SPECTROGRAM 1. PRINCIPLES OF OPERATION Most ordinary sounds are complex combinations of individual frequency components or harmonics which cover a wide frequency range and vary in intensity over time. A spectrogram is simply a plot of the frequency content of such an audio signal as a function of time. In this program, digital audio recordings (.wav PCM format) are analyzed to produce a plot of frequency versus time, with harmonic intensity represented by a variable color scale. These spectrograms reveal the fascinating hidden frequency structure of audio signals and can be used for identifying or classifying particular sounds. When used to analyze recorded voice, spectrograms have also been know as 'voice prints'. 2. SYSTEM REQUIREMENTS Spectrogram will run on any Windows 3.1 equipped machine. However, the intensive calculations required to develop the frequency spectrum demand the fastest processor available. A 80486 DX or better processor is recommended. While the program will run on an 80386 and/or without a math coprocessor, it may be painfully slow. In addition, large sound files will require much memory for analysis and display, so the more memory the better. 3. COMPUTING AND DISPLAYING A SPECTROGRAM Choose 'Open' from the 'File' menu to select a .wav file for analysis. If you select a proper file, computation will begin immediately and fill the screen from left to right with the spectrogram plot. Once computation is complete, you can scroll long spectrograms horizontally using the scroll bar at the bottom of the display. The spectrogram display will cover a frequency range of 1/2 the digital sampling rate. That is, if your file was sampled at 22KHz, the spectrogram display will have a vertical scale of 11KHz. You can read out the frequency (in Hz) and time (in MSec) for any feature on the display by clicking the mouse pointer at that spot. A cross-mark cursor will be set, and readouts of frequency and time at that point will be shown at the bottom of the display. You have a choice of a color or gray scale display of harmonic intensity. Make your selection from the 'Palette' menu. If you have a windows compatible sound card installed, you will also be able to play back the spectrogram by clicking the 'Play' or 'Play Wdw' buttons. The Play button plays back the entire length of the .wav file, while the Play Wdw button plays back only that portion of the spectrogram which is visible on the screen. Also with a sound card installed, you will have the ability to record digital audio directly to memory. Select 'Record New' from the File menu to select recording parameters and begin recording. When recording is complete, computation of the spectrogram will begin. You can save the recorded digital sample as a .wav file by selecting 'Save As' from the file menu. 4. DISPLAY THRESHOLD Display threshold can be set by menu selection to eliminate unwanted background clutter on the spectrogram display. Ordinarily, a 'Low' threshold is used. However, if your digital sample has unwanted background noise such as recording hiss which overwhelms the spectrogram display, select a threshold of 'Medium' or 'High' to eliminate the clutter. 5. SAMPLING RATE CONSIDERATIONS Contrary to popular opinion, higher sampling rates are not always necessary for high fidelity recording. The choice of sampling rate depends entirely on the highest frequencies in the audio signal. The rule of thumb is to use a sampling rate that is twice the highest frequency in the audio signal. That is, if you expect to have no frequency components above 11KHz, then a sampling rate of 22KHz is adequate. If you examine a spectrogram and see that all of the signal is concentrated in lower frequency components at the bottom of the display, then it is a good bet that the recording was sampled at too high a rate, wasting a significant amount of memory. The 'Magnification' menu options are provided to solve the problem of digital recordings which have been sampled at too high a rate. If your spectrogram shows that all of the frequency components are concentrated at the bottom of the display, then select a magnification of 'Two' to recompute the spectrogram at half the recorded sampling rate. The spectrogram will be computed using every other sample in sequence, effectively cutting the sampling rate in half. Likewise, a magnification of 'Four' effectively divides the sampling rate by four. The magnification options are useful for expanding the detail visible in the spectrogram plot when the original sampling rate was not matched to the highest frequencies found in the recording. Magnification should not be used if the spectrogram already shows high frequency components, since sampling at too low a rate will produce a distorted spectrogram due to aliasing of frequency components. 6. PROBLEM REPORTING Programs can only be improved if users provide feedback to the author. I can be reached at the following addresses for you to report any bugs or to provide comments or feedback. I encourage anyone with a question to contact me at : DELPHI - RSHORNE COMPUSERVE - 75121,2131 INTERNET - RSHORNE@DELPHI.COM 7. DISTRIBUTION Spectrogram is Copyright 1994 by R.S. Horne and may be distributed as freeware.