Note that the current implementation of this actor only reads its parameters during initialization, so the filter cannot be changed during execution.
When the decimation (interpolation) parameters are different from unity, the filter behaves exactly as it were followed (preceded) by a DownSample (UpSample) actor. However, the implementation is much more efficient than it would be using UpSample or DownSample actors; a polyphase structure is used internally, avoiding unnecessary use of memory and unnecessary multiplication by zero. Arbitrary sample-rate conversions by rational factors can be accomplished this way.
To design a filter for a multirate system, simply assume the sample rate is the product of the interpolation parameter and the input sample rate, or equivalently, the product of the decimation parameter and the output sample rate. In particular, considerable care must be taken to avoid aliasing. Specifically, if the input sample rate is f, then the filter stopband should begin before f/2. If the interpolation ratio is i, then f/2 is a fraction 1/2i of the sample rate at which you must design your filter.
The decimationPhase parameter is somewhat subtle. It is exactly equivalent the phase parameter of the DownSample actor. Its interpretation is as follows; when decimating, samples are conceptually discarded (although a polyphase structure does not actually compute the discarded samples). If you are decimating by a factor of three, then you will select one of every three outputs, with three possible phases. When decimationPhase is zero (the default), the latest (most recent) samples are the ones selected. The decimationPhase must be strictly less than the decimation ratio.
Note: in this description "sample rate" refers to the physical sampling rate of an A/D converter in the system. In other words, the number of data samples per second. This is not usually specified anywhere in an SDF system, and most definitely does NOT correspond to the SDF rate parameters of this actor. This actor automatically sets the rates of the input and output ports to the decimation and interpolation ratios, respectively.
For more information about polyphase filters, see F. J. Harris, "Multirate FIR Filters for Interpolating and Desampling", in Handbook of Digital Signal Processing, Academic Press, 1987. @author Edward A. Lee, Bart Kienhuis, Steve Neuendorffer @version $Id: FIR.java,v 1.95 2006/10/24 23:50:35 cxh Exp $ @since Ptolemy II 0.2 @Pt.ProposedRating Yellow (neuendor) @Pt.AcceptedRating Yellow (neuendor) @see ptolemy.data.Token
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