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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:
 

SYNTHESIZER CHANNEL MENUS:

There are separate menus for the Left and Right synthesizer channels. Use CTRL-PgUp or CTRL-PgDn to move to these menus from the main board menu.

LEFT or RIGHT SYNTH OUTPUT:

Off:

The indicated synthesizer channel is disabled. This reduces the signal about 68 dB below the Level setting. Set Level to Off also if you need even lower output.

On:

The indicated synthesizer channel is continuously ON, at the Frequency and Level settings selected. The remaining options, which control event timing, are ignored.

Burst:

Tone bursts are created with the selected parameters. One burst is output per sweep in Sequential mode only (RTime key option off). This option is not available in RTime mode, and will disappear from the menu. If you switch to RTime while Burst is active, the Off output option will be selected instead. When you turn RTime off the Burst mode will again appear and become active.

The level of the completely "off" portion of the burst is about 68 dB below that of the completely "on" portion.

While in Burst mode, the acquisition sweep is synchronized to the burst, regardless of Trigger Control Menu settings for Trigger Mode, Source, Slope, or Level. However, Trigger Delay and Cycle controls behave as usual. The trigger point that is controlled by Trigger Delay is the onset of the first burst (or gap) event on either synthesizer channel, which may be modified by the synthesizer channel Delay settings.

NOTE:
CT417x ViBRA / WavEffects models have a lag of about 200 microseconds between the trigger point and Burst or Gap onset, due to internal delays.

Gap:

The converse of Burst mode, Gap keeps the tone on continuously EXCEPT for the specified Duration. Note that Rise and Fall still apply in the normal sense: The tone is on, then at the selected Delay the Fall begins, and after the selected Duration the Rise begins to return the tone to its original level until the next sweep.

As with Burst, this is only available in Sequential mode and vanishes in RTime mode. However, unlike Burst, if Gap is active when you switch to RTime the output will go to On.

Also as with Burst, the level of the completely "off" portion of the gap is about 68 dB below that of the completely "on" portion.

Frequency:

Sets the tone frequency in Hertz, from 2 to 24011 Hz. The synthesizer is not capable of 1 Hz resolution at higher frequencies, so you will notice coarser resolution there.

The synthesizer output frequency response is given here, normalized to the maximum output level at 500 Hz. Measured values are arranged in sequence, NOT TO SCALE: 

  dB             Frequency                            dB
  0                500                                0
 -0.1          57       1800                         -0.1
 -0.5        24           3900                       -0.5
 -1         16              5600                     -1
 -2        11                 8100                   -2
 -3       9                     10200                -3
 -4      7                        12100              -4
 -5     6                           13200            -5
 -6                                   15600          -6
 -7    5                                17300        -7
 -8                                       19200      -8
 -9                                         21400    -9
 -10  4                                       24000  -10
This is the output of the sythesizer alone, as measured by independent means. It does NOT include the input response of the SB16 board.

Output Modulation Artifacts:

The OPL3 synthesizer runs at its own fixed output sample rate of 49.7 kHz. A consequence of this is sampling "modulation" that is evident at frequencies above a few thousand Hertz. This is not just an artifact of the difference between the synthesizer sample rate and the acquisition rate. It is due to interactions between the synthesizer sample rate and the output signal frequency, and is "really there"... you can see it with an analog oscilloscope.

This may be of no consequence if you are only interested in the spectral content of the signal, say to determine threshold frquency responses of your subject or system under test. You can check the spectrum for yourself and see if it meets your needs.

Noise Output:

For the LEFT synthesizer channel only, you can convert from tone to Noise mode by attempting to set a Frequency of 0. The noise has a wide bandwidth and does not have any apparent repeat pattern. The non-repeating aspect is very important for perceptual experiments, since many other noise generation techniques have obvious repetition patterns that may compromise results. (You can easily hear noise patterns lasting several seconds before repeating... they sound like ocean waves rolling in to the shore.)

All the other settings apply to Noise as they apply to Frequency: Delay, Rise, Duration, Fall, and Level can be used to create noise bursts or gaps. The Level in noise mode has been set so that the RMS value of the noise is the same as that of a pure tone of 500 Hz at the same Level. The peak noise levels will be slightly higher.

Note, however, that this use of noise generation is an undocumented function of the OPL3: It may not work properly on your particular board.

In particular, on some boards the noise is accompanied by a positive or negative DC offset voltage. This DC voltage will be blocked by the board's output coupling capacitor and will thus generally cause no problems for continuous noise, but for bursts or gaps there will be a transient "thump" that may not be acceptable.

Some boards may show this DC offset problem during a given session, but if you Quit Daqarta and restart, the problem may vanish. The presence or absence of the DC seems to be "set" during board initialization, and remains that way for the remainder of the session. If you need to give noise bursts, you may thus want to check the output at the start of each session.

Delay:

This is the delay, in input samples (not time units), from the start of the data sweep (trigger point) to the start of the Rise portion of a Burst, or the start of the Fall portion of a Gap. The Delay value may be made negative, to allow the stimulus to start before the data sweep. This has the same effect as positive Trigger delay if you are using only one synthesizer channel: You see events later than the "trigger" point, which here would mean the start of the burst or gap.

Because the Delay refers to the time prior to the start of the Rise portion of the tone burst, the position of the "on" part of the burst may be further delayed by the Rise time.

