How analogue television carries an image

The path at a glance

Here analogue means that picture information is represented by a voltage that changes continuously with time, rather than by a sequence of stored numbers.

The system turns each picture row into an electrical waveform, adds colour and timing, places that waveform on a radio carrier, and reverses the process in the receiver.

A picture source scans or supplies each row as changing electrical values over time.
Y′ carries a complete black-and-white picture; two colour differences describe the colour to add to it.
Composite video carries picture and timing together, then controls a radio-frequency carrier.
The receiver performs those changes in reverse and uses timing to scan the rows onto the display.

A picture becomes a sequence in time

A television system does not send a complete rectangular picture all at once. It reads one horizontal row from left to right, then the next row, until the frame has been covered. Position across a row therefore becomes time in an electrical waveform: a different part of the picture is represented at each successive moment.

Reading the picture and generating timing are coordinated operations. The picture branch supplies changing image level; the timing branch supplies the reference that lets the receiver find each line. Neither is derived from the other.

This serialization is the first idea to keep in mind throughout the article. Every waveform below describes what happens while the system moves through a row of the picture. Synchronizing and blanking intervals tell the receiver where rows and frames begin; active-picture intervals carry their visible content.

Conventional analogue television normally transmitted each frame as two interlaced fields: one contained alternating rows, and the other filled the gaps between them. The diagrams use consecutive rows because that is easier to follow; the representation changes are the same.

The signal changes representation

Analogue broadcasting does not send three RGB voltages through the air. The same serialized picture passes through several representations: encoded colour components, composite baseband video, a modulated radio-frequency signal, recovered baseband video, and decoded colour components.

The picture information persists through the chain, but the physical signal carrying it changes. A video voltage exists in the cable; a modulated electrical signal feeds the transmitting antenna; an electromagnetic field crosses the space between antennas.

From R′G′B′ to luma and colour difference

The simple idea is to carry a complete black-and-white picture together with two additional colour instructions. Their technical names are luma and colour differences. PAL and NTSC carry both colour coordinates in every active line; SECAM alternates them between lines and recovers the missing coordinate from the adjacent line.

R, G, and B mean red, green, and blue signal values. The prime mark in R′G′B′ says that a specified transfer function has already reshaped them for the signal; they are not values directly proportional to scene light. At each moment within the row, a weighted sum forms luma Y′: the signal that contains the complete black-and-white picture. Subtraction then describes colour relative to that grey base level:

Imagine that Y′ is the grey level already carried for one picture point. If red and blue equal that level, both colour instructions are zero and the point is grey. Raising red above the grey level makes the red difference positive; lowering blue below it makes the blue difference negative. The receiver later adds those signed differences back to the same grey base. The equations simply perform that operation for every point along every row:

\begin{aligned} Y^{\prime} &= K_RR^{\prime}+K_GG^{\prime}+K_BB^{\prime} \\ D_B &= B^{\prime}-Y^{\prime} \\ D_R &= R^{\prime}-Y^{\prime} \\ K_R+K_G+K_B &= 1 \end{aligned}

The three coefficients specify how strongly red, green, and blue contribute to Y′. Their sum is one, so equal R′, G′, and B′ values remain the same neutral level in Y′. When both colour differences are zero, the sample is neutral grey. The sign of either difference only says on which side of that grey reference the colour lies; it does not mean negative light.

Together, the two differences locate colour around that grey reference. Their direction is related to hue: changing their proportion and signs selects a different kind of colour. Their distance from the centre is related to colour strength: small differences lie near grey, while larger differences describe a more strongly coloured sample. Y′ separately moves the sample lighter or darker. The exact axes and scaling depend on the television system.

Swipe horizontally to inspect the diagram

The named hues illustrate different directions at a fixed luma; their positions are not universal standard coordinates. NTSC, PAL, and SECAM scale and orient their axes differently, but all need two numbers to locate colour away from neutral grey.

This arrangement preserves a monochrome-compatible picture in Y′ and lets the colour information use less bandwidth: a smaller range of signal frequencies. Human vision notices fine spatial detail more strongly in luma than in colour. The two colour differences are later encoded into chroma—the colour-carrying part of the composite signal. Chroma can therefore be made less detailed while the picture still appears sharp. The coefficients and the scaling of the two differences are properties of the named system; the equation above shows the shared structure rather than one universal matrix.

