bw_sr_reduce

Sample rate reducer.

This is purely an audio effect, it doesn't actually produce an output signal at a lower sample rate. The algorithm is deliberately crude to obtain the characteristic aliasing due to lo-fi downsampling.

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_sr_reduce_coeffs

typedef struct bw_sr_reduce_coeffs bw_sr_reduce_coeffs;

Coefficients and related.

bw_sr_reduce_state

typedef struct bw_sr_reduce_state bw_sr_reduce_state;

Internal state and related.

bw_sr_reduce_init()

static inline void bw_sr_reduce_init(
	bw_sr_reduce_coeffs * BW_RESTRICT coeffs);

Initializes input parameter values in coeffs.

bw_sr_reduce_set_sample_rate()

static inline void bw_sr_reduce_set_sample_rate(
	bw_sr_reduce_coeffs * BW_RESTRICT coeffs,
	float                             sample_rate);

Sets the sample_rate (Hz) value in coeffs.

bw_sr_reduce_reset_coeffs()

static inline void bw_sr_reduce_reset_coeffs(
	bw_sr_reduce_coeffs * BW_RESTRICT coeffs);

Resets coefficients in coeffs to assume their target values.

bw_sr_reduce_reset_state()

static inline float bw_sr_reduce_reset_state(
	const bw_sr_reduce_coeffs * BW_RESTRICT coeffs,
	bw_sr_reduce_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.

bw_sr_reduce_reset_state_multi()

static inline void bw_sr_reduce_reset_state_multi(
	const bw_sr_reduce_coeffs * BW_RESTRICT              coeffs,
	bw_sr_reduce_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.

bw_sr_reduce_update_coeffs_ctrl()

static inline void bw_sr_reduce_update_coeffs_ctrl(
	bw_sr_reduce_coeffs * BW_RESTRICT coeffs);

Triggers control-rate update of coefficients in coeffs.

bw_sr_reduce_update_coeffs_audio()

static inline void bw_sr_reduce_update_coeffs_audio(
	bw_sr_reduce_coeffs * BW_RESTRICT coeffs);

Triggers audio-rate update of coefficients in coeffs.

bw_sr_reduce_process1()

static inline float bw_sr_reduce_process1(
	const bw_sr_reduce_coeffs * BW_RESTRICT coeffs,
	bw_sr_reduce_state * BW_RESTRICT        state,
	float                                   x);

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

bw_sr_reduce_process()

static inline void bw_sr_reduce_process(
	bw_sr_reduce_coeffs * BW_RESTRICT coeffs,
	bw_sr_reduce_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 coeffs and both using and updating state (control and audio rate).

bw_sr_reduce_process_multi()

static inline void bw_sr_reduce_process_multi(
	bw_sr_reduce_coeffs * BW_RESTRICT                    coeffs,
	bw_sr_reduce_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 the common coeffs and both using and updating each of the n_channels states.

bw_sr_reduce_set_ratio()

static inline void bw_sr_reduce_set_ratio(
	bw_sr_reduce_coeffs * BW_RESTRICT coeffs,
	float                             value);

Sets the output to input sample rate ratio in coeffs.

Valid input range: [0.f, 1.f].

Default value: 1.f.

bw_sr_reduce_coeffs_is_valid()

static inline char bw_sr_reduce_coeffs_is_valid(
	const bw_sr_reduce_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_sr_reduce_coeffs.

bw_sr_reduce_state_is_valid()

static inline char bw_sr_reduce_state_is_valid(
	const bw_sr_reduce_coeffs * BW_RESTRICT coeffs,
	const bw_sr_reduce_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_sr_reduce_state.

C++ wrapper

Brickworks::SRReduce

template<size_t N_CHANNELS>
class SRReduce {
public:
	SRReduce();
	
	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 setRatio(
		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_sr_reduce_process() to bw_sr_reduce_process_multi().
  • Added debugging checks in bw_sr_reduce_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 bw_sr_reduce_set_sample_rate(), bw_sr_reduce_reset_coeffs(), bw_sr_reduce_update_coeffs_ctrl(), and bw_sr_reduce_update_coeffs_audio().
  • Added initial input value to bw_sr_reduce_reset_state().
  • Added bw_sr_reduce_reset_state_multi() and updated C++ API in this regard.
  • Now bw_sr_reduce_reset_state() returns the initial output value.
  • Added overloaded C++ reset() functions taking arrays as arguments.
  • bw_sr_reduce_process() and bw_sr_reduce_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.
  • Added debugging code.
• Version 0.6.0:
  • Removed dependency on bw_config.
• Version 0.5.0:
  • Added bw_sr_reduce_process_multi().
  • Added C++ wrapper.
• Version 0.4.0:
  • Fixed unused parameter warnings.
• Version 0.3.0:
  • First release.