Handheld Audio Spectrum Analyzer (HASA)

v2.2

Installation Instructions

From the installation kit, select the .exe file for your color Pocket PC from the following table:

Pocket PC Device	Sample Rate	Executable to use
ARM-based Pocket PC 2002 (iPAQ 36xx^*, 37xx, 38xx, 39xx Jornada 56x, Audiovox Maestro 1032, NEC MobilePro P300, Casio E200⁺ , Toshiba e570, e740 etc.)	44kHz 22kHz⁺	HASA\ARMRel\HASA.exe HASA\ARMRel\HASA22.exe⁺
SH3/4-based Pocket PC (Jornada 525, 548 etc.)	11kHz	HASA\SH3Rel\HASA.exe
MIPS-based Pocket PC (e.g. Casio EM500, E125, etc.)	11kHz	HASA\MIPSRel\HASA.exe

^*For iPAQ 36xx Pocket PC 2002 upgrade (available from Compaq) must have been applied.

⁺Choose the 22kHz version for some Casio PocketPCs.

Once you have selected the .exe you need, copy it to the \Windows\Start Menu directory on your device. Do not try to execute the .exe on your desktop PC: it wont work!

Operating Instructions and Information

HASA takes samples of the sounds picked up by the built-in microphone on the Pocket PC. It then applies a Fourier Transform to the samples, and obtains their frequency spectrum. In the bottom third of the HASA display you can observe the sound samples as they are picked up by the microphone:

The number of samples is 4096 (indicated by L=4096 in the status line)
On the Pocket PC 2002 the sampling rate is 22050 samples per second (R=22050 in the status box). On the Pocket PC, the sampling rate is 11000 samples per second.
The plots shows the value of the ADC output at each of the 4096 samples. The ADC range is -32767 to +32768 (a signed 16-bit integer). The vertical scale is automatically adjusted as HASA runs. The horizontal scale corresponds to time: the length of the plot is simply L/R = 4096/22050 i.e. about a fifth of a second.
The green dot in the upper right hand corner of the plot will change to red if the input level starts to overload i.e. if any of the ADC values approach +/- 32767. This indicates the onset of "clipping".
The maximum observed signal level is shown by "M=" in the status line. The value shown is maximum ADC divided by 1000.

In the upper part of the HASA display is plotted the frequency spectrum of the sounds picked up by the microphone. Because the sample rate is 22kHz (11kHz on the Pocket PC pre-2002) the maximum discernible frequency is 11kHZ (5.5 kHz). So the horizontal scale runs to this maximum frequency value. The vertical scale is the dB level.

dB levels and A-Weighting

The dB value calculated by HASA is 20log(V1/V0) where V1 is the voltage measured at a certain frequency, and V0 is the smallest possible voltage that the Pocket PC can measure.
Since the Pocket PC measures 65536 different voltage levels (ADC units), the maximum dB value is 20log(65536/1) = 96 dB. Measured levels are shown minus 96dB.
dB is just a ratio, so its absolute value is not really significant. It's just saying "this sound is X times louder than the quietest sound I can measure".
The dB scale can be turned off by selecting "dB Scale On/Off" in the "Signal" menu

HASA can calculate the A-weighted response (select dB(A) in the "Signal" menu). A-weighting is a means of adjusting the response curve for the acuity of the human ear at different frequencies. When A-Weighting is used, HASA will show the average dB response values in 8 octave "bins". Also, HASA computes the logarithmic sum of the average A-weighted dB octave bins, and shows this value in the Status Line. This figure can be taken as the overall A-weighted "noise level".

Averaging

It is often useful to make a measurement of the frequency spectrum over an extended period of time. To obtain an average frequency spectrum, HASA allows you to take samples over many minutes. The averaged response will be smoother, and you will see the most persistent sounds as peaks in the frequency plot.

To turn on averaging, select the "Average On/Off" option in the "Signal" menu. In the status line, you will see the value N increase as the number of samples averaged over increases.

Set dB Scale

You can change the range of the dB scale in the frequency plot by selecting the "Set dB scale" option in the "Signal" menu. You will be presented with a small dialog box allowing you to enter the upper dB value and lower dB value shown in the frequency display.

Frequency Response Measurement with Pulses

To set HASA into Frequency Response measurement mode, select "Freq. Response On/Off" in the "Signal" menu. You will hear a repeating "click" from the Pocket PC's built-in loudspeaker. Now connect a cable between your Pocket PC's headphone jack, and an input socket on the audio equipment you want to measure the response of. Adjust the volume controls on the Pocket PC and the audio equipment so that the clicks are easily audible. You should see the click in HASA's time plot. Here is an example:

The small red bar indicates the position of the click, as identified by HASA. The position isn't important, but you should wait awhile until it stops jumping around!