Reduced chroma bandwidth makes fine colour transitions softer. Unequal filtering or delay between the luma and chroma paths can additionally shift colour relative to brightness. Later effect articles use these separate causes to explain colour softness, coloured fringes, and luma–chroma misregistration.

A monochrome receiver uses the luma component of the complete composite waveform without reconstructing the colour differences. A colour encoder transforms those two differences into the chroma form required by NTSC, PAL, or SECAM.

For a neutral grey sample, R′ = G′ = B′. Both colour differences are then zero, so the active-picture part contains only Y′. A monochrome receiver still accepts the same complete composite waveform as a colour receiver. It uses the Y′ picture but does not reconstruct the two colour differences, so the result appears in black and white.

PAL, NTSC, and SECAM carry colour differently

Chrominance is produced from the two colour-difference coordinates, but the three colour families do not place it into composite video in the same way.

PAL and NTSC carry colour by changing the strength and timing of a rapid oscillation around Y′. Stronger oscillation generally represents stronger colour; its position within the cycle represents a hue-related direction. SECAM instead changes the oscillation frequency and sends the two colour differences on alternating lines. The names below describe how each family implements that shared job.

How picture content appears in the signal

The following rows isolate the two variables that are easy to confuse. Each strip is one picture row, and each graph runs through that row from left to right. First, one hue is made darker or lighter. Then colour changes while Y′ is deliberately held constant. The final row lets both vary together. For these examples, the chroma trace is a PAL/NTSC-style oscillation encoded from the two colour-difference coordinates introduced above.

Swipe horizontally to inspect the diagram

The strips use sRGB so their colours can be displayed in a browser. The signal traces use an illustrative PAL/NTSC-style colour-subcarrier scale, not the calibrated levels or transfer function of one broadcast standard. In both PAL and NTSC, luma sets the centre level, chroma oscillates around it, and composite video carries their sum. SECAM is different: it alternates frequency-modulated colour differences by line, so these chroma traces are not SECAM waveforms.

How each colour system forms chroma

For PAL and NTSC, the bridge from two colour coordinates to one chroma waveform is quadrature modulation. One coordinate controls a cosine component and the other controls a sine component 90° away. Adding the two components produces one oscillation; its magnitude and phase jointly preserve both coordinates:

C(t)=D_1(t)\cos(\omega_c t)+D_2(t)\sin(\omega_c t)

Here D₁(t) and D₂(t) are the two slowly changing colour coordinates at the current position in the picture row. t means time, and ωc sets the rapid colour-subcarrier oscillation. Cosine and sine are two copies of that same oscillation shifted by one quarter of a cycle. Scaling and adding them produces the single chroma waveform shown below.

Swipe horizontally to inspect the diagram

In NTSC and PAL, two slowly changing colour coordinates scale cosine and sine versions of the same subcarrier. Adding those perpendicular components produces one oscillation whose magnitude and phase carry both coordinates. This is a coordinate construction, not two separate radio carriers.

The symbols differ because each family scales or rotates the same general colour-difference idea for its own encoder. They are related representations, not additional independent colours.

Family	Colour coordinates	How composite carries them
Shared idea	`R′ − Y′`, `B′ − Y′`	Two differences relative to the monochrome picture
NTSC	`I′`, `Q′`	Rotated and scaled QAM axes
PAL	`U`, `±V`	Scaled QAM axes; the sign of V alternates by line
SECAM	`D′R`, `D′B`	One frequency-modulated difference on each alternating line

NTSC places two colour axes on cosine and sine versions of one colour subcarrier. In engineering terms this is QAM: the oscillation’s angle carries hue-related information and its magnitude carries saturation-related information.
PAL uses the same general QAM method, but reverses one component on alternate lines. A compatible decoder compares the line sequence to reduce a consistent transmission phase error; it cannot undo arbitrary waveform distortion.
SECAM sends one scaled colour-difference component per line using FM. D′R and D′B use different nominal subcarrier frequencies and alternate by line. The receiver uses a one-line delay to combine the current component with the other component recovered from the preceding line.

