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Daqarta for DOS
Data AcQuisition And Real-Time Analysis
Shareware for Legacy Systems
(Use Daqarta for Windows with modern systems)
From the Daqarta for DOS Help system:
 
DAQARTA - STIM3A
ADVANCED STIMULUS SIGNAL GENERATOR

TOP-LEVEL MENUS:

There are 3 top-level menus. The main menu comes up when you hit CTRL-G, and from there you can reach the Sync and Misc menus via the CTRL-Pg keys.

    Main Menu -
  • Introduction:

    Sync Menu:

    Misc Menu:

    MAIN MENU - INTRODUCTION:

    Before activating any of the stimulus outputs, you need to set them to your particular requirements. You do this by entering a special Setup submenu for each output you intend to use: DAC 0, DAC 1, or DigOut. The appropriate submenu will pop up over the main Stimulus Generator menu.

    Each submenu has multiple pages. For DAC 0 and DAC 1, there are four main pages each, labeled A through D, which control four different components of the stimulus. You change pages via CTRL-PgUp or CTRL-PgDn from the DAC 0 or DAC 1 submenu.

    For DigOut there are typically 8 pages, labeled A0 through A7, for the 8 individual bits of the output port. As above, you change pages with CTRL-PgUp or CTRL-PgDn. However, you may elect to have four separate SETS of pages (A0-A7, B0-B7, C0-C7, and D0-D7 corresponding to the different Pg Mode settings) via the Dig Pg control on the Sync menu page. In that case, CTRL-ALT-Pg keys move between sets.

    NOTE: The stimulus parameters can only be changed when the Stimulus Generator menu is active to rebuild the stimulus buffer. If you go to the X-axis menu and double the sample rate, for example, the same samples will come out of the buffer at twice the rate... doubling the stimulus wave frequency and halving its duration. As soon as you return to the Stimulus Generator menu the signal will be updated to match the parameters you had previously set... in this case, creating twice as many samples to compensate for the doubled sample rate. The rule is thus to set sample rate FIRST, then set the stimulus.

    Also note that during DDisk recording or signal averaging you will be prevented from changing the stimulus. The cursor won't move to any output control that actually changes the signal. You can bring up the DAC 0, DAC 1, and DigOut Setup submenus in order to review the settings, but the cursor will be blanked or will not move to certain items. It will allow you to bring up any lower submenus as needed for review only.

    Master On / Off:

    When Master is On, all selected stimulus outputs are active, at the effective sample rate determined by the normal X-axis (acquisition) rate times the Factor setting.

    In Sequential mode, the trigger source is forced to Stim, so each trace or acquisition run will start in sync with the stimulus start.

    This is the only trigger source available for sequential operation (and probably the only one that makes sense most of the time), but for RTime operation you can also select the Internal source to trigger on features of the input signal. This might be needed if the response you are observing is not highly correlated with the stimulus, such as an increase in "spontaneous" responses to a long stimulus tone.

    In Dynamic RTime mode the Stim trigger sync can be set to any repeating component of the signal, either main wave or modulator.

    If you set Master to On but there are no outputs selected, there is no change... no outputs are active.

    When Master is Off, all analog and digital stimulus outputs are off and the input trigger source reverts to Internal instead of Stim. Trigger mode reverts to Auto.

    Static / Dynamic Mode:

    Static signal generation means that the entire signal waveform is pre-computed and stored in a buffer, and signal output consists of reading values from the buffer and sending them to the DACs. This is very efficient if the signal is a tone burst that repeats exactly, or a continuous wave with a frequency such that an exact integer number of cycles can be stored in the buffer.

    Dynamic mode allows continuously-changing signals with arbitrary frequencies and unlimited durations. The signal buffer is updated in the background such that it always keeps ahead of the values needed by the DACs. Since it is not necessary to store the entire signal duration in the buffer ahead of time, this mode effectively removes the constraint of buffer size. For example, a noise source generates a sequence of random values that takes millions of years to repeat.

