Demo transmitter generates invalid data

[sp5wwp]

Demo transmitter available in the examples directory - transmitterPLUTOSDR.grc - doesn't seem to correctly encode data. Additionally, a Throttle block is required in the flowgraph to make the RF stream decode at the receiver.

How to recreate:

• run the flowgraph
• set all the RF params as required
• decode audio with SDR++

Expected result:

• DST, SRC callsigns and CAN are decoded properly
• 1 kHz tone is audible

Actual result:

• DST, SRC callsigns and CAN are decoded properly
• audio is a repetitive squelchy sound

[jmfriedt]

Throttle should never be included in GNU Radio Companion flowgraphs including real hardware as the coarse CPU timing limitation of the Throttle block will conflict with the hardware clock of the Pluto Sink. See the Throttle block Documentation:

"N.B. this should only be used in GUI apps where there is no other rate limiting block. It is not intended nor effective at precisely controlling the rate of samples. That should be controlled by a source or sink tied to sample clock. E.g., a USRP or audio card."

The sink datarate (or RRC and Resampler) should be tuned for the m17_coder to be fed with the expected datarate from the Vector Source.

[sp5wwp]

Yes, I am aware of all that, but for some reason the RF stream generated by a flowgraph with throttle bypassed doesn't even decode in SDR++. All I get is an excellent looking RF signal, but no decode.

[argilo]

For one thing, the forecast function is incorrect:

https://github.com/M17-Project/gr-m17/blob/cbd0e352fde525bf231d07ffa92e4d4eba082bde/lib/m17_coder_impl.cc#L370-L374

16 input bytes produces 192 output symbols, not 384, so the ratio should be 12.

Next, the work function uses a do loop to do its work, checking whether there is sufficient space for a frame only after completing one iteration. If the output buffer is shorter than 192 items, a buffer overrun occurs. Switching to a while loop and putting the condition at the top should fix that.

Since frames are always 192 symbols, the block should just set its output multiple to 192.

[argilo]

I fixed these bugs, removed the throttle from the PLUTO transmitter, and added a missing "Pack K Bits" block, and now the transmitter appears to be working properly.

[sp5wwp]

Confirmed, fixed.