This is an old post, preserved for reference.
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The Minstrel 4th uses cassettes to save and load programs, in the same way as the Jupiter Ace, and many other computers of the era.
Back in the mists of time when the Minstrel 4th was being developed, I had reached the point where it was running, the display was clear, it was running all the code I typed in.
I finally decided to challenge my original assumptions that the files being output from tzxduino, the PC, the laptop, the phone and all the other things that I had tried may have been the problem. Looking around I when through various programs that claimed to do tap to wav conversions.
castool, part of the MAME suite. That was a command line program that took a .tap file and generated a .wav file. That specifically stated that it supported the Jupiter Ace format.
Big Character Scroller Demo' runs very nicely on the Minstrel 4th, with scrolling text and very Amiga like bouncing balls.
I would like the Minstrel 4th to have some other loading options. The first is likely to be serial based, development of that is progressing well.
During testing, I found I had better results recording the programs onto tape using a cassette recorder, and then loading into the 4th from tape. In discussing this, I had suggested it may be having some form of low pass filter effect. George Beckett has tested this further and found that applying a low pass filter at around 8KHz to the wav file within Audacity gives similarly good results.