Functions

Normalized LMS Filters
[Filtering Functions]

Functions

void arm_lms_norm_init_f32 (arm_lms_norm_instance_f32 *S, uint16_t numTaps, float32_t *pCoeffs, float32_t *pState, float32_t mu, uint32_t blockSize)
void arm_lms_norm_f32 (arm_lms_norm_instance_f32 *S, float32_t *pSrc, float32_t *pRef, float32_t *pOut, float32_t *pErr, uint32_t blockSize)
void arm_lms_norm_init_q31 (arm_lms_norm_instance_q31 *S, uint16_t numTaps, q31_t *pCoeffs, q31_t *pState, q31_t mu, uint32_t blockSize, uint8_t postShift)
void arm_lms_norm_q31 (arm_lms_norm_instance_q31 *S, q31_t *pSrc, q31_t *pRef, q31_t *pOut, q31_t *pErr, uint32_t blockSize)
void arm_lms_norm_init_q15 (arm_lms_norm_instance_q15 *S, uint16_t numTaps, q15_t *pCoeffs, q15_t *pState, q15_t mu, uint32_t blockSize, uint8_t postShift)
void arm_lms_norm_q15 (arm_lms_norm_instance_q15 *S, q15_t *pSrc, q15_t *pRef, q15_t *pOut, q15_t *pErr, uint32_t blockSize)

Detailed Description

This set of functions implements a commonly used adaptive filter. It is related to the Least Mean Square (LMS) adaptive filter and includes an additional normalization factor which increases the adaptation rate of the filter. The Cortex-R4 DSP Library contains normalized LMS filter functions that operate on Q15, Q31, and floating-point data types.

A normalized least mean square (NLMS) filter consists of two components as shown below. The first component is a standard transversal or FIR filter. The second component is a coefficient update mechanism. The NLMS filter has two input signals. The "input" feeds the FIR filter while the "reference input" corresponds to the desired output of the FIR filter. That is, the FIR filter coefficients are updated so that the output of the FIR filter matches the reference input. The filter coefficient update mechanism is based on the difference between the FIR filter output and the reference input. This "error signal" tends towards zero as the filter adapts. The NLMS processing functions accept the input and reference input signals and generate the filter output and error signal.

LMS.gif

Internal structure of the NLMS adaptive filter

The functions operate on blocks of data and each call to the function processes blockSize samples through the filter. pSrc points to input signal, pRef points to reference signal, pOut points to output signal and pErr points to error signal. All arrays contain blockSize values.

The functions operate on a block-by-block basis. Internally, the filter coefficients b[n] are updated on a sample-by-sample basis. The convergence of the LMS filter is slower compared to the normalized LMS algorithm.

Algorithm:
The output signal y[n] is computed by a standard FIR filter: 
      
     y[n] = b[0] * x[n] + b[1] * x[n-1] + b[2] * x[n-2] + ...+ b[numTaps-1] * x[n-numTaps+1]
The error signal equals the difference between the reference signal d[n] and the filter output: 
      
     e[n] = d[n] - y[n].
After each sample of the error signal is computed the instanteous energy of the filter state variables is calculated: 
      
    E = x[n]^2 + x[n-1]^2 + ... + x[n-numTaps+1]^2.
The filter coefficients b[k] are then updated on a sample-by-sample basis: 
      
     b[k] = b[k] + e[n] * (mu/E) * x[n-k],  for k=0, 1, ..., numTaps-1
where mu is the step size and controls the rate of coefficient convergence.
In the APIs, pCoeffs points to a coefficient array of size numTaps. Coefficients are stored in time reversed order. 
      
