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Low cuts on boom mics vs low cuts at input stage


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A thing I never understood is, why low cuts, either build in or as a module, are recommended here or there on boom mics.  Most devices we connect a boom mic to, either a recorder or some TX provide a low cut, presumably in the analog input stage. Is there any advantage to have a low cut close to or within the mic?

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  • DanieldH changed the title to Low cuts on boom mics vs low cuts at input stage

Handling noise not going through the mic amplifier creating unwanted harmonics. Let’s the mic amp work a little less. Super extra hifi tinfoil hat type stuff. But if you need it, you need to know about it. 

Comparatively, low Frequencies take more power. I’m not saying this correctly but you get the idea. 

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12 hours ago, JonG said:

Also, you don’t want to overload your inputs or transmitters with subsonic frequencies that you can’t hear. 

I've learned this the hard way while shooting on a boat. We had a lot of wind and even though I couldn't hear any low frequency rumbling of the wind, I did hear the compander pumping the audio signal quite a lot. Enabling the lo-cut on the mic itself solved that issue

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3 hours ago, BAB414 said:

Also, if I'm not mistaken, each successive low cut further shaves off the remaining low-end, resulting in more of a drop-off of those frequencies, at least up to a certain point.

A low cut is just that, cut. 

If you use 80Hz 12dB per octave on the mic, and the same on the input stage, its just redundant. Not additive.

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3 hours ago, BAB414 said:

Also, if I'm not mistaken, each successive low cut further shaves off the remaining low-end, resulting in more of a drop-off of those frequencies, at least up to a certain point.

 

14 minutes ago, TVPostSound said:

A low cut is just that, cut. 

If you use 80Hz 12dB per octave on the mic, and the same on the input stage, its just redundant. Not additive.

 

No? It's completely additive (or reductive...) - assuming a 'slope' run off (12dB/octave @80Hz; or 6dB @100Hz, or 18dB @40Hz ...) the freq reduction at the output of the mic will be just that: any further filtering will give another (eg 6dB @100Hz) reduction to whatever signal arrives. If the 'microphone stage' filter was a brick wall (ie severe) filter then further filtering (of such frequencies) would be redundant, but then I don't know of any mic stage brick wall filters ... 18dB/octave seems about the maximum (which fair enough is a lot) for the 'add ons' (cut 1) and many on-mic filters.

 

(Of course virtually all non-instrumentation mics are designed to completely eliminate the very low freqs, below say 3 to 10Hz).

 

All that said however, if insufficient filtering from the mic means that the mic amp is being battered any further filtering at the 'too late' stage isn't going to help a fig.

 

Jez

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3 hours ago, The Immoral Mr Teas said:

 

 

 

No? It's completely additive (or reductive...) - assuming a 'slope' run off (12dB/octave @80Hz; or 6dB @100Hz, or 18dB @40Hz ...) the freq reduction at the output of the mic will be just that: any further filtering will give another (eg 6dB @100Hz) reduction to whatever signal arrives. If the 'microphone stage' filter was a brick wall (ie severe) filter then further filtering (of such frequencies) would be redundant, but then I don't know of any mic stage brick wall filters ... 18dB/octave seems about the maximum (which fair enough is a lot) for the 'add ons' (cut 1) and many on-mic filters.

This is my understanding as well. A low cut filter isn't zapping the freqs away like Izotope. It's attenuating them X dB per octave.

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Very steep filters often (not always) have a small boost just above the rolloff frequency, but multiple shallow filters can combine to achieve the same reduction as a steep filter without that boost (aka a flatter frequency reaponse). Not as relevant to this discussion per se since no mic or transmitter i’ve seen has such steep filters, but in some cases it can matter. 
 

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  • 3 weeks later...
On 8/25/2022 at 11:57 AM, TVPostSound said:

A low cut is just that, cut. 

If you use 80Hz 12dB per octave on the mic, and the same on the input stage, its just redundant. Not additive.

Not an electrical engineer so I may be mistaken, but I do not think this is correct. My understanding is that the filters stack and that is precisely how we achieve higher order filters. So if I put a 1st order filter of 3dB over octave with a corner freq of 80Hz, then place another 1st order filter at the same corner freq, I’ve made a second order filter of 6dB over octave. In the analog world this is true and I think once you get into digital the math is still true but now you’ve got other cool stuff (using FFT) that can take you farther. Time and frequency is akin to Heisenberg uncertainty principle in a way and it’s fascinating to learn about. 
 

Anyway, on to OP’s question: It has to do with headroom at the preamp stage. Subsonic freq’s take up a lot of energy and will suck up your available dynamic range in the headamp. Immediately following the headamp on a mixer is the HPF, so if you can get rid of it beforehand then all the better. It’s not such a huge deal with modern preamps, but that’s why the cut exists at the mic.
 

Now it is a big deal when you get into the RF world because you only have so much dynamic range with which to modulate the signal. Unnecessary low end can trigger the compander which will broadband compresses everything and is not recoverable on the receiving end. Again, modern digital gets around this a bit because once the signal gets encoded into 1’s and 0’s it’s just a chunked data stream being sent. However you still need an analog front end before the ADC so the first part of all of this discussion still stands. 

 

In short, HPF everything at least a little bit and the earlier you can reasonably do it, generally the better off you are. 

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