antorcha/tornado/doc

Antorcha vs. Tornado feature comparison
---------------------------------------

Number of LEDs		32			64
Pixels			16			64
LED pitch		8 mm			5 mm
1 arc sec / pixel (1)	28 m			17 m
1 arc sec / text (2)	70 m			90 m
LED line length		25 cm			32 cm
Total stick length	63 cm			53 cm (TBD)
Outer radius		~ 90 cm			~ 80 cm (3)
Visible width		~ 100 cm		~ 110 cm (3)
Sweeps per second	~ 4, bidirectional	~ 3, unidirectional
Sweep speed (avg.)	~ 8 m/s			~ 15 m/s (3)

(1) http://en.wikipedia.org/wiki/Visual_acuity
(2) 2 text lines, 1/12 display height
(3) for chain length = 30 cm


Weight
------

Al stick: 52.5 g / 52.7 cm = ~ 1 g/cm
Chain:	44 g / 19 links = ~ 2.3 g/link
	305 mm / 21 links = 14.5 mm/link
	1.6 g/cm
Attachment (screw, washers, nut): 2.3 g
End cap: 1.54 g
Cable:	1.74 g / 382 mm
	group of 3: ~ 0.14 g/cm
LED PCB: 4.1 g
CPU PCB: 4.5 g

Not included in the weight calculation:
- glue
- adhesive tapes
- isolation tapes
- cable binders (if any)
- heat-shrink tubing
- cables/wires for inter-board connections


Update rate
-----------

For square pixels we need one update every 5 mm. With the edge moving
at 15 m/s, that's one update every 1/3000 s. If we assume that,
including overhead, each 64 bit shift and latch takes about 100 clock
cycles, the serial bus needs to run at no less than 300 kHz.

sclk-1123.png shows the ~250 ns clock pulse generated by the current
test firmware. CH2 (blue) is signal entering the first LED module,
CH1 (yellow) is the signal exiting the last module. The absence of
degradation in a 250 ns pulse suggests that we can go as fast as the
MCU is able to.


Image size
----------

With a radius of 80 cm the circumference is roughly 5000 mm.
In order to obtain square pixels at the outer radius, we therefore
need 1000 lines per rotation, irrespective of speed. Each line
consists of 64 pixels or 8 bytes of data. Therefore, the full disc
needs roughly 8 kB of data while a quarter disc needs 2 kB.