This project decodes SMPTE Linear Timecode (LTC) from an audio file (WAV format). SMPTE LTC is a format used to encode timecode data as an audio signal, commonly used in video and film post-production to sync devices.

Sure — let’s do a technical breakdown of how the SMPTE LTC Timecode decoder works, based on the code in the ZIP you provided.

To decode SMPTE Linear Timecode (LTC) from a WAV audio file — this involves detecting square wave audio patterns and translating them into binary timecode data.

This is the core decoding engine.

• Implements a zero-crossing detector (to find changes in the waveform).
• Measures the duration between crossings to distinguish between 0 and 1 bits.
• Identifies 80-bit LTC frames (with a sync word at the end).
• Parses the frame into timecode (hh:mm:ss:ff).

• ltc_decoder_feed(): Accepts PCM samples and detects transitions.
• ltc_decoder_get_frame(): Retrieves a valid LTC frame once found.
• ltc_frame_t: Struct that stores decoded timecode data.

Handles bit-level data buffering.

• Buffers incoming bits.
• Detects the LTC sync word (typically 0011111111111101).
• Builds a complete 80-bit frame for decoding.

Reads 16-bit PCM WAV files.

• Opens and parses WAV headers.
• Reads audio samples into a buffer.
• Converts stereo to mono if needed.

Provides a user interface or test harness to:

• Load a WAV file.
• Feed its audio samples into the decoder.
• Output the decoded timecodes.

flowchart TD
    A[WAV File Loaded] --> B[Read Samples]
    B --> C[Zero Crossing Detection]
    C --> D[Bit Detection]
    D --> E[Bitstream Assembly]
    E --> F[Sync Word Match?]
    F -- No --> C
    F -- Yes --> G[Extract 80-bit Frame]
    G --> H[Parse Timecode]
    H --> I[Return hh:mm:ss:ff]

• Detects when the audio waveform goes from positive to negative (or vice versa).
• Used to determine the start of a bit period.

• Short duration = 1, long duration = 0.
• Based on duration between zero crossings.

• The last 16 bits of a valid frame always form a sync word.
• Decoder waits for this pattern to lock on.

A full LTC frame is 80 bits, structured like this:

Each byte has even parity, and BCD-encoded values (e.g., 4 bits for tens, 4 for units).

This tool:

• Uses audio signal analysis to recover a digital timecode from LTC audio.
• Implements bit parsing, sync detection, and frame decoding entirely in C.
• Has a simple Objective-C interface to test and display results.

Let me know if you want to dive into how a specific part (like the sync word detection or parity check) works line-by-line.