🚀 go-pugleaf

In article <RJM.96Feb7174404@swift.eng.ox.ac.uk>, Bob Manners (rjm@swift.eng.ox.ac.uk) wrote:

: >   ????	Sinclair (various models, do they all use the same format?)

: I think the ZX80/81/Spectrum used the same encoding.

  I'm sorry to say you're wrong. I've no idea what the ZX80 used, but the ZX81
uses something at about 250 baud, and the Spectrum runs at about 1500 baud.
The Spectrum system works by detecting the time between voltage level 
changes, rather than the levels themselves. 

   The tape signals produced by the standard ROM have the following form:
     * A leader tone, five seconds long for a header block, two seconds
       long for a data block;
     * A sync pulse, to denote the end of the leader tone;
     * A series of 0 or 1 bits.

Leader tone:
     ____      __
    |    |    |    Each edge is separated by 619.4us
____|    |____|                      (2168 T-states).

Sync pulse:
   ___
  |   |            The first edge comes after 190.6us (667T);
__|   |            the second after a further 210us (735T).

Zero bit:
    ___
   |   |           Each edge is separated by 244.3us (855T).
___|   |

One bit:
     ____
    |    |         Each edge is separated by 488.6us (1710T).
____|    |


--------------------------- 
John Elliott              | \o_~^~                     \|/      _o  \|/
CHAOS in a sig...         |  |                       ==@V        |\  V
--------------------------- /| _,_                 _//\\|.       A  .|.