I & Q (quadrature)

[pwdrive]

This subject sought of came up via a thread I did on another forum, I posted what's below to that thread.

I'm posting it here in case it might be of interest.....


I & Q, A good video on the basics:


https://www.youtube.com/watch?v=h_7d-m1ehoY

From the same person, a basic circuit walk through for a DCR/SDR job:

https://www.youtube.com/watch?v=BG0rmMGpwMc

I've watched several videos by this person, I've always found them to be good.

Lawrence. EDIT: Part 2 of the first ink posted.

https://www.youtube.com/watch?v=5GGD99Qi1PA&list=PLMca6NKSce5gh0ymm9Y2yv6HzOZx8wfNj

[Brianc]

That is the most clear explanation of quadrature modulation I have seen. Well worth a look if you're rusty - and I was!
Cheers
Brian

[ppppenguin]

For the co;our TV types around here I and Q are indelibly associated with the NTSC colour system. The I and Q axes were indeed at 90 degrees to each other but rotated by some odd amount (33 degrees?) to the reference U and V axes. The idea was to squeeze every last drop out of the spectrum and the properties of human vision by having one colour compon ent narrower bandwidth than the other. In practice few encoders or decoders ever did this. I think the Cox 203 NTSC coder did do it the right way but most others simply enscoded narrow bandwidth on the U and V axes. Or for swithcable PAL/NTSC coders often wideband on both axes.

[pamphonica]

I watched it all through and was intrigued. I have never dabbled much with I/Q mod as I have never been a TV type professionally. I think I managed to understand QAM though, about 40 years ago.

It was used for some 48kbit/sec 4-wire modems in about 1977. I though these modems were pure magic. Up till then modems were 110, 300 or 1200 bits/sec (baud)! The Experimental Packet Switching Service (EPSS), the forerunner of the internet, started off with dial-up at 110 baud from teletype machines. Huge investments in racks of 110 baud modems were almost immediately superceded as ASCII terminals at 300 and 1200 (and later 2400 and 4800) baud took over. Expensive mistake!
I was a hardware test engineer on EPSS the experimental service, and later, on PSS, the public service, used by banks for the first CHAPS payments and inter-bank clearing.

Jeremy