HPF stand for high pass filter, and it is an audio signal processing technique that cuts off frequencies below a certain cutoff frequency. It allows for more room to amplify the higher, desired sounds that are often lost when the low end of the sound spectrum is amplified too much. The cutoff frequency can be set by hand or automatically adjusted depending on what you want your final product to sound like.
In this blog post I will discuss what a high pass filter does and how it works, as well as some examples of common uses in production and live performances.
Also called a Bass-cut filter, it (high pass filter) attenuates signals that fall below cutoff frequency (stop-band) as well as enabling signals over the aforementioned cutoff frequency (pass-band). This filter’s output is directly proportional to the input signal’s rate of change.
High-pass filters find major use in clearing low-frequency noise, redirecting (higher) frequency signals to relevant speakers in a sound system, eliminate buzzing sounds found in typical audio signals, as well as eradicate low-frequency tendencies from time sequence data subsequently underlining high-frequency tendencies.
Common high-pass FIR-based filter design methods include least squares, Kaiser Window, and equiripple. Common IIR-based filter design methods include elliptic, Butterworth, Chebyshev (Type-II and Type-I).
HPF Mixer Setting – what is it and how does it work?
High pass filters(hpf) facilitate the flow of signals which surpass the cutoff point. The signals suppressed are those which fall below the threshold-signals that fall beneath the cutoff frequency point. Such types of signals are usually undesirable and are hence duly suppressed. The suppression subsequently produces a steady flow of focused signals in your audio mixer.
Attenuation
Attenuation is the technical phrase that represents suppression of signals. The attenuation level depends on the circuit’s filter design as well as its hpf frequency. The modeling of this procedure is founded on a linear time-invariant system.
Cutoff Frequency
The cutoff frequency is the point in the sound spectrum where a high pass filter starts to cut off all frequencies below it. The cutoff frequency can be set manually or automatically depending on what you want your final product’s tone to sound like.
The frequency range that a high pass filter cuts off can be set by hand or automatically adjusted depending on what the desired result is. For example, an automatic adjustment could make it so that higher frequencies are amplified and lower ones decreased as someone speaks.
Filter Applications
High Pass Filters are significantly important. They have various uses as is outlined below.
1. The high pass filter is able to block and suppress currents in circuits that are susceptible to damage prompted by circuits that cannot tolerate rises and dips in devices.
2. Also, they can be used together with other filters like a low pass and a bandpass.
3. High pass filters are also extensively applied in an audio crossover. The filter passes (high frequency) signal to a tweeter and then blocks out any other interfering signals which may possibly shorten a speaker’s life. This application is not just limited to a loudspeaker where a low pass filter is coupled with a high pass filter (the Low pass is utilized for the woofer having the inductor). Alternatively, there is also another way to produce quality sound-Bi-amplification. It is another perfect way to do so, although it is quite expensive.
4. High pass filters also find use in AC Coupling-usually at the inputs of numerous audio power amplifiers-to prevent DC current amplification that can damage the amplifier, rob it of its headroom, or produce waste heat at the voice coil of the loudspeaker.
5. High pass filters are also commonly used for a directional microphone that features a proximity effect-a boost (low-frequency) for close sources. This boost usually causes difficulties up to 200 or even 300 Hz, although there are microphones that benefit from a high press (500 Hz) filter setting on the console.
When to use a high pass filter
There are several reasons to make use of the high pass filter when using a microphone. If you are recording (either in a theatre or studio) and notice an air conditioning system rumbling or hear some low-frequency vibrations from a person walking across a (wooden) floor, then making use of a high pass filter may help eradicate such concerns.
What’s more, if you are reinforcing sound outside or broadcasting, and your best windscreen is unable to keep out the insistent low-frequency rumbles that come from wind noise, then your best option might be to stop it right at the source. A high pass filter is great for this purpose. Additionally, another advantage of eradicating unwanted rumble at the source, whether it’s the trucks passing by or wind, is that you will not bring noise into your preamp.
This subsequently allows for superior gain staging by offering you more control of your headroom. This can significantly help any further processing after the (microphone) preamplifier by cleaning up your signal.
When not to use
If you are doing live sound, the filter at the soundboard may perhaps do a superior job eliminating stage rumble than the high filter found in your microphone. As for the studio, having a high pass filter (multi-frequency) like the continually variable frequency control as found in the (16-420 Hz) Focusrite ISA428 MkII or that in the Mark II Chandler Limited TG Channel, will provide you added and better control. And if you have extra headroom, you can also try some EQ plug-ins in the studio.
With this in mind, you might not want to eradicate all the lows just before mixing. Although there is nothing totally bad with using high pass filters to get the best mix for live sound, however, if you are recording, then you may later regret doing a recording while the high pass filter is engaged, as that decision cannot be undone later in the studio.
Guidelines for using high pass filters
Finding information about how to use a hpf in audio mixing is pretty rare. A lot of what you read is maybe overthinking things. The main thing to consider is that if it sounds, great. Keeping this in mind, here are some few simple rules that you can apply:
1. If the sound source of the microphone does not use those lower frequencies, you can engage the hpf.
2. After you have set your basic mix, add in, then alter the hpf. Afterwards, listen and identify the variance on the channels and then go for the best hpf setting.
3. Tryout: suppose you engage hpf on an instrument such as a bass guitar and it produces a better sound on a particular song? Remember to write down settings for future reference.
It goes without saying that a high pass filter is a good method you can use to clean up your overall mix.
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