How to trap input to control the screenshot function?

I want to trap input to control the screenshot function.

vim myscreenshot.sh
screenshot(){
    ffmpeg -video_size 1920x1080 -framerate 25 -f x11grab -i :0.0+0,0   /tmp/out.mp4  
}
close(){
    echo "screenshot is over"
    exit
}    
trap 'screenshot'  ctrl+a #how to fix it?
trap 'close'  ctrl+b #how to fix it?

THe command trap 'screenshot' SIGINT can trap ctrl+c.
My requirement:input ctrl+a start screenshot function,ctrl+b stop screenshot function and exit after running bash myscreenshot.sh & in background.

As an alternative to using readline on a shell prompt, consider the following bash solution. Solution closely matching requirements. Basically:

Loop Forever:

• Wait for Ctrl/A
• Start screenshot in backgroun
• Wait for Ctrl/B
• Stop screen shot, Run external screenshot.sh

It uses bash to read one-character at a time line delimiter is needed.

Side note: Given program running in a window with dedicated terminal, the Ctrl/A and Ctrl/B can be replaced with something easier - just 'A' and 'B'.

#! /bin/bash

screenshot(){
    rm -f /tmp/out.mp4
    exec ffmpeg -video_size 1920x1080 -framerate 25 -f x11grab -i :0.0+0,0   /tmp/out.mp4
}

while true ; do
    a=""
    echo "Type CTRL/A to start"
    while [ "$a" != $'\x01' ] ; do
        read -N1 a
    done
    screenshot &
    sleep 3
    echo
    echo "Type CTRL/B to start"
    while [ "$a" != $'\x02' ] ; do
            read -N1 a
    done
    echo "Processing ..."
    kill $!
    bash myscreenshot.sh &
done