Retrochallenge 2012WW

I’m looking forward to starting on my Retrochallenge 2012WW project. Some of my tasks are:

1. dig out a few Sol-20s
2. see if any work
3. replace keyboard foam pads.
4. connect cassette interface to audio ports of Linux system
5. save a block of memory, and analyze audio on Linux system
6. write a C program to encode object files into audio to load into Sol-20

Thanks to Mike Davis for sending me a set of replacement foam pads!

I’m going to load programs from my Linux system into the Sol-20 using the cassette interface.  The Sol-20 supports two different data rates.  The default is a 1200 bps rate, while the 300 bps rate that can be selected is apparently compatible with the Kansas City Standard.

I’d prefer to use the fast rate, of course, but from the documentation I am somewhat confused about what frequencies the FSK modulation uses for the fast rate.

The Kansas City Standard uses four cycles of 1200 Hz for a zero, and eight cycles of 2400 Hz for a one.

The impression I have from the documentation is that the high speed mode uses a half cycle of 600 Hz for a zero, and one cycle of 1200 Hz for a one.  It seems strange to me that they wouldn’t have used the same FSK frequencies as the KCS, but with one cycle of 1200 Hz or two cycles of 2400 Hz.  I think that would have used simpler circuitry, but perhaps they found that it didn’t perform well.

I’ll confirm the FSK frequencies in high speed mode by writing a block of memory to the cassette interface and capturing it on the Linux system.  That will also allow me to verify that my understanding of the file header format is correct.