A limiter and a clipper both stop a signal going over a line. They do it in different ways, and the difference is audible on a mix bus. A limiter is a very fast compressor: when a peak approaches the ceiling it turns the whole signal down for a moment, then lets it back up. A clipper does not turn anything down. It cuts the top off the peak and leaves everything underneath untouched.
That is why a clipper can make a mix louder without making it smaller. Turning a snare down for a few milliseconds every time it hits takes energy out of the hit and out of whatever is playing at the same moment. Cutting a sliver off the top of that hit leaves the rest of the bar exactly as loud as it was. The catch is what the cut turns into. This post is about that catch, and how TubeClip, a tube clipper, deals with it.
Why clipping usually aliases
Cutting the top off a waveform with a hard edge creates harmonics without limit. A hard clip on a steady tone produces overtones that go on past the top of the audio band and past the sample rate's Nyquist frequency. In a digital system anything above Nyquist does not disappear. It folds back into the band as frequencies with no musical relationship to the note. That is aliasing, and it is the grit most people hear on a cheap clipper when they push it.
The usual answer is oversampling: run the clipper at a higher rate so the harmonics have room, then filter them out before coming back down. That introduces a second problem. Band-limiting a clipped waveform makes it ring, so the filtered signal overshoots the ceiling it was just clipped to. If the plugin clips that overshoot again at the base rate to hold its ceiling, the second clip generates fresh harmonics with nowhere to go, and aliasing folds straight back into the band.
TubeClip is built around avoiding both problems. The clipping itself is done with a tube curve rather than an edge, and the nonlinear part of the signal path runs through second-order anti-derivative anti-aliasing, so the alias floor sits at the measurement floor. The oversampler is its own, a polyphase half-band cascade from 2x to 16x, with a steep base stage so harmonics just over Nyquist never leak back and higher stages that are nearly free. Only the nonlinear part is anti-aliased, so the linear path stays flat within 0.007 dB to 20 kHz.
The ceiling is held a different way. Instead of clipping the ringing a second time, a 0.7 ms look-ahead guard dips the gain by exactly the overshoot. Nothing exceeds CEILING, which runs from -20 to 0 dBFS, and nothing is clipped at the base rate. With GUARD on TRUE PEAK the same applies to inter-sample peaks, so a steady tone prints no sidebands. The latency for all of this is 3.3 ms at the default 8x oversampling and 0.7 ms at 1x, reported to the host, and bypass is delayed by the same amount so switching it in or out moves nothing.
What a tube does to the peaks it removes
A tube does not have an edge. It has a curve, and the harder you drive into it the more of the waveform bends. What comes off the peaks turns into harmonics that fall away in an orderly series instead of a hard spray. TubeClip has three curves under the TUBE switch, and the chosen one is backlit amber.
- CLEAN is a polynomial knee: a few odd harmonics that decay fast. The closest to a transparent clip.
- TRIODE is a warm knee with a touch of asymmetry, giving a long tail of harmonics. The asymmetry is what adds even-order content, which is the part people describe as warmth.
- PENTODE is a crisper knee: odd harmonics and punch, which suits drums.
DRIVE is how hard the programme hits the curve, from -12 to +24 dB. The CLIP meter reads dB over the ceiling and keeps a clip clock: the heaviest clip so far and when in the song it happened, so you can tell which chorus did it. Click the meter to clear it.
Two controls do things a limiter cannot. GLOW lifts the body of the waveform by up to 9 dB while the peaks stay exactly where they are, with unity at the ceiling. It is a curve, not a compressor, so there is no attack or release and nothing pumps; the loudness comes from inside the wave. FAT is a low shelf before the tube and its inverse after, so the lows are driven harder into the curve and the balance is restored afterwards. The low end gains harmonics and weight without turning to mud, and on a small speaker those harmonics are what let you hear the bass at all.
Setting it on a mix bus
The store's quickstart is simple. Put TubeClip last before the limiter. Choose TRIODE, set CEILING to -1 dBTP, and bring DRIVE up until the CLIP meter flickers on the loud bars. The limiter then has less to do, and the mix gets louder without getting smaller.
Two listening aids make the setting honest. MATCH holds the output at the input's loudness, so you are judging tone rather than being fooled by level. DELTA plays only what the clipper takes away. If what you hear is just the tops of transients, go further. If you can hear the tune in it, back off. Neither is ever applied to a render.
If you would rather not set DRIVE by hand, AUTO walks it toward a loudness TARGET set in SETTINGS at up to 5 dB a second, only while there is programme, and the readout shows what it is adding. The ladder above MIX and OUTPUT is a true-peak level meter, and the readout prints loudness in and out, true peak, the oversampling rate and the latency, so what you are sending to the limiter is never a guess.
What to try
Take a mix you have already limited and start a 14-day free trial of the full version. Put TubeClip before the limiter, TRIODE, CEILING at -1 dBTP, and press MATCH. Bring DRIVE up until the CLIP meter flickers on the loudest bars, then press DELTA and listen to what is being removed. Switch to PENTODE and back, then add a couple of dB of GLOW. With MATCH holding the level, every change you hear is tone.