Question regarding multiple sources producing data at different times

[Adrian Musceac]

Hello,

I'm trying to implement a multi-channel DMR transceiver, kind of similar in concept to Osmocom's osmo-trx multi-ARFCN transceiver.

My flowgraph has several sources taking samples from ZeroMQ, each followed by frequency modulation, and the branches are joined with an add block.

My problem is that the sources can produce data (zmq messages) at different times (sometimes not producing anything if the channel is idle). If I return 0 noutput_items from one of the sources, this will block the whole flowgraph (presumably in the add_cc block?). What I've done instead so far is to produce zeros during idle time, but this is not ideal because of the frequency modulator and the resulting constant carrier.

I'd like to know if there's a more elegant solution of which I'm not aware of to this problem. I am also considering tagging the idle time samples and zero-ing them out downstream from the frequency modulator, but it seems too convoluted.

I should also mention that I have complete control of the sources since they are custom C++.

Many thanks,

Adrian