# Basic implementation
Assuming you have successfully implemented the [passthrough](/audio-peripherals/passthrough.md), you can simply copy and paste that project from within the STM32CubeIDE environment. We recommend choosing a name with the current date and `"alien_voice"` in it. Remember to delete the old binary (ELF) file inside the copied project.
## Lookup table
Remember that in the passthrough we set up the system to work with a sampling frequency of 32KHz and a sample precision of 16 bits. Here we will use a modulation frequency of 400Hz for the frequency shifting, so that the digital frequency is a rational multiple of $$2\pi$$as in the [previous example](/real-world-dsp/code-efficiency.md#lookup):
$$
\omega\_c = 2\pi\frac{400}{32,000}=\frac{2\pi}{80}\approx 0.0785398
$$
The values for one period of the digital sinusoid can be encoded in a [circular lookup table](/real-world-dsp/code-efficiency.md#lookup) of length 80, where each element is (in 16-bit precision)
```c
cos_table[n] = (int16_t)(32767.0 * cos(0.0785398 * n));
```
The lookup table is provided here for your convenience. Begin by copying it between the `USER CODE BEGIN PV` and `USER CODE END PV` comment tags.
```c
#define COS_TABLE_LEN 80
static int16_t COS_TABLE[COS_TABLE_LEN] = {
0x7FFF, 0x7F99, 0x7E6B, 0x7C75, 0x79BB, 0x7640, 0x720B, 0x6D22, 0x678D, 0x6154, 0x5A81, 0x5320,
0x4B3B, 0x42E0, 0x3A1B, 0x30FB, 0x278D, 0x1DE1, 0x1405, 0x0A0A, 0x0000, 0xF5F6, 0xEBFB, 0xE21F,
0xD873, 0xCF05, 0xC5E5, 0xBD20, 0xB4C5, 0xACE0, 0xA57F, 0x9EAC, 0x9873, 0x92DE, 0x8DF5, 0x89C0,
0x8645, 0x838B, 0x8195, 0x8067, 0x8001, 0x8067, 0x8195, 0x838B, 0x8645, 0x89C0, 0x8DF5, 0x92DE,
0x9873, 0x9EAC, 0xA57F, 0xACE0, 0xB4C5, 0xBD20, 0xC5E5, 0xCF05, 0xD873, 0xE21F, 0xEBFB, 0xF5F6,
0x0000, 0x0A0A, 0x1405, 0x1DE1, 0x278D, 0x30FB, 0x3A1B, 0x42E0, 0x4B3B, 0x5320, 0x5A81, 0x6154,
0x678D, 0x6D22, 0x720B, 0x7640, 0x79BB, 0x7C75, 0x7E6B, 0x7F99};
```
{% hint style="info" %}
TASK 1: Write a short Python function that prints out the above table given a value for the period of the sinusoid.
{% endhint %}
## Gain
Let's also define a gain factor to increase the output volume. As we said before, we will use a gain that is a power of two and therefore just define its exponent. If you find that the sound is distorted, you may want to reduce this number.
```c
#define GAIN 3 /* multiply the output by a factor of 2^GAIN */
```
## The processing function
In the following version of the main processing function you will need to provide a couple of lines yourself. In the meantime please note the following:
* we are assuming that we're using the LEFT channel for the microphone and we go through the input buffer two samples at a time, while we duplicate the output to produce a signal to both ears.
* `ix` is the [state variable](/real-world-dsp/code-efficiency.md#state_var) that keeps track of our position in the lookup table. Since the alien voice is an *instantaneous* transformation, this is the only global time reference that we need to have
* the function also implements [a simple DC notch](/real-world-dsp/signal-levels.md#removing_dc). Since this filter only requires memory of a single past input sample, there is no need to implement a circular buffer and we just use a single static variable in the function
* [the multiplications should be performed using 32-bit integers](/real-world-dsp/code-efficiency.md#float) and the result is scaled back to 16 bits; we take the gain into account in this rescaling.
```c
void inline Process(int16_t *pIn, int16_t *pOut, uint16_t size) {
static int16_t x_prev = 0;
static uint8_t ix = 0;
// we assume we're using the LEFT channel
for (uint16_t i = 0; i < size; i += 2) {
// simple DC notch
int32_t y = (int32_t)(*pIn - x_prev);
x_prev = *pIn;
// modulation
y = ...
...
// rescaling to 16 bits
y >>= (15 - GAIN);
// duplicate output to LEFT and RIGHT channels
*pOut++ = (int16_t)y;
*pOut++ = (int16_t)y;
pIn += 2;
}
}
```
{% hint style="info" %}
TASK 2: Complete the function to perform sinusoidal modulation.
{% endhint %}
Now place the function between the `USER CODE BEGIN 4` and `USER CODE END 4` comment tags and test the application!
## Going further
You can now try changing the modulation frequency by creating your own lookup tables!
## Solutions
{% tabs %}
{% tab title="Anti-spoiler tab" %}
Are you sure you are ready to see the solution? ;)
{% endtab %}
{% tab title="Task 1" %}
```python
def make_cos_table(period):
c = 0x7FFF * np.cos(2 * np.pi * np.arange(0, period) / period)
print('#define COS_TABLE_LEN {}'.format(period))
print('static int16_t COS_TABLE[COS_TABLE_LEN] = {', end='\n\t')
for n in range(period - 1):
print('0x{:04X}, '.format(np.uint16(c[n])), \
end='' + '\n\t' if (n+1) % 12 == 0 else '')
print('0x{:04X}}};'.format(np.uint16(c[period-1])))
```
{% endtab %}
{% tab title="Task 2" %}
Here is the complete function:
```c
void inline Process(int16_t *pIn, int16_t *pOut, uint16_t size) {
static int16_t x_prev = 0;
static uint8_t ix = 0;
// we assume we're using the LEFT channel
for (uint16_t i = 0; i < size; i += 2) {
// simple DC notch
int32_t y = (int32_t)(*pIn - x_prev);
x_prev = *pIn;
// modulation
y = y * COS_TABLE[ix++];
ix %= COS_TABLE_LEN;
// rescaling to 16 bits
y >>= (15 - GAIN);
// duplicate output to LEFT and RIGHT channels
*pOut++ = (int16_t)y;
*pOut++ = (int16_t)y;
pIn += 2;
}
}
```
{% endtab %}
{% endtabs %}
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