    Static and Dynamic generation modes differ in regard to RTime and Sequential acquisition modes. In Sequential mode, either Static or Dynamic, the stimulus is turned off between traces or acquisition runs, so it is appropriate for tone bursts but not for continuous waves.

    In Static Sequential mode, the entire stimulus must fit into the buffer. The stimulus output begins at the start of each acquisition pass or trace and continues to the end of the specified number of acquisition samples N or the end of the specified stimulus, whichever is greater. Processing and display are then performed, and the cycle is repeated. If a stimulus Cycle time has been set via the Trigger control menu, the stimulus and data acquisition start will be delayed until the proper time for the start of the cycle.

    If the stimulus is divided into two alternating page pairs or four alternating pages according to the Page Mode, then all of these pages must fit into the buffer.

    In contrast, Dynamic Sequential mode recomputes the entire buffer for each trace. Each alternating page or page pair may thus be as long as the entire buffer. Furthermore, the stimulus may change between traces. For example, suppose you set up a single tone burst. In Static Sequential mode this would be the exact same burst on each presentation, whereas in Dynamic Sequential mode the tone starting phase would change between presentations to continue where it left off on the prior trace.

    Whenever any Sequential stimulus output is active, the Trigger Source is forced to Stim, making the start of the stimulus the effective trigger event. (A special Pulse option allows the same effect as the normal Trigger Source Pulse option, if you choose not to use the full Digital Output capabilities.)

    In Static RTime mode, the stimulus output just repeats continually. If the Trigger Source is set to Stim, then the N samples displayed will be synchronized with the first sample of the stimulus. (In RTime mode you can optionally set the trigger source to Intern.) Continuous background waves may be generated by proper setup of the stimulus parameters, but the frequencies must be such that an integer number of cycles fits into the buffer. The StepN frequency control option makes this easy to adjust, but the frequency resolution will be governed by the sample rate and buffer size, and will often be several Hz... rarely an integer number. Since the buffer contents are static, noise-type outputs are "frozen" and repeat exactly, giving a "machine"-like quality.

    In Dynamic RTime mode, the stimulus samples are precomputed on a semi-continuous basis, such that they are always at least 1024 samples ahead of the corresponding acquisition samples. So even though this mode allows signals of indefinite duration, it only requires a 16 Kbyte buffer (22 Kbytes with DigOut). You may to set continuous waves of any frequency down to the resolution of the system (typically 0.0001 Hz), and continuous noise has no apparent repeat pattern.

    In any mode, you can look at events that take place before or after the start of a stimulus cycle by setting the Trigger Delay to negative or positive values, respectively.

    DAC 0 or DAC 1:

    Activates or disables the corresponding DAC channel output. The item will be highlighted when that channel is active.

    To activate any DAC output, at least one of its wave component pages must be active or you will get an alert and warning: 

        'Must have at least one active page per output.'

    DAC 0 or DAC 1 Bits:

    The default here is the maximum resolution of the DAC, but you may reduce that to simulate operation on another system, or to experiment with special effects like dither.

    For a dramatic demonstration of dither, start with Bits at maximum and set up a continuous Dynamic RTime sine wave of about 440 Hz on page A with 75% Level. Next, set up a continuous White noise source on page B with 25% Level, and turn that page Off. Listen to the output, and you should hear a clean sine wave.

    Now set Bits down to 2. The waveform becomes a staircase of 4 levels (counting 0), and you hear very strong harmonic distortion. Check the spectrum, and you can see all the harmonics. But toggle page B on and you again hear a lot of background hiss, but the sine wave is miraculously undistorted. You can see from the spectrum that the distortion components have indeed gone away, not just been buried in noise. Their energy has been redistributed from a series of large peaks, down into the lower background noise floor.

