Monday, 13 July 2009

I have decided to restart my blogging as Tweets
are too small. I tend to never finish a project so
if I tell people what I am doing I will look stupid
if I don't finish it!

The current major project is the development
of an experimental digital radio that will allow
moving video (and other things) using a 70cms
radio on a standard narrowband FM channel.

I have chosen to use full response multi-h CPM
generated by a DDS chip. The waveforms I have
chosen to experiment with are the
MIL-STD 188-181C ones. These are used
on UHF ~250 MHz military satellites,
so are proven technology.

For the prototype radio I have decided to use
a modified T7F packet radio. I will replace the
synthesizer with a DDS so I can have accurate
control of the transmitted phase.
On receive rather than using the hardware
discriminator I will use an A/D converter and
will do the demodulation of the 450 KHz IF
in software.

The demodulator will use the Viterbi algorithm
to estimate the received symbols. This is standard
practice when demodulating CPM.

CPM is a constant envelope modulation so can be
used with class C amplifiers and with receivers
that have limiters in their IF strips.

The approach I have chosen will be scalable to
other frequencies and data rates.

I am going to develop an application I wrote
last year called BSON (Be Seen On Air).
This application reads video from a webcam
compresses it using the BBC's Dirac codec and then transmits it.
Currently BSON uses QAM to transmit the data
but has only been partially successful.

Audio is transmitted using the Speex open
source codec. Speex is not really suitable for
this type of non internet application.

I will log my progress (or lack of it) in this blog
over the next year or so. When I have a basic
system functional I will invite others to help if they
want to.

I don't want to waste peoples time on something
that might never fly!

No comments:

Post a Comment