Write about timecode and loop debugging.

diff --git a/hdmisdi.rst b/hdmisdi.rst
index 55ea0a5158e61b8c8a6298e3350cfb4d84ca6418..064fe85aef8c90cbc6ddf75ec846c31b6d1b14d6 100644 (file)
--- a/hdmisdi.rst
+++ b/hdmisdi.rst
@@ -194,5 +194,34 @@ lowest and highest will be printed. Do note that the measurement is still done
 over a single *output* frame; it is *not* a measurement over the last 100
 output frames, even though the statistics are only printed every 100th.
 
-TODO: Write something about time codes here.
-
+Another trick that can be useful in some situations is *looping* your signal,
+ie., connecting your output back into your input. This allows you to measure
+delays that don't happen within Nageru itself, like any external converters,
+delays in the input driver, etc.. (It can also act as a sanity check to make
+sure your A/V chain passes the signal through without quality degradation,
+if you first set up a static picture as a signal and then switch to the loop
+input to verify that the signal stays stable without color shifts e.g. [#]_)
+
+For this, the *timecode output* is useful; you can turn it on from the Video
+menu, or through the command-line flag *--timecode-stream*. (You can also
+output it to standard output with the flag *--timecode-stdout*.) It contains
+some information about frame numbers and current time of day; if you activate
+it, switch to the loop input and then deactivate it while still holding the
+loop input active, the timecode will start repeating with roughly the
+same length as your latency. (It can't be an exact measurement, as delay is
+frequently fractional, and a loop length cannot be.) The easiest way to find
+the actual length is to look at the recorded video file by e.g. dumping each
+frame to an image file and looking at the sequence.
+
+In general, using Nageru's own latency measurement is both the simplest and
+the most precise. However, the timecode is a useful supplement, since it
+can also test external factors, such as network stream latency.
+
+.. [#] If you actually try this with Nageru, you will see some
+       dark “specks” slowly appear in the image. This is a consequence of
+       small roundoff errors accumulating over time, combined with Nageru's
+       static dither pattern that causes rounding to happen in the same
+       direction each time. The dithering used by Nageru is a tradeoff between
+       many factors, and overall helps image quality much more than it
+       hurts, but in the specific case of an ever-looping signal, it will
+       cause such artifacts.