Reading data from 3.5mm audio jack

[pokmo 0]

I was talking about OOK (on-off keying), while your latest schematic is talking about FSK (frequency shift keying).

 

 

 

Indeed. Could this schematic be used for OOK though?

[enl 227]

@@roadrunner84 The circuit can be used for on-off. When a signal is not present, the period is greater than a boundary value. When it is present, the period is less than the boundary value.

 

The advantages to this type of circuit over many alternatives are simplicity and versatility. It can be used for FSK, OOK, and can be used for standard serial, FM and MFM self timed communication, among others. For FSK and OOK, a minimum number of cycles can be used per bit, often only one, if the channel otherwise doesn't affect phase with frequency or have significant filtering.

 

The drawback is that, in applications where you NEED to use a carrier (such as On-Off or FSK) due to channel properties, the processor has to do timing at higher than bit rate frequency and can't treat the input using standard UART tools.

 

The historical application that comes to mind (1977) for this is the TRS-80 cassette, where, rather than use the Kansas city standard, FM (and MFM) were used with a standard audio cassette recorder. Quite reliable and tolerant to timing variation at one transition per bit. Similar methods were used into the mid-1980's by several other manufacturers.