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All of my TV's (even 6 yr. old CRT) have AV1, AV2, Component and Composite. What are these ports for? I note that only 2 of my TV's have HDMI connections. What is the purpose of the HDMI connection?

RF Connector

This is where one generally connects the coax from one's local cable company to the TV, and then use the TV tuner to select the channel. Old analog equipment, such as VCRs, would use a coax from their RF Output port to the TV's RF (or "cable") port but one would have to tune the TV to Channel 3 or Channel 4 (switchable in the VCR) to see the VCR menu or to watch a recorded show.

I don't recall DVD players having an RF Connector (the old ones might have, but I don't recall any that I owned having one). VCRs have (had?) RF connectors, one for signal in (for recording a TV show on an analog channel), and the other for output to feed the TV (for menus, payback, or even for watching a station with the VCR serving as a tuner).

Video resolution was restricted to standard-definition TV, sometimes noted as 480i (480 lines, interlaced, meaning that every other line would be painted as the electron beam scanned down the screen, and then the other lines painted, so the lines from two scans of the screen were "interlaced" on the screen).

Composite Video + Stereo Audio, or A/V

The composite video port, also known as the A/V (Audio/Video) port, has three RCA color-coded connectors, Red (Audio Right), White (Audio Left) and Yellow (Video composite). Using three cables meant that one didn't have to modulate the signal in the equipment (VCR, DVD player, game console, camcorder, Roku 1 & 2, etc.) nor demodulate it in the TV, so it was cheaper to add A/V ports to a TV than to build in a second and third tuner, and likewise the piece of equipment one was attaching could be less expensive without having to build in a modulator.

Video resolution was restricted to standard-definition TV (480i).

S-Video

An S-Video port has a mini-DIN connector with four pins in it, one for luminance (brightness), one for its ground, one for chrominance (color information), and its ground. I have read that this is a bit better than the composite connections, but I could never see the difference. It seems that S-Video is getting pretty rare. Some of my older equipment has S-Video connectors, but none exclusively so (always also had component or composite connectors along with the S-Video connector), though I imagine it is possible there was some piece of consumer electronics that was S-Video-only, even though I had not seen it..

The S-Video standard does not allow for sound, though sometimes a variant of S-Video will include sound.

Video resolution was restricted to standard-definition TV (480i).

Component Video + Stereo Audio

Component Video uses three RCA cables for video and two RCA cables for audio, again color-coded: Green ("Y" or luminance, i.e., brightness), Blue ("Pb" or difference between brightness and blue, Y-B) Red ("Pr" or difference between brightness and red, Y-R), White (Audio Left), and Red (Audio Right, not to be confused with the "Pr" connection).

An advantage of component over composite is that the color information doesn't have to be modulated onto the composite cable, saving another source of color distortion.

Component video is common with older DVD players and DVD players with progressive scan.

Usually video resolution is standard-definition TV (480i) and often "enhanced definition" TV (480p, i.e., progressive scan so each frame has all 480 or so lines scanned). Typically a progressive-scan DVD player can generate the 480p signal only on the component video cables.

The old Roku 1 units would send 480i during boot-up and then switch to 480p, and I think some even produced 720p over the component cables if one was viewing HD streams, but I couldn't find verification of that. The earlier Roku 2 models also had component video as well as composite, but the later Roku 2's and the Roku 3 do not have component.

HDMI

The HDMI (High-Definition Multimedia Interface) port is an all-digital, high-definition connection that can feed a HD TV in multiple video modes, including these common HD TV formats:

480p (640 pixels x 480 lines, progressive mode "enhanced-definition")

720p (1280 pixels x720 lines, progressive mode "high definition")

1080i (1920 pixels x 1080 lines, interlaced "high definition")

1080p (1920 pixels x 1080 lines, progressive mode "high definition")

One may notice that 720p, 1080i, and 1080p use an aspect ratio of 16:9, contrasted to the standard-definition or enhanced-definition aspect ratio of 4:3. (The 16:9 aspect ratio was chosen as a compromise to make acceptable use of the screen for widescreen movies without completely wasting the screen on the old TV shows, but that is another discussion.)

Several color spaces are supported.

Several digital audio streams are potentially supported as pass-through to the appropriate decoding equipment, the most basic being digital stereo.

Both video and sound travel through the HDMI cable. Also, HDMI supports HDCP (high-bandwidth digital bandwidth protection), which the studios use to enforce digital rights management (meaning it is used by the studios to prevent you from making copies of copyrighted materials)

At this time, HDMI and DVI (Digital Visual Interface, often as a computer connection to the TV to use the TV as a monitor) are the only ways to connect high-definition consumer devices to a HDTV and get a HD image. HD DVRs, blu-ray players, Roku Digital Video Players (various models), upscaling DVD players use the HDMI port to send high-definition images and sound to the TV. While some of this equipment may also have a composite and maybe component ports, these ports are usually limited in current consumer electronics (composite: 480i only; component: usually 480i and/or 480p, less often 720p), forcing higher resolutions to use HDMI with HDCP copy protection.

So the latest widespread accepted form of connection is the HDMI port because it handles high-definition video and various audio encodes, and likely to be the preferred way of connecting new devices to the TV for the foreseeable future. So if you are thinking ahead, you may want extra HDMI ports on your TV.

Other ports

A few TVs may have a USB port for a thumb drive (displaying pictures or downloaded movies, software updates), an Ethernet port (for accessing the Internet or one's LAN for connecting to streaming services such as Netflix, and for software updates), WiFi connection (again, accessing one's LAN or the Internet).
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