Picture and timing form one complete line

During active picture, PAL and NTSC can be understood schematically as luma plus a modulated chroma subcarrier:

v_{\mathrm{active}}(t)=Y^{\prime}(t)+C(t)

That is only the active part of a line. A timing generator supplies horizontal synchronization and blanking; these do not come from the RGB components. The encoder joins timing and active picture into one complete waveform. PAL and NTSC also add a colour burst reference after the sync pulse so the decoder can recover subcarrier phase and amplitude. SECAM keeps the common line timing but uses line-sequential FM chroma and its own chroma-reference conventions.

Normalized level relationships and functional timing; interval widths are not to scale. The line continues from the front porch into the next sync pulse. PAL and NTSC use a colour burst on the back porch. SECAM retains the sync, blanking, and active-picture structure but carries and references colour differently. Black is drawn slightly above blanking so the two concepts remain visible; they coincide in many 625-line systems.

The diagram begins with horizontal sync because that makes the structure easy to read. A continuous signal has no privileged starting point: the front porch drawn at the right belongs immediately before the next sync pulse. Sync, back porch, active picture, and front porch repeat once per line.

Three jobs outside the visible row

The intervals around active picture solve three different problems.

Blanking creates time for retrace. The source and display scan must move from the end of one row to the start of the next. The signal suppresses picture content during this horizontal blanking interval, which contains the front porch, sync pulse, and back porch. Vertical blanking similarly surrounds the field transition and carries the vertical synchronizing pattern.
Sync marks position. The horizontal sync pulse reaches a level outside the picture range. A receiver can therefore recognise it as timing rather than mistake a dark part of the image for the end of a row. Field sync tells the vertical scan when to start the next field.
The porch supplies a reference interval. After horizontal sync, the back porch returns to blanking level before active picture begins. PAL and NTSC place colour burst on part of this porch. The receiver can use the surrounding porch as a level reference and the burst as a colour-subcarrier reference.

Blanking level is the electrical reference used during the blanking interval. Black level is the active-picture value intended to reproduce black. In many 625-line systems black and blanking coincide. Historical NTSC studio practice in the United States commonly placed black at 7.5 IRE while blanking remained at 0 IRE; on that waveform-monitor scale, peak white is 100 IRE and sync tip is −40 IRE. NTSC without setup places black and blanking together. That distinction matters: blanking describes a non-picture interval, while black is still picture information.

It is useful to describe levels relative to blanking instead of attaching one voltage scale to every standard. Picture extends from black toward peak white; the synchronizing pulse extends in the opposite direction. A common 625-line composite interface represents peak white about 0.7 V above blanking and sync tip about 0.3 V below it, producing roughly 1 V peak to peak. Other systems and interfaces specify their own timing, setup, and amplitudes. The figure therefore shows normalized relationships rather than claiming one universal calibration.

How the receiver finds timing and black

After AC coupling and demodulation, the recovered waveform may ride on a changed DC offset or slow drift. A clamp uses a known part of the blanking interval—often the back porch—to restore a stable reference before picture levels are interpreted. It does not create black detail; it decides where the received level scale sits.

A sync separator does not measure picture brightness. It detects the pulses beyond the picture range and turns their leading edges into timing events for the horizontal and vertical scan circuits. The blanking interval then prevents the retrace from becoming visible while those circuits return the scan to its next starting position.

These operations are related but not interchangeable. Moving blanking level changes the reference against which picture values are recovered. Changing sync depth changes the margin with which timing pulses can be detected. A later effect article will vary those quantities separately and state the assumed receiver response; this Principle keeps the correctly formed signal as its subject.

Because Y′ establishes the instantaneous base level and chroma oscillates around it, a nonlinear operation on the complete waveform can alter colour in a way that depends on luma. The transmitter or receiver is not changing an independent “brightness channel” after colour has been added; it is acting on their shared composite excursion.

From composite video to a radio channel

Composite video is baseband voltage: the picture waveform before it has been placed at a radio-channel frequency. Put simply, the transmitter turns changes in this video voltage into changes in a radio wave; the receiver performs the reverse conversion.

The two similarly named carriers belong to different levels. The colour subcarrier is the fast colour ripple already inside composite baseband video. The much faster vision carrier belongs to the assigned radio channel. It is the unmodulated RF reference whose amplitude the picture waveform controls.

