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I was reading on wikipedia this:

the subscriber's television or a set-top box provided by the cable company translates the desired channel back to its original frequency (baseband)

But when I checked the block diagram of the receiver it doesn't talk about a baseband but (IF) http://www.brainkart.com/article/Monochrome-TV-receiver_3017/ so what could the baseband here or what is the baseband of video streaming?

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  • $\begingroup$ Hello Sumer, and welcome to ham.stackexchange.com! $\endgroup$
    – rclocher3
    Jan 26 at 17:57
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Baseband video is the DC to about 6 MHz signal that encodes the brightness channel, and sync. If it includes colour too then it's called Composite video.

Brightness and sync are truly baseband, a specific voltage range represents the black to white brightness over time as the beam scans the display, with sync represented by pulses to a different voltage. Colour is actually modulated onto a carrier, even at 'baseband'.

In the diagram you quote, the baseband is effectively the signal just after the detector block. In a receiver+display there's no need to generate a standards-compliant video signal.

IF, as you correctly surmise, contains the baseband video but modulated to some intermediate, frequency, perhaps about 50 MHz, suitable for further filtering and amplification before detection. It is solely a choice of the receiver designer, and is invisible from the outside. The receiver you're thinking of, takes in RF and produces baseband video.

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  • $\begingroup$ ok, so like the band width of voice signal is 4 KHz as I remember, the band width of video is 6 Mhz..nice, thnx $\endgroup$ Jan 27 at 7:23
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I think that Wikipedia article fragment that you quoted is misleading or incorrect, but I'm not an electrical engineer and I've never designed a cable TV box. The article implies that the cable TV box plugged into the customer's television translates the frequency back to the original frequency that was used before the signal for that channel was multiplexed with the signals for the many other channels.

I think that your intuition is correct. I don't see any reason that the customer's cable TV box should translate the signal back to whatever frequency was used as an intermediate frequency in the transmitting equipment. It's far more likely that the customer's cable TV box translates the frequency of the signal to an intermediate frequency that is convenient for the downstream electronic components that further process the signal and send it to the customer's television.

Of course it is possible that the IF used in the receiver is the same frequency in the IF used in the transmitting equipment, either because of coincidence or because that frequency is commonly used as an IF by the television industry. But the IF of the receiver was surely chosen by the engineers who designed the receiver, and not by the engineers who designed the transmitting equipment.

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The TV receiver reference you cite is for an (ancient) analog television. Since the migration from the analog NTSC to the digital ATSC standard for high-definition content, much has changed.

As you see in this diagram showing the connections to a modern cable TV set-top box, the RF signal coming from the service provider is converted into a wide range of signals to support connection to not only an HDTV monitor but also to A/V receivers, digital video recorders, etc. enter image description here

The "baseband" signal to which your ancient reference points is the "composite video" signal number 8 above. But, since the "Cable In" signal from the service provider is digital, the composite video signal must be separately produced from the digital information, not simply translated in frequency from the input. Note that this baseband signal is modulated onto Ch 3/4 to produce the "RF Out" signal 2 in the diagram.

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  • $\begingroup$ ok, so it was the base band of composite video which is 6 Mhz ,but also HD shouldn't be converted to a base band ? how much is it ? $\endgroup$ Jan 30 at 11:29

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