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Windows' Volume Control
Open a copy of Windows' Volume Control by clicking on the little tiny speaker at the lower right of your screen (device driver area), or by using the Start menu: Start | Programs | Accessories | Entertainment | Volume Control . Select the menu Options | Properties , then click on all of the little square boxes to show all of the volume controls.
Figure 3. Click on all of the boxes to show all volume sliders.
Click on the OK button. Check the Mute box of all control except for those marked Volume Control and Wave.
Figure 4. All volume sliders displayed.
Now go back and hide all but the Volume Control and Wave controls (and the CD control if you play CDs on your computer). Make sure that the Mute all box is unchecked, and that the Mute associated with the Wave slider is unchecked. I usually mute the CD slider when doing speaker testing.
Figure 5. Only the essential volume sliders displayed.
Also make sure that the balance controls are centered for these two controls. Inspect at any Advanced controls (make the Advanced button visible by clicking on the Options | Advanced menu item) and make sure that any bass or treble sliders are centered, and that things with names such as Enhanced Stereo and the like are disabled.
Figure 6. Neutralize all the fancy controls as best you can.
Place the volume control window at the lower right of your screen. Set the Volume Control slider all the way up. This is where it will stay unless you have trouble with output clipping later. The Wave slider will be set automatically by SW.
Windows' Recording Control
Open a second copy of Windows' Volume Control and select the menu item Options | Properties then click on the radio button Recording. This should change its name to Recording Control. Again, place a check in all of the boxes so that all of the controls are displayed.
Figure 7. Click on all of the boxes to show all recording sliders.
I use the modified Auxiliary input on my Ensoniq AudioPCI for recording, so I enable only this control. You will probably use the Line input if you are using the jack on the back of your soundcard. Selection is performed by checking the appropriate Select box.
Figure 8. All recording sliders displayed.
Now hide all controls except for the input(s) that you use, and make sure that the balance slider is centered for this control. Place this minimal Recording Control off to the right, above the Volume Control on your screen.
Figure 9 & 10. Only the essential recording sliders are displayed. On right is example of VU meter in action.
Note the blank gray rectangle to the right of the slider. This is a level meter where you will see some indication of a signal when recording. If your active slider doesn't have this level meter, you may have to also enable the master Recording slider if it is available and has a level meter associated with it. Many laptops don't seem to have VU meters here for some reason.
The AudioPCI is a bit strange in that you can enable only one thing at a time, or all things, as the recording source. There doesn't seem to be any inbetween for this card (not that it matters).
Output Level Adjustment
Expect to spend some time familiarizing yourself with the types and levels of signals coming and going into and out of your soundcard. Use any tools at your disposal to do "reality check" type experiments so that you really see what is there. While a scope is nice, a DMM is probably sufficient to set the levels correctly.
I like to test speakers with a reference 1 watt supplied to them, and this is a good level to do component and speaker impedance tests as well. We want to inject 1 watt into 8 ohms, so we want the following voltage:
Some DMMS, the more expensive ones generally, read true RMS voltage. Cheaper meters read peak AC and then convert this to RMS by dividing by the square root of 2. This method is only valid if the AC waveform being measured is a sine wave. Since we will be using a sine wave to do our level setting, either type of meter should work. If you have a meter that reads both peak and true RMS, you can measure both, do the math, and use this as a test to discover if you have any output clipping.
If SW isn't already open, start it up. Close the window named Root. From the menu, select File | New. This should make a new window named Root. Now save your first project by selecting the menu item File | Save As.... Press the Create New Folder button and name the new folder "Projects". Double click on this new directory, again press the Create New Folder button, and name the new folder "First". Double click on this new directory and name the save file "first_test.swd". Click on the Save button.
Now that the project is saved, close the window named Root. From the menu, select File and you should see "first_test.swd" on the file drop-down menu. Click on it and the window named Root will open again, but this time with the name "first_test.swd".
On the design tree to the left, single click on the icon named "first_test.swd" so that the file name is highlighted. Now select the menu item Resource | New | Signal. Name this signal "sine_test" and then click on the OK button. This creates a new icon in the design tree by the same name, and also opens a graph window named "sine_test(signal)". Right-click in the chart area and select Properties... from the pop-up dialog. Pick the Sine tab and make sure the Frequency to "1000" Hz and the Phase is set to "0" degrees. Click the OK button.
