bw_comb

Comb filter / delay effect with feedforward and feedback paths.

Essentially an implementation of the structure described in

J. Dattorro, "Effect Design, Part 2: Delay-Line Modulation and Chorus", J. Audio Eng. Soc., vol. 45, no. 10, pp. 764-788, October 1997.

Unlike in the original paper, the feedback signal is not subtracted but rather added to the input.

Examples

Here you can download one or more example VST3 plugins for Windows, macOS and Linux. Source code of the audio engine(s) is included in the archive(s).

VST® is a trademark of Steinberg Media Technologies GmbH, registered in Europe and other countries.

API

Module type: DSP

bw_comb_coeffs

typedef struct bw_comb_coeffs bw_comb_coeffs;

Coefficients and related.

bw_comb_state

typedef struct bw_comb_state bw_comb_state;

Internal state and related.

bw_comb_init()

static inline void bw_comb_init(
	bw_comb_coeffs * BW_RESTRICT coeffs,
	float                        max_delay);

Initializes input parameter values in coeffs using max_delay (s) as the maximum delay time.

max_delay must be finite and non-negative.

bw_comb_set_sample_rate()

static inline void bw_comb_set_sample_rate(
	bw_comb_coeffs * BW_RESTRICT coeffs,
	float                        sample_rate);

Sets the sample_rate (Hz) value in coeffs.

bw_comb_mem_req()

static inline size_t bw_comb_mem_req(
	const bw_comb_coeffs * BW_RESTRICT coeffs);

Returns the size, in bytes, of contiguous memory to be supplied to bw_comb_mem_set() using coeffs.

bw_comb_mem_set()

static inline void bw_comb_mem_set(
	const bw_comb_coeffs * BW_RESTRICT coeffs,
	bw_comb_state * BW_RESTRICT        state,
	void * BW_RESTRICT                 mem);

Associates the contiguous memory block mem to the given state using coeffs.

bw_comb_reset_coeffs()

static inline void bw_comb_reset_coeffs(
	bw_comb_coeffs * BW_RESTRICT coeffs);

Resets coefficients in coeffs to assume their target values.

bw_comb_reset_state()

static inline float bw_comb_reset_state(
	const bw_comb_coeffs * BW_RESTRICT coeffs,
	bw_comb_state * BW_RESTRICT        state,
	float                              x_0);

Resets the given state to its initial values using the given coeffs and the initial input value x_0.

Returns the corresponding initial output value.

If parameter coeff_fb has value -1.f or 1.f, then x_0 must be 0.f.

bw_comb_reset_state_multi()

static inline void bw_comb_reset_state_multi(
	const bw_comb_coeffs * BW_RESTRICT              coeffs,
	bw_comb_state * BW_RESTRICT const * BW_RESTRICT state,
	const float *                                   x_0,
	float *                                         y_0,
	size_t                                          n_channels);

Resets each of the n_channels states to its initial values using the given coeffs and the corresponding initial input value in the x_0 array.

The corresponding initial output values are written into the y_0 array, if not BW_NULL.

If parameter coeff_fb has value -1.f or 1.f, then x_0 must only contain 0.f.

bw_comb_update_coeffs_ctrl()

static inline void bw_comb_update_coeffs_ctrl(
	bw_comb_coeffs * BW_RESTRICT coeffs);

Triggers control-rate update of coefficients in coeffs.

bw_comb_update_coeffs_audio()

static inline void bw_comb_update_coeffs_audio(
	bw_comb_coeffs * BW_RESTRICT coeffs);

Triggers audio-rate update of coefficients in coeffs.

bw_comb_process1()

static inline float bw_comb_process1(
	const bw_comb_coeffs * BW_RESTRICT coeffs,
	bw_comb_state * BW_RESTRICT        state,
	float                              x);

Processes one input sample x using coeffs, while using and updating state. Returns the corresponding output sample.

bw_comb_process()

static inline void bw_comb_process(
	bw_comb_coeffs * BW_RESTRICT coeffs,
	bw_comb_state * BW_RESTRICT  state,
	const float *                x,
	float *                      y,
	size_t                       n_samples);

Processes the first n_samples of the input buffer x and fills the first n_samples of the output buffer y, while using and updating both coeffs and state (control and audio rate).

bw_comb_process_multi()

static inline void bw_comb_process_multi(
	bw_comb_coeffs * BW_RESTRICT                    coeffs,
	bw_comb_state * BW_RESTRICT const * BW_RESTRICT state,
	const float * const *                           x,
	float * const *                                 y,
	size_t                                          n_channels,
	size_t                                          n_samples);

Processes the first n_samples of the n_channels input buffers x and fills the first n_samples of the n_channels output buffers y, while using and updating both the common coeffs and each of the n_channels states (control and audio rate).

bw_comb_set_delay_ff()

static inline void bw_comb_set_delay_ff(
	bw_comb_coeffs * BW_RESTRICT coeffs,
	float                        value);

Sets the feedforward delay time value (s) in coeffs.

