Family BASIC Data Recorder
"Manufactured by Matsushita/Panasonic for Nintendo, the recorder was released in 1984 only in Japan as an addition to the Family BASIC Keyboard to save data from BASIC programs created by users. Also, Castle Excellent, Excitebike, Mach Rider and Wrecking Crew can use this device in order to save tracks/stages created by users." — from wikipedia
Input ($4016 write)
7 bit 0 ---- ---- xxxx xExS | | | +- 1-bit DAC audio to audio cassette +--- When 0, force audio readback to always read as binary 0 (5V)
The audio to the cassette recorder goes through a first-order highpass at 100Hz and is attenuated to 5mVPP at the input to the recorder.
Output ($4016 read)
7 bit 0 ---- ---- xxxx xxAx | +-- 1-bit ADC audio from audio cassette
Because of how magnetic tape works, playing back the tape will produce a signal that is the lowpassed derivative of the original. Then this audio from the cassette recorder goes through a highpass with corner frequency of 800Hz before being discretized. In simulation, square waves of frequency 600 Hz up to the bandwidth of the tape appear to be recovered by this processing.
For an emulator, just play back the same bit stream as it was emitted.
It is not known whether Family BASIC uses Kansas City Standard encoding, Bell 103 or 202, or some other arbitrary home-grown convention for encoding the audio on the tape. Castle Excellent's recorder handling code mostly exists between $8000 and $80FE, and provides save games using the 1200 baud ('CUTS') and bit-reversed variant of Kansas City Standard. Wrecking Crew uses the tape format from the Sharp MZ personal computer.
Reverse-engineered schematics by Enri: