FIR Filter

Functionality

FIR Filters offer the Halogen system designer additional flexibility that is not easily obtainable with the standard IIR filters offered in Halogen.

It is assumed that the FIR Filter will be designed by a means external to the Halogen software. The FIR filter block simply provides a mechanism to import the FIR filter coefficients, select the number of channels and display the result in a meaningful way once imported. The HAL DSP engine executes the imported filter.

The delay associated with the FIR block is obviously controlled by the coefficient design of the filter. Using a linear phase design will add delay equal to approximately half the number of FIR taps processed at our 48 kHz sampling rate. You may view this by selecting the group delay response graph in the FIR Filter properties dialog. In addition to the designed delay, the FIR block adds one sample of processing delay.

How to Use

Adding the Block to Your System

Click the Processing tab to open the Processing Workspace.
In the palette area, click the DSP tab.
Expand the Filters category of blocks.
Click and drag the FIR Filter block into your Processing Map.
Wire it into your system in the appropriate location.
(Optional) Customize the names of the block and the input and output node by clicking their current name and then typing the custom name in the text box that appears. Click the X to save the name.

Configuring the Block

Open the FIR Filter block's properties by double-clicking the block or hovering and clicking the properties icon that appears in the upper right of the block's title bar.
Click the Import FIR Taps button located at the bottom right of the block and browse to the desired file of FIR Taps and select open.

The file of FIR Taps to be imported should use one of the following formats:

a set of comma separated values
a set of values separated by white space
simply one Tap per line with or without a comma

Each Tap may be numerically formatted in one of three ways:

standard decimal point form (e.g. 3.14159)
no decimal point form (e.g. 124)
scientific notation as exported by excel (e.g. 1.154E-6 or 1.154e-6).

The file selected for importing may have either a .csv extension or a .txt extension. All FIR filter Taps are stored internally and in the DSP using a single precision IEEE floating point format.

note: The maximum number of Taps supported by the FIR filter block is 1024.

User Interface Elements

FIR Filter Block

UI Element	Purpose
Input nodes	Connection point for wiring inputs to the FIR filter block
Output nodes	Connection point for wiring the FIR filter outputs to other blocks
<Add> nodes	Click to add another Input/Output pair of nodes, or wire to the <Add> node to automatically create a new channel.

FIR Filter Block Properties

(Hover over the thumbnail below to view the properties dialog box.)


UI Element	Purpose
Response graph	Graphical representation of the FIR filter that was last imported. Use View options to change graph content.
View options	Selecting Magnitude (selected by default) displays the FIR filter's Magnitude response on the graph. Selecting Group Delay displays the filter's group delay on the graph. Selecting Impulse displays the filter's impulse response on the graph.
FIR Size (# taps imported)	Contains the number of taps of the FIR filter (as imported by the user). If you have not imported any Taps, the FIR Size box displays ---.
Import FIR Taps	Click this button to select and import an external file of FIR Taps. See Configuring the Block above for file format details.
Export FIR Taps	Click this button to export the current set of Taps to an external file. Exported Taps are saved in scientific notation (e.g. 1.154E-6)
Bypass	Check this box to bypass the FIR filter, which effectively turns the filter into a wire.