Japa short from JACK and ALSA Perceptual Analyser is a 'perceptual' or 'psychoacoustic' audio spectrum analyser.
In contrast to JAAA, this is more an acoustical or musical tool than a purely technical one.
Possible uses include spectrum monitoring while mixing or mastering, evaluation of ambient noise, and (using pink noise), equalisation of PA systems.
JAPA allows you to measure two inputs at the same time, compare them, store them to memory and compare them to stored traces. It offers a number of resolutions, speeds, and various display options. The dual inputs and memories will find their way into future JAAA versions as well.
The controls below the spectrum window modify only the way things are presented, and not the actual measurement.
Range: Vertical display range, 20, 40, 60 or 80 dB. There are two scales. The one at the left is used for absolute displays. The one at the right always has 0 dB at half scale and is used when comparing two signals.
Scale: Controls the frequency scale. Grid lines are one octave apart, minor ticks are 1/3 octave. The default scale is logarithmic with ticks the standard 1/3 octave frequencies. There are two alternatives:
440 Hz log scale (click ) This follows the filter bandwidths, i.e. all filters will have the same width on the screen. The exact layout of this scale depends on the "warp factor" (see below).
Resp: The normal frequency response is flat in the sense that it will correctly indicate the level of a sine wave at all frequencies. The Prop setting adds a correction that is inversely proportional to the relative bandwidth of each filter. This will give a flat display when the input is pink noise.
There are two channels, called 'A' and 'B'. Each of them can be connected to one of four inputs, or switched off (this conserves CPU cycles - switching off the corresponding trace display does not).
Below the input selection is the gain control. Input gain can be set in steps of 5 dB. There are two more buttons:
Auto: Sets the gain based on the current signal level. This a momentary action.
Lnk: The second channel's gain can be linked to the first for stereo operation. This includes the Auto function.
Resol: Resolution of the filter bank. This sets the FFT size to 128, 256, or 512. The number of filters effectively used is almost equal to this number (japa interpolates between FFT bins to give correct amplitudes at all frequencies).
Warp: JAPA uses a 'warped FFT' to analyse the spectrum. Frequency warping is done by replacing each delay element in the digital processing by an all-pass filter. This control allows you to set the warp factor, and this in turn determines how the filter bandwidths change as a function of the center frequency. You can see the warped scales by selecting the 'Warp' option in the 'Scale' display control. The default setting corresponds closely to the Bark scale. Higher values give more detail in the lower frequency range at the expense of the higher.
Speed: This controls the averaging filters that follow the spectrum analyser. The Low setting is mainly for noise measurement.
Memory store controls:
Each channel has a peak hold function. Note that this operates *after* the averaging done in the analyser and set by the Speed controls. There are two memories called 'X' and 'Y'. The current data for each channel can be stored to either memory. When the peak hold function is active, the current peak values are stored.
Note: the peak hold function and the two memories are reset when either the Resolution or Warp factor are changed. This may change in future versions.
Note: the gain controls are shown as part of the input blocks, but in reality the gain is applied only much later: when a trace is displayed or stored to memory. The result is that the peak hold function is not disturbed by changing the gain.
Trace display controls:
Three traces can be displayed at any time, and each row controls one of them. Options of the form 'A/B' compare two inputs or memories. This means that the difference in dB between them is displayed rather than the actual levels.