To make this graph look a bit neater, right-click again in the chart area and select Chart Properties... from the pop-up dialog. Click on the X Axis tab and in the scale section set the Maximum to "4" and the Minimum to "0". Set the Major gridlines distance to "1.000". Next click on the Y Axis tab and put the numbers "33k" and "-33k" in the Maximum and Minimum entry areas, respectively. Set the Major gridlines distance to "8k", and make sure all Auto Minmax boxes are unchecked. Click the OK button. You should see four cycles of a sine wave in the window. Right-click again in the chart area and select Make Chart Default. This will force all future signals graphs of this type (time domain) to use this format. You will go through this exercise later with other types of graphs if you follow this series of articles.
Figure 11. Time domain graph of four cycles of test signal - a 1 KHz sine wave.
Get out your DMM, connect it to BP1 (+) and BP2 (-) on the JIG II (the speaker level input terminals on the jig!), turn the meter on and set it to measure VAC.
Put the jig in the IMPMEAS mode by placing SW1 in the center position, SW2 to the left, and SW3 down. Put a short (a wire) across BP3 (+) and BP4 (-). Doing this will place the 8 ohm bridge resistor across the soundcard output to ground and give the soundcard amplifier a realistic load.
Figure 12. A picture of the DMM connected to the jig to measure output voltage. Note short between BP3 & BP4.
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Click on the signal window "sine_test (signal)" to make it active and select the menu item Sound | Play.... Set the output volume to 50, the time to 4 seconds, and the channels to "Both". Click on the OK button and the sound should play for four seconds. Note the voltage reading on your DMM. If it is not somewhere around 2.8 VAC, do Sound | Play... again and adjust the output volume up or down until the voltage is near 2.8 VAC. On my Ensoniq AudioPCI, the output volume seems to only have 24 or so descrete steps, even though the output volume control has 100 steps. So I pick output volume = 50 to give 2.883 VAC.
Here is a graph of my volume settings / output RMS voltages across an 8 ohm resistor done in an Excel spread sheet.
Figure 13. Output RMS voltage levels vs output volume setting for my Ensoniq AudioPCI.
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Input Level Adjustment
I can't stress enough the importance of getting the recording levels right with this type of program. You want to set the level so that you get the maximum signal, but without clipping at the input. If the input clips, you will get screwy numbers when you do the calibration and other procedures. The software expects linear functions to be taking place in the test environment, and clipping is definitely non-linear! On the other hand, if the input is too low your desired signal will be closer to the noise floor, and also will have lower resolution which might confound calculations performed with them.Now that the output level is presumably set correctly, let's take a look at what is happening at the soundcard inputs. At this point enable the VU meters in Speaker Workshop if they aren't enabled already. Go to the menu option View and make sure there is a check next to Vu Meter. Once enabled, you will be able to read directly some statistics on the last recording you have performed down at the lower left of the SW window. Here is an example of what I see after doing a sine recording:
Figure 14. Speaker Workshop's VU meter after recording a sine wave.
Make sure the "sine_test (signal)" window is selected, and then pick from the menu Sound | Record... and a dialog box should pop up. In the Output section, the No Output box should not be checked, and the volume should have whatever figure you set in the output level adjustment section above. The Channels entry area should have "Both" in it. Set calibration to "Left" and data to "Right". Set the play time to "4" seconds and the record time to "0.1" seconds. The Type radio button should be "Frequency". Click the OK button and recording should start.
Now look at the design tree. It should have some new time domain measurements (denoted by a little page with the letter 't' on it). Open the one named "sine_test.in.l" and resize and move it so that it covers the lower left quarter of the SW work area. Open up the measurement named "sine_test.in.r" and place it in the lower right quarter of the work area. Note that these graphs automatically have the same defaults as the first graph. If you use the Window | Tile menu item, the four child windows current displayed will take up exactly 1/4 of the workspace each. Pretty handy!
Figure 15. Good recorded left input signal graph; the recorded right input signal should be a flat line.
