Kris Kirby

That sounds reasonable since the 16550A UART has a 16-byte transmit FIFO buffer and a matching 16 byte RX FIFO buffer. https://media.digikey.com/pdf/Data%20Sheets/Texas%20Instruments%20PDFs/PC16550D.pdf

Not really. A telephone modem has all the tricks for getting the most speed out of a 0-4 kHz speech window. It'd be easier to take the resistive hybrid out...

Can you reproduce this by using an input file like the example shows? It's possible that the program doesn't handle data below a certain size. You may be able to...

If it's too loud, the harmonics will cause issues; the higher ones are probably being masked by the Nyquist limit and/or anti-alias filters. Any other issue is likely to be...