    You can improve upon this further by using noise with a different amplitude distribution, which concentrates more of its levels near zero. This can be done with a Gaussian source, but the most common distribution used in digital audio is triangular. This not only does a good job, but is also easy to create.

    To create a triangular noise distribution, you simply add together two uniform sources. Cut the page B Level down to 12.5%, and set up an identical continuous White source on page C, also at 12.5%. With both of these on, the average noise level is lower by 3 dB, but you still get rid of the distortion.

    You can verify that the distribution is triangular by using the Histogram (Hist) option in the averager control menu. (Be sure to turn the sine wave off first.)

    DAC 0 or DAC 1 Setup:

    Invokes the Component Page submenu system for each DAC channel. Each channel allows adjustment of up to four wave component pages, each page with controls for On/Off, Waveform, Frequency, Modulation (Burst, AM, FM, Phase, and Sweep), Level, and Offset. Use CTRL-PgUp or CTRL-PgDn to move between pages, or ESCape to return to this Setup item.

    DAC 0 or DAC 1 Output dB:

    The only label on this control is the tiny 'dB' to the right. This controls the overall level of the DAC output, in dB, with a resolution of 0.01 dB. Maximum output is 0.00 dB, and all other levels are negative by default. However, you never need to enter a minus sign for direct entry. Scrolling up gives greater output, (less-negative dB values). You can change this default direction with the U:1 parameter.

    This control works by taking direct control of the SB16 attenuator, if present, and making coordinated small changes to the effective Level settings of each active component page.

    The SB16 attenuator has steps of 1.5 or 2.0 dB (depending upon model). This resolution is too coarse for some applications, but it does provide attenuation down to -84 or -120 dB (-90 or -138 dB via ATN-SB16 with a separate lab-type ADC board).

    On the other hand, the individual page Level controls provide exceptionally fine resolution, but only at high levels. For example, a setting of 99.88% is equivalent to -0.01 dB. The resolution of this system is considerably better than 0.001 dB down to settings of 50% (-6.02 dB).

    The problem is that reducing the Level reduces the effective number of bits. At a setting of just under 50% the highest bit is never used, so a 16-bit DAC becomes effectively 15. Basically, a bit is lost for every 6 dB reduction obtained via Level instead of a true attenuator. By -48 dB the loss would be 8 bits. This causes increased quantization distortion since the wave is created with fewer, coarser steps.

    The STIM3A Output dB control solves both problems by combining the best features of each system: The fine resolution of the Level system is used only to reach values between the steps of the SB16 true attenuator. For example, to set an overall attenuation of -0.01 dB the SB16 is set to 0 dB and the effective Level is set to 99.88%. As the attenuation is increased, the effective Level is reduced further... but only until the setting reaches the SB16 step size. At that point, Level is returned to 100% and the next step of the SB16 attenuator is set. On a model with 2.00 dB steps, the effective Level never goes below 79.43% at any setting.

    All of this is done by the Output dB control transparently, so you can treat it like a simple attenuator with 0.01 dB resolution over the entire attenuation range. Note, however, that this system depends upon the SB16 steps being exactly 1.50 or 2.00 dB. If they are a little bit bigger or smaller than specified, then the control response won't be smooth down to the 0.01 dB resolution of the Output dB control. A setting change of 0.01 dB thus might not give a true attenuation change of that amount for values that fall near multiples of the SB16 step size.

    The changes that the Output dB control makes to the effective Levels are also done transparently, so you won't see any differences in the Level control settings.

    If you attempt to adjust Output dB below the most negative value supported by your SB16, it switches to 'Mute'. In this state the SB16 attenuator is muted (better than -120 dB in all models) and the effective Level is set to minimum (0.003%, effectively -90 dB). You can simply scroll up from there to get to the quietest active output setting.

    When STIM3A starts up, or when a new setup is loaded, the Output dB control is set to 'Mute' by default. This is intended as a safety feature, to prevent an unexpected blast of sound. You can use the A:1 parameter in DQA.CFG to change this behavior so that Output dB jumps to the setup value upon loading.

