Last updated 11-19-2021

The Raspberry Pi 4B is the only SBC that has two HDMI outputs. This makes it especially economical for digital signage. The board itself only costs $35 plus shipping in the 2GB memory size. Heatsinks or an aluminum case that acts as a heatsink will be needed as well as a power supply, micro HDMI to full size HDMI cables, and a microSD card for its OS. It is capable of simultaneously playing two independent commercial DVD movies on its separate two displays at 1920×1080 each. It also has separate HDMI audio and analog audio devices which can be assigned separately to two video streams.

Screenshot of 2 displays playing different commercial DVD movies simultaneously

Alternately, program content can be displayed at 3840 x 1080 spread across each two displays per Pi 4B. Content can also be rotated for 1080 x 3840, 2160 x 1920, or 1920 x 2160 per Pi 4B, or independently for each of its two displays. Pi 4Bs can be aggregated for video wall grids of more than 2 displays.

In addition to playing movies, it is possible to play a URL including Youtube, play a slideshow on one or more displays, or run apps for weather or web cams.

The Raspberry Pi 4B clients have graphic hardware support for H.264 video. By playing on the client instead of a server, network traffic is reduced to close to zero if the content is on the Pi 4B(s), and still greatly reduced if the content is on a cloud compared to playing on a server and forwarding pixels to each of the clients. For telling the clients what to play, another Pi 4B or even a smart phone would be sufficient.

The RaspiPlayer consists of an identical OS on a microSD card for each Pi 4B and of server software to remotely configure the Pi 4B clients for unique hostnames so that program content can be configured to target each specified display connected to specific Raspberry Pi4Bs. The configuration/control server communicates with the clients with a unix socket over ssh. microSD cards can be eliminated on player devices by using piserver to PXE boot them. Pisever in the distribution OS has not yet been updated to see the Pi 4B model, but its source has been updated and can be built for multiple architectures.

If your business only needs one signage display, then you can use the other display for controlling the signage as well. For instance a food cart with one signage display to show menu and prices could use the rest of the same Pi 4B as a desktop computer that printed receipts and kept track of sales.

CLI remote control

In the screenshot above, an x96max is being used to remotely control a Raspberry Pi 4B playing the two movies you see reporting their current AV: position. Although I did not have a 1920×1080 monitor for HDMI digital audio, if I did, one of the movies could have been configured to play its audio through HDMI cable making it independent of the other movie playing audio through its analog audio output. Both movies are being played using hardware H.264 decompression.

The bottom 70% of this screen is showing resource usage on the Raspberry Pi 4B client (not on the x96max that is the controlling what the clients are playing). All software required above is opensource and free, so the CLI version requires no license fees.

The tmux unix socket can be written to by tmux inside of php using shell_exec. Therefore, a php web server could provide a GUI like application to control the display of content on all screens.

X96 Max Android TV Box repurposed with Linux as media file server. Serving two commercial DVD ISOs (as files on microSD in USB media reader) simultaneously. Power requirement while doing this only 1.9 – 2.3 watts.

Now screenshots of the two client displays playing these different movies

Raspberry Pi 4B client, full screen commercial DVD playing on HDMI 1
Raspberry Pi 4B client, full screen commercial DVD playing on HDMI 2
Resource usage at Raspberry Pi 4B client playing two simultaneous commercial DVD ISOs locally. Power use 4.4-5.2 watts not counting the two 1920 x 1080 displays connected to the Raspberry Pi 4B.

The video quality is indistinguishable from playing the same DVD films in a Blu-ray Disc Player. Clearly, video should be played on the client, not the media server. The Phistek ZE7000 network attached zero client requires at least a Intel i3 media server for one client 1920 x 1080 display. The x2go client approach is a little choppy even with an Intel i3 media server. Playing the content on the client eliminates the step of compressing changed pixels in the server video frame buffer and then decompressing them again at the client end. Network bandwidth is conserved because the original video content is compressed at a much higher ratio than is possible in real time for forwarding changes in a video frame buffer. There is hardware support for the best of the content compression schemes.

While in the examples above, the content was first read from two USB DVD drives on the server, to make two ISO files on a microSD card in a USB media reader for the x96max, you can use handbrake to create a more compressed MP4 file for just the main title and serve it from a web server on the x96 max to the raspberry pi 4B client and view it in a browser instead of in VLC.

I will update this page when I have data on Pi 4B clients PXE booting (no microSD card) from a server running piserver, and providing a web server as well as eMMC storage under piserver for content to be controlled and displayed on the clients at the displays and connected only by network cable to the media server.