Communication devices:

Watch the videos on Communication devices (youtube.com)
 

https://www.youtube.com/watch?v=SvIrLC7bCjQ

https://www.youtube.com/watch?v=T6TOq4APDKo

https://www.youtube.com/watch?v=-LO3hCjT50U

https://www.youtube.com/watch?v=7xnNRQjmTMg

https://www.youtube.com/watch?v=QnCBCQa-2XU

https://www.youtube.com/watch?v=IpHLPDmebWE

https://www.youtube.com/watch?v=q6u0AVn-NUM