LeadIn

The lead-in has the following important functions.

1. Skip over unpredictable delays, ignoring intermediate tape noise.
2. Synchronise to an exact predictable bit-accurate position.
3. Correct for any phase inversion in the audio system.
4. Correct for changes in playback speed due to tape stretch or motor variations.

This is achieved by a two stage waveform.

Synchronisation pulsetrain

 _   ___
| |_|   |___|     (Repeated twice) Short and long pulses of 220µs and 440µs periods
 ___   _   _
|   |_| |_| |___| (Repeated four times) Symmetry breaking pulse, 220µs and 440µs again
 _   ___
| |_|   |___|     (Repeated twice) Short and long pulses of 220µs and 440µs periods

This pulse train is measured by timing between each level change on the audio input. It is detected using a 32 bit shift register. The symmetry breaking pulses help reduce false triggering from normal data. Once a match occurs, the next pulse is commencement of the calibration pulsetrain.

Calibration pulsetrain

 _______         _______         _______         _______
|       |_______|       |_______|       |_______|       |_______| 4 x 880µs high, 880µs low

This pulse train typically takes 3520µs, but it takes exactly 32 'unit' periods, where a unit is typically 110µs, and all pulses in the loader are a multiple of that timing. So by dividing the pulsetrain timing by 32 and then multiplying by 2.5, 3.5, 4.5, 5.5, 6.5, 7.5 and 8.5, the loader calculates thresholds to discriminate between all further pulses in the block. This is done in real time by the loader. Once the calibration pulsetrain is complete, the payload block commences.