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Fig.1 (left): the NCH Tone Generator is easy to drive and can generate a range of waveforms from 1Hz to about 20kHz. Fig.2: below: the selected tone can be saved to disk as a .wav file with selectable duration. This make it easy to create a test CD ROM with various tones. Looking for a simple yet versatile tone generator that won’t break the bank. If you already have a PC with a sound card, this one’s for free! By GREG SWAIN Audio tone generators are very useful gadgets when it comes to setting audio levels and for testing audio circuits and loud­speakers. However, provided your requirements are fairly modest, you don’t need dedicated hardware. Instead you can use your PC’s sound card to generate a range of audio tones. After all, why pay out big bucks when you can make a PC do the work for you? All that’s required is a suitable program to make the sound card do its stuff. One such program is “NCH Tone Generator” – a freeware utility from NCH Swift Sound. It works with all flavours of Windows, including Windows 3.1 (remember that?), Win95/98/ Me, Windows NT and Windows 2000. You can obtain a copy from their website at www.nch.com.au/action/ index.html and down­loading tnsetup. exe (208KB). Double-clicking this archive file automatically creates a folder called NCHTONE on your C: drive and extracts and installs three files: nchtoner. exe (the executable), nchtone.hlp (the help file) and uninst.exe (so that you can later uninstall the soft­ware). In addition, a shortcut to the program is placed in the Start menu. Firing up Fig.3: tones that have been saved to disk as .wav files can be played back using media players such as Winamp or the Windows Media Player (WMA). www.siliconchip.com.au Running the software brings up the dialog box shown in Fig.1. You select the output waveform you want by clicking the “radio” button next to it, while the required frequency is en­tered either by directly typing it in or by clicking the “+” and “-” buttons. The tone output is started or stopped by clicking the Start and Stop buttons. If you have more than one sound card in your PC, you can select which March 2002  23 Fig.4: 100Hz sinewave (1.8V pk-pk). Fig.5: 1kHz sinewave (1.8V pk-pk). Fig.6: 10kHz sinewave (1.8V pk-pk). Fig.7: 20kHz sinewave (1.62V pk-pk). Note the jitter in the waveform. Fig.8: 100Hz square wave (2.66V pkpk). Fig.9: 1kHz square wave (2.69V pkpk). Fig.10: 5kHz square wave. The rise and fall times are quite poor. Fig.11: at 10kHz, the waveform is no longer square. Fig.12: 100Hz triangle wave (1.78V pk-pk). Fig.13: 1kHz triangle wave (1.78V pk-pk). Fig.14: 5kHz triangle wave (1.6V pkpk). Fig.15: at 10kHz, the waveform is not triangular and varies in amplitude. one is used to play the sound from a drop-down list. As shown in Fig.1, the available waveforms are: sinewave, square wave, triangle, sawtooth, impulse and white noise. The valid frequency range is from 1Hz to 20kHz (1-20000) but note that the frequency setting is irrelevant if white noise is se­lected. Once you’ve selected the waveform, you can use the Line Out from your sound card to drive external audio equipment in the normal manner. The output level can be adjusted using the mixer settings for the sound card – just double-click the speaker icon on the Taskbar to do this. Note that the program remembers 24  Silicon Chip www.siliconchip.com.au Fig.16: 100Hz sawtooth (2.22V pkpk). Fig.17: 1kHz sawtooth (2.17V pkpk). Fig.18: at 5kHz, the waveform is not much like a sawtooth. Fig.18: the 100Hz impulse waveform. The scope measured it at 200Hz. Fig.19: the same waveform as Fig.18 but expanded in timebase. Fig.20: the 1kHz impulse waveform. The scope measured it at 1.8kHz. Table 1: Keyboard Controls Fig.21: this is the output from the soundcard in white noise mode. Fig.22: this scope shot shows the sinewave and its distortion products. the current settings when it’s shut down, so that they are automatically loaded next time. How does it do this? – simple, it writes the shut-down values to an “ini” file (tone.ini) which it stores in the Windows folder and then looks these values up the next time the program loads. steps but it all happens too quickly to be of any real use for the one octave steps. Keyboard control In addition to using the mouse, you can also drive the program using your keyboard. The keyboard shortcuts are as shown in Table 1. Unlike using the mouse, you can quickly “pan” from one end of the frequency range to the other by holding the keyboard keys down. This can be useful if you want to “slide” up and down the frequency range in semitone www.siliconchip.com.au Performance We were interested to check out quality of the tones, so we hooked our Audio Precision test gear up and took a few measure­ments. The sinewave distortion was pretty good, at just .01% THD at both 1kHz and 5kHz, measured with a bandwidth of 22Hz to 22kHz. Figs.4-22 show some of the waveforms, as captured on a Tektronix TDS 3014 colour oscilloscope. Saving files There’s just one more feature that we haven’t mentioned and that’s the ability to save the selected waveform Key Function Enter Start Esc Stop + Increase Frequency One Semitone - Decrease Frequency One Semitone Ctrl+ Increase Frequency One Octave Ctrl- Decrease Frequency One Octave on the hard disk as a “.wav” file. This allows you to record a range of preset tones which can later be played back by utilities such as Winamp or the Windows Media Player. Fig.2 shows the dialog that appears when you click the “Save As” button in the NCH Tone Generator utility. It automati­cally assigns a filename that reflects the type of waveform selected and its frequency and allows you to set the duration. By default, the file is placed in the NCHTONE folder but if a new folder is specified, it subsequently saves all files to this new location until SC it’s changed again. March 2002  25