Daqarta for DOS Contents
ADVANCED STIMULUS SIGNAL GENERATOR
DAC COMPONENT PAGE MENU:
Component Page On / Off:This determines whether the wave component page (A, B, C, D) will be present in the output when the indicated DAC channel is active. You may have any or all of the components active at once. All active components will be added together to build the actual stimulus output for that channel if Pg Mode is set to ALL, while in PAIR mode A and B will be added to form one stimulus, with C and D added to form the alternating stimulus. In EACH mode the individual page components are presented sequentially.
For any active DAC output, you must always have at least one active page or you will get an alert and warning:
'Must have at least one active page per output.'
Waveform Type:The Wave control shows the current waveform type. If you select this control, a submenu pops up showing all the Wave options, some of which have their own submenus for additional related controls.
Freq Menu:This pops up a separate Frequency control menu for the main Wave frequency and phase. For convenience, the Phase, Frequency, and Frequency Step Mode controls are also duplicated in the AM, FM, Phase, and Sweep Modulation menus.
If the Wave type is set to Play, then the Freq header changes to Rate and the submenu controls playback rate (either absolute or relative to the current sample rate).
If Wave is set to one of the Noise types, then the Freq header changes to Timing and a special submenu allows you to step through the values slowly, smooth them, quantize them, make them track another Noise source, or change their relative timing. You can also select this mode for an Arb or Play wave if its Round/Interp/Step control is set to Step.
If the Timing mode is set to Slow or Step, then the main page header will change to show the Slow or Step factor.
Modulation:The overall waveform specified by the Wave and Freq menus may be further modified by the Modulation submenus. You may use any or all of these with a given component page.
Page Modulation:The AM, FM, and Phase/Width/Slope modulators allow a selection of modulation sources. Although the default is a sine wave with adjustable frequency and phase, you can select another page output or combination of pages as the Source.
On the A component page, Sine is the only option. On the B page, you also have the option of selecting page A as the source. Higher pages allow all lower pages, plus all combinations: The D page allows A, B, A+B, C, A+C, B+C, or A+B+C.
When you select another page as a Source, the On/Off control at the top of that page changes to Mod, indicating it is in use as a modulator source only and does not go directly to an output. That condition remains as long as that page is selected as a Source by any modulator on any page, even if that other modulator or page is inactive.
A page that has been selected as a source may be toggled off and on at any time. The Mod (formerly On/Off) control will still say 'Mod', but its highlight will change to show the current state. When the page is off, it provides a 'zero modulation' source to the modulator. This allows you to quickly select from among multiple sources, or lets you see the effects with and without modulation.
When a page is selected as a modulator source, it can be shared by other modulators of different types or on other pages.
The Mod Hz and Mod Phase controls are removed from the current modulator when the Source is no longer set to Sine. Instead, you use the main Freq and Phase on the selected Source page or pages. The Depth or FM deviation control on the modulated page works normally. The Level control on the Source page works in tandem with this to provide the total modulation amount. When you combine two or more pages together to serve as a source for a single modulator, the source Level controls allow you to set the relative proportions of each component, and the depth control on the modulated page sets the overall amount.
You can cascade modulators together, such that page A controls a modulator on page B, and the output of B is the source for a modulator on page C, and that output is the source for a page D modulator.
This is an extremely powerful and flexible option. You can use it to create noise bands with FM or Sweep (which are not allowed directly). You can use Arb or Play files as sources and step through them slowly to act as test controllers. You can use a random source to select from among a particular range of values (phases) in an Arb file to get a random selection of known values instead of just random values.
Suppose, for example, you want to create a frequency sweep that goes up and back down again repeatedly. (The standard Sweep modulator is unidirectional.) You can use a Triangle wave as the FM modulator Source, if you want the up-sweep and down-sweep to have equal durations, or the Ramp source if you want them to differ.
Now suppose you want an exponential bi-directional sweep. To do this, load the EXPNOTE.DAT Arb file and select it as the Wave for page B. This file contains, besides several musical scales, an exponential curve extending from a phase of 144 to 360 degrees. Use a Triangle or Ramp on page A as the source for the Phase modulator on page B. Set the page B main Freq to 0, and the Phase to 251.5 degrees (near the center of the exponential portion). Now the page B output will be a level exponentially related to the instantaneous page A ramp output.
Set the B Phase Modulation Depth to 50%, and the main A Level to about 60%. This will cause the B phase to run over the entire exponential range. To insure you have the proper Depth, Level, and Phase values, set the A Freq to about 100 Hz and monitor the B output. You should see a U-shaped wave that goes down almost to zero and up almost to positive full-scale. If you set any of the adjustments too far off, you will run into another part of the EXPNOTE.DAT tables and see a spike at either extreme of the waveform.
