Spectrum tape interface: Difference between revisions

Spectrum tape interface (view source)

849 bytes added , 30 July 2018

→‎Pulses: Add speed calculation.

49

edits

@@ Line 16: / Line 16: @@
 ''(Does initial polarity matter?)''
+=== Speed ===
+Due to the fact that the different bits take different time on tape, the Spectrum's tape routine does not have a well-defined speed. We can calculate some limits and typical values though; as noted above, a '0' bit takes 2 × 855 = 1710 T-states and a '1' bit takes 2 × 1710 = 3420 T-states. Assuming a 48K Spectrum which runs at 3.50 MHz (the 128K machines run at 3.54 MHz or about 1% faster; that difference is largely irrelevant here), this means:
+* A stream of pure '0' bits loads at 3500000 ÷ 1710 ≈ 2046 baud
+* A stream of pure '1' bits loads at 3500000 ÷ 3420 ≈ 1023 baud
+* A mixed stream with equal numbers of '0' and '1' bits loads at 2 × 3500000 ÷ (3420 + 1710) ≈ 1364 baud
+Real world Spectrum data tended to have more '0' bits than '1' bits, so the typical baud rate was actually a bit higher than 1364 baud.
 == Blocks ==