FIR RF Filtering

Background
There are three RF filtering modes that the Sonix systems can take advantage of:
 * No Filtering
 * Frequency Compounding Filtering
 * Depth Dependent Filtering

No filtering allows the entire signal to pass through unbanded, resulting in a raw looking B image.

Frequency compounding splits the beamformed digital RF signal into 3 paths after it gone through a constant digital gain filter. Each path performs an FIR of varying center frequency and bandwidth that is selectable on the research menus. Also included is an adjustable filter gain level that is controlled through coefficient shift values, found on the research menus as well. After the FIR, each path performs it's own envelope detection, and is compounded into one signal before going through a lookup table that converts the data from 16 to 8 bits before being sent to the PC for scan-conversion and other processing.

Depth dependent RF filtering does not use compounding, but rather applies a different filter at certain image depths. As with the compounding method, there are 3 selectable filters that can be selected. This method may be more applicable when scanning tissue that is very sensitive to higher resolution in the near field and deeper penetration for the far field. Harmonic imaging uses this mode as to enhance the effectiveness of the cancellation techniques required.

No Filtering
To work with this method, the following parameters should be set as specified:
 * RF FIR Type must be set to 0

Frequency Compounding
To work with this method, the following parameters should be set as specified:
 * RF FIR Type must be set to 1
 * FC Disable must be set to 0
 * RF FIR A is the selection into the index file for the first frequency to be compounded
 * RF FIR A Shift is the gain control for the FIR A coefficients
 * RF FIR B is the selection into the index file for the second frequency to be compounded
 * RF FIR B Shift is the gain control for the FIR B coefficients
 * RF FIR C is the selection into the index file for the third frequency to be compounded
 * RF FIR C Shift is the gain control for the FIR C coefficients
 * FC Ampl A is set at a level between 30 and 60, depending on the probe and preset

Depth Based Filtering
To work with this method, the following parameters should be set as specified:
 * RF FIR Type must be set to 1
 * FC Disable must be set to 1
 * RF FIR A is the selection into the index file for the first frequency to be compounded
 * RF FIR A Shift is the gain control for the FIR A coefficients
 * RF FIR B is the selection into the index file for the second frequency to be compounded
 * RF FIR B Shift is the gain control for the FIR B coefficients
 * RF FIR C is the selection into the index file for the third frequency to be compounded
 * RF FIR Depth A is the depth for the FIR A to work until (ie. 0 cm to A cm)
 * RF FIR Depth B is the depth for the FIR B to work within (ie. A cm to B cm)
 * FIR C will work within B cm to the bottom of the image
 * RF FIR Overlap is the number of samples to overlap in between the seams of the filters
 * FC Ampl A needs to be increased to get proper gain levels (approximately 3 times the value in frequency compounding mode)

With no FIR Overlap set, the image will have significant boundaries shown, as can be seen below.

Filter Design
All the filters in the filters.txt file on the system were created with the ScopeFIR program. These are standard band-pass filters ranging from 15 to 63 taps. The text file also has space for automatic gain compensation parameters, so preset adjustment on the screen does not necessarily need to take place.

More information can be found here.