FPK data mode

This is a reverse-engineered description of the data mode used by Flare Pepper Kitchen option 8.

FPK uses binary audio frequency-shift keying (AFSK). The default parameters are as follows:

• Symbol rate: 300 Bd
• Space (0) frequency: 1270 Hz
• Mark (1) frequency: 1070 Hz

FPK transmissions consist of a series of bytes framed in the 8-N-1 configuration. That is, each transmitted byte begins with a single 0 bit, followed by the eight data bits ordered least significant bit first, followed by a single 1 bit.

The following sections describe unframed bytes in the conventional most significant bit first order. All multi-byte fields are big endian.

Each FPK transmission is a single packet. All packet types share the following structure:

• CRC (16 bits): CRC-16-MODBUS computed from all preceding bytes in the packet, including the initial 0x5A5A5A5A sequence. CRC-16-MODBUS is defined by the polynomial x¹⁶+x¹⁵+x²+1, initial state 0xFFFF, reflected input, and reflected output; for input bytes 5A 5A 5A 5A 04 00 00, the correct result is 0xEBE6.

• PC (16 bits): Number of data packets in the file descriptor that have payload present.
• MD5 (128 bits): Binary MD5 hash of the transmitted file.
• Filename: ASCII string terminated with a single 0x00 byte.

• BR (16 bits): Number of bytes remaining at this point in the transmission, excluding the current packet. In practical terms:
  • For the first data packet, this field is equal to the size of the file.
  • For the last data packet, this field is equal to the packet's payload length.
• Payload: Bytes to copy into the file unmodified. Data packets with no payload are valid.

Each file descriptor is a series of numbered packets:

• Packet 0 is always an info packet with metadata on the transmitted file.
• Packets 1, 2, 3, etc. are always data packets with fragments of the file, in order from first to last.
• Beyond the final data packet with payload present are empty data packets.