ALSA Programming

[MarkBeauchene]

I can nicely output a sine wave with the RPI using ALSA as long as the samples are 1 channel, 8 bit. When I try 16 bit or 2 channels it isn't pretty.

Here are the 8 bit details:
if ((err = snd_pcm_set_params(handle, SND_PCM_FORMAT_U8, SND_PCM_ACCESS_RW_INTERLEAVED, 1, 48000, 1, 100000 ) ) < 0)
{

unsigned char buffer [ 96 ]; /* samples to play */

int tempint = 255 * sin ( baseVal [ keyPlaying ] * keyPos );
buffer[innr] = tempint & 0xff;

Here is my best guess (doesn't work) at 16 bit, one channel:
if ((err = snd_pcm_set_params(handle, SND_PCM_FORMAT_U16_LE, SND_PCM_ACCESS_RW_INTERLEAVED, 1, 48000, 1, 100000 ) ) < 0)

unsigned char buffer [ 2 * 96 ]; /* samples to play */

int tempint = 37763 * sin ( baseVal [ keyPlaying ] * keyPos );
buffer[innr + 1] = ( tempint % 256 ) & 0xff;
buffer[innr] = ( tempint / 256 ) & 0xff;

Anyone see what I am doing wrong?

Additionally, to play silence in 8 bit (works great) I use:

for ( innr = 0; innr < sizeof ( buffer ); innr++ )
{
buffer[innr] = 0 & 0xff;
}

But in 16 bit there is a tone output:
for ( innr = 0; innr < sizeof ( buffer ); innr++ )
{
buffer[innr] = 0 & 0xff;
}

The ALSA site has tons of info, but I think I need "ALSA for Dummies"

Mark

[LdB]

Yes you have 16bit issue here

Code: Select all

int tempint = 37763 * sin ( baseVal [ keyPlaying ] * keyPos );

I am also guessing 37763 is supposed to be 65535 being a full 16 bit range

Then you store them little endian

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buffer[innr + 1] = ( tempint % 256 ) & 0xff;
buffer[innr] = ( tempint / 256 ) & 0xff;

Consider the other endian

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buffer[innr] = ( tempint % 256 ) & 0xff;
buffer[innr+1] = ( tempint / 256 ) & 0xff;

The fact this makes noise as you commented

But in 16 bit there is a tone output:
for ( innr = 0; innr < sizeof ( buffer ); innr++ )
{
buffer[innr] = 0 & 0xff;
}

Probably tells you that you can't do 8 bit writes

Okay so how do we fix this
I am going to make a suggestion you google stdint.h the C/C++ defined bit length unit
using int16_t makes life a whole lot easier

I will modify your code to use stdint.h and it will then look exactly like the 8 bit code

Code: Select all

#define <stdint.h>
int16_t [96 ]; /* samples to play */

int16_t tempint = 65535 * sin ( baseVal [ keyPlaying ] * keyPos );
buffer[innr] = tempint;

for ( innr = 0; innr < sizeof ( buffer ); innr++ )
{
buffer[innr] = 0;
}

If you want your 8 bit code in the same way

Code: Select all

#define <stdint.h>
int8_t [96 ]; /* samples to play */

int8_t tempint = 255 * sin ( baseVal [ keyPlaying ] * keyPos );
buffer[innr] = tempint;

[PeterO]

I am going to make a suggestion you google stdint.h the C/C++ defined bit length unit
using int16_t makes life a whole lot easier
I will modify your code to use stdint.h and it will then look exactly like the 8 bit code

+1
PeterO

[MarkBeauchene]

Thanks. I’ll look into into this in the morning. I just found I needed to make a change in the 8 bit code. The value needs to be
127 + 127* sin (.....). This way the values will be 0 to 254. It sounds way better.

[PeterO]

You could also have done it by changing
SND_PCM_FORMAT_U8
to
SND_PCM_FORMAT_S8
and
int tempint = 255 * sin ( baseVal [ keyPlaying ] * keyPos );
to
int tempint = 127 * sin ( baseVal [ keyPlaying ] * keyPos );

BUT ISTR that might depend on the sound device having to support signed samples (and not all of them do that). I can't remember if the alsa code will do the conversion on the fly for you..

PeterO

[blackshard83]

BUT ISTR that might depend on the sound device having to support signed samples (and not all of them do that). I can't remember if the alsa code will do the conversion on the fly for you..

AFAIK signed 16 bit integers are the standard for 16 bit samples

[MarkBeauchene]

In case anyone has been following this - This RPI does have the equivalent of a sound card built in, and the ALSA objects are great. I have now programmed a synth with 8 note polyphony that generates a very acceptable organ, still in just 8 bits. The Linux system made connecting to a MIDI keyboard very easy as well and C is a very reasonable language. (I did have some experience with C++ Builder several years ago.)

Thanks to the forum members who "held my hand". Perhaps one day I'll write up a tutorial on how this has come together that could prove helpful to others.