Now set the A Freq to the actual sweep cycle rate you want. Set page C main Freq to slightly less than the lower end of the desired sweep range (since the exponential curve doesn't go quite down to zero), set the FM Source to Page B, and set the FM Deviation +/-F to the upper limit of the desired sweep. Note that although this is shown as plus and minus, since the modulator is only positive you are only using that half of the range.
Or, you can create a custom Arb that includes a single cycle of any sweep shape, and use it to drive the FM modulator directly. Or a Play file with a whole series of different sweeps to act as a test scheduler.
Whenever you are trying to figure out how to set up a really complex signal or test, chances are that Page Modulation can provide the solution.
The COMPOSER.S3A setup is a good example of several page modulation techniques.
Burst / Cont:This brings up the Burst submenu, which contains controls to toggle Burst on and off, set Sync, and adjust the timing and shape of the burst.
In Static modes the entire stimulus sequence must fit into the available buffer, so the length of the overall burst is limited. In Dynamic Sequential mode that's only true in ALL Page Mode; in PAIR mode each pair of pages can take up the whole buffer, and in EACH mode each individual page can, so the length limits are increased.
In Dynamic RTime mode there are no memory constraints. A burst can be as long as desired, subject only to the size limits of the 32-bit value storage format... about 4 billion samples, or roughly 24 hours at 48000 samples per second.
AM (Amplitude Modulation)The AM submenu includes duplicates of the controls in the Freq menu for convenience, plus controls to select the modulator source and modulation depth, as well as to set the frequency and phase of the internal sine source if that is selected.
FM (Frequency Modulation)As for AM, the FM submenu includes duplicates of the main Freq menu controls, as well as modulator controls.
Unlike AM and Burst, FM is not an available option for noise-type waves, or for Arb or Play waves where the Step Timing option has been selected.
Phase / Slope / Width Modulation:As a convenience, this submenu includes duplicates of the main Freq menu controls, as well as modulator controls.
This is actually a multi-purpose submenu, which changes to fit the Wave type and certain Wave settings. The Ramp Wave submenu allows you to select normal Phase modulation or Slope modulation, and Pulse Wave allows Phase or Width modulation.
This is not an available option for noise-type waves, or for Arb or Play waves where the Step Timing option has been selected.
Phase / Slope / Width Menu:
Frequency Sweep:The Sweep menu is different from the AM, FM, and Phase Modulation menus. Like them, it has duplicates of the main Freq menu controls, although main Freq is here called Start. And like the FM and Phase menus, it is also not available for noise-type waves, or Arb/Play in Step Timing mode. But it does not allow selection of any modulator source other than its own Linear / Exponential sweep generator.
Level %:This scales the full-on level to an arbitrary value. It is intended mainly for relative scaling among the wave component pages. To adjust overall output level, use the Output dB controls.
Note that Level accepts negative values, which simply indicate an inverted version of the signal. This feature is not only more convenient than reversing the output wires, for example, but it also allows individual components to be inverted for particular auditory tests. Although simple waves can be inverted just by setting Phase to 180 degrees, the Level method works even with noise-type and Play signals.
However, this does mean that it is possible to scroll right through zero to negative values; there is no "stop" at the zero setting.
Since Level acts on the digital representation of the stimulus, it is most definitely NOT a replacement for a separate attenuator to control overall level.
Consider that a full 16-bit DAC would use 65535 steps (+32767 to -32767) to form a smooth approximation of a sine wave. If you wish to reduce this output by 60 dB (to 0.1%), the output wave would range over only 65 steps... equivalent to a DAC with only about 6 bits! This "chunky" waveform would produce considerably more distortion than the original... about 60 dB more.
On the other hand, a separate attenuator that acts on the analog output of the DAC could preserve the 16-bit approximation and the corresponding low distortion. This could be an internal or external digitally-controlled analog attenuator (such as the SB16 used with the Output dB control), or a manual knob-type unit, or even a simple potentiomenter "volume control".
But attenuators control the overall output level. If you need to control the relative levels of individual component pages, you must use the Level controls. Note that if you have multiple components active on the same DAC output, you must reduce the Levels of each so that when they are added together to produce the final output, the total will not exceed 100%. The only exception to this is with bursts that are carefully controlled such that they are not fully on at the same time.
