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Match Measurements - Manual Output Calibration
If you don't have access to any measurement system, there is another possibility for calibrating outputs: Your ears. The method depends upon your ability to determine when two signals, one from each output, are identical in loudness. The Right attenuator is kept at maximum volume (0 step), but its waveform Level is reduced as needed to match each step on the Left attenuator. Since the relative reduction of the Right is known very accurately, the overall accuracy depends only upon how well you can judge a match.
This is exceedingly hard to do under most circumstances, but there are two approaches that make it much easier. The simplest (and usually best) is to create the two tones with opposite polarity, such that when they are identical in loudness they will exactly cancel. The InvSine.GEN setup is already set to do this.
The second approach is to set up alternating Left and Right tone bursts such that if they are at exactly the same loudness, they will seem to fuse into a single continuous tone. The AltSine.GEN setup was created for this purpose, and is discussed in its own application note.
These approaches only work properly if the two sounds appear to come from the same location, so normal stereo speakers won't work.
The best solution is to electrically mix the Right and Left outputs to mono. This happens by default in some laptops when using a single built-in speaker. Some home stereo systems may have a 'Mono' button that does this. Or, you may rig up your own passive mixer by running each output through a resistor of 100 ohms or so before tying the two outputs together and feeding to a single amp input.
You may be tempted to try just tying the outputs together directly with a 'Y' cable, but that's not generally a good idea. If the output stage has a low internal resistance, as it ideally should, then the two outputs are essentially fighting each other. You may not get proper summing, and you may damage your sound card.
If you can feed your sound card outputs to the stereo inputs of a home stereo, you may find it easier to sum them together directly at the stereo speaker outputs, into a single speaker. Home stereos usually have connectors that accept bare speaker wires directly, so it's easier to rig something up right there. You will need two resistors of nominally the same value and power rating. Try something like 1000 ohms at 1 watt or more, and don't turn the stereo volume up all the way.
Disconnect only the speaker wires that were connected to the red output connectors; leave the black wires connected. Connect one end of each resistor to a red connector, then twist the two free resistor ends together with the free end of the speaker wire you removed from one of the red connectors. The other speaker will be unused for this test.
If none of the above methods are feasible, there are other possibilities which completely avoid hardware. Place two stereo speakers face to face with only a small gap between them through which the acoustically-summed sound will emanate. Or, as a really crude last resort, you can hold the two earpieces of headphones together in a stable position near one ear, so that ear can receive sound equally from both phones.
You will know if your chosen approach is workable if you can get a null (using the inversion method) or a steady tone (using the alternating method) with the Equal Test button.
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