Add some documentation about tally display.

diff --git a/monitoring.rst b/monitoring.rst
index 8e64307e4ef9ce1b9f8e3ad2ecdf18906ae2f2b7..204c37a885956706769fbe96eeb5a47272f3d7f1 100644 (file)
--- a/monitoring.rst
+++ b/monitoring.rst
@@ -31,3 +31,31 @@ backward compatibility in metrics, so if Nageru internals change significantly,
 some graphs could stop working. Of course, you can change the dashboard as you
 see fit; if you don't use e.g. HDMI/SDI output, you will probably want to
 remove the panels related to it.
 some graphs could stop working. Of course, you can change the dashboard as you
 see fit; if you don't use e.g. HDMI/SDI output, you will probably want to
 remove the panels related to it.

+
+
+Tally display
+-------------
+
+Not related to Prometheus per se, but yet an important part of monitoring is the
+*tally light*, or just tally for short. The tally is a talkback signal from the
+mixer to the camera, showing the cameraperson that they are indeed on air (or are
+about to get to). Professional studio cameras will often not only have a tally
+light for the operator, but also for the subject (to know which camera to look
+into). However, most cheaper cameras will have none at all.
+
+Nageru does not have functionality to talk to tally lights directly (there are too
+many kinds of interfaces and standards), but it does have functionality to expose
+tally _data_. Using this, you can build your own tally light, be in through injecting
+the right bits on an SDI cable, having a single-board computer set some GPIO pins
+for a LED, or simply show a red or green dot on a mobile phone with a web browser.
+
+Nageru's tally data is automatically exposed over HTTP, just as any other monitoring.
+You can get one JSON file with a list of all channels (at “http://yourserver.example.org:9095/channels”),
+or you can ask for a specific channel and get just its color in plain text
+(at “http://yourserver.example.org:9095/channels/2”, where 2 is the lowest
+channel ID, since live and preview don't have tally). The tally color values
+are taken automatically from the theme's *channel_color* function, so that they
+match the borders around the input in the producer's mixer display—typically
+green for on preview and red for live. This also means they can be CSS textual
+colors (like “red”), although you can of course make the theme return only
+“#ff0000” or similar if this makes your tally application simpler.