Hi Mr Hans Van Pelt
First off I want to thank you for the very informatic link you’ve just given me, I’m a Vietnamese, Vu is my family name and Khanh is my proper name
Yes I took into consideration the band width I wanted it’s from 350 to almost 2500 herzts quite modest by any standard I ‘ve also put band pass filter at the in put as well as out put , when I monitor speech on one setup eg. Adc convert send it to the on board dac and out the tx port it sounds intelligible with the tx board but on the rx board the sound becomes garbled I’ve tried null cable as well as laser link they all sound the same: garbled ! I guess when my mcu sending data during which time the out put from adc was partially lost which results in high distortion of the received signal because when I put the board in duplex mode in sound much worse monitoring data stream out of rs232 port on my dual trace scope showed that it only transmit half the time and the other half it receives.
Any how I’m going to download and read the infos on the link you gave me to have better insight into digital data

Ps: very nice English knowledge you have, can’t help thinking English is your native language, I’m a retired electronics engineer these projects are to satisfy my desire in learning things, stay in touch avoid being a relic you know, but none are for profits
Great thanks and lots of luck (next week is our lunar new year)
Vu Nhu Khanh