Decoding the RF Output of a VCR with an RTL-SDR
Over on YouTube use Scelly has uploaded a video showing how he has used an RTL-SDR dongle and the TVSharp SDR# plugin to decode video from the RF output of an old VCR (videocassette recorder). VCR players were designed to output the same PAL or NTSC signal that old analog TV transmissions used, and the RF output of the VCR was connected directly to the TV's antenna input.
The TVSharp plugin for SDR# can be used to decode these signals, however as the bandwidth of PAL/NTSC signals is much larger than the 2.4 MHz provided by the RTL-SDR, only a black and white image can be received. Scelly writes:
RF Output from VCR connected directly to input of my RTL-SDR. The RF output is tuned to channel 22 (487.25 MHz), and as the signal is so wide, my RTL-SDR can only display the luminance data (black and white video) and audio, although not at the same time. If I had two RTL-SDRs or an SDR with a larger bandwidth, I could have both audio and video playing at the same time.
The video playing is "The Prince of Egypt" on VHS Video Cassette.
In the statements there seems to be a lack of understanding of analogue TV in the post.
There was also SECAM in addition to NTSC and PAL color system available in TV and VCR.
Color TV is basically a black and white TV to which at a later time the standard was amended to add color information. The color information requires actually less bandwidth, than the black and white TV signal.
The black and white information requires at max. about 5.5 MHz bandwidth (single side band AM with reduced carrier) if white and black alternates, that is without audio. Black and white TV picture signal vary in resolution, and consists of the for the standard defined number of lines (horizontal) and refresh rate for vertical sync pulses for interleaved odd-/even-lines pictures (50 Hz or 60 Hz).
For audio, mono and/or stereo, additional FM subcarrier are defined.
Color TV depends on the black and white picture for luminance and sync and adds color information. The color information is interleaved in the black and white spectrum, therefore if you receive black and white you can also receive color. There are three Color system which add the color information in different ways, e.g. PAL (Phase Alternating Lines) the color was stable but required a color reference signal, while the initial NTSC was and you had to correct e.g. green faces yourself.
Due to the varying resolution, odd-/even-picture-frame rate, varying carrier frequencies for audio and color types, TV and VCR supporting multiple -systems were offered only decades after color TV.
This is just a quick and dirty version of how complictae TV is.