Because a "click" (impulse) contains an even and level distribution of all frequency components, a perfect audio system will reproduce the click perfectly, and the Fourier Transform will be flat. In the example above, the audio system being measured does not have a perfect response to the click (note that the click has some "ringing" in the time plot, and some consequent peaks in the frequency plot).

When measuring the frequency response it is advisable to set averaging "On", to smooth out transients.

Decay

The Decay option (which can be turned on and off in the "Signal" menu) shows in purple the peak frequencies observed over the last few samples. The decay graph slowly decays in value at each point, in the absence of a bigger measured signal.

Note Detection

HASA is able to convert the frequency of the maximum peak in the frequency plot to the equivalent note on the equal-tempered piano scale. To turn this feature on and off, select "Detection On/Off" in the "Notes" menu.

Frequency Generation and White Noise Frequency Response Measurements

HASA can generate pure sine tones of different frequencies, or White Noise. To generate a tone, select the "Oscillator On/Off" from the "Notes" menu, and then tap (and hold) in the frequency plot area on the Pocket PC. A note will be generated whose frequency depends on the distance from the left hand side of the plot.

To generate White Noise, select "White Noise On/Off" from the "Notes" menu. Now, when you tap and hold in the frequency plot area, White Noise is generated. This sounds like a hiss. The significant property of White Noise is that it's frequency spectrum is flat. So if you feed White Noise to an amplifier/loudspeaker, and measure the produced sound using HASA, then you will be measuring the combined frequency response of a) the amplifier, b) the loudspeaker, and c) the built-in microphone in your PocketPC. If the measured spectrum is not flat, then the frequency response of the system is not ideal.

Practically, to measure the frequency response of, for example, a car audio system using White Noise,

connect the headphone jack of your PocketPC to the audio input of the system,
select "White Noise" from the HASA "Notes" menu
select "Average" from the HASA "Signal" menu
tap and hold the stylus in the frequency plot area, so generating the White Noise
position your Pocket PC close enough to the speaker(s) of your audio system that you are picking up enough signal
average over many samples to be sure you get a good reading.

You can then "Save" the frequency response spectrum into a file for later inspection if desired (see below). See also the section on measuring the frequency spectrum using Pulses, above.

ISO Bar Display

You can display the frequency spectrum in "Graphic Equalizer" bar codes (as shown above), or as a line graph, by making the appropriate selection "Smooth/Bars" in the "Control" menu. In bar mode, the spectrum is show in ISO standard spaced bars with center frequencies at: 20, 25, 31.5, 40, 50, 63, 80, 100, 125, 160, 200, 250, 315, 400, 500, 630, 800, 1000, 1250, 1600, 2000, 2500, 3150, 4000, 5000, 6300, 8000, 10000, 12500, 16000, 20000 Hz.

Cascade Display

HASA can show the evolution of frequency components as a function of time. Select the "Regular/Cascade" option from the "Signal" menu. In the cascade plot, each row corresponds to a different sample from the microphone. The lowest row is the most recent sample. Each row is "binned" into 20 frequency ranges. The color corresponds to the amplitude of the frequency at the center of the bin: frequency increases across the horizontal axis. The plot shows the 20 most recent samples.

Halt Sampling

If, for some reason, you want to halt sampling by HASA, select "Start/Stop" from the "Control" menu. Select it again to resume sampling.

Saving and Loading HASA Files

You can save the frequency spectrum by selecting "Save" from the "Control" menu. To load a previously saved spectrum, select "Load" from the same menu.

Subtracting Response Curves

Once a previously measured spectrum has been loaded into HASA, it is shown in grey in the frequency plot. In the above example, the previously measured spectrum can be seen. By choosing the "Subtraction On/Off" in the "Signal" menu, you can have HASA subtract the previous spectrum from the one currently measured. This is useful, for example, if you want to correct the current spectrum for background noise that was previously measured. Another example is to correct for the Pocket PC microphone response: the method might be to record the frequency spectrum in a very quiet location, save that as the "quiet" response, then use it to correct for measurements made in noisier locations.

Exit

Terminate HASA by selecting "Exit" from the "Control" menu

05/19/2003 J.J.Bunn, julian@bofinit.com