[Laser] serial port PDM and PWM

[Chuck Hast]

On 12/7/06, TWOSIG at aol.com <TWOSIG at aol.com> wrote:
> As a follow on to the idea of optimizing sound card ( radio communication )
> modes for optical communication, I stated that the output should be two state
> because much of the equipment to emit light tends to be highly non-linear.
> This is at odds with the sound card which is trying to generate accurate
> analog  complex sine wave output.
>
> An alternative is to use the computer serial port which is specifically
> designed to output "ones" and "zeros".  Recently, computers have gotten  away from
> providing the classic serial ports that are well suited for what I am  about
> to describe.  This is particularly true of new laptop computers that  might be
> better tools for field work with optical communication links.   However, even
> USB and IEEE 1394 (such as i.Link from Sony and Firewire  from Apple ), which
> are optimized for bursts of high speed data to and from  multiple devices,
> may provide ways to use this idea.
>
> The computer serial port provides a means to transmit and receive data from
> other devices.  For this application, we need only look at the transmission
> system.  Data from the computer memory is sent to the port.  The  actual
> mechanism is not important, but the essence is that the contents of  memory, such as
> an ASCII character, is sent to the port, which then sends it out  over wires
> one bit at a time.  To do that, the port needs to  add leading zero bit(s) (
> START ) and trailing one bit(s) ( STOP  ).  The port can be configured, and for
> this example I am going to use  eight (8) data bits, no parity, single start
> bit, single stop bit.  I am  also going to assume that the data is transmitted
> most significant bit to least  significant bit.  ( Those of you who know
> wheither that is Big Endian or  Little Endian can explain the difference. )  I am
> also going to use 115,000  bits per second for this example, and assume that
> the computer and software  being used can stuff enough data at the port that it
> will run continuously for  the time that we are interested.
>
> The electrical interface for this should be isolated and leave the optical
> device emitting when the port is in its idle state.  Data transmission will
> cause the output to blink off for one bit time, about 8.7 microsecond.  The
> data bits follow, as does one stop bit.  If the data is being sent  continuously,
> there should be no lag between the end of one byte's stop bit and  the next
> byte's start bit.  Each eight bits of data requires ten bit times  to transmit.
>  The number of bytes being sent is therefore 11,500.  For  the PWM and PDM
> systems this is the basic sample rate.  Since there are  eight data bits that
> are under the control of the software, there can be only  nine states being sent
> in any one sample.  ( There are 256 ways that the  eight bits can be
> arranged, but for this purpose we only care how many are  "ones".  The hexidecimal
> values X'33", X'55", X'66", X'99', X'AA',  and X'CC' are all equivalent having
> four bits in the "one" state.   From the standpoint of the average value of an
> individual sample the minimum  value is 1 of 10, or 0.1 density.  The maximum
> is 9 of 10, or 0.9  density.  Zero out of ten cannot be sent because the must
> always be a stop  bit sent by the port.  Similarly ten out of ten cannot
> because there must  always be a start bit. )
>
> That gives us a sample rate of 11,500 per second and a granularity  of nine
> states.  It might be possible to send acceptable voice data  under those
> conditions.  If you were to send 16 consecutive X'FF' followed  by 16 of X'00', you
> get a a 359.375 Hz square wave.  If you reverse the  order, you have all that
> is needed to create BPSK.  A slightly more eligant  way to do the same thing
> is to approximate the shape of a sine wave, rising  for 16 samples, then
> falling for 16. ( Reverse for the other phase. )
>
> QPSK needs four phase states. For a square wave implimentation you  need to
> change the phase by a value of 0, 90, 180, or 270 degrees.  To  change by 0,
> continue the current sequence.  To change by 180, reverse the  order of the 16
> X'FF' and 16 X'00'.  To change by 90, send X'AA' for 8  samples and continue
> the current sequence.  To change by 270, send X'AA'  for 8 samples and reverse
> the X'FF' and X'00'.  ( Or you could send 0, 8,  16, or 24 samples of X'AA' for
> the 0, 90, 180, or 270 degree phase shifts.
>
> For a pseudo sine wave the sequence runs so:
> Start at minimum, rise for 16 samples reaching maximum, then fall for 16
> samples returning to minimum.
> Start at mid-value, rise for 8 samples reaching maximum, then fall for  16
> samples reaching minimum, the rise for 8 samples returning to  mid-value.
> Start at maximum, fall for 16 samples reaching minimum, then rise for 16
> samples returning to maximum.
> Start at mid-value, fall for 8 samples reaching minimum,  then rise for 16
> samples reaching maximum, the fall for 8  samples returning to mid-value.
>
>
> In a similar manor you can create frequency and phase relationships that  you
> may need for FSK, IFSK, multitone..... Whatever.
>
>
>
> The nine state granularity can be extended by grouping samples. Two
> consecutive samples allow you to have 17 states, but only 5750 samples per  second.
> Three sample groups have 25 states at 3833.333.  Four, 33 at  2875.  ( This is
> where higher bit rates on the port help.  Perhaps a  simple interface board
> for a USB or 1394 to UART would help. )
>
>
>
> If the intend is to produce PWM, the 8 datat bits to be sent to provide the
> output states are:
>
>
> State 1        X'00'   ASCII  NUL
> State 2        X'01'    ASCII SOH
>
> State 3        X'03'    ASCII ETX
> State 4        X'07'    ASCII BEL
>
> State 5        X'0F'    ASCII SI
> State 6        X'1F'    ASCII US
>
> State 7        X'3F'    ASCII ?
> State 8        X'7F'    ASCII DEL
>
> State 9        X'FF'   (  I think the ASCII is a y with double dot over it. )
>
>
>
>
>
>
>
>
>
> The PDM data for each state is open to some interpretation.  I suggest  the
> following:
>
>
> State 1        X'00'   ASCII  NUL
> State 2        X'10'    ASCII DLE
>
> State 3        X'24'    ASCII $
> State 4        X'92'    ASCII '
>
> State 5        X'AA'    ASCII ( superscript a with underscore )
> State 6        X'6D'    ASCII m
>
> State 7        X'DB'    ASCII U with a carret
> State 8        X'F7'    ASCII ( divide sign )
>
> State 9        X'FF'   (  I think the ASCII is a y with double dot over it. )
>
>
>
> Getting the hooks into existing software to use these data samples via the
> serial port may be more difficult than adapting the sound card output, but I
> hope that I have suggested something that can be used for beacons or newly
> developed software that is appropriate for optical emitters.
>
>

