I drilled a hole in my #Commodore modem to add a headphone jack because I couldn't stand the sound of the built-in speaker. An unintended benefit is that now I can record the incoming audio signal directly. Here's a video of my #C64 receiving the first few paragraphs of Pride and Prejudice sent at 300 baud over VOIP, including the audio signal received by the modem:
https://www.youtube.com/watch?v=3qS3ArCV24s
300 baud is painfully slow, but that makes it easier for a plain old sound card sampling at 44.1kHz to record the signal with enough fidelity to clearly see the individual bits. In the attached image the Y axis is frequency and the X axis is time. The higher frequency represents 1 and the lower frequency represents 0. I've manually decoded the first few characters just for fun. Characters are ASCII-encoded and sent least-significant-bit-first, and each byte is preceded by a low "start" bit and followed by a high "stop" bit (ie. the terminal is configured with "8n1"). The signal may remain high between characters such as between the "R" and "I".
#RetroComputing #Commodore64 #JaneAusten