Can I ask if the US sounders are like the ones we used (still use) on the railways where key down generates a clunk, and then a different sound when the armature releases and returns - which is how you know when the dot and dashes end. While I happily concur with the spacing differences on the US version of International Morse (and the different letter versions), the critical component is the ability to know when the character components end, and that isn't the spacing or gap, it's how long the key is down. Without that information it doesn't work reliably. Many people do use the Morse slang to do numbers with lots of conjoined dahs - like zero, just send a long tone instead. A friend tells me the fizz I mentioned is actually just key down armature vibration where another set of contacts works a bit like a buzzer.