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184 Signal processing
Section 6: Meter operation Reference, Installation, and Operations Manual
June 2013 3-9000-743 Rev S
6.1.2 Signal processing
The signal at the receiving transducer is digitized and digital signal processing techniques are
used to provide accurate transit time measurement.
A measure of the signal “goodness” is the signal-to-noise ratio (SNR). The higher the SNR, the
better the signal. In general there are two types of noise: asynchronous “white” noise and
synchronous “colored” noise. Asynchronous noise is noise that occurs across the frequency
spectrum. Synchronous noise is concentrated around a particular frequency. The meter
provides two methods for improving the received signal waveform SNR by reducing the noise
energy: Stacking and Filtering.
Stacking
Stacking is a method of firing a transducer multiple times and averaging the received signals on
a point-by-point basis. Stacking is effective on asynchronous noise such as is typically seen with
valve noise. This method is not useful for removing synchronous noise and should not be used
when there is a great deal of signal “jitter.” Meters are configured by default with a stack size of
2. Care should be taken when turning off stacking and it is recommended to consult with Daniel
Customer Service if you are unsure of how stacking a signal can effect the meter's operation.
Refer to Technical Support under the Help menu of Daniel MeterLink for contact information.
The number of consecutive times to fire each transducer is specified via the StackSize data
point. Available stack sizes are 1 (None), 2, 4, 8, and 16. A stack size of 1 selects no stacking (i.e.,
stacking disabled). Stacking is only available when the standard update rate is selected (see
Section 6.1.3 below).
Filtering
Filtering applies a bandpass filter that removes noise that is above and below the transducer
frequency. Filtering is effective on noise outside of the frequency passband of the filter (e.g.,
filtering works on any noise outside of the passband of the filter). Filtering is enabled/disabled
via the Filter data point (TRUE=enable filtering, FALSE=disable filtering). Meters are configured
by default for filtering disabled. Care should be taken when turning on filtering and it is
recommended to consult with Daniel Customer Service if you are unsure of how filtering a signal
can effect the meter's operation. Refer to Technical Support under the Help menu of Daniel
MeterLink for contact information.