How to downmix 5.1 and 7.1 channels to 2 channels

From: Amanda Ava Koci Pereira <arsoftware10@gmail.com>
To: alsa-devel@alsa-project.org
Subject: How to downmix 5.1 and 7.1 channels to 2 channels
Date: Mon, 19 Oct 2020 23:32:12 -0300	[thread overview]
Message-ID: <CALAz8EWAKP-LaeDyQZW0BauwYBthSbgJgxRT8hypuXEWEOfPhA@mail.gmail.com> (raw)

Hi guys...

We need some clues about to downmix 5.1 and 7.1 channels to 2 channels

for the moment we are using the following code ->

It works but fails miserably if all channels have data at same time

#define AR_K_P_ATTENUATION_LEVEL 1.0

int p = 0;
int i;
int save_channel_0_k_p;
int save_channel_1_k_p;
double save_channel_0_data_double_k_p;
double save_channel_1_data_double_k_p;
double sample_double_temp_k_p;
int number_of_samples_k_p = decoded_frame->nb_samples;
int ricardo_deslocador__ = 0;
frame_processed_k_p = 1;

int outputBufferLen = number_of_samples_k_p * 2 * 2;
*size_out=outputBufferLen;
short *outputBuffer = (int16_t *)buf;
int16_t * ar_buffer_ = (int16_t *) decoded_frame->data[0];

for(i=0; i < number_of_samples_k_p; i++)
{
//channel 0
int16_t sample_k_p = ar_buffer_[ricardo_deslocador__++];

save_channel_0_k_p = p;

save_channel_0_data_double_k_p = (double) sample_k_p;

p++;

///////////////////////////////////////////////
//channel 1
sample_k_p = ar_buffer_[ricardo_deslocador__++];

save_channel_1_k_p = p;//so usa no final...

save_channel_1_data_double_k_p = (double) sample_k_p;

p++;

////////////////////////////////////////////////
//center
sample_k_p = ar_buffer_[ricardo_deslocador__++];

sample_double_temp_k_p = (double) sample_k_p;

sample_double_temp_k_p *= AR_K_P_ATTENUATION_LEVEL;

save_channel_0_data_double_k_p += sample_double_temp_k_p;

if(32767.0 < save_channel_0_data_double_k_p)
{
save_channel_0_data_double_k_p = 32767.0;
}

if(-32768.0 > save_channel_0_data_double_k_p)
{
save_channel_0_data_double_k_p = -32768.0;
}

sample_double_temp_k_p = (double) sample_k_p;

sample_double_temp_k_p *= AR_K_P_ATTENUATION_LEVEL;

save_channel_1_data_double_k_p += sample_double_temp_k_p;

if(32767.0 < save_channel_1_data_double_k_p)
{
save_channel_1_data_double_k_p = 32767.0;
}

if(-32768.0 > save_channel_1_data_double_k_p)
{
save_channel_1_data_double_k_p = -32768.0;
}

//////////////////////////////////////////////////////
///////subwoffer
sample_k_p = ar_buffer_[ricardo_deslocador__++];

sample_double_temp_k_p = (double) sample_k_p;

sample_double_temp_k_p *= AR_K_P_ATTENUATION_LEVEL;

save_channel_0_data_double_k_p += sample_double_temp_k_p;

if(32767.0 < save_channel_0_data_double_k_p)
{
save_channel_0_data_double_k_p = 32767.0;
}

if(-32768.0 > save_channel_0_data_double_k_p)
{
save_channel_0_data_double_k_p = -32768.0;
}

sample_double_temp_k_p = (double) sample_k_p;

sample_double_temp_k_p *= AR_K_P_ATTENUATION_LEVEL;

save_channel_1_data_double_k_p += sample_double_temp_k_p;

if(32767.0 < save_channel_1_data_double_k_p)
{
save_channel_1_data_double_k_p = 32767.0;
}

if(-32768.0 > save_channel_1_data_double_k_p)
{
save_channel_1_data_double_k_p = -32768.0;
}
////////////////////////////////////////////////////
/////surround left 4
sample_k_p = ar_buffer_[ricardo_deslocador__++];

sample_double_temp_k_p = (double) sample_k_p;

sample_double_temp_k_p *= AR_K_P_ATTENUATION_LEVEL;

save_channel_0_data_double_k_p += sample_double_temp_k_p;

if(32767.0 < save_channel_0_data_double_k_p)
{
save_channel_0_data_double_k_p = 32767.0;
}

if(-32768.0 > save_channel_0_data_double_k_p)
{
save_channel_0_data_double_k_p = -32768.0;
}
///////////////////////////////////////////////
//////////surround right 5
sample_k_p = ar_buffer_[ricardo_deslocador__++];

sample_double_temp_k_p = (double) sample_k_p;

sample_double_temp_k_p *= AR_K_P_ATTENUATION_LEVEL;

save_channel_1_data_double_k_p += sample_double_temp_k_p;

if(32767.0 < save_channel_1_data_double_k_p)
{
save_channel_1_data_double_k_p = 32767.0;
}

if(-32768.0 > save_channel_1_data_double_k_p)
{
save_channel_1_data_double_k_p = -32768.0;
}
/////////////////////////////////////////////
//finalizing

outputBuffer[save_channel_0_k_p] = (signed short)
save_channel_0_data_double_k_p;
outputBuffer[save_channel_1_k_p] = (signed short)
save_channel_1_data_double_k_p;
}

/*

if possible point us to the files in ALSA that make the downmix and it will
be enough

Thanks a lot

*/

Ricardo