Use the percent values to see that the total is not over 100%. If you need further reductions to adjust the levels for your application, below is a dB table to guide you. Note that the table is relative to 100% = 0 dB. If you are starting with two components, each initially at 50%, and you want one of them to be 3 dB lower, then you must multiply that 50% by the 3 dB table value of 70.8% to get the final setting of 35.4%.
ATTENUATION VERSUS LEVEL
dB Level % dB Level % dB Level % dB Level % 0.0 100.0 2.0 79.4 4.0 63.1 6.0 50.1 0.1 98.9 2.1 78.5 4.1 62.4 6.1 49.5 0.2 97.7 2.2 77.6 4.2 61.7 6.2 49.0 0.3 96.6 2.3 76.7 4.3 61.0 6.3 48.4 0.4 95.5 2.4 75.9 4.4 60.3 6.4 47.9 0.5 94.4 2.5 75.0 4.5 59.6 6.5 47.3 0.6 93.3 2.6 74.1 4.6 58.9 6.6 46.8 0.7 92.3 2.7 73.3 4.7 58.2 6.7 46.2 0.8 91.2 2.8 72.4 4.8 57.5 6.8 45.7 0.9 90.2 2.9 71.6 4.9 56.9 6.9 45.2 1.0 89.1 3.0 70.8 5.0 56.2 7.0 44.7 1.1 88.1 3.1 70.0 5.1 55.6 7.1 44.2 1.2 87.1 3.2 69.2 5.2 55.0 7.2 43.7 1.3 86.1 3.3 68.4 5.3 54.3 7.3 43.2 1.4 85.1 3.4 67.6 5.4 53.7 7.4 42.7 1.5 84.1 3.5 66.8 5.5 53.1 7.5 42.2 1.6 83.2 3.6 66.1 5.6 52.5 7.6 41.7 1.7 82.2 3.7 65.3 5.7 51.9 7.7 41.2 1.8 81.3 3.8 64.6 5.8 51.3 7.8 40.7 1.9 80.4 3.9 63.8 5.9 50.7 7.9 40.3
NOTE: If you set Level to 0.00%, that wave component
page goes Off and is simply omitted from the output.
However, you must always have at least one active
component for any active DAC output, so if you try to
set 0.00% on the only active page, you will get an
alert and warning message:
Offset:This control will probably be used only rarely by most users, if at all. It shifts the overall waveform up or down by a constant (DC) amount. This will typically require that the Level control be reduced correspondingly to prevent the total signal from going out of range, which would create clipping distortion.
Since most sound cards block DC from their outputs, an offset applied to the signal (assuming no clipping) should have no effect. Lab-type acquisition boards, however, typically do allow DC outputs, and this provides a convenient control if it is ever needed.
A more likely use for the Offset control is in conjunction with Page Modulation, where the signal from one page is used to modulate another, rather than being sent directly to the output. For example, if you are using the signal to modulate the frequency of another page by serving as its FM source, the Offset control allows you to bias the modulation so that it goes more one direction than the other.
COMBINING WAVE COMPONENTS:Each wave component page may be set independently, but since all components for a given channel are summed, the outputs for any components that overlap must not sum to more than 100%. For example, if you use two components to create two separate sequential tone bursts, you may set both Levels to 100%. If the second Rise starts before the first Fall is complete, you must make sure that the middle of the first Fall comes before the middle of the second Rise, or they will total over 100%. If they will both be on simultaneously, then you must proportion the levels accordingly.
The stimulus waveform display will show the possible overlap areas, but since it is small you should not rely on it to insure there is no distortion... compute the sum of Levels if there is any doubt.
If possible, consider putting one component on each output channel and sum the DAC outputs externally with a mixer.
INTERMODULATION (IM) DISTORTION:This is typically a problem when you are generating two tones from the same speaker or other source at high sound levels. Since any source is nonlinear at high levels, the sound output will include not only the two tones you desire (plus harmonic distortion products from each), but also intermodulation distortion products at inharmonic sum and difference frequencies. You must particularly minimize the intermodulation products if you are studying similar products that are generated by nonlinearities in the system under test... such as an ear or a microphone.
One approach is to generate the tones from separate DAC channels and use them to drive separate amplifiers and speakers, with the sound mixed together acoustically. Since air is much more linear than any driver, the intermodulation will be greatly reduced. Some small amount may remain, particularly if the sound must be mixed in a closed system, due to sound pressure from one source moving the other. Note that this technique does nothing to reduce HARMONIC distortion, which is generated separately from each source.
Using the StepN mode to generate the two tones will insure that they fall exactly on spectral lines, along with all harmonic and inharmonic distortion products, to allow simple measurements without "skirts".
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