This parameter is internally limited to avoid buffer overflows.

value must be finite.

Default value: 0.f.

bw_comb_set_delay_fb()

static inline void bw_comb_set_delay_fb(
	bw_comb_coeffs * BW_RESTRICT coeffs,
	float                        value);

Sets the feedback delay time value (s) in coeffs.

This parameter is internally limited to avoid buffer overflows.

value must be finite.

Default value: 0.f.

bw_comb_set_coeff_blend()

static inline void bw_comb_set_coeff_blend(
	bw_comb_coeffs * BW_RESTRICT coeffs,
	float                        value);

Sets the blend coefficient value in coeffs.

value must be finite.

Default value: 1.f.

bw_comb_set_coeff_ff()

static inline void bw_comb_set_coeff_ff(
	bw_comb_coeffs * BW_RESTRICT coeffs,
	float                        value);

Sets the feedforward coefficient value in coeffs.

value must be finite.

Default value: 0.f.

bw_comb_set_coeff_fb()

static inline void bw_comb_set_coeff_fb(
	bw_comb_coeffs * BW_RESTRICT coeffs,
	float                        value);

Sets the feedback coefficient value in coeffs.

Valid range: [-1.f, 1.f].

Default value: 0.f.

bw_comb_coeffs_is_valid()

static inline char bw_comb_coeffs_is_valid(
	const bw_comb_coeffs * BW_RESTRICT coeffs);

Tries to determine whether coeffs is valid and returns non-0 if it seems to be the case and 0 if it is certainly not. False positives are possible, false negatives are not.

coeffs must at least point to a readable memory block of size greater than or equal to that of bw_comb_coeffs.

bw_comb_state_is_valid()

static inline char bw_comb_state_is_valid(
	const bw_comb_coeffs * BW_RESTRICT coeffs,
	const bw_comb_state * BW_RESTRICT  state);

Tries to determine whether state is valid and returns non-0 if it seems to be the case and 0 if it is certainly not. False positives are possible, false negatives are not.

If coeffs is not BW_NULL extra cross-checks might be performed (state is supposed to be associated to coeffs).

state must at least point to a readable memory block of size greater than or equal to that of bw_comb_state.

C++ wrapper

Brickworks::Comb

template<size_t N_CHANNELS>
class Comb {
public:
	Comb(
		float maxDelay = 1.f);

	~Comb();

	void setSampleRate(
		float sampleRate);

	void reset(
		float               x0 = 0.f,
		float * BW_RESTRICT y0 = nullptr);

# ifndef BW_CXX_NO_ARRAY
	void reset(
		float                                       x0,
		std::array<float, N_CHANNELS> * BW_RESTRICT y0);
# endif

	void reset(
		const float * x0,
		float *       y0 = nullptr);

# ifndef BW_CXX_NO_ARRAY
	void reset(
		std::array<float, N_CHANNELS>               x0,
		std::array<float, N_CHANNELS> * BW_RESTRICT y0 = nullptr);
# endif

	void process(
		const float * const * x,
		float * const *       y,
		size_t                nSamples);

# ifndef BW_CXX_NO_ARRAY
	void process(
		std::array<const float *, N_CHANNELS> x,
		std::array<float *, N_CHANNELS>       y,
		size_t                                nSamples);
# endif

	void setDelayFF(
		float value);

	void setDelayFB(
		float value);

	void setCoeffBlend(
		float value);

	void setCoeffFF(
		float value);

	void setCoeffFB(
		float value);
...
}

Changelog

• Version 1.2.0:
  • Added support for BW_INCLUDE_WITH_QUOTES, BW_NO_CXX, and BW_CXX_NO_EXTERN_C.
  • Added debugging checks from bw_comb_process() to bw_comb_process_multi().
  • Added debugging checks in bw_comb_process_multi() to ensure that buffers used for both input and output appear at the same channel indices.
• Version 1.1.0:
  • Now using BW_NULL and BW_CXX_NO_ARRAY.
• Version 1.0.0:
  • Added initial value argument in bw_comb_reset_state().
  • Added bw_comb_reset_state_multi() and updated C++ API in this regard.
  • Now bw_comb_reset_state() returns the initial output value.
  • Added overloaded C++ reset() functions taking arrays as arguments.
  • Now using size_t instead of BW_SIZE_T.
  • bw_comb_process() and bw_comb_process_multi() now use size_t to count samples and channels.
  • Added more const and BW_RESTRICT specifiers to input arguments and implementation.
  • Moved C++ code to C header.
  • Added overloaded C++ process() function taking C-style arrays as arguments.
  • Removed usage of reserved identifiers.
  • Clearly specified parameter validity ranges.
  • Added debugging code.
• Version 0.6.0:
  • Removed dependency on bw_config.
• Version 0.5.0:
  • Added bw_comb_process_multi().
  • Updated mem_req/set API.
  • Added C++ wrapper.