Friday, January 25, 2008

A primer on TV terminology

After reading discussion threads from many forums, it appears that a lot of people are unfamiliar with the terminology used to describe the different features of modern TVs. As a service to them, I present the following primer.

Audio terminology

Unbalanced Analog Unbalanced analog audio (often just referred to as analog audio) is the oldest form of audio input/output. Audio is carried on two wires: one carrying the signal, and one attached to ground. In TV and stereo applications, the connector is usually an RCA connector. By convention, most stereo audio cables use a red connector for the right-side channel and a white connector for the left-side channel.

Other applications sometimes use other connectors. For instance, computers usually use a 1/8" tip-ring-sleeve (TRS) connector that carries two channels of audio sharing a common ground. Musical instruments usually use a 1/4" 2-wire TRS connector (also known as a "TS" connector).

Balanced Analog Balanced audio is not often used for consumer devices, but is commonly used for professional audio. Audio data is carried on three wires: one carrying the signal, one carrying the inverse of the signal, and one attached to ground. Inside devices, the two signals are combined (inverting the inverse-signal), eliminating much nosie and interference from the audio.

Many different connectors are used for balanced audio. Microphones typically use a 3-pin "XLR" connector. Also common is a 1/4" TRS connector.

S/PDIF S/PDIF stands for "Sony/Philips Digital Interconnect Format". This is a mechanism for encoding multiple channels of digital audio (using multiple formats, including uncompressed PCM, Dolby Digital and DTS) into a single digital stream of bits. For consumer audio, S/PDIF data is either carried on 75-ohm coaxial copper cable with RCA connectors (usually called called "coaxial") or on optical fibers with TOSLINK connectors.

Note that RCA connectors are used on a wide variety of audio, video and data cables. These cables may or may not be coaxial, and have a wide variety of impedances. For this reason, you should make sure to use a cable designed for S/PDIF audio and not any old cable with RCA connectors that you may have on-hand. The wrong kind of cable may degrade the signal, resulting in dropouts or a total loss of audio.

When connecting devices together, they obviously have to use the same standard. Trying to send S/PDIF data to a device that expects analog audio will obviously not work (and may damage your equipment.) Modern televisions and home theater receivers usually support multiple audio formats.

In general, S/PDIF audio is recommended, whenever possible. Because it is digital, signal quality is usually much better than analog audio. The multi-channel capability of S/PDIF allows a single wire (or optical fiber) to carry all the channels of a surround-sound production. With analog audio, multiple connections are necessary to carry surround sound. (e.g. 6 RCA connectors or 3 TRS connectors.)

Video terminology

Interlaced A video format where each frame consists of two fields - one containing the odd-numbered lines and one one containing the even-numbered lines. Odd and even fields are alternated to produce the sequence of frames. The human eye usually does not notice the image alternating between the odd and even fields, but some people can notice flickering images or other minor distortions caused by interlaced video.
Progressive scan Also known as non-interlaced, progressive scan is a video format where each frame consists of only one field, containing all of the frame's lines. Progressive scan images often look better than their interlaced equivalents, eliminating the flicker that some people perceive.
NTSC Short for "National Television System Committee", NTSC is a system for encoding video data on an analog signal. It is the standard for analog video in the United States, Canada and a few other countries. Other analog video standards are used in other countries, including PAL and SECAM.

NTSC video has a vertical resolution of 486 lines (each frame of signal contains 525 lines - 486 containing the image, and 39 rows of "vertical blanking" which may contain non-image information.) As an analog signal, there is no precise horizontal resolution - the signal continuously varies across each scan-line.

Black-and-white NTSC signals are interlaced, with a refresh rate of 60 fields per second. Color NTSC signals use a slightly slower refresh rate (59.94 fields per second) in order to accommodate the slightly more complex signal in a way that is backward-compatible with black-and-white TV sets.

RF Short for "Radio Frequency", RF video is a video signal (analog or digital), combined with an audio signal and modulated onto a radio carrier. This is typically done for over-the-air broadcasts (receivable with an antenna), and for some other kinds of broadcasts (like cable TV). Many video-producing devices, like VCRs and set-top-boxes, may also generate RF video, in order to be compatible with televisions that have no other way of receiving a signal.

For analog video, RF signals tends to produce a low quality image. For digital video, RF signals look just as good as other formats.

ATSC ATSC is the "Advanced Television Systems Committee", an organization that defines digital television standards in the USA. All televisions capable of receiving a digital signal over the air (that is, with an antenna) in the US must have an ATSC-compliant tuner
QAM Digital cable TV systems do not typically use ATSC. They use a different standard called QAM. A television capable of receiving digital cable TV without a set-top box must have a QAM tuner. Cable companies tend to encrypt signals, however, so a QAM tuner by itself will typically not be able to tune in many channels.
CableCARD CableCARD is a device, typically leased from a cable TV provider, that, when installed in a TV with a QAM tuner, will decrypt the video content, eliminating the need for a set-top box, for many purposes.

CableCARD technology is still being developed, and is being improved. In its current form, CableCARD can not be used for two-way communication (where signals are sent back to the cable company). This means a set-top box is still required to use the interactive features of digital cable TV services (like video-on-demand.)

