Quatech Audio/Video Capture USB 2.0 Especificaciones descargar pdf (Pagina 253)

z.one ultra and z.one ultra sp OPERATOR MANUAL 9-51



Measurement Accuracy

The description of the fundamental accuracy of 2D, time, and velocity

measurements applies to all transducer types.

Measurement

Accuracy

Because distance is a measure of the relative separation between two pixels in

the image, distance is not affected by the delay tolerances in the front-end echo

acquisition system, including variations in the response times of the transducer

and analog electronics.

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Two main sources of error may affect distance measurements:

 Sound speed assumed by the system does not match the actual sound

speed of the medium

 Algorithmic error associated with floating point versus fixed point math

■

The conversion of time delay to distance is based on a sound speed of 1540

m/s for all transducers and application types.

 This is independent of the Speed of Sound Correction, which, if activated,

affects only the focus quality (and not the scaling) of the image.

 Due to sound speed variations between different types of tissue, caliper

measurement errors in the order of 5% are expected for images of the

human body.

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With respect to algorithmic error, the entire digital signal processing chain in

the System is designed with sufficient precision to ensure that the cumulative

digital processor rounding errors from detection to scan conversion will not

exceed one pixel in the final XY display.

 While the display 2D image size varies with the image depth setting and

zoom state, the image generally takes up more than 100 pixels along the

range dimension.

 Therefore, one pixel corresponds to a distance error of less than 1% of the

maximum range of the image.

■

Based on the above considerations, it should be safe to assume that for a

medium whose sound speed is close to the 1540 m/s, the caliper distance

measurement fractional error is:

Tolerance in distance < +/-2% plus (1% of maxRange/distance)

■

The area and circumference accuracies can be derived from the distance

accuracy as follows:

Tolerance in area =

((1+lateral distance tolerance) * (1+axial distance tolerance) -1)

< +/-4% plus (2% of maxRange/smaller of 2 dimensions)

Tolerance in circumference =

sqrt(2)*(maximum of 2 distance tolerances)

< +/-3% plus (1.4% of maxRange/smaller of 2 dimensions)

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Quatech Audio/Video Capture USB 2.0 Especificaciones Pagina 253

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