Re: Passing Tags or Additional Port Data to Change Parameters of Downstream Blocks

Matt,

Take a look at my DVB-S2 OOT. This is a different version of the DVB-S2
transmitter that implements VCM (as opposed to the regular CCM version
included in GNU Radio).

https://github.com/drmpeg/gr-dvbs2

It implements exactly what you're talking about to achieve VCM (Variable
Coding and Modulation). In VCM, the coding and modulation parameters can
change on a frame by frame basis. To accomplish this, the parameters are
sent as stream tags.

To get you started, here's where the stream tag is first created. It bit
packs a 64-bit PMT with all the parameters.

https://github.com/drmpeg/gr-dvbs2/blob/master/lib/bbheader_bb_impl.cc#L496

Then it's sent downstream and all the subsequent blocks use it for
configuration (and also pass it downstream to the next block).

Ron

On 4/18/24 09:39, Matt Clemons wrote:
> Hello,
>
> I've been using the dtv/dvbt blocks in GnuRadio Companion to decode
> dvbt signal recordings. I noticed a lot of the blocks in the RX chain
> require the user to select the modulation parameters (Constellation,
> Hierarchy, Code Rate, etc.). My goal is to remove a lot of these user
> parameters with the use of Transmission Parameter Signals (TPS) data
> called out in the DVB-T spec. This would make the DVBT receiver more
> autonomous, and be better than brute forcing all the modulation
> settings until you get a recovered transport stream in the cases where
> you don't know the transmitter settings. This solution seems to be a
> worthwhile improvement to the current DVBT blocks by utilizing the TPS
> data called out in the spec. Implementing this in the c++ code is
> where I'm running into issues and where I'm looking for help/advice.
>
> In the DVB-T spec the Transmission Parameter Signals (TPS) data can be
> recovered which will tell you all the modulation parameters of the
> main signal. As long as you know the TX mode and Guard Time, you can
> configure the OFDM Symbol Acquisition block and the FFT block, which
> are the only 2 required blocks before the Demod Reference Signals
> (DRS) block. The DRS code already decodes the TPS data but is only
> looking at 2 of the total 68 bits to determine what the frame index
> is, and then clears the std::deque of TPS data.
>
> My goal is to use all of the TPS data and either pass it in a tag or
> pass through a new port so that all the remaining downstream blocks
> can get the modulation parameters from that data rather than a user.
> I've dug into the tags and the PMT types required to pass data, but I
> can't wrap my head around how I would configure the modulation
> parameters of the downstream blocks from the port or tag TPS data
> passed to it.
>
> For example, a lot of the DRS modulation parameters are not needed for
> the DRS block to work, and when you recover Constellation Type,
> Hierarchy Type, and Tx Mode you could then pass these parameters to
> the DVB-T Demap block instead of having to create the block with those
> parameters.
>
> If there are any examples of other blocks using tags or ports to
> change the functionality of another block, that would be useful. Since
> I am fairly new to GnuRadio and C++ these modifications seem difficult
> to me, hoping someone with more experience in both could point me in
> the right direction.
>
> thanks!