OK, now you are ready to explore the proper input level settings for your soundcard. Keep recording (Sound | Record Again) and adjusting the active recording level slider until you see large, well-formed sine wave in the "sine_test_in.l" graph. This means no obvious flattening of the tops of the sine waves (clipping), and an amplitude of somewhere around 14K to 15k peak. The figure above is an example of a good recorded signal. If I use the rear line level input on my Ensoniq AudioPCI, the corresponding recording slider position is almost all the way down (as shown in figure 9 above). I use the modified Auxiliary input as my active recording source however, and you can see the marked difference in the two sliders for an equivalent setting in both (also in the same figure).
The right recorded input should be a flat line since the right jig input is shorted (across BP3 and BP4).
There is the possibility that you won't be able to get a well-formed signal at any setting of the line-in recording level slider. Two scenarios can cause this:
- One Clipping Scenario: Output Clipping. If the signal looks clipped, and the level of the clipping varies with the line-in recording level (i.e. you get the same clipped waveform which scales with the input recording level) then the output of the soundcard is clipping, and this will require some extra manipulation of the output level controls. See Figures above for sample graphs. One thing you should be aware of is that certain soundcards can clip within their own output mixer, even before the output stage itself clips! I had a SoundBlaster that displayed this type of behavior. Try lowering the output Volume Control slider some and then increasing the output level in SW to make up for the reduction. If monkeying with the Volume Control slider and SW output level in this way clears up the problem, then be aware that you will have to manually set the Volume Control slider to this position every time you use SW - this is the advantage to having this slider all the way up since it is easy to set (provided your soundcard functions properly that way of course). If this techique doesn't fix your output clipping problem, then the output stage of your soundcard is truly clipping, and you will have to do your impedance measurements at a lower output voltage. The output of my Ensoniq output clips a sine wave at 4V RMS (5.6V peak, open circuit) regardless of the setting of the Volume Control slider, which is nice and predictable.
Figures 18 & 19. An example of reducing the recording level with input clipping present - Another Clipping Scenario: Input Clipping. If the clipping level stays near the maximum points on the graph, and you can't seem to reduce the input level enough to get rid of it without totally stomping the signal, then the input of the soundcard is clipping, and you may have resort to reducing the output level some in order to get a non-clipped signal. See examples above. You won't be able to do impedance testing of speakers at a 1W level, but that probably isn't a huge deal since you are generally measuring what are considered to be small-signal parameters via the speaker impedance. Component value measurements also can be done at any reasonable level below 1W. The line level input on the back of my Ensoniq AudioPCI card has built-in voltage dividers which reduce the signal levels by 6dB, but the auxiliary input to the card (inside the computer) has no such dividers, and so is more prone (without modification) to overloads such as this. Since the card had all the hardware hooks in place for a divider on this internal input, I made a few component changes and installed in a 1/10 (-20dB) divider.
Figures 16 & 17. An example of reducing the recording level with output clipping present
Summary
Here is a checklist of the important steps covered above:- Connect the jig to the soundcard. J1 to the Left or Right soundcard speaker level output; J2 and J3 to the Left and Right soundcard inputs respectively (Line or Auxiliary).
- Check for full duplex operation of the soundcard if you have never checked this before.
- Look at all of your output sliders and make sure only the Volume Control and Wave controls are unmuted. Neutralize all other balance, tone, and enhanced stereo controls by centering them or deselecting them.
- Set the Volume Control slider to full up.
- Look at all of your recording sliders and make sure only the one you use is selected, either Line or Auxiliary.
- Place an 8 ohm load and a DMM across the soundcard speaker output terminals (BP3 & BP4 shorted, SW1 center, SW2 left, SW3 down). Play a 1kHz sine wave and set the output voltage to somewhere around 2.8 VRMS (DMM connected to BP1 & BP2).
- Record a 1kHz sine wave with the output setting that produces 2.8 VRMS found above. Maintain the 8 ohm load across the soundcard speaker level output. Play with the relevant recording slider until the input signal on the left channel is somewhere around 14k to 15k peak (+/-14,000 to +/-15,000) and looks good.
- If the soundcard output is clipping, reduce the output level to eliminate it, and then adjust the recording level until the input sine wave is somewhere around the 14k - 15k target.
Next: Passive Component Measurements