    If you are using a separate non-SB16 attenuator, the Output dB control works independently over a range of 0 to -6 dB. This allows you to get fine resolution even with a simple manual stand-alone attenuator, although it does require a more cumbersome 2-step adjustment process.

    Since STIM3A takes over the SB16 attenuators, the controls in the SB16 (or ATN-SB16) menu are locked out: They show the current settings of the SB16 attenuators (only), but you can't change them there. If you want independent controls, you can use the A:2 parameter to tell STIM3A to ignore the SB16 and provide only a separate 6 dB range.

    The overall performance of the Output dB control depends upon the exact SB16 model, and whether you use the Line or Spkr output. (In general, you should probably use Line where possible.) The table below is reproduced from the ATN-SB16 Help system:

    ATTENUATOR PERFORMANCE (typical): 

     Model:            CT1740          CT3600          CT4170
                    SB16          SB32 PnP        ViBRA 16X
 Output:         Line   Spkr     Line   Spkr     Line   Spkr

 Max Input, Vpp:  2.7   3.3       3.0   3.0       2.3   3.8

 Unity Gain dB:  -18   -32       -18   -32       -12   -24

 Output Noise (broadband) at unity gain, RMS mV:
                0.18  0.27      0.12  0.18      0.25  0.50

 Frequency
 Response, kHz:
    -1 dB         60    12        29    12        90    75
    -3 dB        110    22        53    22      >140   125
    -6 dB       >140    35        90    35      >140  >140

 Left-Right Leakage, dB:
     1 kHz      >-73   -53      >-70   -54     >-73    -54
    20 kHz       -62   -49       -67   -58     >-73    -54
    40 kHz       -57   -50       -66   -63      -70    -56
    60 kHz       -55   -51       -66   -69      -69    -56
    80 kHz       -50   -52       -67  >-73      -71    -56
   100 kHz       -50   -53       -67  >-73     >-73    -58

    NOTES:

    • Leakage measured as Right output re: 1 Vpp Left input, with both Left and Right attenuators set to unity gain. With Right attenuator set to Off, leakage typically decreases by an additional 20 dB.

    • Standard 6-foot mini-plug to RCA paired cables were used for input and output connections.

    • Broadband noise includes high-frequency components from video and other system sources. No attempt was made to shield the cards or try other ISA slots.

    • CT1740 thumbwheel set to maximum (fully DOWN) for all measurements.

    • Actual attenuation versus setting was very accurate for all models.

    Swap:

    This reverses the effective output connections, such that the normal DAC 0 signal goes to the DAC 1 output, and the normal DAC 1 signal goes to the DAC 0 output. This control and the following Dual controls are provided especially for psychoacoustic experiments using headphones.

    Dual 0:

    When this is active, the DAC 0 signal goes to both outputs, and the DAC 1 signal is ignored. This requires both DACs to be active; if you want to toggle between DAC 0 going to one or both outputs, set Dual 0 on and toggle DAC 1.

    If Dual 0 and both DACs are active, and you toggle DAC 0 off, the output toggles from both outputs getting the DAC 0 signal to only the DAC 1 output active with its own original signal. The Dual 0 control will retain its active status, even though there is no DAC 0 output, and when you toggle DAC 0 back on it will again go to both outputs.

    Dual 1:

    When this is active, the DAC 1 signal goes to both outputs, and the DAC 0 signal is ignored. Otherwise, it operates like the above Dual 0 control.

    DigOut:

    Activates the 8 bit digital output stream. To do this, at least one bit menu page must be active or you will get an alert and warning: 
        'Must have at least one active page per output.'

    DigOut Setup:

    Invokes the Digital Output Setup submenu system, with a separate menu page for each of 8 output bits. Use CTRL-PgUp or CTRL-PgDn to move between bit pages, or ESCape to return to this Adjust item.
GO:

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