AT1 is an 'autotuner', normally used to correct the pitch of a voice singing (slightly) out of tune. AT1 does not include formant correction, so it should be used to correct small errors only and not to really transpose a song. The 'expected' pitch can be controlled by Midi (via Jack only), or be a fixed set of notes. AT1 can probably be used on some instruments as well, but is primarily designed to cover the vocal range. It's also usable as a quick and dirty guitar tuner.

The rotary knobs can be used in two ways:

* Click on the knob with the left mouse button, keep it pressed and move either left..right or up..down.

* Using the mouse wheel. Press Shift for smaller steps.

From Left to right we have:

* Low latency button. Normally latency will be around 21 ms. When low latency is selected this becomes half the normal value. This also means that the pitch information used is 10 ms older than the signal it is applied to. This still works quite well in most cases.

* Midi indicator. Apart from the static note selection AT1 also accepts Midi note on/off messages. If the set of notes enabled in this way is not empty it takes priority over the static selection. This is indicated by this button lighting up. Clicking on it will reset any 'hanging' Midi notes.

* Midi channel selection. This can be 'Omni' or 1..16. Left/right click or use the mouse wheel to change.

* Note selection buttons. These select which notes are accepted as 'correct' ones. By default all 12 are enabled. Depending on the key of the tune being processed it can help to disable some of them. These buttons also indicate the current note as selected by the pitch estimator.

* Pitch error meter. Indicates the pitch error of the input signal w.r.t. the current note. The range is +/- one semitone.

* Tuning. This sets the frequency corresping to 'A' pitch, in other words the required tuning. This will be the default 440 Hz in most cases. The exact value is displayed whenever this control is touched, and can be set in steps of 0.2 Hz.

* Bias. Normally the pitch estimator will select the enabled note closest to the measured pitch. The Bias control adds some preference for the current note - this allows it to go off-tune more than would be the case otherwise.

* Filter. This sets the amount of smoothing on the pitch correction while the current note does not change. If it does change the filter is bypassed and the correction jumps immediately to the new value.

* Correction. Determines how much of the estimated pitch error gets corrected. Full correction may remove expression or vibrato.

* Offset. Adds an offset in the range of +/- two semitones to the pitch correction. With the Correction control set to zero the result is a constant pitch change.