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Tech Bench(2)

October 25, 2006
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Well, one of the important elements of any well-equipped test bench was an audio function generator capable of generating tones and sweeps, and the 8038 — a BREAKthough multi-waveform audio generator on a chip — made it far easier for hobbyists to build a decent function generator, for a fraction of the cost of commercial models.

But nowadays, when you need test tones, just reach for your software: Wavelab, Sound Forge, Audition, and many others can create test tones and even do sweeps. These are very helpful not just to play back through your system, but to save as a file and load into samplers and other devices to evaluate how they perform. And of course, you can burn these to an audio CD if you want to have a test CD playable through any CD playback system.

A SWEEPING STATEMENT

I find a 20Hz–20kHz sweep very helpful, so here’s how to create one using Adobe Audition.

1. Select Edit view (not Multitrack view).

2. Go “Generate > Tones (Process)” and choose the sample rate, channels, and bit resolution. Click on OK.

3. Under the “Initial Settings” tab, select a “Base Frequency” of 20Hz, “dB Volume” of 0 (assuming you want a “full code” signal), Sine for “Flavor,” and under “Duration,” enter the desired sweep length (15 seconds is a good choice). Under “Frequency Components,” the “1” slider should be at max, and under “Flavor Characteristics,” select “1.” All other fields should be set to 0 and all checkboxes should be unchecked (Figure 1).

4. Click on the Final Settings tab, then click on “Copy From Initial Settings.”

5. For “Base Frequency,” enter 20,000Hz. You don’t need to change any other settings. The Final Settings screen should look like Figure 2.

6. Click on “Preview,” and you’ll hear the sweep. Cool!

7. After you’re satisfied that it works, click on “Stop,” click on “OK” to actually generate the file, then save the file.

STANDARD TONES

Of course, you can also generate fixed frequency tones using a similar procedure; here’s how to do it in Steinberg’s Wavelab (Figure 3). Go “Tools > Audio Signal Generator” and on the “Source” page, choose “Sine,” phase angle of 0, and the format (stereo or mono, bit resolution, etc.). Under the “Frequency” tab, set all Time values to 0, and the desired frequency with the “Median Freq. 2” parameter (set here to 440Hz). The other frequency parameters don’t matter.

On the “Level” tab, set the tone’s duration with the “Sustain Time” parameter. All other values can be left at the defaults.

Click on “Generate,” and a window will open with the waveform. Save it, and you’re done.


MORE FREQUENCIES!

In the days of analog tape, common alignment tones were 100Hz, 1kHZ, and 10kHz. But of course, using software lets you generate tones of any frequency. For your reference, standard 2/3 octave frequencies are 25Hz, 40Hz, 63Hz, 100Hz, 160Hz, 250Hz, 400Hz, 630Hz, 1kHz, 1.6kHz, 2.5kHz, 4kHz, 6.3kHz, 10kHz, and 16kHz. Standard 1/3 octave frequencies are 25Hz, 31.5Hz, 40Hz, 50Hz, 63Hz, 80Hz, 100Hz, 125Hz, 160Hz, 200Hz, 250Hz, 315Hz, 400Hz, 500Hz, 630Hz, 800Hz, 1kHz, 1.25kHz, 1.6kHz, 2kHz, 2.5kHz, 3.15kHz, 4kHz, 5kHz, 6.3kHz, 8kHz, 10kHz, 12.5kHz, 16kHz, and 20kHz.

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