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GNRC Tutorial Wiki Part 4: GNRC Instrumenation and Feedback

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tandersn edited this page Nov 26, 2022 · 9 revisions

Instrumentation and display.

There are a number of "blocks" in GNRC that simulate instrumentation (like an oscilloscope) and/or display information about the signal, or the signal itself. We use this to inspect the integrity and properties of a signal. (Note: using a CX card in raw mode, you can actually make a crude oscilloscope or spectrogram). Lets take a look at some of these:

Instrumentation:

Different kinds of instrumentation is useful in different settings. In some cases a waterfall sink (spectrogram) gives you more information, in other cases a Time Sink is more appropriate. For instance, to look at the "levels" of a CVBS file (a .tbc file), the Time Sink gives an informative readout, but the waterfall sink, not so much (waterfall on the left, time sink on the right):

Above, you can tell in the Time Sink that the sync level of the signal is at the bottom. The Waterfall Sink shows you have signal and roughly where it is.

Alternately, to look at the RF captures directly, the reverse might be true:

Above, what can we tell from this RF readout on the Waterfall Sink?

  1. It's an LD RF signal (because of where the 4 main signal components are in the spectrum)
  2. It has an EFM track (the lower ~1.5Mhz is showing signal)
  3. It is not an AC3 disc (the 2 analog audio channels seem fairly symmetrical, an AC3 disc will show significant difference between the R and L analog channels.

From the Time Sink we get a better feel of the signal amplitude.

There are lots of other types of instrumentation that can be useful, test them out on your own.

Visual Display

There is one "odd ball" GUI block in GNRC, which i am very glad for its existence, the Video SDL Sink. This block can be used to display video directly in GNRC. There are a few things to note about the VideoSDL:

  • It's relatively simplistic
  • No matter which of the 2 data types it supports that you select for the input, it wants values in the 0 - 255 range.
  • It will slow the graph down, so is good for preview, but disable it and it's requisite blocks for full processing.
  • You'll want to use a throttle block with the Video SDL Sink, or you will end up with too high of a frame rate.

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