diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..ea8c4bf
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1 @@
+/target
diff --git a/Cargo.lock b/Cargo.lock
new file mode 100644
index 0000000..4e404bc
--- /dev/null
+++ b/Cargo.lock
@@ -0,0 +1,7 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 4
+
+[[package]]
+name = "adpcm-pwm-dac"
+version = "0.1.0"
diff --git a/Cargo.toml b/Cargo.toml
new file mode 100644
index 0000000..0f74483
--- /dev/null
+++ b/Cargo.toml
@@ -0,0 +1,6 @@
+[package]
+name = "adpcm-pwm-dac"
+version = "0.1.0"
+edition = "2024"
+
+[dependencies]
diff --git a/README.md b/README.md
deleted file mode 100644
index 2bd9d23..0000000
--- a/README.md
+++ /dev/null
@@ -1 +0,0 @@
-This branch is used by entomologist to track issues.
\ No newline at end of file
diff --git a/e4c811653781e69e40b63fd27a8c1e20/description b/e4c811653781e69e40b63fd27a8c1e20/description
deleted file mode 100644
index 6524fee..0000000
--- a/e4c811653781e69e40b63fd27a8c1e20/description
+++ /dev/null
@@ -1,3 +0,0 @@
-fix scaling on DpcmDecoder::output_next()
-
-currently it scales to u8, but pwm outputs can use more! use usize!
diff --git a/notes.md b/notes.md
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index 0000000..61faa61
--- /dev/null
+++ b/notes.md
@@ -0,0 +1,8 @@
+# ADPCM CODEC
+ref: 1, 2
+
+
+# RESOURCES
+1. [wikipedia: adpcm](https://en.wikipedia.org/wiki/Adaptive_differential_pulse-code_modulation)
+2. [st-micro: an3413](https://www.st.com/resource/en/application_note/an3143-audio-software-codec-for-the-stm8s-stmicroelectronics.pdf)
+3. [st-micro: an4453](file:///home/lex/Downloads/an4453-implementing-the-adpcm-algorithm-in-stm32l1xx-microcontrollers-stmicroelectronics.pdf)
diff --git a/src/dac.rs b/src/dac.rs
new file mode 100644
index 0000000..3053783
--- /dev/null
+++ b/src/dac.rs
@@ -0,0 +1,221 @@
+use crate::interface::DacInterface;
+
+pub enum Direction {
+    Up,
+    Down,
+}
+
+pub struct DpcmSample {
+    direction: Direction,
+    // only 3 bits
+    step_count: u8,
+}
+
+impl DpcmSample {
+    pub fn new(byte: u8, sub_index: usize) -> Self {
+        let data = if sub_index == 0 {
+            byte & 0x0F
+        } else {
+            byte >> 4
+        };
+        let direction = match data & 0x1 {
+            0x1 => Direction::Up,
+            0x0 => Direction::Down,
+            _ => Direction::Up,
+        };
+        let step_count = data >> 1;
+
+        Self {
+            direction,
+            step_count,
+        }
+    }
+}
+
+pub struct Dac<'a, T: DacInterface> {
+    output: T,
+    data: Option<&'a [u8]>,
+    index: usize,
+}
+
+impl<'a, T: DacInterface> Dac<'a, T> {
+    pub fn new(output: T) -> Self {
+        Self {
+            output,
+            data: None,
+            index: 0,
+        }
+    }
+    pub fn load_data(&mut self, data: &'a [u8]) {
+        self.data = Some(data);
+    }
+    pub fn seek_to_sample(&mut self, index: usize) {
+        self.index = index;
+    }
+    pub fn output_next(&mut self) {
+        if let Some(data) = self.data {
+            self.index = self.index + 1;
+            // reset the index to 0 if we roll over
+            if (self.index >= data.len()) {
+                self.index = 0;
+            }
+            self.output.write_amplitude(data[self.index]);
+            // self.output.write_amplitude(100);
+        }
+    }
+}
+
+pub struct DpcmDac<'a, T: DacInterface> {
+    output: T,
+    data: Option<&'a [u8]>,
+    index: usize,
+    prev_amplitude: usize,
+    step_size: usize,
+    min: usize,
+    max: usize,
+}
+
+impl<'a, T: DacInterface> DpcmDac<'a, T> {
+    pub fn new(output: T) -> Self {
+        Self {
+            output,
+            data: None,
+            index: 0,
+            prev_amplitude: 0x80,
+            step_size: 0x3,
+            min: 0x25,
+            max: 0xc8,
+        }
+    }
+    pub fn load_data(&mut self, data: &'a [u8]) {
+        self.data = Some(data);
+    }
+    pub fn seek_to_sample(&mut self, index: usize) {
+        self.index = index;
+    }
+
+    // output the next sampe. returns true if there are samples remaining, false if there are no samples remaining.
