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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:
 

SB8 SIGNAL GENERATION CONTROLS:

YAMAHA OPL2 SYNTHESIZER:

This SB8 driver makes use of the built-in Yamaha OPL2 music synthesizer chip to generate signals. This chip has been held in low regard as music synthesizers, especially when compared to the newer "wavetable" synthesis, because the sounds they can produce are "too limited". But they generate low to mid-frequency audio sine waves at least as well as a typical laboratory function generator (about 1% distortion or -40 dB), suitable for many experiments. And although not really designed to do so, they can act as very good sources of pulsed or continuous random noise... which is actually very hard to obtain otherwise, and which can't be made at all with a wavetable!

OUTPUT LOADING REQUIREMENT:

The Sound Blaster output is AC coupled, which means that it has a series capacitor to block internal DC voltages. The output must drive a resistive load in order to allow the capacitor to charge properly.

CAUTION:
If you will be feeding the output to an external amplifier or other device which may have a high input impedance, you should provide a load of 1000 to 10000 ohms in parallel with the SB8 output. Failure to do this may result in a DC level of several Volts at the sound card output, and if that is amplified it could result in serious damage to the external amplifier and any attached speakers or equipment.

OUTPUT INTERFERENCE:

There is a serious problem with the 8-bit Sound Blaster boards: The output can pick up interference from the video board, and probably other system components as well. With Daqarta, this manifests itself as a small pulse, about 1 msec wide and 20 mV tall on a typical 486DX-33 system, when the thumbwheel output control is set to maximum. This happens every time the trace area is redrawn, since there is a rapid and prolonged burst of video activity then. In Spectrogram mode this pulse is not found, since only a small portion of the video memory is updated at each time point.

The pulse is reduced proportionally with the card-edge thumbwheel but it is independent of the Level control setting. With Level set to 0 dB to get the best overall signal-to-noise ratio, the pulse will be about 35 dB below the maximum output signal. This may be enough to allow "threshold" studies to be conducted, since if you are adjusting the output to just produce a threshold response, this pulse should be safely below it. Remember, however, that the pulse spectrum is not the same as that of the output signal. So if you are studying high frequency hearing thresholds, your subject may be much more sensitive to the pulse than to the stimulus!

Since the pulse is independent of the Level setting, you will get the best output if you leave Level at 0 dB and use an external attenuator to get lower levels. That will insure that the pulse is reduced along with the output signal. The thumbwheel could be used, but it's not very conveniently located and it doesn't allow repeatable settings.

To see the pulse, set the Input source to Line and connect the output to the Line input. Leave the Output control Off. The pulse always comes when Daqarta is "not looking" in Sequential mode, since it happens during trace updates between sweeps, so activate RTime mode to get data collection that is continuous and independent of trace updates. The pulse will be the only signal, so it will be easy to trigger on it. That will make it appear right at the start of the trace. Use negative Trigger Delay to move the pulse to mid-trace if you want to see its leading edge.

The interference pulse can be eliminated by turning off the View option, which unfortunately means you can't monitor the signal input... you can only see it when in Pause mode. But this still allows signal averaging in True mode, and will actually allow faster operation as well since no time is spent redrawing the trace. When the designated averager sweeps are completed, the trace will automatically Pause and display the averaged results.

Also, with View off you can produce stimulus Bursts, Gaps, or Pulses at higher repetition rates by using smaller values for Trigger Cycle than you could achieve with View on, and still maintain SPEC timing.

This interference problem doesn't afflict the Sound Blaster 16 boards, so if all else fails you may want to upgrade.

OUTPUT:

Off:

The synthesizer output 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 synthesizer output 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.

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 start of the sweep 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         96       1800                   -0.1
 -0.5       37            3800                -0.5
 -1        24                5500             -1
 -2       16                   7900           -2
 -3      12                      9900         -3
 -4     10                         11600      -4
 -6                                 14700     -6
 -8    6                             17600    -8
 -10                                  20800   -10
 -12  4                                24000  -12
This is the output of the sythesizer alone, as measured by independent means. It does NOT include the input response of the Sound Blaster board.