SDTV Short for "Standard Definition Television", SDTV defines several digital video formats that are similar to existing analog formats. In the United States, SDTV consists of the following formats:
  • 480i - This is 640x480, 704x480 or 720x480 pixels, interlaced, with a refresh rate of 59.94 fields per second (compatible with NTSC analog signals). 480i images may have a 4:3 aspect ratio or a 16:9 wide-screen aspect.
  • 480p - This is 640x480, 704x480 or 720x480 pixels, non-interlaced (progressive scan). 480p has three sub-variants: 480p24, 480p30 and 480p60, corresponding to refresh rates of 24, 30 and 60 frames (not fields) per second, respectively. 480p24 is commonly used for movies (since the original film was produced at 24 frames per second). 480p60 is often defined as an "EDTV" mode, because it uses twice the bandwidth of SDTV broadcasts.

Televisions sold as "SDTV" devices with digital tuners often have circuitry that can receive HDTV signals and display them at SDTV resolutions.

EDTV Short for "Enhanced Definition Television", EDTV is a marketing term commonly used to refer to non-HD televisions capable of displaying a 480p60 image.
HDTV Short for "High Definition", HDTV defines several resolutions and refresh rates that offer image quality better than SDTV and EDTV. In the United States, these are:
  • 720p - This is 1280x720 pixels, non-interlaced (progressive scan). 720p has three sub-variants: 720p24, 720p30 and 720p60, corresponding to refresh rates of 24, 30 and 60 frames per second, respectively.
  • 1080i - This is 1920x1080 pixels, interlaced. 1080i (aka 1080i60) has a refresh rate of 60 fields per second.
  • 1080p - This is 1920x1080 pixels, non-interlaced (progressive scan). 1080p has two standard sub-variants: 1080p24 and 1080p30, corresponding to refresh rates of 24 and 30 frames per second, respectively.

    Many HDTVs also support a non-standard sub-variant of 1080p60, which has a refresh rate of 60 frames per second. ATSC broadcasts never use 1080p60, but some other signal sources (like HD disc players and some video game consoles) offer it as an option.

Composite video In the United States, Composite video is NTSC analog video, transmitted without an RF carrier, over a wire. Consumer electronics devices almost always use an RCA connector for composite video. By convention, the connector is usually colored yellow.
S-video S-Video is NTSC analog video that has had its image-brightness data ("Luma") separated from its image-color data ("Chroma"). The luma and chroma signals are transmitted on separate wires. This produces an image somewhat better than composite video.
Component video Component video is an analog video format capable of carrying analog versions of all the SDTV, EDTV and HDTV resolutions (note, however, that not all TVs with component video inputs can display all these resolutions!). Component video signals are typically carried on three wires, using RCA connectors on each.

Two different signal formats are usually supported for component video: Y/Pb/Pr and RGB. RGB carries the red, green and blue image components on separate wires. Y/Pb/Pr is conceptually similar to S-video: one wire ("Y") carries image-brightness ("Luma") information, one ("Pb") carries the blue component of the color information and one ("Pr") carries the red component of the color information.

By convention, component video cables color the connectors red, green and blue. When Y/Pb/Pr format is used, the green connector is usually used for Y, the blue for Pb and red for Pr.

VGA VGA is a computer video format that is occasionally found on HDTV sets and some consumer video equipment. It uses an HD-15 connector to carry analog video. All of the video components (red, green, blue, horizontal sync and vertical sync) are carried on separate wires. VGA connections can support resolutions far in excess of HDTV standards (for example 2048x1536 progressive-scan at 75 frames per second), although not all displays with VGA connectors can display such images.
DVI DVI is a digital video format. The DVI connector carries one or two channels (also known as "links") of digital video information. One link is sufficient to carry the full range of HDTV resolutions, and most computer resolutions. When both links are used, the maximum resolution is doubled, allowing high resolution displays, like 30" LCD panels with their 2560x1600 resolutions.
HDMI HDMI is the current standard for getting digital video into HDTV sets. The HDMI connector combines digital video signals with digital audio signals and some remote-control features into a single convenient cable. HDMI is backward-compatible with single-link DVI video.
HDCP HDCP is short for "High-bandwidth Digital Content Protection". HDCP is a system for encrypting digital video and audio as it is transmitted between devices. The purpose is to prevent someone from recording the video and audio in violation of some interpretations of copyright law. High definition video players (including Blu-Ray Disc players and digital cable TV receivers) use HDCP when sending audio and video data over DVI and HDMI connectors. If the receiving device does not have an HDCP decoder, it can not display the data.

In consumer products, almost all devices with HDMI ports support HDCP. Devices with DVI ports may or may not support HDCP. This is very important, because a TV without HDCP capability can not display an HDCP-encoded signal.

Today, most displays sold as HD televisions support HDCP, but most displays sold as computer monitors do not. Note that I said "most" - there are exceptions in both cases. There are still some TVs that do not support HDCP, and there are some computer displays that do.


Anonymous said...


I only got a few sentences in when I noticed what I feel is an error: "S/PDIF data is either carried on copper wire with RCA connectors (usually called called "coaxial")"

RCA connectors are decidedly NOT related to any kind of coax connectors (F-type, BNC, etc.). RCA connectors/cables are typically referred to as "phono" jacks/cables.

Just thought I'd mention it, as I feel you might confuse some readers.

Thanks for the useful primer!

Shamino said...

You are correct that not all cables with RCA connectors are coaxial. S/PDIF over copper is supposed to use coaxial cable.

I mentioned the name "coaxial" because that's what is typically printed on the backs of DVD players, amplifiers and other devices that accept S/PDIF audio over copper.

I'll clarify the text of my article to avoid confusing readers. Thanks for helping me out.