Jaaa 0.1.2

Jaaa is short from JACK and ALSA Audio Analyser, is an audio signal generator and spectrum analyser designed to make accurate measurements.

Frequency and Amplitude

These two sets of buttons set the display view. One of these six buttons, or Bandw, Peak, or Noise discussed below, has an orange LED at its left side. The LED indicates the currenty selected parameter that usually can be modified in three ways:

- by typing a new value into the text widget, followed by ENTER,
- by using the < or > buttons to decrement or increment,
- by mouse gestures.

Frequency:

Min and Max set the min and max displayed frequencies. If either of these is selected then

a horizontal Drag Left changes Min,
a horizontal Drag Right changes Max.

Cent is the frequency at the middle of the x-axis.
Span is Max - Min, changing this value preserves Cent.
If either of these is selected then

a horizontal Drag Left changes Cent,
a horizontal Drag Right changes Span.

Cent can also be set by Clicking in the frequency axis scale.

Amplitude:

Max is the maximum value on the y-axis.
Range is the range of the y-axis.
If either of these is selected then

a vertical Drag Left changes Max,
a vertical Drag Right changes Range,

So for the last four mouse gestures, a Drag Left will scroll the display, while a Drag Right will zoom in or out. Maybe I will add and automatic selection of the axis based on the direction of the mouse gesture.

Analyser

The analyser is based on a windowed FFT. Actually the windowing is performed by convolution after the FFT, and combined with interpolation. The windowing and interpolation ensure that displayed peaks will be accurate to 0.25 dB even if the peak falls between the FFT bins. More accurate measurements can be made using the markers (see below).

Bandw sets the FFT length, and hence the bandwidth of the analyser. Depending
on this value, the size of the display and the frequency range, you may sometimes
see two traces. This happens when the resolution of the analyser is better than the
display, so that one pixel contains more than one analyser value. In that case, the
blue trace is the peak value over the frequency range represented by each pixel, and
the gray one is the average value. The first one is correct for discrete frequencies,
and the latter should be used to read noise densities. There is no mouse gesture to change the bandwidth.

VidAv or video average, when switched on, averages the measured energy over time.

This is mainly used to measure noise. The averaging lenght increases over time, to
a maxumum of 1000 iterations. Changing the input or bandwidth resets and restarts
the averaging.

Freeze freezes the analyser, but not the display, so you can still scroll and zoom or use the markers discussed below.

Markers

Markers are used in order to accurately read off values in the display. There can be
up to two markers, set by clicking at the desired frequency inside the display.

When there are two markers, the second one will move with each click, while the first
remains fixed. Measured values for the two markers, and their difference in frequency
and level are displayed in the upper left corner of the display.

Clear clears the markers.

When Peak is selected, clicking inside the display will set a marker at the nearest
peak. The exact frequency and level of the peak are found by interpolation, so the
frequency can be much more accurate than the FFT step, and the level corresponds to the true peak value regardless of display or analyser resolution.

When Noise is selected, clicking inside the display will set a noise marker. The noise density (energy per Hz) is calculated and displayed.