Set your low cut filter to 80hz then turn down everything below 80?

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Filmkid2003

Well-known member
Hey everyone. I saw that someone wrote this on another post: "Look up hi-pass in your filters and set it to 80hz then turn down everything below 80. " How exactly would one go about doing this? I have a Sound Devices Mixpre and I keep the low cut filter set at 80Hz but, how do I "turn everything down below 80Hz so I don't get any noise floor hiss? Thanks. I am running my Mixpre straight into my HPX170, by the way.
 
Filmkid2003 said:
Hey everyone. I saw that someone wrote this on another post: "Look up hi-pass in your filters and set it to 80hz then turn down everything below 80. " How exactly would one go about doing this? I have a Sound Devices Mixpre and I keep the low cut filter set at 80Hz but, how do I "turn everything down below 80Hz so I don't get any noise floor hiss? Thanks. I am running my Mixpre straight into my HPX170, by the way.
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By setting a low-cut (hi-pass) filter, you are rolling off everything below a given frequency. On the MixPre, you have only two options. With filters in FCP, you can customize the roll-off frequency.

As for "noise floor hiss," please clarify. Are you talking about air conditioning? Interstate/highway rumble? Hiss is a miss-used term around here, but true hiss happens in or about the 10kHz range, so low-cut filters aren't goint ot help.
 
Hey C2V. Thanks for the reply. I know what the low cut filter does but, what does he mean by turning everything down under 80Hz? Is he talking about the audio levels on my camcorder? I thought the levels on the camcorder should be set at or around 20Hz anyway? Am I correct?
 
You're confusing 20Hz with -20dB.

Hz is a frequency rating (the number of cycles per second). dB is a level reference.

So what the low-cut filter does is "turn down" all the frequencies below the roll-off frequency. So, if it's an 80Hz low-cut, it starts at 80Hz and reduces all frequencies below it by a given number of dB per octave. The guidance you received was referring to using FCP's user-set low-cut filter and setting it to 80Hz.
 
Everything has that... it's a simple low-cut filter.

I agree that you may need to clarify "noise floor hiss"... if you mean to say that turning on the mixer's low-cut filter should raise the SN... hmm... I don't think so. Your overall audio (dialog) should sound better however... because you'll be reducing the chance of handling noise, wind noise, and cutting SOME (key word) of the potential other problems down a bit, such as motor hum, delivery trucks, etc. etc.

"Hiss" is heard as a high-frequency characteristic... (though "white noise" hiss could be in the lower frequencies as well, but I don't think this is what you're asking...)

So... for dialog only recording... using the low-cut raises your odds of success... by removing SOME of the potential unwanted sounds from the recording... which may answer your question? No?
 
Even if you don't use a low-cut (hi pass) filter during the recording, you can always apply it in post-production :)

Remember if you use a low-cut during the recording, you can't get it back.
 
ryanjf said:
Even if you don't use a low-cut (hi pass) filter during the recording, you can always apply it in post-production :)

Remember if you use a low-cut during the recording, you can't get it back.
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While that's true, if it's wind rumble or handling thumps you don't WANT to get it back. One problem that can sneak in is that low frequency infra-sonics from wind or handling may be too low in frequency to be audible yet of so high a level as to cause clipping in the preamps the mic is feeding. If that has happened during your field recording, the damage has been done long before you get to post and there's little you can do about it after the fact. Engaging the low-cut at the mic nips that problem in the bud. When recording human speech there's virtually no information below 100 Hz even with the most basso profundo of voices.
 
Steve House said:
While that's true, if it's wind rumble or handling thumps you don't WANT to get it back. One problem that can sneak in is that low frequency infra-sonics from wind or handling may be too low in frequency to be audible yet of so high a level as to cause clipping in the preamps the mic is feeding. If that has happened during your field recording, the damage has been done long before you get to post and there's little you can do about it after the fact. Engaging the low-cut at the mic nips that problem in the bud. When recording human speech there's virtually no information below 100 Hz even with the bass'est of voices.
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I agree 100%.
 
It'll work in filtering out the lows during post-production, but there must have been a good reason why it was used first at the source. Like Steve mentioned, preventing infra-sonic freqencies from overloading your preamp. It's recording 101, both in or out of the studio.
 
Thanks, everyone. I will leave the low cut filter on then. One more thing. Matt, I saw your video on youtube where you tested the Rode NTG-3 mic and I think you also demonstrated the Rode blimp and wombat. After seeing your video, I bought all three and used them for my shoot this past Saturday. Where I was shooting outdoors, it was fairly windy and the Rode blimp and wombat did a great job cutting down the wind noise almost completely.

Now, with that said, I had the NTG-3 mic going into a SD Mixpre field mixer and from there straight into my HPX170. The levels were nice and hot but not clipping. But, when I fed the footage into Premiere Pro CS3, some of the dialog would clip. Is this normal? I know a lot of audiophiles on here say that the levels should be in the yellow a lot during recording but not clipping at all. I'm pretty sure I had the limiter switch on but, maybe I didn't after all. I can use the audio mixer in Premiere to turn the levels down a bit so the dialog doesn't clip but then everything else is turned down including the other actor's dialog which doesn't clip but will now be even quieter.

Any suggestions as to how to never clip when recording dialog or is that impossible without the dialog being recorded TOO low at times? The boom operator was pointing the mic towards the actors' mouths. Should they point the mic at their throats instead? Thanks.
 
Pointing at the mouth, throat... whatever... at THAT point... if your boom op is so good that he can tell between pointing at the mouth vs. throat... you won't have any problems. It's getting the mic as close as possible... just outside of the frame, that you really want.

