[LAD] crosstalk between jack clients/ports?

[Vaclav Mach]

Hi,
I have the following system based on Jack API already working:

/**** single PC configuration ***
Jack Connections:
system_capture_port->jack_port_in (jack_client_in)
jack_port_out (jack_client_out)->system_playback_port

process of jack_client_in:
jack_port_in buffer->ringbuffer

thread1:
ringbuffer -> RTP stream

process of jack_client_out:
ringbuffer->jack_port_out buffer

thread2:
RTP stream -> ringbuffer
*******/

Both of the clients are maintained by the same application.
Sound transmission between two PCs is working well, sound is nice, we 
can talk to each other (something like VoIP).
However, a person speaking at PC1 hears his own echo delayed and 
attenuated. I have found out, that the echo comes from PC2 through the 
RTP stream (reason of the delay). And now, the most interesting 
observation is that when I disconnect the Jack connection between 
jack_port_out and system_playback_port at PC2, the echo at PC1 is no 
more audible.

I want to avoid the echo and I have two ideas where the problem can be:
1) there is a crosstalk between jack ports/clients
2) there is a crosstalk in my HW (mainboard sound device with intel_hda 
driver)

Any advice or suggestion?

Thanks,
Vaclav