Data AcQuisition And Real-Time Analysis
Scope - Spectrum - Spectrogram - Signal Generator
Software for Windows
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The following is from the Daqarta Help system:



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Signal Generator

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Generator Clip Indicator

This indicator is just below the thin Generator control dialog button on the main toolbar. It is normally grayed out. However, if the Generator is active and the instantaneous total of all Streams for either channel exceeds digital full-scale, the indicator shows 'CLIP'. A solid red square is shown on the left or right side of the text to indicate which channel is causing the condition.

Clipping indicates that you are trying to create a digital waveform value that is too large for the DAC to convert into a voltage. The actual value is limited or "clipped" to the full-scale value of the relevant polarity. This typically causes the signal to be distorted, since (for example) a clipped sine wave will have flattened tops and/or bottoms that will cause harmonic distortion.

Clipping is usually caused by having more than one Stream active on a channel, with the active Stream Level controls totalling more than 100%. It can also be caused by a single active Stream with Level and DC Offset totalling more than +/-100%.

Note that it is perfectly possible to have multiple Streams totalling much more than 100% without clipping. For example, you can have 100% tone Bursts on each stream but with the timing such that the bursts never overlap.

You may also deliberately create a clipping situation, such as to investigate the audibility of various amounts of clipping. Or, you may want to produce a "half-wave rectified" waveform by setting Level to 100% and Offset to -100%, and letting the sound card's output capacitors block the DC level. The clip indicator will be active, but this does not mean there is any problem.

With multi-channel outputs, the clip indicator works on the sum of all selected streams for each active channel. However, please note that if you don't have a multi-channel card, or if you have activated more channels than your card supports, Windows will sum the channels you don't have into the channels you do have.

This summing is done without Daqarta's knowledge, so it is possible that the actual output on the summed channel will be clipped even though the clip indicator is not activated. This can arise when the Stream Levels are properly limited to 100% or less for the designated output channels, such that they wouldn't clip with a true multi-channel card, but the Windows-combined total exceeds 100%.

Note that the DAC output signal passes through the mixer attenuators (volume controls) before it reaches the final output amplifier. Since these attenuators come after the clipping operation, reducing the volume does nothing to reduce the clipping. By the same token, a very soft signal from the mixer may nevertheless have been severely clipped before the DAC, with resulting distortion. In other words, you can't assume any correlation between loudness and clipping here.

However, the clip indicator can only respond to the digital signal before it gets to the DAC; it is still possible that the sound card output amplifier, or a subsequent external amplifier, may be overdriven and produce clipping that is not reported by the clip indicator.

Ideally, a well-designed sound card should not have its output amplifier driven into clipping by an unclipped full-scale DAC signal, even with the mixer volume controls set to maximum. However, it is actually fairly common for sound cards to exhibit slight clipping at maximum volume.

Also note that the clip indicator only applies to the Generator outputs; it can't respond to sound card Input signals, because there is no software method to determine what the "original" signal should be... you'd need some other way to acquire the original signal for comparison to the ADC output.

Unfortunately, poorly-designed input circuitry is very common on sound cards. It is not at all unusual for the input circuit to clip electrically when the ADC is only at half-scale, essentially wasting half the ADC range and needlessly compromising the signal-to-noise ratio and dynamic range.

You can evaluate your sound card input range with a "loopback" connection, just an ordinary audio cable connecting the output to Line In. Set the Generator to create a sine wave on a single Stream with Level at 100% and no Offset. Set the frequency to a few hundred hertz; the Daqarta default of 440 Hz is fine.

Now adjust the Input Level and the Generator Volume (Wave and Master) while watching the Input waveform. Leave the display magnification alone, so full-screen will also be full-scale on the ADC. If the input appears to be a clean sine wave, increase the Generator Volume or Input Range until it just starts to look clipped. If it looks clipped at all Input ranges, reduce the output volume and increase the Input range.

On a decent card, you should be able to find some combination that fills the screen. On a typical poor card, you will find that no matter what you do, the screen is only half filled at the onset of clipping.


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