GNRC Tutorial Wiki Part 2: Sampling Rate

Sample Rate

Each new graph you create in GNRC comes with one variable block already added because it is so ubiquitous: samp_rate. The "samp_rate" variable defines the sample rate of your project. Most of the time, it is the sample rate of your source input file. It is auto populated into a lot of the blocks we will be using for convenience. Things get a little complicated when you start to "resample" your data, but that's a topic for another day.

• When using hardware devices, like direct input from cx card, samp_rate will not change the sampling rate of said device. Setting samp_rate higher than the HW device sampling rate will have no effect on sampling rate, setting it lower will cause buffer overflows and dropped samples.
• When not using hardware devices, samp_rate is mainly used for the advanced blocks of GNRC to process the data.
• When not using a hardware device, using a "throttle" block helps keep the stream at a reasonable rate so as not to overload your computer.

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Common sampling rates:

• 40msps, can be written as 40e6 in GNRC. DdD and mod'd CX card sampling rate
• 14318181, 4fsc NTSC, 4 times the NTSC color sub carrier rate, used for NTSC .tbc files, and the default sampling rate of cx card in 16 bit mode (stock xtal)
• 17734475, 4fsc PAL, 4 times the PAL color sub carrier rate, used for PAL .tbc files
• 17897727, 5fsc, 5 times the NTSC color sub carrier rate, and the 10fsc sampling rate of cx card in 16 bit mode (stock xtal)
• 28636363, 8fsc, 8 times the NTSC color sub carrier rate, and the default sampling rate of cx card in 8 bit mode (stock xtal)

NOTE: PAL 4fsc and NTSC 5fsc are very close, but they are not the same. It is sometimes overlooked, so be attentive to this issue.