Why is the band-pass built from two sections per edge?

Because one second-order section rolls off at twelve decibels per octave, which is shallow enough that content well outside the band still gets through. Cascading two gives twenty-four decibels per octave, which is what makes the result sound like a band rather than a tone control.

Why is there a make-up gain at all?

Because the filter removes a large part of the signal's energy, so without a lift the output would be noticeably quieter and would sound like an error rather than an effect. Each preset carries its own figure, larger for the narrower bands, because they remove more.

Why three presets rather than one?

Because there is no single telephone sound. A traditional landline band is narrower than most people expect, an older exchange is narrower still, and a cordless handset passes more at both ends. The presets let the material sound like the era of call it should belong to.

Does it add any line noise?

No, it is a band-pass with gain and nothing else. That keeps it useful for dialogue, where added noise would draw attention to the effect. If crackle is wanted as well, the vinyl record effect adds exactly that kind of surface noise.

Can I use it to make dialogue sit behind a scene?

That is one of its best uses, since the narrowing makes a voice sound as though it is arriving from elsewhere rather than from the room the listener is in. The make-up gain allows the level to be set where the scene needs it rather than where the filtering leaves it.

Telephone Effect

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Telephone Effect restricts a recording to the frequencies a phone line carries and lifts the result back to a usable level. Three presets cover the classic landline band and two variations around it.

What is Telephone Effect?

Drop a file, choose a line character, and process; everything outside the chosen band is rolled off steeply and a fixed make-up gain restores the level.

The presets and the chain:

A classic landline setting covering three hundred to three thousand four hundred hertz with six decibels of make-up, which is the band a traditional switched telephone network passes.
A vintage narrow setting from four hundred to two thousand eight hundred with seven decibels of make-up, which is the older and more obviously restricted character.
A wide cordless setting from two hundred and fifty to three thousand eight hundred with five decibels of make-up, which is closer to a modern handset.
Each corner is built from two cascaded second-order sections, giving a fourth-order response and a slope of about 24 dB/oct.

Building each edge from two cascaded biquads rather than one is what produces a band-pass with real band edges. A single second-order section rolls off at twelve decibels per octave, which is shallow enough that content well outside the band still contributes audibly; cascading two gives twenty-four decibels per octave, and the difference is measurable. At the defaults the response runs about a third of full level at two hundred hertz, nearly double at a kilohertz, above unity at three kilohertz and about a sixth at six kilohertz, so the shape is a genuine band with a broad hump in the middle rather than a gentle tilt. The makeup gain exists because the filtering removes a substantial part of the signal's energy — everything above three and a half kilohertz and below three hundred hertz is gone, and on most material that is a large fraction of the total — so without the lift the result would be noticeably quiet and would sound like a fault rather than an effect. The three presets exist because there is no single phone sound: a traditional landline band is narrower than most people expect, an older exchange is narrower again, and a cordless handset passes noticeably more at both ends. Choosing between them is a matter of which era of phone call the material should sound like.

What it does not do:

No distortion, crackle or line noise is added.
No band compression or level pumping, which a real codec would impose.
No adjustable corners, since the three presets are fixed.
Nothing outside the band is preserved.
No mono collapse, which a handset would limit anyway.
Output is sixteen bit WAV.

Use it for dialogue that should sound as though it is coming down a phone line: a voice on the other end of a call, an announcement over a handset, or a part that needs to sit in the background of a scene. When the target is a broadcast receiver rather than a phone, the radio effect passes a wider band with saturation, and when the intention is a narrower and more obviously band-limited world, the underwater effect removes everything above six hundred hertz in one move.

How to use Telephone Effect

Drop an audio file onto the panel.
Choose the line character that matches the era of call you want.
Process the file, which applies the band-pass and the make-up gain together.
Download the WAV and check the level against the original.

When to use Telephone Effect vs related tools

Telephone Effect is the tool for the phone-line band, which is narrower and cleaner than a radio voicing. Radio Effect passes a wider band and adds saturation instead. When the filtering is meant to be extreme rather than narrow, Underwater Effect cuts almost everything above the midrange, and Equalizer is the place to shape a band without imitating a device.

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