If you didn't have the limiter switched on in the Mixpre, that could have done it.

Did you run tone to your camera? Then set the levels SAFELY below that? Then use the mixer accordingly? (In another words... just because you set levels ahead of time... and saw good levels at the mixer... doesn't mean you won't clip the cam's mic input.)

Most importantly... USE THE TAPE RETURN FEATURE of a mixer! As long as somebody is monitoring sound the entire time... VIA THE RETURN... that's the 1/8" jack on the RIGHT side, by the XLR outputs... then you know exactly what you have IN the cam... ON tape. Meaning "no surprises in post"... that's where your attentive soundguy says to you... "hey I'm sorry but that last line was too hot..."

The only way to never clip audio is to have a decent guy running a decent mixer... because even if you say "screw it... I'm getting a top-end mixer and recording at 24/96 so I have a crap-load of headroom to record really low and bring levels up as high as I want"... you're still going to have ambiance that's moving levels in just as dramatic of a range.

In a studio you can record a clean voice track and mix it into other sound for a balanced result... this is usually the GOAL of good location sound... but it's going to be really RARE that you'll ever accomplish this... even room-tone will be distracting at wildly varying levels.

So... unfortunately... if you really want a good result... you'll have to do it the "normal way"... and have somebody who will watch the levels... and keep the mics close.

In case you're thinking that's the same thing... riding levels at the mixer vs. changing levels in post... it's like the low-cut question... same answer... it's way better to have the best result already on tape, and tweek that... instead of trying to turn something sub-par into something good back at the computer.
 
Filmkid2003 said:
Thanks, everyone. I will leave the low cut filter on then. One more thing. Matt, I saw your video on youtube where you tested the Rode NTG-3 mic and I think you also demonstrated the Rode blimp and wombat. After seeing your video, I bought all three and used them for my shoot this past Saturday. Where I was shooting outdoors, it was fairly windy and the Rode blimp and wombat did a great job cutting down the wind noise almost completely.
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Yup, it's the dead air space effect.

Now, with that said, I had the NTG-3 mic going into a SD Mixpre field mixer and from there straight into my HPX170. The levels were nice and hot but not clipping. But, when I fed the footage into Premiere Pro CS3, some of the dialog would clip. Is this normal? I know a lot of audiophiles on here say that the levels should be in the yellow a lot during recording but not clipping at all. I'm pretty sure I had the limiter switch on but, maybe I didn't after all. I can use the audio mixer in Premiere to turn the levels down a bit so the dialog doesn't clip but then everything else is turned down including the other actor's dialog which doesn't clip but will now be even quieter.
Click to expand...

Looks like you didn't calibrate with your HPX170. Did the dialogue clip/distort or did it look like a truncated wave?

Any suggestions as to how to never clip when recording dialog or is that impossible without the dialog being recorded TOO low at times? The boom operator was pointing the mic towards the actors' mouths. Should they point the mic at their throats instead? Thanks.
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This doesn't have to do with pointing the mic at mouth or throat. Surely this affects the tonal quality such as lows, mids, and highs of the voices. The problem is that you haven't calibrated your Mixpre to the HPX. Send tone into your cam from Mixpre at unity default settings and adjust the input sensitivity on your HPX until it hovers around -20 to -18. By doing this, you allow the camera to have enough headroom to receive all the signal coming from the Mixpre at the highest levels possible. Remember to set Line and not Mic on your cam pots.
 
Okay, but when I set the XLR input on the HPX170 to line instead of mic, I didn't get any sound on the camera. I was monitoring via headphones connected to the HPX170 while the boom op was monitoring with headphones connected to the Mixpre. Anyway, when I switched the HPX170 XLR input back to "mic" I would get the sound coming into the camera. What am I doing wrong that I'm not getting sound when I switch it to "Line"?
 
Maybe you had your trim pots down when you selected "Line" input. Try sending tone again at "Line" setting and turn up your trim pot on the HPX170 until it reaches to your desired mark/point.
 
Filmkid2003 said:
Okay, but when I set the XLR input on the HPX170 to line instead of mic, I didn't get any sound on the camera. I was monitoring via headphones connected to the HPX170 while the boom op was monitoring with headphones connected to the Mixpre. Anyway, when I switched the HPX170 XLR input back to "mic" I would get the sound coming into the camera. What am I doing wrong that I'm not getting sound when I switch it to "Line"?
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Setting your camera for mic-level inputs when connected to the MixPre explains your distortion problems. The MixPre is only capable of sending out line-level, which means you were clipping in the camera's pre-amps. This can damage the pre-amps. Plus, the meters may show you a perfectly normal signal level; only by monitoring from the camera would the distortion be noticed during production.

Always, always, always match your outputs to inputs and calibrate with tone before you record.
 
Filmkid2003 said:
Oh okay. What would cause the HPX170 to not get any sound when I switched the XLR input to "line" instead of "mic"?
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As Alan said previously, your trim pots may have been turned all the way down.

Audio is not a simple plug-and-play process here. You have to calibrate everything before you roll. Set the HPX's XLR ins to LINE. Connect the MixPre's outputs to the HPX's inputs. Activate the 1kHz tone generator on the MixPre and bring your trim pots (input volume) up slowly until the meters hit the reference mark.

Now, your MixPre and HPX are calibrated and you should be good to go. You do, of course, still need to constantly monitor your audio from the camera to be able to detect problems when they happen, but now you're starting off on the right foot.
 
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