise generating circuitry--->VARIABLE volume knob
With a hotter (louder) mics or source material, distortion is the issue, not noise (the hot signal will literally drown out the noise). The end result should be the same though: volume knobs at 45-65%. But the gain structure situation described above is *reversed* to avoid distortion:
FIXED GAIN loud mic level (or loud source)--->VARIABLE gain menu setting--->FIXED circuit distortion threshold--->VARIABLE volume knob
So the GAIN MENU setting can decrease the signal *before the distortion generating bits* as the VOLUME KNOB SETTING restores the signal to achieve manageable record levels.
In the first example (lower level mic/signal), distortion is largely irrelevant and in the second example (higher level mic/signal) noise is largely irrelevant.
May all your audio be clean, quiet and hot!
-----------------
First a little background:
In a mixing console or mic preamp, a "trim pot" (pot = potentiometer, or gain control) is a level control at or near the earliest stage of a signal path. Its purpose is to adjust gain before it hits the later stages of the signal path (equalizer, output stages, A/D converter, etc.) in order to A: prevent distortion, and/or B: optimize signal-to-noise ratio.
A mixer or preamp's "pad" does the pretty much the same thing, but is a switch as a rule, not a pot, most often reducing gain by about -20db or so. It is 'ahead' of the trim pot in the preamp/console's signal path, and is thus the first line of defense against preamp overload/distortion. Thus the GAIN setting in a camcorder's menu would be somewhat more accurately described as a variable pad, not a trimmer, since the settings are fixed.
So much for semantics... : )
-------------------
The next stage of gain control is the main volume control - the channel volume fader or knob on a mixer, or the knob you see on the outside of a camcorder (mixers usually have yet another control for the output stage, but not cameras). The HVX200a's audio block diagram (presumably similar to the HPX300's) appears to indicate that the "gain" menu item is indeed an early, "pad" type control and that the outside volume knobs are much later in the circuit. So the setup drill for these types of camcorders would be:
-------------------
-Set the "gain" menu item with a given mic (mics vary hugely in nominal output) so that a proper record level (average meter indication of about -12 to -16db, and all peaks safely below clipping) is achieved with the physical volume knob position at 45-65% of its rotation.
-If that knob position doesn't produce enough gain, change the gain menu item to a *lower* number (from -40 to -50 for example). If the knob position produces too much level, switch the gain setting to a higher number.
-Then use the volume knob again to fine tune the level so it's ideal. The gain menu changes go in big jumps, so the knob is there to allow you to make the smaller adjustments between the steps.
-If you've got the level knobs at about 45-65% of their travel and the level indicated on the meters is as described above, you're done.
-Regarding the GAIN menu numbers: they indicate the *nominal output level of the microphone* (-30, -40, -50, etc.). A mic that puts out a nominal -30db is 10db *hotter* than a mic that puts out -40db (remember we're talking negative numbers here). So, changing the menu GAIN setting from a LOWER number (like -40) to a HIGHER number (like -30) DECREASES gain. You're telling the camera to expect a 10db hotter mic than at the -40 setting.
---------------
Note: the noise in any device like a camera or mic always exists in a *fixed* amount.
No individual gain change will actually increase or decrease noise - it and the signal will stay in exact proportion wherever the gain is set. But *combinations of more than one gain control* can. So, for example, if you've got the gain menu set very low (a higher number, like -30), and you have to 'make up' the gain by turning up the volume knob way up, you will increase noise because you are amplifying noise-generating electronic components inbetween the two gain controls (low gain early, noisy bits in the middle, high gain later).
The solution is to raise early gain (menu) and lower late gain (knob). Now, the noise-generating stages inbetween, which are still making the same amount of noise they always do, are effectively made less so because the signal has been increased *before* the noisy bits get in the game, and then reduced *after* them. All the noise generated by them thus decreases with the lowering of the volume knob. The chain of events looks like this:
FIXED GAIN, normal or low level mic (or quiet source)--->VARIABLE gain menu setting--->FIXED noise generating circuitry--->VARIABLE volume knob
With a hotter (louder) mics or source material, distortion is the issue, not noise (the hot signal will literally drown out the noise). The end result should be the same though: volume knobs at 45-65%. But the gain structure situation described above is *reversed* to avoid distortion:
FIXED GAIN loud mic level (or loud source)--->VARIABLE gain menu setting--->FIXED circuit distortion threshold--->VARIABLE volume knob
So the GAIN MENU setting can decrease the signal *before the distortion generating bits* as the VOLUME KNOB SETTING restores the signal to achieve manageable record levels.
In the first example (lower level mic/signal), distortion is largely irrelevant and in the second example (higher level mic/signal) noise is largely irrelevant.
May all your audio be clean, quiet and hot!
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