Gain level vs. input volume level?

[Denez McAdoo]

A number of digital records have a gain switch that boosts the signal from low, to medium, to high. But my question is how is this different from the input volume level?

[Roger Gunkel]

The gain controls the level of signal coming into the input in relation to the impedance of that input. This means that if you try interface too high a gain signal with a particular input impedance, you will overload that input no matter how low you take the volume. Conversely, if the input gain level is too low, then there will be too little signal level to give useful volume, and probably with a high level of system noise.

The gain control therefore enables you to match the recorder's input to the incoming signal, to maximise the use of the volume control and minimise the noise level and distortion. There will frequently be a clipping light near the gain control to show when the signal is optimised. Take the gain control up until it just starts to clip on the highest input, then back it off slightly. The volume control is then used to set the record level on the meters. If there is no clip level, then set the volume control to the 0db position and using the loudest sound that you expect to record, turn up the gain until the volume meter just starts to peak, then back it off slightly. That will give you he best signal to noise ratio.

A final point is to ensure that the gain level is always set before the volume control is used. Failure to set up the gain correctly results in poor sound quality and is a common fault with inexperiienced sound engineers.

Roger

[Greg Miller]

I more or less agree with the above, but with a few points of clarification.

I personally think the term "gain level" is confusing. "Gain" is the amount of amplification provided by an electronic device. "Level" is the the amplitude of a sigal at some point within a given device, or at some point where two devices are interconnected.

For example, if an amplifier has an input signal whose level is 1 volt, and the amplifier's gain is +6 dB, then the output signal will have a level of 2 volts.

Gain is measured in dB, and it can be a positive number (amplification) or a negative number (attenuation). A gain of 0 dB means there is no change in level.

Level of a signal within a recorder is also measured in dB but it's always a negative number, since zero dB (more correctly zero dBFS -- Full Scale) represents the clipping level of the device. No signal can be louder than that, and you need all your audio to be lower than that so it won't clip.

Level of an electrical signal where two devices are interconnected can be measured in dBm (where 0 dBm = 1 milliwatt of power in a 600 ohm circuit), or dBV (a voltage measurement) or less often some other reference level.

You can't really measure milliwatts directly, which is why dBm is used less and less with passing time. To know the milliwatts in a circuit, you first need to know the impedance of the circuit, then do some math. E^2 (voltage) = P (power level in watts) * R (impedance in ohms). So for 1 milliwatt in a 600 ohm circuit, you find that E^2 = .001 * 600 = 0.6, therefore E = square root of 0.6 = 0.775 volts. So you then get a voltmeter, find out where the needle is pointing when you apply 0.775 volts, and mark that as "0 dBm" on the meter scale... etc. And if you later use that meter to measure a circuit with some impedance other than 600 ohms, the dBm reading will be wrong!

In the final analysis, what really counts is the voltage, not the power, and not the impedance. If the gain is set too high for a given input voltage level, then clipping will occur despite the setting of volume controls later in the system... but that isn't really related to impedance (as was stated by a previous poster), it's just a function of voltage. Impedance mismatch can cause some problems, but it doesn't directly cause clipping.

[Roger Gunkel]

You are of course right Greg re voltage rather than impedance, it's been over 30 years since I used any electrical theory so I am getting rusty :-) The practice though is as I described, which should enable the OP to get his basic settings correct.

I also agree with you regarding the use of the term 'Gain', which often seems to get interchanged with 'volume'. You often hear people say 'bring the gain up', when they mean volume. The alternative and older name was 'trim', which seems to much better describe the use of the control.

Roger

[Rick Reineke]

In most of the budget digital recorders, The gain switch is usually just an input trim, whilst the record volume is only the digital level. Many have a fixed analog gain, so a hot source signal will 'clip' the input stage and distort regardless of the operator lowering record volume. The limiters on most of these recorders are in the digital realm as well and do not protect the analog input stage. On the other hand, if the input source is low, boosting the digital volume just brings up the noise from the inadequate pre amps.

[Greg Miller]

Hello Roger,

Ah, sounds as if you're another old-timer like myself (meaning anyone over 20 years of age).

Seems to me that there is a lot of "creep" in terminology.

I remember reading somewhere (and this probably 40 years ago) that the term "volume" originally applied to PA systems, and strictly speaking referred to the volume of air being moved by the loudspeakers.

I dislike the term "volume" in relation to electronics. Strictly speaking you change the "gain" of an amplifier; doing so will change the "volume" of sound in the room. But if you're recording, you're not moving air, you're just storing bits, so to my [overly literal] mind, "volume" does not apply.

Indeed, as you point out, most better mixers and recorders have more than one control. There is an input "trim" control, which ideally* adjusts the gain of the first preamp stage, thus affecting the input level that will produce clipping. (On more expensive gear, the "trim" control will be a continuously variable potentiometer; on cheaper "pocket recorders" it may be a two- or three-position switch.)

* I say "ideally" above, because with some cheaper gear, it's just an attenuator between the input jack and the first preamp stage. The disadvantage of this attenuator configuration is that the input stage's gain remains fixed, so the noise level remains fixed also. When the input gain is actually variable, lowering the gain should also lower the noise level.

At any rate, your procedure is absolutely correct: Get the "talent" to make some sound at the loudest possible level, and adjust the trim control to slightly below the point where the input clip light blinks. You shouldn't need to touch the trim thereafter, unless the input level becomes a lot louder in actual performance than it was during the set-up procedure.

[Roger Gunkel]

@Greg- yeah 66 now and 50 of those years involved in a mix of music, audio engineering and video production and still love it :-)

I also seem to remember something in the dim and distant past about audio volume meaning movement of a volume of air, so technically mass rather than decibels. Of course with no air mass there would be no decibels anyway! Its a bit like varying the volume of water flow with a tap.

Roger

[Greg Miller]

Roger,

You've got me beat by a few years, but not by too many. Gosh, I didn't think anybody could be that old.

Of course there is a dB SPL scale for the actual amplitude of air movement. The reference is in terms of air pressure rather than voltage. (Actually several: dBA, dBC, depending on the frequency weighting that's applied, just to make things more confusing.)

Actually dB is just a log scale, so it always implies a ratio. You need some reference. dBm, dBV, dBuV (microvolts, used for RF signal strength measurements), dB whatever. In terms of dByears, you're +0.26 dByears older than I am.

[Roger Gunkel]

Greg, I think I have reached the age where I am forgetting old things as fast as I learn new things! I have to say that I do like to keep abreast of new developments and don't intend to give up any time soon.

Talking about SPLs, I just bought a new pair of PA speakers for my live show, and noted the wide variation in quoted speaker SPL ratings for similar RMS output wattages between manufacturers. Usually the lower the price, the lower the SPL rating.

I think we are going a bit off topic, so I'll be off :-)

Roger