Sunday, 14 August 2016

DATV-Express on Windows update

I have been keeping busy even though I have not posted for a couple of months.
After the AMSAT-UK conference it became obvious that DVB-S2 would take
a staring role in the new Es'hailsat 2 Satellite (which apparently is named after the
Arabic name for the star we know as Canopus). The satellite is now scheduled for
a Q3 2017 Launch.

To this end I have ported my Linux DVB-S2 implementation over to Windows.
The actual implementation is a C++ class and runs almost entirely on the host.
This means it uses more CPU than the DVB-S implementation does. I am still
ironing out bugs, by that I mean stuff I have working fine, when given to others
to test is not quite as successful.

While we have receiver support for normal symbol rates it may be a while before
we have RBTV (Reduced Bandwidth TV) support in place. Still we have a year
to get this all up and running.

I have also started a new challenge and that is the development of a GPU based
DVB-S2 receiver. I have done some similar work in the past but not at these
high symbol rates. DVB-S2 was specifically designed to take advantage of
parallel processing. I have been looking for an excuse to use the 2816 cores on
my graphics GTX 980 TI card. Originally I had planned on just implementing
the LDPC decoder on the GPU using the "Belief Propagation" algorithm but
bit by bit it is looking like most of the modem will in fact be on the GPU as
it is a better fit than on the CPU.

After having struggled with Graphics Drivers when installing the NVIDIA
Cuda 7.5 toolkit on my Linux machine it came as  a relief when I found how
simple the Windows Toolkit was to install. My only  mistake was not installing
Visual Studio 2013 before the toolkit. The current Toolkit does not support
the 2015 edition I was using for development.

The GPU modem is more an academic exercise than something that would be
practical for most people.

I am currently looking at ways to estimate the noise variance on the
communications channel as this is required to calculate the Log Likelyhood
Ratios LLRs which are required for the BP algorithm.
This is really pushing my maths skills. Wish me luck!

Saturday, 11 June 2016

EVM measurements of DATV-Express

Above is the EVM measurement taken of DATV-Express using an Agilent E4406A.
It is of an unmodified board with no IQ offset applied. Some of the error is due to
a symbol rate mismatch between Express and the test gear. Express derives it's
symbol rate from a cheap xtal oscillator (the one used for the USB2 clock).
When generating the same waveform using an Agilent E4437B using a common
reference the EVM is only a couple of percent better.

The tests were done using the W-CDMA option that allows QPSK EVM
measurements to be taken provided the symbol rate is around
4 MSymbols per second.

The maths

EVM (%) = √(Perror/Pref) x 100

Where  Perror is the power of the error vector and Pref is the power of the ideal reference.

Tuesday, 7 June 2016

Couple of mods that I have done to DATV-Express

External reference added

C91 change
The first was to modify one of my boards to accept an external frequency reference
in this case 10 MHz. The reference must be greater than 0.3V pk-pk and less
than 4v pk-pk.

The second change I made was to change C91 in the PLL filter from 4n7 to 47n.
As you can see from the phase noise plot this reduces the noise at 100 KHz by
around 17 dB. I very much doubt this makes much difference to the DVB-S
performance but as I intend using the Express board for SSB as well, the
2 changes I have made should make a difference.

I am currently waiting for my SRP (Special Research Permit) for 71 MHz to
arrive, more on that when it turns up.

I am also waiting on some eBay 'bargains' to arrive as well. These include some
tunable 50 - 80 Mhz BPFs from Poland that I will put on the VNA as soon as
they turn up.

Finally I have added an E4437B 4 GHz signal generator to my rack of test gear.
It is the unit at the top of the pile. I offered the dealer around half what he was
asking and I was surprised when he accepted the offer. The guy on the
Signal Path Blog recently torn down and repaired one of these units, so I had
an idea of the risk before I bought.

I was getting a bit frustrated with my old Marconi 1 GHz signal generator as
most of my activities are above 1 GHz now.The generator has lots of neat
features including the ability to do multi tone testing of amplifiers and of
playing back test waveforms downloaded over GPIB.

Other projects in the pipeline include 70 Mhz PA re-vamp and a universal
receive converter for DATV (it uses eBay surplus so isn't reproduceable at
low cost).

That is all for now.

Wednesday, 4 May 2016

SDR part 2

Work on the SDR continues but it is proving to be quite complicated to
get to work. I am currently working on the ADC interface to the software.

Originally I tried to implement it with plain logic but that did not prove fast
enough. I am now doing the job properly using ISERDESE2 and IDELAYE2

The ISERDESE2 block does the serial to parallel conversion but
it is necessary to use IDELAYE2 blocks to delay the sample clocks so they
are centred at the eye of the symbol. The reason for this is the time uncertainty due
to a delays caused by buffering and routing of the high speed data clock.

