Embedded SBC (Single Board Computers)
SBC can be low cost, noise, space and power usage option for 24/7 sdr decoding and streaming.
These can optionally be run with no monitor (headless) with remote control via ssh console and due to low dc power requirements can even be run off battery. Trunk recorder provides pre-made Docker packages to run without having to compile from source saving much time and effort when installing or upgrading. Installation#docker
The 2 most popular, affordable and commonly available and used models are Raspberry Pi 3 and 4. Featuring quad core cpu running between 1.2ghz and 1.5ghz.
The main draw back with this is that trunk recorder and sdr can be very cpu intensive, particularly so as your tuned bandwidth and number of recorders increases, so you need to monitor your system once setup to check it is not being overloaded and dropping data. FAQ#i-am-seeing-a-lot-of-0000s-or-oooos-in-my-console & Monitoring system performance, activity & email alert
I have successfully run a pi3 with up to 2Mhz bandwidth and 6 recorders, but more comfortably 1.5mhz and 4 recorders, while also running several liquidsoap streams, but i suspect how you go will also be relative to how active your sources are. While I have not tried the performance of pi4 looking at its specs and cpu benchmarks, its not a huge upgrade.
One thing to watch out with raspberry pi, is that by default they are not sold with cooling heatsinks or fan and you should watch your temperatures as under high load the cpu will throttle (reduce) its speed once over certain temperatures. You can buy after market cooling solutions though, but running a fan (which being small can be quite noisy) may work against one of the reasons you bought a SBC.
Another thing to consider, SBC commonly use micro SD cards for hard drives, but they have limit write cycle life, so if you have a very active system or just want to preserve the sd anyway, you can have your logs moved to a ram drive, here is an example rpi3/4 script
Unless you have a fairly small system to monitor you might be looking for something more powerful, while there are many options out there, at the time of writing this (2021), I feel one of the best value v performance is the latest model by HardKernel ODROID-N2+, as its not a big jump in price over rpi but is a significant boost in performance and cooling capacity.
It features 6 cores running at 4x 2.4Ghz and 2x 2Ghz and a massive stock heatsink that works excellent without a fan even at high load, but even if you do need/want a fan it fits a larger one.
Here are some graphs showing system performance of rpi3 (w/ heatsink no fan) vs n2+ (stock heatsink no fan) both running the exact same system config: 2x SDR, 1.35Mhz (trunked p25, 4 recorders) & 0.250Mhz (1 conventional p25 recorder):
(Keep in mind with the following that the quad core rpi3 is maxed out at 4.0 load or 400% usage, while 6 core N2+ would be 6.0 load or 600% usage.