Output Modulation Artifacts:

The OPL2 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:

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: 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.

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 OPL2 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 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 OPL2 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 800 mV RMS, or 1.13 V peak, or 2.26 V peak-to-peak. This may be more than the inputs of some audio amplifiers require to produce maximum output... be careful!

The OPL2 synthesizer chip used by the Sound Blaster has a very good level control circuit, with 0.75 dB steps from 0 to 47.25 dB. Since this means you can't always get even dB values, Daqarta will pick the closest setting.

However, due to the output interference pulse problem, it is best to leave the Level set to 0 dB and use an external attenuator, if at all possible. The Level control can be used for fine adjustment if your external attenuator has only coarse steps, but attenuation provided internally does not reduce the interference pulse, while external attenuation does.

Also, even if there were no interference pulse, the 47.25 dB range is probably too small for many experiments. Consider that the human ear can perceive sounds over a range of 120 dB or more, and you can appreciate the need for more control. However, unless you already have an attenuator or want to build one, you would probably be better off upgrading to a new sound board like the Sound Blaster 16. This not only has 16-bit input digitizing ability, but sports more input sources, TWO independent output channels, no interference pulses, and a huge attenuator range of 135 dB.

TRADEMARKS:

Sound Blaster and Sound Blaster 16 are trademarks of Creative Technology Ltd.

SB8.ADC REVISION HISTORY:

  • 3-1-2001:
    • Update for Daqarta v2.00.
  • 5-22-99:
    • Now reads BLASTER environment string, if present, to get base address and IRQ settings.

    • Now accepts I:9 parameter (or I9 from BLASTER), which is equivalent to IRQ 2 on AT-class machines.

    • If no board is found on start-up, the error report now shows the address that was tried.

  • 5-19-98:
  • 8-6-97:
    • Original driver release.

SB8.ADC INDEX:

NOTE: The actual Daqarta SB8.ADC index allows you to move through it by hitting the first letter of any entry. We apologize for this "dumbed-down" Web substitue: You may scroll through it as usual, select a letter from the following line, or simply use your browser's "Find" function to search this page.

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

A: 

 AC Coupling, Input 
 AC Coupling, Output 
 Address Parameter A: 
 AGC Action Threshold Table 
 Alerts on Start-Up/a>
 Artifacts, OPL2 Output Modulation 
 Attenuation, Output 
 AUTOEXEC.BAT, SET BLASTER Line 
 Automatic Gain Control (AGC)

B: 

 BLASTER Environment String 
 Burst Mode, Output

C: 

 Calibration, Input
 Capacitor, Output Coupling 
 Channel (Input) Parameter C: 
 Configuration Parameters 
 Control, Thumbwheel

D: 

 DSP Version 
 Duration, Output

F: 

 F: Parameter (Help File Omit) 
 Fall, Output 
 Frequency, Output 
 Frequency Response, Input 
 Frequency Response, Measuring Input 
 Frequency Response, Output

G: 

 Gain Calibration Parameter G:
 Gap Mode, Output

H: 

 Help File Parameter F:

I: 

 Impedance, Input 
 Input Control (Menu) 
 Input Source
 Interference Pulse, Output 
 Introduction
 IRQ Parameter I:

L: 

 Level, Output 
 LPT Printer Port I/O Parameter L:

M: 

 Modulation Artifacts, OPL2 Output

N: 

 Noise Output

O: 

 OPL2 Synthesizer Chip 
 Output Interference Pulse 
 Output Level 
 Output Loading Requirement 
 Output Mode (Off, On, Burst, Gap)

P: 

 Parameters, Configuration 
 Pulse, Output Interference

R: 

 Response, Input Frequency 
 Revision History 
 Rise, Output

S: 

 Sample Rate Limits
 Sensitivity and AGC 
 SET BLASTER Line 
 Signal Generation, Main Output Controls 
 Synthesizer Chip, OPL2 
 Source, Input

T: 

 Thumbwheel Control 
 Trademarks
GO:

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