DVB-T 6, 7 or 8 Mhz Channels ?

[G4GUO]

The subject line pretty much sums it up.

Is there a standard for Amateur DVB-T ?

The reason I ask is that

8 Mhz channels need a 64/7 MHz clock
7 MHz channels need a 8 MHz clock
6 MHz channels need a 48/7 MHz clock

As I am attempting to implement ODFM on my USRP2
and the USRP2 needs a clock rate that divides into 100 MHz
I need to use interpolation/decimation to scale the
output to 10 MHz if there is already a standard I
will code that otherwise I guess I will choose the
easiest one to code as I am too lazy to do them all

- Charles G4GUO

[G8ADM]

Hi Charles
Well there are very few amateur DVB-T transmissions.
The only one I know of is in Australia.
They are allowed to use the UHF broadcast band and therefore have plenty of bandwidth available.

Elsewhere we use DVB-S. QPSK. This is because the RF bandwidth is proportional to the symbol rate.
So for a symbol rate of about 2 MS/Sec this requires a bandwidth of about 2 MHz. which is what we use in the 70cm band.
In the higher bands we tend to use a symbol rate of 4 MS/s.
Another advantage is that the transmitter linearity is not so critical as with C-OFDM

The symbol rates mentioned above are not set in concrete, some people use other symbol rates.

Please let us know how you get on.

Regards,

Dave.
G8adm

[G4GUO]

Hello Dave,

I already have working DVB-S TX code so I thought
I would have a crack at doing DVB-T TX.

7 MHz spacing is probably the easiest as I can
use simple interpolation / decimation to get to
a sample rate that is suitable for the USRP2 SDR.

I think I have figured out the bit and symbol interleaving
and the position of the pilot tones and TP encoding but until
I can get a signal up at the right rate I won't know.

I bought one of those cheap USB receivers you see
on eBay last week for £10. It claims to do 6/7/8 MHz
and the various -T modes QPSK,16QAM and 64QAM
as well as 2k and 8K so I will use that for
testing (if it works).

I still have not figured out how best (cheapest) to convert
down to the TV band from 24cms yet!

Of course if I was really clever I would implement the
receive side as well and use the USRP2. I think I will
give that a miss for now.

DVB-S receive would be much easier but I don't know
enough about Linux to interface between the demodulator
and something like MythTV

- Charles G4GUO

[G8ADM]

Hi Charles

Well that sounds great fun.
Good luck and keep us informed.

Dave.
G8adm

[G4GUO]

I do have my blog http://www.g4guo.blogspot.com/
which currently records the trials and tribulations of
my attempts to get on DVB.

It is a real shame the USRP2 is so expensive as the
potential is huge for these types of applications. Still
with the march of technology this stuff will get cheaper.

I think someone has already implemented a receiver
for 8VSB, currently it only works on the USRP1 though.

I was thinking this morning that I could one day
add pre-distortion of the TX waveform to compensate
for the characteristics of the PA or if I was to implement
a DVB-S receiver I could add blind equalisation to make
my system more robust to multipath.

For the moment I would just settle for a stable 4M tx signal!

I managed to get the DVB-T USB device to work after a
fair amount of work searching the internet for better
drivers. There is some manual control over the rx parameters
but not as much as I would have liked. However it does seem
to use a standard interface (BDA) so I might be able to find some
better software to use with it.

- C

[g8gtz]

Hi

I did play around with 8 MHz DVB-T a few years ago but the need for a low phase noise very linear (very hot) 8 MHz wide amplifier makes it difficult for amateurs to implement.

Also the fact that the power is spread over over 8 MHz means the reciever threshold is much higher than 2 or 4 MHz QPSK and surprisingly the QPSK did not suffer from multipath as much as anticipated, which is the main benefit of OFDM.

This plus we can get QPSK recievers already on 23cms and doesn't need any downconversion to UHF means QPSK seems to be the best bet for DATV.

BTW - Australia uses 7 MHz OFDM

[G4GUO]

Hi,

What triggered my thoughts about DVB-T was someone locally to me has some
commercial gear for it, with an SDR it is only software, a lot of the code in DVB-S and DVB-T
is the same i.e same Reed Solomon code, same Convolutional code. Of course the modulation
is different and DVB-T adds two extra interleavers but I have pretty much got that code written now.

I was reading a report from the FCC comparing the U.S 8VSB system and OFDM, in
theory while OFDM should perform better than 8VSB in practice, provided a decent
antenna is used up in the clear it seems 8VSB is better as it works at a lower S/N.
The main advantage of OFDM was for mobile reception. Also it seems that the
receiver performance (8VSB) has improved markedly as new equalisation techniques are
implemented with each new generation of receiver.

I know DVB-S and 8VSB are not the same but it does show (to someone without
any practical experience as yet) that things might not be what they seem.

If anyone is interested the report is here
http://www.fcc.gov/Bureaus/Engineering_ ... vreprt.pdf

It is nice to see some activity on these forums.

- Charles

[KenW6HHC]

Hi Charles G4GUO,

The only other DVB-T station that I have information on is the DB0SRS repeater, operated by Stefan DG8FAC, that uses 8 MHz. However Stefan DG8FAC is pushing the edge of ham technology by combining six TransportSteams with a total of twelve video pictures at the same time.

Your choice of 7 MHz seems very reasonable. Most hams will not need to push out 12 video steams.

Some details of the DB0SRS repeater can be found in an article called TechTalk80 at www.W6ZE.org/DATV/

73...de Ken W6HHC

[G4GUO]

Hello Ken,

7 MhZ is the simplest to do as it uses the same up/down ratios
that I am using in my DVB-S software.

If anyone is interested I have been blogging away as I develop
http://g4guo.blogspot.com/
I am always open to receiving feedback as it helps me gauge the
level of technical detail I should go into in the blog.

- Charles

[KenW6HHC]

Hi Charles G4GUO,

Big congrats on sorting out your SI Tables and posting a PIX of your low-power video transmission!!

Yes, your blog is interesting to follow. Keep it going.

Do you have a Block Diagram of your transmitter set-up? (how much is software...how much is hardware?)

73...de Ken W6HHC