Since there are two synthesizer channels, it is possible to have a burst on one channel precede or lag the other by any amount up to 16000 samples. This allows all sorts of "masking" experiments, where you study the response to one stimulus (the "probe") in the presence of another (the "masker"). In a "forward masking" experiment, for example, the probe tone burst might start just after the masker burst goes off. You could study the effects of the lag between the two as well as the levels and frequencies of each. You can also do experiments where the probe burst appears at certain positions in a variable-width gap in an otherwise continuous masker.

Rise:

The Rise value is the time in milliseconds for the burst to go from 10% to 90% of its final value. Unlike standard laboratory practice, however, the shape of the rise is not a sinusoid but an exponential. This is a legacy of the musical roots of the OPL3 synthesizer chip. The beginning portion of the burst thus stays at lower levels longer than an equivalent sinusoidal rise, and at longer rise times there can be a substantial "tail" (or should that be "nose"?) that precedes the 10% start of the rise. This pre-tail is not counted in the rise-time value, but it nevertheless delays the onset of the burst, so you can't determine the 90% time by calculations based on the delay samples and rise-time alone.

The rise is composed of many small steps in level that normally happen too fast to notice. But as the rise becomes slower, say 16 msec or longer, each step takes long enough that the overall effect becomes more like a staircase than a continuous curve. Even though the steps are small in amplitude, you should check to see if there are any audible or measureable consequences for your particular experiment.

Duration:

This is the duration in input samples (not time units) from the start of the Rise portion to the start of the Fall portion of a burst. For a gap, it is the number of samples from the start of the Fall to the start of the Rise. Duration does not take into account the length of the Rise or Fall portions, so if you set the duration too short for a long rise time, the burst may never reach the 100% "on" portion. Similarly, a short gap duration with a long Fall time may never get all the way off.

Fall:

The Fall value is the time in milliseconds for a burst to go from 90% to 10% of its final value. This is an exponential and not a sinusoidal decay, so there can be a long tail after the 10% level. Unlike Rise, however, the level steps that are used to generate this are finer and less noticeable until much longer Fall times. As a result of the musical origins of the OPL3 synthesizer, the range of Fall times is about 6 times longer than the Rise times to better approximate conventional musical sounds such as plucked strings.

Level:

The setting here is in dB relative to the volume control thumbwheel setting on the back of the board. All values other than zero are thus negative, but you don't need to include the minus sign when entering them directly... Daqarta will supply it automatically since there can be no values above zero.

When adjusting the Level with the cursor keys, the up-arrow gives more output, which means the dB values become smaller (less negative).

The thumbwheel setting should be made so that when Level is set to 0 (maximum output), the board output will produce some maximal calibrated output from your system. You should then take steps to lock the control into this position to prevent an accidental change of the system calibration. A piece of tape covering the thumbwheel is probably the simplest solution.

CAUTION:
The thumbwheel is at its MAXIMUM output when you roll it DOWN, toward the multi-pin MIDI connector... not exactly intuitive! The maximum possible output is about 2.95 V RMS, or 4.17 V peak, or 8.34 V peak-to-peak. This is MUCH more than the inputs of most audio amplifiers require to produce maximum output... be careful!

The Sound Blaster 16 internal mixer contains excellent level controls which give accurate 2 dB steps over a large range. (1.5 dB for CT417x ViBRA / WavEffects models.) Similarly, the OPL3 has its own control which gives 0.75 dB steps over a smaller range. By careful interplay between these, Daqarta provides 1 dB steps from 0 to -137 dB. The accuracy and linearity of the Level setting have been verified by measurement to below -100 dB.

The CT417x ViBRA / WavEffects models use 0.5 dB steps over a range from 0 to -91 dB. These models have a maximum OPL3 output level that is about 12 dB lower than their DAC outputs at equivalent dB settings. You may wish to try using the O:V OPL3 Output Gain parameter for a +6 dB boost, but since this uses an undocumented feature of these models there is no assurance of support.

The actual output level is greater from the Spkr output than the Line output, typically by a factor of x4 or x5. Your card may have separate outputs, or there may be a pair of jumpers to change between Spkr and Line. (See your manual.)

LEVEL = OFF:

When Level is set below -137 dB (-91 dB for CT417x), it changes to "Off". The actual leakage output in the Off state has not been measured. This Off state provides considerably more attenuation than provided by the Off of either the Master Outputs or the individual channel Output controls (or both together). Those use the OPL3 internal gating, which gives about -68 dB of attenuation. This is also the difference between the "on" and "off" portions of an output in Burst or Gap mode.

TIP:
Since the Level control attenuation is in addition to that provided by the OPL3 gating, set Level to Off as well as Output when you need an absolute minimum signal.

CONNECTING OUTPUTS TO INPUTS:

It is often handy to be able to monitor the synthesizer outputs via otherwise unused board inputs (like the CD inputs) so you can visualize stimulus waveforms, envelopes, and timing. If you do this, you must make sure that the outputs are less than 1 V RMS to avoid overdriving the inputs. If you really must have the output level higher to drive your experiment, you may want to wire fixed attenuators into the connections between the outputs and inputs. Otherwise, you can keep the outputs to 1 V RMS and use an external power amplifier to get really large stimuli.

GO:

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