ZX spectrum loading sound

Question

Apologies for the vagueness of this question; I'm remembering back to the early 1980s! When I loaded programs from cassette tape into my 48k ZX Spectrum, the sound (and bar animation) was quite distinctive, and followed a fairly standard pattern for perhaps the first 10 seconds. What was it about the structure of programs that led to this standard loading approach?

Answer 1

The program data was stored as audio on tape.

It's for pilot signals (cyan/red) and data signals (thinner stripes). The pilot signals help indicate the speed of the tape so the data signals can be read correctly as different players may run at different speeds.

From Wikipedia:

The standard method of storing files on tape used pilot signals, headers, and data blocks. Pilot signals are used to calibrate the system to the speed of the tape, both in terms of how it was written and of natural slight variations between different tape decks. Headers have a short file size of 19 bytes (17 for header information, 1 for flag and 1 for checksum), and the loader generally presents one of these messages depending on their type: Program: for programs written in BASIC; Bytes: for machine code, screen dumps, etc.; or Character array: for an ASCII-encoded file.

During standard loading and saving processes, the border flashes with cyan/red stripes for the pilot signal and yellow/blue stripes for the header and data blocks; which colour of the pair is used depends upon the bit that was last read from the tape. Pilot signals are usually represented with a thick stripe size; on header and data blocks, the stripes are thinner (depending the baudrate).