PWM Audio

Description

The PicoVGA library includes support for audio output using PWM modulation. By default, the audio output is done on the GPIO19 port - defined in the pwmsnd.h file. The port can be connected to directly (e.g. headphones) or better via a simple RC filter with low pass. Audio output using PWM modulation has the advantage that 1 output pin is sufficient and the output circuitry is very simple. The disadvantages are the noise of the sound modulation frequency and the low bit depth of the sound (8 bit depth is used). Higher depth is not possible because of the limited processor frequency. This output is sufficient for most common, undemanding applications (such as retro games). For higher sound quality, another method must be used.

• Sounds are at format 8-bit unsigned (middle at 128), 22050 samples per second, mono
• PWM cycle is 256: TOP = 255
• Required PWM clock: 22050*256 = 5644800 Hz
• GP19 ... MOSI + sound output (PWM1 B)

Functions
void	PWMSndInit ()
	Initialize PWM sound output. More...
void	PlaySound (const u8 *snd, int len, Bool rep=False, float speed=1.0f)
	Output PWM sound. More...
void	StopSound ()
	stop playing sound More...
void	SpeedSound (float speed)
	Update sound speed. More...
Bool	PlayingSound ()
	Check if playing sound.
void	SetNextSound (const u8 *snd, int len)
	Set the next sound to play. More...

Function Documentation

PWMSndInit()

void PWMSndInit

(

)

Initialize PWM sound output.

Note

If the system clock changes, you must reinitialize the audio output settings by calling this function again.

PlaySound()

void PlaySound	(	const u8 *	snd,
		int	len,
		Bool	rep = False,
		float	speed = 1.0f
	)

Output PWM sound.

The audio must be in uncompressed PCM format, mono, 8 bits, frequency 22050 Hz. The parameters can be used to specify whether the sound will be repeated and the relative speed at which it will be played (the 'speed' parameter can be used to speed up or slow down the sound).

Parameters

snd	Pointer to sound
len	Length of sound in number of samples
rep	True to repeat sample
speed	Relative speed (1=normal)

StopSound()

void StopSound

(

)

stop playing sound

The output to the output pin will continue to be output (it will appear as a low noise), but the value of zero 128 will be used.

SpeedSound()

void SpeedSound

(

float

speed

)

Update sound speed.

Used when repeatedly playing a sound to change the pitch (e.g. the sound of a car engine).

Parameters

speed

Relative speed (1.0 represents the standard playback speed)

SetNextSound()

void SetNextSound	(	const u8 *	snd,
		int	len
	)

Set the next sound to play.

Used for repeated sounds to finish playing the current sound and continue with the next sound.

Parameters

snd	Pointer to sound
len	Length of sound in number of samples