mirror of https://github.com/merbanan/rtl_433.git
58 lines
3.9 KiB
Markdown
58 lines
3.9 KiB
Markdown
# Primer on SDR concepts
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A brief introduction to some concepts, most questions really only need a keyword to look it up.
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> What is a signal?
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In the simplest way think of it as a sound file. Load a sample in [Audacity](https://www.audacityteam.org/) and play it! But that's only the [sampled](https://en.wikipedia.org/wiki/Sampling_(signal_processing)), [quantized](https://en.wikipedia.org/wiki/Quantization_(signal_processing)) [baseband](https://en.wikipedia.org/wiki/Baseband) of the [radio wave](https://en.wikipedia.org/wiki/Radio_wave)...
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> 433 MHz is 433920000 Hz? and so listening to 433920001 Hz is a different frequency that would not catch any 433 MHz signals?
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"433" really means the 433.92 MHz [ISM-band](https://en.wikipedia.org/wiki/ISM_band). "434" would be more appropriate. It's a band of "sample rate"-width. E.g. 433.795 to 434.045 MHz at the default 433.92 "center frequency" and 250 kHz sample rate.
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> How can you decrypt a signal? I am getting some signals with my SDR and it would be nice to know a bit more about them?
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Mostly it's not encrypted but only coded and [modulated](https://en.wikipedia.org/wiki/Modulation).
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> Is the data transmitted in binary code? Always?
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There are multiple layers. Look up [ASK](https://en.wikipedia.org/wiki/Amplitude-shift_keying), 2-ASK / [OOK](https://en.wikipedia.org/wiki/On-off_keying), [FSK](https://en.wikipedia.org/wiki/Frequency-shift_keying), 2-FSK then [PCM](https://en.wikipedia.org/wiki/Pulse-code_modulation), [PWM](https://en.wikipedia.org/wiki/Pulse-width_modulation), [PPM](https://en.wikipedia.org/wiki/Pulse-position_modulation), [MANCHESTER](https://en.wikipedia.org/wiki/Manchester_code), [DMC](https://en.wikipedia.org/wiki/Differential_Manchester_encoding).
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> Can I know the strength of the signal to know if the transmitter is close or far?
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Only if there is no [AGC](https://en.wikipedia.org/wiki/Automatic_gain_control) or the current AGC value is reported (not with rtl-sdr).
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> What is the meaning of the information I capture with rtl_433? It would be nice to know the meaning of each line or the different parts it shows?
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It's the decoded transmission and some meta-data about the signal.
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> Why was I getting all zeros when I was listening to my device? How could I change this to get the correct information?
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Wrong assumption about decoding parameters. The suggested flex decoder is just a simple statistical heuristic.
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> What is the information provided by the -A switch? pulse width, gap width, pulse period.....
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See PWM.
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> Can I buy and install a bigger antenna to get better signal? Or is there some limitation on the signal that will not allow this?
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More metal is in fact the only reliable way to get better reception. But there are limits, s. wave length. And you could add directionality.
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> Can I transmit a signal to my receiver thermometer and make it think that I am the transmission device? Does this mean that I can impersonate any transmitter device?
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Sure. This almost always possible.
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> Does a sensor need some kind of computer/arduino to send signals or can it work with just the sensor and some transmitter hardware?
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There are specialized chips like the [EV1527](https://www.sunrom.com/get/206000) that do this. A general-purpose [MCU](https://en.wikipedia.org/wiki/Microcontroller) isn't likely to be used.
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> And I would like to know a bit about other frequencies outside 433 MHz.
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868 is also interesting. Mostly FSK is used, where 433 mostly uses OOK (ASK).
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> I would like to send a doorbell button push to MQTT. rtl_433 doesn't decode it, but it's a simple button, so I only need a trigger event. With rtl_433 -a -R 0 I am able to catch the button with {25}... code. What is the correct way to send the raw data to MQTT?
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Use `-A` and note the `-X` line. Then use that to write a flex decoder. See e.g. [EV1527-PIR-Sgooway.conf](https://github.com/merbanan/rtl_433/blob/master/conf/EV1527-PIR-Sgooway.conf).
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Have fun.
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