    {b[numTaps-1], b[numTaps-2], b[N-2], ..., b[1], b[0]}
pState points to a state array of size numTaps + blockSize - 1. Samples in the state buffer are stored in the order: 
      
    {x[n-numTaps+1], x[n-numTaps], x[n-numTaps-1], x[n-numTaps-2]....x[0], x[1], ..., x[blockSize-1]}
Note that the length of the state buffer exceeds the length of the coefficient array by blockSize-1 samples. The increased state buffer length allows circular addressing, which is traditionally used in FIR filters, to be avoided and yields a significant speed improvement. The state variables are updated after each block of data is processed.
Instance Structure
The coefficients and state variables for a filter are stored together in an instance data structure. A separate instance structure must be defined for each filter and coefficient and state arrays cannot be shared among instances. There are separate instance structure declarations for each of the 3 supported data types.
Initialization Functions
There is also an associated initialization function for each data type. The initialization function performs the following operations:
  • Sets the values of the internal structure fields.
  • Zeros out the values in the state buffer.
Instance structure cannot be placed into a const data section and it is recommended to use the initialization function.
Fixed-Point Behavior:
Care must be taken when using the Q15 and Q31 versions of the normalised LMS filter. The following issues must be considered:
  • Scaling of coefficients
  • Overflow and saturation
Scaling of Coefficients:
Filter coefficients are represented as fractional values and coefficients are restricted to lie in the range [-1 +1). The fixed-point functions have an additional scaling parameter postShift. At the output of the filter's accumulator is a shift register which shifts the result by postShift bits. This essentially scales the filter coefficients by 2^postShift and allows the filter coefficients to exceed the range [+1 -1). The value of postShift is set by the user based on the expected gain through the system being modeled.
Overflow and Saturation:
Overflow and saturation behavior of the fixed-point Q15 and Q31 versions are described separately as part of the function specific documentation below.

Function Documentation

void arm_lms_norm_init_f32 ( arm_lms_norm_instance_f32 *  S,
uint16_t  numTaps,
float32_t *  pCoeffs,
float32_t *  pState,
float32_t  mu,
uint32_t  blockSize 
)

Initialization function for floating-point normalized LMS filter.

Parameters:
[in]*Spoints to an instance of the floating-point LMS filter structure.
[in]numTapsnumber of filter coefficients.
[in]*pCoeffspoints to coefficient buffer.
[in]*pStatepoints to state buffer.
[in]mustep size that controls filter coefficient updates.
[in]blockSizenumber of samples to process.
Returns:
none.
Description:
pCoeffs points to the array of filter coefficients stored in time reversed order: 
      
    {b[numTaps-1], b[numTaps-2], b[N-2], ..., b[1], b[0]}
The initial filter coefficients serve as a starting point for the adaptive filter. pState points to an array of length numTaps+blockSize-1 samples, where blockSize is the number of input samples processed by each call to arm_lms_norm_f32().
Examples:
arm_signal_converge_example_f32.c.

Definition at line 55 of file arm_lms_norm_init_f32.c.

void arm_lms_norm_f32 ( arm_lms_norm_instance_f32 *  S,
float32_t *  pSrc,
float32_t *  pRef,
float32_t *  pOut,
float32_t *  pErr,
uint32_t  blockSize 
)

Processing function for floating-point normalized LMS filter.

Parameters:
[in]*Spoints to an instance of the floating-point normalized LMS filter structure.
[in]*pSrcpoints to the block of input data.
[in]*pRefpoints to the block of reference data.
[out]*pOutpoints to the block of output data.
[out]*pErrpoints to the block of error data.
[in]blockSizenumber of samples to process.
Returns:
none.
Examples:
arm_signal_converge_example_f32.c.

Definition at line 151 of file arm_lms_norm_f32.c.

void arm_lms_norm_init_q31 ( arm_lms_norm_instance_q31 *  S,
uint16_t  numTaps,
q31_t *  pCoeffs,
q31_t *  pState,
q31_t  mu,
uint32_t  blockSize,
uint8_t  postShift 
)

Initialization function for Q31 normalized LMS filter.

Parameters:
[in]*Spoints to an instance of the Q31 normalized LMS filter structure.
[in]numTapsnumber of filter coefficients.
[in]*pCoeffspoints to coefficient buffer.
[in]*pStatepoints to state buffer.
[in]mustep size that controls filter coefficient updates.
[in]blockSizenumber of samples to process.
[in]postShiftbit shift applied to coefficients.
Returns:
none.

Description:

pCoeffs points to the array of filter coefficients stored in time reversed order: 
      
    {b[numTaps-1], b[numTaps-2], b[N-2], ..., b[1], b[0]}
The initial filter coefficients serve as a starting point for the adaptive filter. pState points to an array of length numTaps+blockSize-1 samples, where blockSize is the number of input samples processed by each call to arm_lms_norm_q31().

