I understand this question is about the Tandy, so I'll move this to a self-answered question if necessary, but I believe you'd be interested in this information. On a ZX Spectrum (at least, the popular +2A revision with integrated Datacorder and modulated RF sound through the TV), the audio waveform on the tape is played through the TV's speakers during loading as various (unpleasant) sounds. ('Like a donkey drowning in something viscous.' it's been called.)
Because the different parts of a recorded block of data on a Spectrum tape have a uniform layout and timing, these phases not only had predictable colours on screen but also familiar tones - the result is a slowly cycling combination of constant tones, screeches and noise.
This video is the full loading of Jetpac for the Spectrum:
The 'pilot tone' is shown as red and cyan bars and sounds like a constant square wave. Block headers consist of a pilot tone followed by some data, so when a game begins to load, you'll hear 'beeeeep - squish' as the block is synced to then loaded.
The content of longer data blocks will have long random crunching sounds accompanied by blue and yellow dense bars as the data is loaded.
Periodic data will have its own characteristic periodic sounds while loading . This is particularly noticeable when the full content of the Spectrum screen is loaded directly -uncompressed- from the cassette into screen memory to display a title screen during the remainder of the loading process1. This memory block is sometimes called a SCREEN$ after the BASIC token representing it. A SCREEN$ has the 1bpp graphic INK data for the screen (in a non-intuitive layout of lines) first followed by the colour attribute values as a block. This means that a game title screen with a lot of repeating patterns in its graphic will have distinctive repeating whirrs, chirps, or crunches as the visual patterns are read. The attribute table also has its own unique sound since most of the screen will have repeating/common sections of colour.
Due to these effects, when a game is first loaded on a Spectrum, the initial load of the title screen has a very recognisable sequence:
- boooop - bip - silence (loading and displaying the program name)
- boooop - crunch - silence (loading the BASIC loader from the tape that will control the rest of the loading - the screen may go black at this point if the loader code chooses to do so)
- boooop - cru-cru-cru-crunch - cru-cru-cru-crunch - cru-cru-cru-crunch (this process is lengthy as the picture dot data is loaded)
- nya-nya-nya-nya-nyaaaaaaaaaa - silence (the attribute table is loaded and the title screen is 'coloured in')
Then the main game content and assets will be read as longer data blocks consisting of protracted (ten minutes upwards) continuous random harsh high pitched crunching sounds preceded by pilot beeps.
As Doc Brown would say 'Please excuse the crudity of this model.', but this is the best way I can convey it in words. :)
What should be the waveform for ZX Spectrum tapes?
1. You may have noticed that if it's loading directly from tape to screen memory, the Spectrum isn't doing anything else while the title screen loading is taking place. The game isn't loading in the background! A full one minute of loading is just to show a pretty picture before the real loading begins.