DSD+ and FMPx use Transmission Control Protocol (TCP) networking ports to communicate. Link here for
Wikipedia definition and history of TCP/IP protocol.
From RTL-SDR.COM see below:
December 30, 2015
DSD+ Updated to Version 1.101
DSD+ (Digital Speech Decoder+) is a popular decoding tool that can be used to listen to P25, DMR and other unencrypted digital speech signals. Recently DSD+ has been updated from version 1.074 to version 1.101.
The new version brings several changes, including the ability to decode Hytera Extended Pseudo Trunk (XPT) systems, Airspy compatibility, performance improvements and a TCP/IP link from FMP to DSD+ (no longer need to use a virtual audio cable). The full change log is as follows:
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DSD+: DSD+ can get its raw audio source from FMP via a TCP link instead of via Virtual Audio Cable or VB-Cable
-i<TCPport> FMP TCP link port number (256-65535)
linking FMP to DSD+ via VAC or VBC is deprecated; please use the TCP
link feature instead; any port number between 10000 and 65000 should be fine
I don't pretend to totally understand how TCP/IP works or how and why it works in this application. All I know is it does work and is not difficult to set up if the information in the available materials is followed. The RTL-SDR.COM site has a lot of good information.
I hope this helps answer your question.
PS: In my opinion, none of the SDR decoding software packages are plug and play so to speak. Batch files, configuration files and data files need to be created/modified to fit the user's situation. It takes work to get set up but it is well worth it in the end. One can really build a great system with multiple dongles and enough USB ports, thankfully USB hubs exist. I have three dongles and I run three instances of DSD+ and FMP24 on an old Dell notebook to simultaneously monitor three sites, in different counties, on the MPSCS in Michigan. One site through the left speaker, one site through the right speaker and the third in mono through both speakers to enable me to sort out in my head which site the audio is coming from. It took time and some trial and error to set up but it works for me.