Definition at line 54 of file arm_lms_norm_init_q31.c.

void arm_lms_norm_q31 ( arm_lms_norm_instance_q31 *  S,
q31_t *  pSrc,
q31_t *  pRef,
q31_t *  pOut,
q31_t *  pErr,
uint32_t  blockSize 
)

Processing function for Q31 normalized LMS filter.

Parameters:
[in]*Spoints to an instance of the Q31 normalized LMS filter structure.
[in]*pSrcpoints to the block of input data.
[in]*pRefpoints to the block of reference data.
[out]*pOutpoints to the block of output data.
[out]*pErrpoints to the block of error data.
[in]blockSizenumber of samples to process.
Returns:
none.

Scaling and Overflow Behavior:

The function is implemented using an internal 64-bit accumulator. The accumulator has a 2.62 format and maintains full precision of the intermediate multiplication results but provides only a single guard bit. Thus, if the accumulator result overflows it wraps around rather than clip. In order to avoid overflows completely the input signal must be scaled down by log2(numTaps) bits. The reference signal should not be scaled down. After all multiply-accumulates are performed, the 2.62 accumulator is truncated to 1.32 format and then saturated to 1.31 format. The output signal and error signal are in 1.31 format.
In this filter, filter coefficients are updated for each sample and the updation of filter cofficients are saturted.

Definition at line 60 of file arm_lms_norm_q31.c.

void arm_lms_norm_init_q15 ( arm_lms_norm_instance_q15 *  S,
uint16_t  numTaps,
q15_t *  pCoeffs,
q15_t *  pState,
q15_t  mu,
uint32_t  blockSize,
uint8_t  postShift 
)

Initialization function for Q15 normalized LMS filter.

Parameters:
[in]*Spoints to an instance of the Q15 normalized LMS filter structure.
[in]numTapsnumber of filter coefficients.
[in]*pCoeffspoints to coefficient buffer.
[in]*pStatepoints to state buffer.
[in]mustep size that controls filter coefficient updates.
[in]blockSizenumber of samples to process.
[in]postShiftbit shift applied to coefficients.
Returns:
none.

Description:

pCoeffs points to the array of filter coefficients stored in time reversed order: 
      
    {b[numTaps-1], b[numTaps-2], b[N-2], ..., b[1], b[0]}
The initial filter coefficients serve as a starting point for the adaptive filter. pState points to the array of state variables and size of array is numTaps+blockSize-1 samples, where blockSize is the number of input samples processed by each call to arm_lms_norm_q15().

Definition at line 55 of file arm_lms_norm_init_q15.c.

void arm_lms_norm_q15 ( arm_lms_norm_instance_q15 *  S,
q15_t *  pSrc,
q15_t *  pRef,
q15_t *  pOut,
q15_t *  pErr,
uint32_t  blockSize 
)

Processing function for Q15 normalized LMS filter.

Parameters:
[in]*Spoints to an instance of the Q15 normalized LMS filter structure.
[in]*pSrcpoints to the block of input data.
[in]*pRefpoints to the block of reference data.
[out]*pOutpoints to the block of output data.
[out]*pErrpoints to the block of error data.
[in]blockSizenumber of samples to process.
Returns:
none.
Conditions for optimum performance
Input, output, state buffers should be aligned by 32-bit
Restrictions
If the silicon does not support unaligned memory access enable the macro UNALIGNED_SUPPORT_DISABLE In this case input, output, state buffers should be aligned by 32-bit

Scaling and Overflow Behavior:

The function is implemented using a 64-bit internal accumulator. Both coefficients and state variables are represented in 1.15 format and multiplications yield a 2.30 result. The 2.30 intermediate results are accumulated in a 64-bit accumulator in 34.30 format. There is no risk of internal overflow with this approach and the full precision of intermediate multiplications is preserved. After all additions have been performed, the accumulator is truncated to 34.15 format by discarding low 15 bits. Lastly, the accumulator is saturated to yield a result in 1.15 format.
In this filter, filter coefficients are updated for each sample and the updation of filter cofficients are saturted.

Definition at line 68 of file arm_lms_norm_q15.c.

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