The software has to go through a training sequence where it moves the sample
point to find the bit transition points. This is controlled by a state machine.
An ISERDESE2 block is used in the clock sync subsystem were it
correlates the clock against itself to find the optimum sample point. More
ISERDESEs are used for the actual de-serialisation of the data lanes.
There are 2 data lanes per channel and four channels.
The channels operate at 500 MHz (4 x 125 MHz). Data is read on both
the rising and the falling edges of the data clock.
Because of the delay added by the IDELAYE2 to the data clock it is also
necessary to use bit slipping to regain frame alignment of the bitstream.

The good news is that according to the Xilinx data sheet what I am trying to do
is well within the abilities of  my Zynq, it is just going to take a bit longer than
I had hoped.

On the bright side this exercise is forcing me to finally learn how to write code
for Zynq based systems as I have had to use so many aspects of the device.

I have FreeRTOS working as the embedded OS, DMA from PL to PS working.
SPI comms to the ADC working and of course TCP/IP/Ethernet using
lwip for comms working.

I have included an image of the prototype system talking to SDR# using the
NetSdr protocol used by RFSpace just to prove I have been busy.

Monday, 18 April 2016

Back to my Diversity SDR project

4 Channel Diversity Hardware

Vivado Block Diagram

I finally took some time out from DATV-Express to work on my SDR.
The hardware consists of a Digilent Zedboard (Zynq 7020) an Analog Devices
interposer board (which required a slight modification) and an Analog Devices
AD9253 ADC evaluation board. The Zedboard was bought used from a
seller on eBay so I paid about half the retail price but as I didn't get the
Xilinx licence I am using the free webpack version.

The OS I plan to run is FreeRTOS I think I have the FPGA code just about
finished and am now starting on the C code for the ARM part of the Zynq.
This will primarily just be taking data DMAed from the PL (FPGA) fabric
and sending it out over Ethernet. 

The board is currently running a demo program that uses the lwIP
stack which I plan to use as a framework for my own application.
I am calling it Diversity4SDR (because it has 4 channels).

The AD9253 is a 4 channel 125 MS/s 14 bit ADC. The FPGA code which uses
mainly free Xilinx AXI4 IP blocks decimates the 125 MS/s into 4x 1MS/s
channels so they will fit over 1Gbit Ethernet.

As SDRs are pretty simple to do I am using this as a training exercise to
get up to speed with Xilinx Vivado . Eventually I plan to use Vivado
to implement a DPD system.

What has spurred me on to get back to this is the excellent work done by
Pavel Demin examining what he has done has help tremendously with the
FPGA code.

Now you know why I seldom ever get on the radio.

Thursday, 11 February 2016

Express DVB-S Transmitter

A couple of days ago I released a preliminary version of the software on the
BATC Forum. Results so far have been encouraging. A couple of bug fixes
down the road and it seems we have something usable.

Top picture is the control panel of the software itself. The picture below is a
smugshot of me transmitting an MPEG2 SD picture using vMix and vMix
social media. The signal is being received using DVB Dream software.

I would like also to say that the program does not need vMix to work, it
can take video from a range of capture devices including webcams but
the SD version of vMix is free and makes for a very professional looking

It still needs some rough edges sorted out and I still have to make a lot
of fundamental design decisions, like should I add support for the Intel video
compression toolkit which provides hardware acceleration on Haswell
chipsets for the codecs.

For ease of use this will probably become the preferred solution for
my DATV station and I am even thinking of building a bespoke PC
dedicated to it.

Monday, 1 February 2016

Complete DVB-S Transmitter Windows Application

DVB-S Transmitter Application
Now that I have reached a stage were things are starting to work I thought
I would give you all a preview of the program I am currently working on.

It looks simple but a lot of work has gone into this. What is it you ask?

Well it is a Windows based program that can capture video/audio from a source,
compress it using the FFMPEG codec libraries, turn it into a valid MPEG2
Transport Stream and then send it to the DATV-Express board where it is
turned into DVB-S by the FPGA and then transmitted.

Currently it supports H.262 (MPEG 2), H.264 (MPEG4-AVC / MPEG4 part 10 )
and H.265 HEVC. At the moment it only supports MPEG1 Layer II sound although
I will be adding AAC.

I have tested it with 2 different Webcams, an Ezcap capture dongle and of course vMix.
It supports the usual range of symbol rates 200K to 8M and the usual frequency range
65 MHz to 2.48 GHz.

The capturing is done using Windows DirectShow so in theory just about any device
that has DirectShow support will work with it. In practice there is bound to be some
device format I don't have support for.

I am currently doing the boring stuff, adding GUI support via the menus to configure
the large range of parameters.

On my reading list at the moment is the DTG D-Book 7 which describes the standards
that digital TV works to in the UK. Part A covers terrestrial transmission, the majority
of the spec is taken up with standards for connected TV, something that is not really
relevant to Amateur TV.

Everyone on the DATV-Express team is on holiday (except me) at the moment so not
much is happening. Hopefully I will have an  initial version of the software available
in about 2 weeks time.