BATC Forum

I have been using this neat little app when I have needed to access Freeview (e.g no internet access) and it works really well.
So I asked the developer...

"Can you add low symbol rate DVB-T? Like 66/125/333 KS?"

To which he replied...."Gareth the driver relies on hardware support (it does not do SDR) so this would depend on the hardware capabilities of the devices. I am unsure if the supported devices by the DVB-T driver have low symbol rate DVB-T feature. If you know whether this is possible do let me know."

I'm not entirely sure what info he needs as we know the RTL does just turn RF into a data stream and it works at low SR's into e.g. SDRAngel.

If anyone can shed light on what he might need to add/do to his software to facilitate the addition of low DVB-T SRs, please contribute.

Gareth

Hi Gareth,
No one else has commented so here is a contribution. DVB-t specification is for 6,7 or 8MHz channels. Full details can easily be found on line. (EN 300 744)
As far as I am aware, reducing the bandwidth for amateur purposes is basically just slowing the whole thing down. The 2K mode in 8MHz has carrier spacing ~4kHz and symbol period ~0.25 mS (plus guard interval). So for 20kHz bandwidth that becomes 10Hz and 0.1 seconds; that is 400 times slower.
The coding and interleaving uses a "frame" of 96 symbols, (so about 10 seconds or more) and 4 frames in a "superframe" (so about 40 seconds or more).
I am not sure if you need a complete superframe, or just one complete frame, to sync and decode the signal, but in 20kHz it is always going to be a bit slow.
To someone who knows only about DVB-T, talking about a reduced rate of 66 kS/S will not mean much, as the maximum symbol rate in DVB is only 4 kS/s.

Ken

Thanks Ken, you've shed a bit of light on the issues. I mistakenly assumed it was possible just as with DVBS which has been reduced to a mere 18ks.
Don't think there is much likelyhood of AerialTV being tweaked to suit.
'Every day is a school day'....
Gareth

I had a look at the RTL2832U datasheet. That is the DVB decoder chip in the dongles. It allows to set the FFT mode and guard interval from a menu of standard values (2k or 8k FFT and 1/32 etc. GI)
However the Symbol rate and hence bandwidth are set in by a division ratio variable in a register. If I am interpreting it correctly that could be set to something other than the standard values for 6,7,or 8MHz. It seems to be a 26 bit number so maximum 3FF FFFFh which is only 5 times the value required for 8 MHz, so that might mean no hope of setting it to less than 1.6 MHz .

Ken