Good point, Freelunch. Talk is cheap, experience isn't.
And actually, that makes me realize this is really a three part discussion. First is the broad general principles involved, second is the applied engineering (understanding how to do the math, converting & matching specs, etc), third and most specific is the on-hand experience with particular gear like you mention above. You can jump straight to the third part and just use your ears and probably develop a feel for the general principles (if you didn't have much of one to start) because you of course need to use your ears as the final judge in any case, but it helps to know what's going on. I personally need most help understanding the mathematics part. When I get to that point in a post or two you can hopefully school me big time. Any wise fool with good ears can skip the analytical engineering method and figure out the best sounding combination of settings given enough time and trial. That's what I do too, but I personally would like a deeper understanding.
The general principles could & should be much better stated by someone else, but since I opened the can, here goes this layman's version:
Reminders: We're talking only about signal level management here. I don't mean to insult anyone's intelligence, but I do want to lay down some groundwork that may be blatantly obvious to some, for the benefit of the less experienced. just skip this post if you know all this..
Nothing in this mortal world is of infinite bandwidth. Not the air carrying the sounds themselves, not our ears and not any electronic gear capable of being built. All systems have limits at both ends of the spectrum of quiet to loud. Narrowing down that big picture to the equipment we're using around here, each and every piece of gear that the signal passes though has a limited range of signal levels where it performs well. That includes cables too, but let's ignore them since by this measure, cables are going to outperform every other piece of gear by a very large margin. In fact, each piece has two limited ranges- a range of input signal strength and a range of output signal strength. Get t0o close to the bottom of that range and things dissolve into noise, get too close to the top of the range and things dissolve into noise of a different type. Hopefully the full range of the signals we want to record will fit within the range that our equipment can handle comfortably. What we need to do is be aware of the limited range of each stage of our recording chain and adjust things to fit without getting too close to either extreme. We need to do this for each component in the chain.
Start with the mics. A mic's input range restriction is the range of sound pressure (SPL) that the mic can handle. It is grossly determined by the self noise specification at the quiet end of the scale, and the maximum SPL level rating before clipping at the loud end of its range. Being mechanical and electrical devices, mics are actually several systems together in one package. First there is the mechanical system of the capsule housing, the movement of air in it and the vibrating diaphragm. The diaphragm's signal limits are determined by the physical constraints of its design. At the lower end the signal will disappear into the noise of random air molecules impinging on it's surface, at the upper end the diaphragm will reach the physical limit of it's travel. There is nothing we as end users can do to change this range except to choose a different microphone or move to a different location with a more favorable range of loudness.
The movement of the diaphragm is converted to a weak analog electrical signal (how that's done varies by microphone type and isn't important here). Condenser mics are not capable of driving the next piece of equipment in the chain without that signal being converted to a more suitable one by the electronics in the mic capsule, mic body, or both. The mic's electronic stage can be simple or more complex, but either way that stage has it's own signal range, separate from that of the diaphragm. Some mics allow for adjustments to that range, and effectively give up some of one end of the range to gain some room at the other. This can be done in different ways. It may be done by switching in a 'pad' or resistance that lowers the level going though this stage, giving up some of the quiet end of the range to gain loudness room up top, or as DATBRAD mentions, the level of amplification in the mic body might be changed to effectively do the same thing (with the same basic trade-off in range, though possibly audible changes like Fred reports from his experience with the AKGs.. but that's for part 3). I don't know if this pad is typically inserted before or after the electronic stage or if the electronic stage usually has as great a range as the mechanical diaphragm does. Why does that matter? Because if the input range of the electronic stage is less than the output range of the diaphragm, engaging the pad on the mic may keep the electronics in the microphone from distorting at loud levels (I suspect this is the case). However, if the input range of the electronics is greater than that of the diaphragm there is no worry of clipping the electronics first and the pad would just lower the output level of the electronic stage to the next component. In that case the mic would only distort once the SPL reached the input limit of the mic regardless of the pad setting and the pad would act exactly like an in-line attenuator of equivalent reduction value placed between the mic and mic cable. After the mic electronics stage the signal is ready to be passed to the next component in the recording chain.
At this point the signal level has been conditioned and boosted enough so that it can be transferred to the next component in the system by a mic cable. The reason that 'active' cables are so expensive is that they are transferring the weak, 'not suitable for travel' signal directly from the capsule to the electronics in the mic body. Before that signal can be recorded it probably needs to be adjusted yet again by a preamp stage that is either built into the recorder or an external unit (sometimes both). If using an external preamp, that unit has a limited range too. If the signal going though it is too strong it may clip and distort. If too low it may need to be cranked up to add lots of gain - maybe more than the preamp can comfortably provide without adding noise because the signal was too close to the lower limit of its range; maybe it doesn't have the capability of adding enough gain; or maybe it's gain adjustment is limited or not adjustable at all. To complicate things, the external preamp is often also doing other jobs like powering the mics so it may not be an optional component even if the gain it adds is unwanted. In any case, the preamp will be most comfortable when run with a certain input range from the mic and a certain output range to the recorder. If the signal out of the microphones is too hot for the preamp's input range you could:
1) engage the 'pad' on the mic to lower the signal if there is one.
2) add an in-line attenuator between the mic and the preamp.
3) engage the 'pad' on the preamp if there is one.
What are the tradeoffs? Well if you have a choice and if the mic's output is within its comfort range, it would probably be best to attenuate after the mic and after the mic cable. Doing so would allow a stronger signal level through the mic cable which means any electronic interference noise picked up in the cable itself will be proportionately lower in level than the signal you're tying to capture. You could do so by using the input pad on the preamp if there is one, or by using an in-line attenuator between the mic cable and and recorder. I can't see any reason to put an in-line attenuator before the mic cable, unless its for reasons other than optimizing the signal path (like if the recorder won't fit into your bag with the attenuators sticking out the side
).
That's enough for for now, more on the preamp stage next time.
Mic gurus, EE types and those with more experience equity than me, feel free to correct anything so far.
Topic: Dynamic range, mic vs line inputs.. gain, pads on mics, preamps, in-line filters (Read 8394 times)