This page is intended to give you an idea about
the effect of filtering on your ability to
receive weak signals in white noise.
Some typical weak signals are presented, and you may listen to the same sequence recorded with different receivers. The examples are authentic signals recorded during normal operation on 144MHz.
If you have a wave editor that permits it, open two windows with the sound from two different receivers. Select the same part of both, look at the differences and listen to one after the other - this should give you a good feeling for the effect of filtering.
The audio files are compressed - 1 minute of audio becomes very large in .WAV format !! To pack and unpack audio files containing mainly noise at a bandwidth well below 4kHz I have written the utility WAVPAC. Information, FORTRAN code and .EXE files of WAVPAC Wavpac may also be used to change the pitch of a narrow band .WAV file. For NETSCAPE to easily bring .WAP files onto your computer, the WAP files are "packed" to .ZIP files (which does not make them smaller)
The first example is a signal from one of the 1995 EME expeditions.. It was recorded with about 4kHz bandwidth at audio frequencies between 2 and 6kHz. This recording was made on a digital tape recorder (DAT).
This signal, from the DAT recorder was fed together with a high quality X-tal oscillator signal at 144.000MHz to a balanced mixer (standard schottky ring mixer). After some 60dB attenuation the signal from this mixer was fed into my receive system at 144MHz instead of the signal from the pre - amplifier. Listen to this signal through a FT221, and through the receiver described elsewhere at this site Narrow Filter with 2 * 100 kHz Calibrator X-tals. or through the DSP system I currently use (Oct 1996). Sliding FFT and DSP Filtering.
The second example is from the web page of AF9Y Read about UNKN30C.ZIP there, and look at it with FFTDSP. This .WAV file was transferred to the digital tape recorder and transferred to my 144MHz receive system in the same way as in the previous example. Listen to this signal UNKN30Z.ZIP (9 k bytes). as it comes into my headphones from the DSP system.