And, could we see ratio of on to off time as the inverse of scan frequency?
No not at all. Supposed you multiplexed at a refresh rate of 1KHz. Then you changed it to refresh at 100Hz what would be the difference in on / off ratio of those two situations. Answer absolutely nothing, it would be the same. In one case it would spend 0.5mS high and 0.5mS low. In the other case it would spend 10mS high and 10mS low. In both cases the LED is on for half (50%) of the time.
is it linear relation with duty rate too
no but then neither is the brightness / current of the LED.
I think human eyes might have some weakness distinguishing multiplexing trick
No it doesn't that is the whole reason why TV works.
a controlled experiment is needed
No the persistence of vision has been well researched and documentation for over ONE HUNDRED YEARS you just need to look at the literature.
Back to julienb
what do you advice me
If you are going to multiplex then any LED will be off for 7 units of time and on for one unit of time. Or in other words you can reduce the number of LEDs in the current calculation. Given you have 100R resistors and are going to feed it with 5V and the LEDs in question drops 2V for red and 3.2V for the others you will have:-
I=E/R ...... I= (5-2)/100 .... I=0.03A or 30mA per Red LED.
and 18mA for the other colours.
Add up all the LEDs 8 by 8 = 64 times by 3 (RGB) = 192
But only one in 8 will be on at any one time so the number of effective LEDs = 192 / 8 = 24
One third of them are red so
So total red LED current = 8 * 30 = 240mA
plus the Green & Blue = 16 * 18 = 228mA
Giving a total of 528mA
Therefore this is too much to supply from the USB power (500mA Max)
Note this is just LED current you probably need to add about another 40mA for the other chips. So you are probably using about 570mA.
If anyone spots something wrong with this maths then please point it out.
A guy uses exactly the same stuff and uses only the usb power without problem
He is fooling himself, he is putting his USB port at risk. It doesn't blow if 500mA is just exceeded but it it operating it outside the limits and while it might work for a short time in the long term damage is being done. He has probably not run the matrix with all the lights on for a prolonged period of time but that is what you have to assume if you want to make things properly.
My advice for the well being of your USB connection is to use a separately regulated 5V power supply capable of at least 1A.