Editing Commodore Datasette (section)

== Encoding ==
[[File:Commodore-Datasette-C2N-Mk1-Open-Back.jpg|thumb|Inside the Datasette]]
[[File:c2n waveform.png|thumb|left|The resulting waveform from storing data]]
{{Listen|filename=Datasette.ogg|title=Datasette sound sample|description=Commodore Datasette 18-second sound sample|format=[[Ogg]]}}

To record physical data, the [[zero-crossing]] from positive to negative voltage of the analog signal is measured. The resulting time between these positive to negative crossings is then compared to a threshold to determine whether the time since the last crossing is short (0) or long (1).<ref name="wav-prg_sf-tape">{{cite web|title=How Commodore tapes work|url=http://wav-prg.sourceforge.net/tape.html}} 091205 wav-prg.sourceforge.net</ref> Note the lower amplitude for the shorter periods.

A circuit in the tape unit transforms the analog signal into a logical 1 or 0, which is then transmitted to the computer via the [[Commodore 64#Input/output (I/O) ports and power supply|tape connector]]. Inside the computer, the first [[MOS Technology CIA|Complex Interface Adapter]] (6526) in the C64 senses when the signal goes from one to zero. This event is called trigger and causes an [[interrupt request]]. This event can be handled by a handler code, or simply discovered by testing bit 4 of location $DC0D. The points that trigger this event are indicated by the black circles in the figure.<ref name="wav-prg_sf-tape" />

Inside the tape device the [[tape head|read head]] signal is fed into an [[operational amplifier]] (1) whose output signal is [[direct current|DC]]-filtered. Op-amp (2) amplifies and feeds an RC filter. Op-amp (3) amplifies the signal again followed by another DC filter. Op-amp (4) amplifies the signal into [[clipping (signal processing)|clipping]] the [[sine wave|sine]]-formed signal. The positive and negative rails for all op-amps are wired to +5V DC and [[Ground (electricity)|GND]]. The clipped signal therefore fits into the [[transistor–transistor logic|TTL]] electrical level window of the [[Schmitt trigger]] step that in turn feeds the digital cassette port.<ref name="c2n_schematic">Datasette service manual model C2N/1530/1531, preliminary, Oct. 1984 PN-314002-02</ref>

On the [[PAL]] version of the C64, the time granularity is {{nowrap|1.014 [[microsecond|μs]]}} (for NTSC {{nowrap|0.978 μs}}). Since each bit uses {{nowrap|3284 [[Clock signal|clock cycles]]}} this means {{nowrap|3284 * 1.014 μs}} = {{nowrap|3330 μs/bit.}} or a {{nowrap|300 bit/s}} data rate.

Once the bits can be decoded, they are fed into a [[shift register]]<!--MSB or LSB first?--> and are continuously compared to a special bit sequence. This bit sequence can also be seen as a byte. A bit-sequence match means that the stream is byte-synchronized. The first byte to compare with is called the ''lead-in byte''. If matched, it's compared to the ''sync byte'' as well.<ref name="wav-prg_sf-tape" />

An example: Turbo Tape 64 has a ''lead-in byte'' $02 (binary 00000010), ''sync byte'' $09 (binary 00001001) and a following sync sequence of $08, $07, $06, $05, $04, $03, $02, $01.<ref name="deceukelaire198502" /><ref name="wav-prg_sf-tape" />