File farrow_core.h¶

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Farrow fractional-delay interpolator — linear / parabolic / cubic. More...

• #include "clib_common.h"
• #include "jm_perf.h"
• #include "dp_state.h"
• #include <complex.h>

Classes¶

Public Types¶

Public Functions¶

Macros¶

Detailed Description¶

A selectable-order Lagrange interpolator in Farrow (Horner-in-µ) form — the lean alternative to a full polyphase resampler when all you need is a fractional-delay tap for a timing loop. All three orders share one 4-tap delay line and interpolate at the SAME point — between the two middle taps — so the group delay is 2 samples regardless of order, which keeps a driving symbol-timing loop order-agnostic. Push input samples with farrow_push(); evaluate the output at a fractional offset µ ∈ [0,1) with farrow_eval(). The fractional offset is meant to come from an integer timing NCO (the post-wrap accumulator value), so the timing stays drift-free while only the interpolation itself is floating point.

order: 0 = linear (2-tap Lagrange), 1 = parabolic (4-tap symmetric piecewise-parabolic Farrow, α = 0.5), 2 = cubic (4-tap cubic Lagrange). All three are symmetric about the interpolation point, so the phase (delay) response is linear — no timing bias. Linear and cubic are exact for degree 1 and 3 polynomials; the piecewise-parabolic trades exactness for a flatter magnitude response than linear at no delay cost.

Lifecycle: farrow_create -> (push / eval / reset)* -> farrow_destroy, or embed by value with farrow_init().

farrow_state_t f;
farrow_init(&f, FARROW_CUBIC);
for (size_t i = 0; i < n; i++) farrow_push(&f, x[i]);
float complex y = farrow_eval(&f, 0.3f);   // x interpolated 0.3 past tap[1]

Public Types Documentation¶

enum farrow__core_8h_1a06fc87d81c62e9abb8790b6e5713c55b¶

enum farrow__core_8h_1a06fc87d81c62e9abb8790b6e5713c55b {
    FARROW_LINEAR = 0,
    FARROW_PARABOLIC = 1,
    FARROW_CUBIC = 2
};

Public Functions Documentation¶

function farrow_create¶

Create a Farrow interpolator.

farrow_state_t * farrow_create (
    int order
) 

Parameters:

• order 0 = linear, 1 = parabolic, 2 = cubic.

Returns:

Heap-allocated state, or NULL on allocation failure.

Note:

Caller must call farrow_destroy() when done.

function farrow_delay¶

size_t farrow_delay (
    farrow_state_t * state,
    const float complex * x,
    size_t x_len,
    double mu,
    float complex * out,
    size_t max_out
) 

function farrow_delay_max_out¶

size_t farrow_delay_max_out (
    farrow_state_t * state
) 

function farrow_destroy¶

Destroy a Farrow interpolator.

void farrow_destroy (
    farrow_state_t * state
) 

Parameters:

• state May be NULL.

function farrow_eval¶

Interpolate at fractional offset mu ∈ [0,1) between d[1] and d[2].

JM_FORCEINLINE  JM_HOT float complex farrow_eval (
    const farrow_state_t * s,
    float mu
) 

Horner-in-µ evaluation of the order's Lagrange polynomial. µ = 0 returns d[1] (= input at i - 2); µ → 1 returns d[2].

Parameters:

• s State. Must be non-NULL.
• mu Fractional offset in [0,1).

Returns:

The interpolated sample.

function farrow_get_group_delay¶

size_t farrow_get_group_delay (
    const farrow_state_t * state
) 

function farrow_get_state¶

void farrow_get_state (
    const farrow_state_t * state,
    void * blob
) 

function farrow_init¶

Initialise in place: set order, clear the delay line.

JM_FORCEINLINE void farrow_init (
    farrow_state_t * s,
    int order
) 

function farrow_push¶

Push one input sample into the delay line (oldest drops out).

JM_FORCEINLINE  JM_HOT void farrow_push (
    farrow_state_t * s,
    float complex x
) 

function farrow_reset¶

Clear the delay line; keep the order.

void farrow_reset (
    farrow_state_t * state
) 

function farrow_set_state¶

int farrow_set_state (
    farrow_state_t * state,
    const void * blob
) 

function farrow_state_bytes¶

size_t farrow_state_bytes (
    const farrow_state_t * state
) 

Macro Definition Documentation¶

define FARROW_GROUP_DELAY¶

#define FARROW_GROUP_DELAY `2u`

define FARROW_STATE_MAGIC¶

#define FARROW_STATE_MAGIC `DP_FOURCC ('F', 'R', 'R', 'W')`

define FARROW_STATE_VERSION¶

#define FARROW_STATE_VERSION `1u`

The documentation for this class was generated from the following file native/inc/farrow/farrow_core.h