Transmission with more than two channel plugins does not work

[f4exb]

When more than two channel plugins are running concurrently SDRangel enters a blocking state and/or signal is severely distorted.

With two plugins running concurrently some lockups can also occur leading to excessively long refresh periods.

This is a showstopper for version 3.

[f4exb]

I might have some explanation on this problem

In receive mode the device feeds a FIFO and whenever data is available in the FIFO the chunk of available samples is passed to the channel receivers that process these samples independently on their own thread.

In transmit mode the picture is quite different. The device pulls (reads) data from the FIFO and when available samples get below a certain depletion threshold channel transmitters are called to feed the FIFO. The trouble is that mixing needs to be made between channels to produce the sample that will be written to the FIFO. The DSPDeviceSinkEngine performs this task of pulling a sample from each of the channels in turn to mix them and write the result to the FIFO. Although channels run on their own thread they are now tightly tied together which makes a nice opportunity for locking up!

So mixing transmission channels needs more thinking if at all possible and cannot be derived from the reception side just turning the data flow the other way round.

[f4exb]

While the above remarks are valid the lockup problem was actually introduced with this commit: c5f1acd

This commit implements the audio input in the AM modulator (the only existing at the time) so I suppose the lockup is related to competition in the handling of the audio input device.

[f4exb]

This is actually a problem with how the AudioFifo should be implemented to run successfully. The writes and the reads should be approximately balanced. Here the problem is that the writes are done by chunks and only a single sample is read at a time in a loop. The fix consists in implementing an audio buffer in the modulators so that a chunk of audio samples can be read at once from the AudioFifo.

[f4exb]

Fixed in release v2.5.2