How do I decode Motorola Two-Tone?

[KF7ZNI]

I am attempting to write software to parse and decode Motorola Two-Tone signals, but I'm having a heck of a time figuring out how to get from having the A and B frequencies to getting a usable code out of it. Honestly its probably due to me not understanding how these codes are used normally...
A lot of people have recommended using this chart here but it seems that I am expected to know which "Reed group" is being used before I parse the tones. Is it even possible to parse just the A and B frequencies into a usable code?

For example, if I have tones:
A: 398.1hz and
B: 912.0hz

Could I use that to parse a alphanumerical code?

 

[krokus]

I am attempting to write software to parse and decode Motorola Two-Tone signals, but I'm having a heck of a time figuring out how to get from having the A and B frequencies to getting a usable code out of it. Honestly its probably due to me not understanding how these codes are used normally...
A lot of people have recommended using this chart here but it seems that I am expected to know which "Reed group" is being used before I parse the tones. Is it even possible to parse just the A and B frequencies into a usable code?

For example, if I have tones:
A: 398.1hz and
B: 912.0hz

Could I use that to parse a alphanumerical code?

Have you looked at the old open source version of Two-Tone Detect?

 

[aaknitt]

Is it even possible to parse just the A and B frequencies into a usable code?

Ugh. Sometimes? Maybe? I think that "back in the day" there was pretty strict adherence to the tone plans put out by specific manufacturers (Motorola, Plectron, etc.) since pager frequencies were set by physical tone reeds and the tone plans were designed to accommodate the physical limitations that those presented. Now that everything is microcontroller-based and programmable, there's a lot more mixing and matching going on.

I would say about 70% of the time I can use the tone tables to figure out what the original cap codes were, but sometimes the frequencies just don't seem to match much of anything, like someone just picked some random numbers.

Honestly in the modern world the cap codes are a relic...only used because that's the way they used to be. Pretty much all modern pager programming software allows you to just enter the frequencies and not even care about the cap code.

With that being said, the tone plans were put together with a lot of good reasoning and engineering behind them (prevent falsing from other reed groups, harmonics, etc. etc.), so sticking to them still makes sense, even if the cap codes don't need to be used.