+    pub fn output_next(&mut self) -> bool {
+        if let Some(data) = self.data {
+            let sub_index = 1 - self.index % 2;
+
+            let sample = DpcmSample::new(data[self.index / 2], sub_index);
+            let new_amplitude = match sample.direction {
+                Direction::Down => self
+                    .prev_amplitude
+                    .saturating_sub(sample.step_count as usize * self.step_size)
+                    .max(self.min),
+                Direction::Up => (self.prev_amplitude
+                    + (sample.step_count as usize * self.step_size))
+                    .min(self.max),
+            };
+
+            // calculate normalized amplitude and write out
+            let normalized_amplitude = (new_amplitude - self.min) * 100 / (self.max - self.min);
+            self.output.write_amplitude(normalized_amplitude as u8);
+            self.prev_amplitude = new_amplitude;
+
+            // increment the sample index
+            let mut samples_remaining = true;
+            self.index = self.index + 1;
+
+            // reset the index to 0 if we roll over
+            if (self.index >= data.len() * 2) {
+                self.index = 0;
+                self.prev_amplitude = 0x74;
+                samples_remaining = false;
+            }
+
+            // return whether or not we have samples remaining
+            samples_remaining
+        } else {
+            // if the sample failed to load, we (duh) have no more samples remaining
+            false
+        }
+    }
+
+    pub fn disable_output(&mut self) {
+        self.output.disable();
+    }
+}
+
+pub struct DpcmDecoder<'a> {
+    data: Option<&'a [u8]>,
+    index: usize,
+    prev_amplitude: usize,
+    step_size: usize,
+    min: usize,
+    max: usize,
+}
+
+impl<'a> DpcmDecoder<'a> {
+    pub fn new() -> Self {
+        Self {
+            data: None,
+            index: 0,
+            prev_amplitude: 0x0,
+            step_size: 0x3,
+            min: 0x50,
+            max: 0xaa,
+        }
+    }
+    pub fn load_data(&mut self, data: &'a [u8]) {
+        self.data = Some(data);
+    }
+    pub fn seek_to_sample(&mut self, index: usize) {
+        self.index = index;
+    }
+
+    pub fn is_done(&self) -> bool {
+        if let Some(data) = self.data {
+            self.index >= (data.len() * 2) - 1
+        } else {
+            false
+        }
+    }
+
+    // output the next sample's amplitude
+    pub fn output_next(&mut self) -> usize {
+        if let Some(data) = self.data {
+            if self.index >= (data.len() * 2) - 10 {
+                return 0;
+            }
+
+            let sub_index = 1 - self.index % 2;
+
+            let sample = DpcmSample::new(data[self.index / 2], sub_index);
+            let new_amplitude = match sample.direction {
+                Direction::Down => self
+                    .prev_amplitude
+                    .saturating_sub(sample.step_count as usize * self.step_size)
+                    .max(self.min),
+                Direction::Up => (self.prev_amplitude
+                    + (sample.step_count as usize * self.step_size))
+                    .min(self.max),
+            };
+
+            // calculate normalized amplitude
+            // TODO: e4c811653781e69e40b63fd27a8c1e20
+            let normalized_amplitude =
+                (new_amplitude.saturating_sub(self.min)) * 100 / (self.max - self.min);
+            self.prev_amplitude = new_amplitude;
+
+            // increment the sample index
+            let mut samples_remaining = true;
+            self.index = self.index + 1;
+
+            // reset the index to 0 if we roll over
+            // if (self.index >= data.len()  * 2) {
+            //     self.index = 0;
+            //     self.prev_amplitude = 0x74;
+            //     samples_remaining = false;
+            // }
+
+            // return the normalized amplitude
+            return normalized_amplitude;
+        } else {
+            // otherwise just output 0
+            0
+        }
+    }
+}
diff --git a/src/interface.rs b/src/interface.rs
new file mode 100644
index 0000000..8224c4d
--- /dev/null
+++ b/src/interface.rs
@@ -0,0 +1,5 @@
+pub trait DacInterface {
+    // write the amplitude (0->100%)
+    fn disable(&mut self);
+    fn write_amplitude(&mut self, amplitude: u8);
+}
\ No newline at end of file
diff --git a/src/lib.rs b/src/lib.rs
new file mode 100644
index 0000000..d34c0f8
--- /dev/null
+++ b/src/lib.rs
@@ -0,0 +1,3 @@
+#![no_std]
+pub mod interface;
+pub mod dac;
\ No newline at end of file