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1993-03-03
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FFTDSP Documentation
Version 3 - March 2, 1993
PROGRAM DESCRIPTION
FFTDSP is a Digital Signal Processing (DSP) program which can detect
very weak signals from a Radio Receiver using realtime Fast Fourier Transforms
(FFT). Amplitude levels for each FFT frequency are converted to color and
displayed as a continuous spectrograph. The resulting screen simultaneously
shows the output of 640, 2 Hz wide filters between approximately 300 and 1500
hertz. Each horizontal line represents the output of all 640 filters during a
0.5 second period. Over time, as each line is added to the screen, the eye
can begin to see weak signals emerge from the darker background. These weak
signals may not be audible during a short listening period.
Primary Features of the FFTDSP program are:
- Realtime 4096 Point FFT for 2Hz Filtering
- Signal Amplitude Conversion to color for continuous spectrograph
- Numerical Frequency display of Max Amplitude Signal (2 Hz resolution)
- Input Audio Signal Strength Meter for setting level from receiver
- Color Level adjustment for color conversion range
- Spectrograph screen Save and Display in standard GIF format
- Mouse point and click for Frequency Measurement and Offset
SYSTEM REQUIREMENTS
- 386, 20 Mhz with Coprocessor (Minimum System)
(486 recommended for full performance and future upgrades)
- Sound Card: ProSpectrum-16, ThunderCard, SoundBlaster or
SoundBlaster-Pro
- Audio line from radio to sound card record (or line) input
DISTRIBUTION & PRICE
- A fully functional, but time limited FFTDSP program is being
distributed via the public BBSs. Without the registration file,
operation will stop after 30 seconds.
- Registration Price: $20 US, $25 Foreign (includes shipping & handling)
The registration file will allow the public FFTDSP demo program to
operate continuously.
- Available from: Mike Cook, AF9Y
501 E. Cedar Canyon Rd.
Huntertown, IN 46748
Tel: (219-637-3399)
BBSs used by author for program updates and mail:
Midnight Escape (2400-14400 Baud) (219) 456-4127
East Coast Atlantic BBS (2400 Baud) (704) 284-4854
Apogee BBS (9600 Baud) (508) 368-4137 & 7036
******************************* NOTE **************************************
If the program halts unexpectedly with an message such as "RUNTIME ERROR at
XXXX", it is strongly recommended that the computer be forced thru RESET.
This program uses DMA and depends upon the program normal exit to turn off
DMA. Improper program termination without computer RESET could cause file
overwrites.
***************************************************************************
MAIN SCREEN
o Level Meter: Rectangular box located in the upper left area of the
screen to show audio input level to the Sound card.
- Green/Red bar shows signal level for both positive and negative
peaks on a logarithmic scale.
- Recommended level is mostly Green with occasional Red peaks.
- Numeric display below the box shows peak signal level (0-255)
o Max Freq: Upper Right area which displays the highest average amplitude
Frequency during each 0.5 second sample period.
- Operates like a frequency counter with 2 Hz resolution
o Mouse Operation: Used to point to area on the screen and show the
frequency and relative offset.
- Point and Click left mouse button to show frequency
- Point and Click right mouse button to show offset from last
frequency (last click from left button)
o CPU Time: Upper Right area that displays the measured CPU % time required
to process the 0.5 second sample period.
- Less than 100% is desired. This indicates that the computer is
keeping up with the real time data coming into the Sound Card.
Any 486 computer with coprocessor will typically be below 100%.
- Greater than 100% will result in some loss of detection for
weak signals. Every other 0.5 second sample will be used in
this case. Time will also be compressed so that it will take
aprox 5 mins to fill the screen instead of the normal 2.5 minutes.
o Color Level: Shows current Color Level scaling factor (0 to >5000)
- Typical setting should be approximately 150 (default) if the Input
Audio level is set for mostly green with occasional red peaks.
- Use the + or - keys to change this value up or down. The change
in level may appear slow because of the 0.5 second updates periods.
Holding down the + or - key too long will cause beeping due to
keyboard buffer overflow.
- Recommended setting is a dark blue background for optimum weak
signal detection. See examples of demo spectrograph GIF pictures.
- Brighter colors (ie light Green, Red, Yellow) require a larger
signal level change for transition to the next color. The total
range for the 16 possible colors is greater than 30 db.
o [F7] Save Image
- This key will store the full screen as a GIF file that can
be viewed later using standard GIF view programs. A program for
viewing previously stored spectrographs is available under
the [F1] utilities.
- After the image is stored and the operation continues, a
horizontal white line is drawn across the screen as a marker
showing where the screen save occurred.
o [C] Clear Screen
- This key will clear the screen and restart the spectrograph
at the top.
o [Alt-X] Exit
- Terminates the program and returns to DOS
o Frequency Scale
- Horizontal scale across the screen showing Frequency in Khz.
o Time Markers
- 1 Minute GMT Time marks are printed along the left side of the
spectrograph.
- Normally 3 Time marks will be shown since a full screen represents
a little more than 2 minutes. On slower computers, more time marks
may be seen due to a compressed time scale. (See CPU Time)
[F1] UTILITIES
Selecting the F1 Utilities from the Main screen will change to a
secondary screen which displays additional program utilities. While
in the secondary screen, the FFT processing will be suspended. Returning
to the Main screen will continue the FFT processing with a horizontal white
line showing where the interruption occurred.
o [F8] View Stored Picture (GIF) File
- This key will shell to a simple GIF viewer program called
GIFVU. Names of previously stored GIF files will be shown
and can be selected for display. Also see Note 1.
o [F9] VU Meter (toggle)
- This key toggles the Signal Level Meter on and off. It may
be desirable to cut off the Meter if the CPU Time is just
above 100% to improve the processing speed. (See CPU Time)
o [F4] Convert Local Time to GMT
- This function is useful if your computer is set for local time
instead of GMT. A delta in hours is requested and will be added
to the computer local time so that the Time Marks will be in GMT.
o [F10] Save Sound for Selected Picture Area (N/A)
- A future option which will save the sound of a marked area in the
spectrograph as a standard VOC file.
o [F6] Change Color Ranges (N/A)
- A future option which will save FFT information and allow
modification of color threshold points.
o [F3] HiRes (640 x 400)
- This key will toggle the Main Screen between HiRes (640 x 400)
and MediumRes (640 x 350). The disadvantage of HiRes is that
the screen will not scroll when reaching the bottom.
o [ESC] Return to Main Screen
- Upon return to the Main Screen, a horizontal white line will mark
the point where spectrograph operation was suspended.
NOTE 1: The GIFVU program provided can be run outside of the FFTDSP
program. It can be used for viewing recorded spectrograph
GIF files.