Oscillator implementation

Data Structures
struct	sstvenc_oscillator

Macros
#define	SSTVENC_OSC_PHASE_FRAC_BITS   (29)
#define	SSTVENC_OSC_PHASE_FRAC_SCALE    ((double)(1 << SSTVENC_OSC_PHASE_FRAC_BITS))

Functions
double	sstvenc_osc_get_frequency (const struct sstvenc_oscillator *const osc)
void	sstvenc_osc_set_frequency (struct sstvenc_oscillator *const osc, double frequency)
void	sstvenc_osc_init (struct sstvenc_oscillator *const osc, double amplitude, double frequency, double offset, uint32_t sample_rate)
void	sstvenc_osc_compute (struct sstvenc_oscillator *const osc)

Detailed Description

This is a simple module that can be used to produce a single sinusoid tone at a given frequency for a given sample rate.

The phase is computed at each sample step, and increments modulo 2-Pi. Frequency, amplitude and phase can be modified at any time, they take effect on the next call to sstvenc_osc_compute.

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Data Structure Documentation

sstvenc_oscillator

struct sstvenc_oscillator

Oscillator data structure. This must remain allocated for the lifetime of the sinusoid.

Use sstvenc_osc_init to initialise this structure.

Definition at line 29 of file oscillator.h.

Collaboration diagram for sstvenc_oscillator:

Data Fields
double	amplitude	The amplitude of the sinusoid: range 0-1.0. This may be adjusted to ampiltude-modulate the oscillator. The new amplitude takes effect on the next call to sstvenc_osc_compute.
double	offset	Phase offset in radians: range 0-2*M_PI. This may be adjusted to phase-modulate the oscillator. The new phase offset takes effect on the next call to sstvenc_osc_compute.
double	output	The last computed output of the sinusoid. The audio output should be read from this field.
uint32_t	phase	Fixed-point phase of the sinusoid oscillator. Do not manipulate directly, use sstvenc_osc_init or sstvenc_osc_compute to update. If just re-starting a sine with identical settings, you may set this to 0.
uint32_t	phase_inc	Fixed-point phase increment each iteration. Use sstvenc_osc_set_frequency to adjust this or sstvenc_osc_get_frequency to convert back to a frequency in Hz.
uint32_t	sample_rate	Sample rate for the sinusoid in Hz. Must not be changed after initialisation.

Macro Definition Documentation

SSTVENC_OSC_PHASE_FRAC_BITS

#define SSTVENC_OSC_PHASE_FRAC_BITS   (29)

Fixed-point phase bit allocation. For a uint32_t, this gives us a range of 0-7.999 with a precision of ~1.86 nano-radians.

Definition at line 20 of file oscillator.c.

SSTVENC_OSC_PHASE_FRAC_SCALE

#define SSTVENC_OSC_PHASE_FRAC_SCALE    ((double)(1 << SSTVENC_OSC_PHASE_FRAC_BITS))

Fixed-point scaling factor, computed from the number of bits.

Definition at line 25 of file oscillator.c.

#define SSTVENC_OSC_PHASE_FRAC_SCALE                                         \
        ((double)(1 << SSTVENC_OSC_PHASE_FRAC_BITS))

Referenced by sstvenc_osc_compute(), sstvenc_osc_get_frequency(), and sstvenc_osc_set_frequency().

Function Documentation

sstvenc_osc_compute()

void sstvenc_osc_compute

(

struct sstvenc_oscillator *const

osc

)

Compute the next sinusoid value and store it in the output field. A no-op if the sample rate is set to zero.

The next sample is made available in sstvenc_oscillator::output.

Parameters

[in,out]

osc

Oscillator context being computed.

Definition at line 52 of file oscillator.c.