This is the sort of idea I was thinking about, I had not really got into the
formating, but one thing I have several of around the house are old TNC's
from back when I was quite involved in packet radio (still am and indeed
am working on short haul links between packet sites using optical) I am
going to build the optical heads to link two TNC's and use the TTL output
from the comms chip to drive the laser, and feed the RX output into the
chip input. The data is HDLC and the underlaying protocol is AX25, but
I have done some bench testing with both LED's and laser diodes and it
worked just fine.

There are also AX25 drivers that can turn a serial port into a HDLC formated
data stream, since the data stream is synchronous the TX and RX data
lines are of no use, what the code writer did was to use some of the control
lines as the data lines and bit bang them. That source is available and
could probably be modifed to do what you are talking about above but it
would of course not be the TX and RX lines.

I am very interested in this sort of thing, I have some links that I want to try
using optical in order to see how well they stand up to weather, sunlight
and other interference over long periods of time. Obviously they probably
will fail in some of the heavy rain we have here, but that is just for a short
period of time, and the whole idea is to test the concept out and see how
well it can be used for long term linking.


-- 
Chuck Hast  -- KP4DJT --
To paraphrase my flight instructor;
"the only dumb question is the one you DID NOT ask resulting in my going
out and having to identify your bits and pieces in the midst of torn
and twisted metal."