Bit Angle Modulation (BAM) in a PIC

[Darrel Taylor]

I've been working with this on and off for quite some time now.
I've also had it working for quite some time now ... (with 1 exception).

It creates some of the smoothest fading effects I've seen so far, so I really want to figure out this one last problem with BAM theory.

The problem happens when passing in either direction of 128 DutyCycle.
Before that point, after that point, even continuously at that point, all works perfectly. But as it transitions from either above or below 128 there is a visble BLINK from the LEDs.

It's taken me Waaaay too long to figure out why, but I'm now pretty sure this is the reason ...

With anything below 128, the waveform looks something like this ...

<img src="http://www.picbasic.co.uk/forum/attachment.php?attachmentid=3127" /> <!-- -->
<br>
And with anything equal or above 128, it looks like this ...
<img src="http://www.picbasic.co.uk/forum/attachment.php?attachmentid=3125" /> <!-- -->

A continuous stream of either of the above pulses works perfect.
It's only in the transition from below to above 128 that 1 out of the 100 pulses that second looks like this ...

<img src="http://www.picbasic.co.uk/forum/attachment.php?attachmentid=3126" /> <!-- -->

The change in pulse positions creates an ON period equal to 255, just for 1 period. Then it's fine again.
But that 1 period, is extremely visible.

Going from 127 to 128 it's a bright blink. From 128 to 127 it's a Dim blink, because they line up the other way (combined period = 0 dutycycle).

The visual appeal and resulting reduction in processor requirements are too great to just give up on BAM.
So I ask for your thoughts.
<br>

[RadikalQ3]

You can invert the value of the bits in each cycle.

In odd cycles, do first the normal order retard for each bit:
1,2,4,8,16,32,64,128

and, in the even cycles, reverse the value of each bit:
128,64,32,16,8,4,2,1

Edit:
Oops this can generate the problem even without transtitions of the dutty cycle used... hummm... maybe rearranging the value of the bits, not just reversing them :?

[Darrel Taylor]

Hmmmm, such a Radikal idea.
Very interesting ...

I had thought of putting the 7th bit in the middle, but it always came up lop-sided since it's Half of the period.

But making a mirror image of the pulses (at twice the frequency) would put it smack dab in the middle of a nice symetrical waveform that might actually work.

I must try this ...

Oh, and Hi there Q3.
Welcome to the forum!

[anonymouse]

Hmmmm, such a Radikal idea.
Very interesting ...

I had thought of putting the 7th bit in the middle, but it always came up lop-sided since it's Half of the period.

But making a mirror image of the pulses (at twice the frequency) would put it smack dab in the middle of a nice symetrical waveform that might actually work.

I must try this ...

Oh, and Hi there Q3.
Welcome to the forum!

Just my musings can you do a check if duty cycle = 127 + 1 then bit 7=0 (0.005s interrupt) then 128 duty cycle. I.e cause a timer interupt delay before starting on with the BAM again.

[anonymouse]

Just my musings can you do a check if duty cycle = 127 + 1 then bit 7=0 (0.005s interrupt) then 128 duty cycle. I.e cause a timer interupt delay before starting on with the BAM again.

Thats as clear as mud, => 128 begin reverse read byte and vice versa

[Darrel Taylor]

RadikalQ3,

Thanks for the Mirror Image idea!
It has completely removed the blinking problem.

Unfortunately the higher frequency and extra code to reverse things has created some new ones, but I think I can work through them.

For your first post here, it was an awesome one.

Thanks again,

Also, thanks anonymouse,
But with multiple channels at different dutycycles all using the same timer, changing the interrupt sequence at 127 isn't possible.
I hadn't mentioned the multi-channel part yet ... oops.

[RadikalQ3]

RadikalQ3,
Thanks for the Mirror Image idea!
It has completely removed the blinking problem.

Oh, it's nothing... just was an idea.
I have spent the last days reading all I found in internet about PWM, BAM, Frecuency Modulation, etc.
I have a project to do and I am gathering all the information about this topic.
Simply... I found your topic (and this forum) and I thought that was a nice challenge solve your blinking problem O
I am waiting the recepcion of some hardware and in a few days I can do test with this for my own.

Later, I realized that this is a forum on basic! (and I just use assembler)