                                                               {
        if (osc->sample_rate) {
                /* Compute output */
                osc->output = osc->amplitude
                              * sin(osc->offset
                                    + (((double)osc->phase)
                                       / SSTVENC_OSC_PHASE_FRAC_SCALE));
 
                /* Increment phase, modulo 2Pi */
                osc->phase += osc->phase_inc;
                osc->phase
                    %= (uint32_t)(2 * M_PI * SSTVENC_OSC_PHASE_FRAC_SCALE);
        }
}

References sstvenc_oscillator::amplitude, sstvenc_oscillator::offset, sstvenc_oscillator::output, sstvenc_oscillator::phase, sstvenc_oscillator::phase_inc, sstvenc_oscillator::sample_rate, and SSTVENC_OSC_PHASE_FRAC_SCALE.

Referenced by sstvenc_cw_handle_state_mark(), sstvenc_modulator_next_fall_sample(), sstvenc_modulator_next_hold_sample(), sstvenc_modulator_next_rise_sample(), sstvenc_psosc_fill_buffer(), and sstvenc_sequencer_compute().

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sstvenc_osc_get_frequency()

double sstvenc_osc_get_frequency

(

const struct sstvenc_oscillator *const

osc

)

Get the oscillator frequency in Hertz.

Parameters

[in]

osc

Oscillator context.

Returns

The oscillator frequency in Hertz.

Definition at line 28 of file oscillator.c.

                                                                             {
        return ((double)(((uint64_t)osc->phase_inc) * osc->sample_rate))
               / (2 * M_PI * SSTVENC_OSC_PHASE_FRAC_SCALE);
}

References sstvenc_oscillator::phase_inc, sstvenc_oscillator::sample_rate, and SSTVENC_OSC_PHASE_FRAC_SCALE.

sstvenc_osc_init()

void sstvenc_osc_init	(	struct sstvenc_oscillator *const	osc,
		double	amplitude,
		double	frequency,
		double	offset,
		uint32_t	sample_rate )

Initialise an oscillator with the given amplitude, frequency and phase offset. Use this when starting a new sinusoid.

Parameters

[in,out]	osc	Oscillator context being initialised.
[in]	amplitude	Starting oscillator amplitude (range 0.0-1.0).
[in]	frequency	Starting frequency of the oscillator (range 0.0 - ½ of sample_rate).
[in]	offset	Starting phase offset in radians.
[in]	sample_rate	Output sample rate in hertz.

Definition at line 42 of file oscillator.c.

                                                                             {
        osc->amplitude   = amplitude;
        osc->offset      = offset;
        osc->output      = 0.0;
        osc->sample_rate = sample_rate;
        osc->phase       = 0;
        sstvenc_osc_set_frequency(osc, frequency);
}

References sstvenc_oscillator::amplitude, sstvenc_oscillator::offset, sstvenc_oscillator::output, sstvenc_oscillator::phase, sstvenc_oscillator::sample_rate, and sstvenc_osc_set_frequency().

Referenced by sstvenc_cw_init(), sstvenc_modulator_init(), and sstvenc_sequencer_begin_tone().

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sstvenc_osc_set_frequency()

void sstvenc_osc_set_frequency	(	struct sstvenc_oscillator *const	osc,
		double	frequency )

Set the oscillator frequency in Hertz.

Parameters

[in,out]	osc	Oscillator context being updated.
[in]	frequency	The frequency in Hertz. This MUST be at least 0Hz and less than the nyquist frequency (half the sample rate).

Definition at line 33 of file oscillator.c.

                                                                           {
        assert(frequency >= 0);
        assert(frequency < (osc->sample_rate / 2));
 
        osc->phase_inc = (2 * M_PI * frequency * SSTVENC_OSC_PHASE_FRAC_SCALE)
                         / osc->sample_rate;
}

References sstvenc_oscillator::phase_inc, sstvenc_oscillator::sample_rate, and SSTVENC_OSC_PHASE_FRAC_SCALE.

Referenced by sstvenc_modulator_next_tone(), sstvenc_osc_init(), and sstvenc_sequencer_begin_tone().

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