The transmitter uses it to modulate a vision carrier at the assigned television channel frequency. For amplitude-modulated vision the idealized RF signal is

s_{\mathrm{RF}}(t)=A\!\left(v(t)\right)\cos\!\left(2\pi f_v t\right)

Here v(t) is the instantaneous composite-video value, A(v) turns that value into carrier amplitude, and f_v is the vision-carrier frequency. The cosine is the rapidly alternating carrier itself, while A(v) forms its envelope. With negative modulation, a greater composite-video level produces a smaller RF envelope.

The picture information has not been encoded into another kind of picture at this point. The same complete composite waveform now controls the envelope of a much faster RF oscillation. The RF carrier moves that information into an assigned radio channel so an antenna can radiate it.

In the common negative vision modulation convention, increasing picture level reduces carrier amplitude. A conventional negative-modulation signal therefore places synchronization near high carrier amplitude and peak white near low carrier amplitude. Some systems use positive modulation instead; colour family and RF polarity are separate choices. For example, System L paired SECAM colour with positive vision modulation.

The resulting modulated vision signal is filtered for VSB transmission, combined with the associated sound signal, amplified, and delivered to the antenna. NTSC, PAL, and SECAM describe the colour formation; the complete television system specification also defines line and field timing, channel bandwidth, vision modulation, sound, levels, and RF relationships.

The receiver reverses the representation changes

The radio channel transports an electromagnetic field, not composite-video voltage. A receiving antenna converts part of that field back into an RF electrical signal. The tuner and intermediate frequency stages select the channel, a vision demodulator recovers composite video, and the colour decoder separates Y′ and chroma before reconstructing R′G′B′.

How the receiver separates luma and colour

In PAL and NTSC, Y′ and chroma arrive as one summed waveform, not on two separate wires. A basic decoder uses low-pass or notch filtering for luma and band-pass filtering around the colour subcarrier for chroma. That simple split loses or confuses detail where their frequency content overlaps. More capable comb filters compare the repeating structure of neighbouring lines or fields, which can separate overlapping luma and chroma detail more accurately. SECAM similarly separates luma from its frequency-modulated chroma before its one-line memory combines the alternating colour differences.

No separator is perfect. Fine monochrome detail near the colour-subcarrier frequency can be mistaken for colour—cross-colour. Chroma left in the luma estimate can appear as a fine moving pattern—cross-luminance or dot crawl. These are separation errors: they can couple visible brightness and colour even when the original Y′ and colour differences were formed correctly.

How the receiver recovers RGB

Only two colour differences are needed. Once the colour decoder has recovered Y′ and those two differences from composite video, its matrix adds them back to recover red and blue. Green has not been discarded: the same luma equation determines the remaining component:

\begin{aligned} R^{\prime} &= Y^{\prime}+D_R \\ B^{\prime} &= Y^{\prime}+D_B \\ G^{\prime} &= Y^{\prime}-\frac{K_R}{K_G}D_R-\frac{K_B}{K_G}D_B \end{aligned}

That recovered sequence still has to become a picture again. Sync controls the horizontal and vertical display scan, while the decoded video continuously controls brightness and colour as the scan moves. A classic CRT receiver does not first store a complete digital frame:

The recovered timing locates each scan line. The coloured blocks mark successive moments in the continuously changing video drive, not stored digital pixels. In a classic CRT receiver, scanning and video drive happen continuously: the set does not first store a complete digital frame.

The simplified active-picture path can now be inspected without skipping its representation changes. The explorer starts with one encoded source colour, forms Y′ and the two system-specific colour coordinates, shows composite video controlling an idealized RF carrier, detects its envelope, recovers composite video, and ends with the decoded colour. Line timing, VSB filtering, the radio channel, tuner, and IF stages remain in the surrounding article rather than in this interactive model.

First choose Neutral grey to set R′, G′, and B′ to the same value: both colour differences fall to zero. Then choose Saturated colour or move one component and watch luma and chroma change together. The final swatch uses an ideal decoder; SECAM reconstruction includes the other colour difference retained from the adjacent line.

Signal explorerOne colour through the broadcast path

Colour encodingNTSC

Vision AM

Historically used colour / AM combination

Encoded source colour R′G′B′R′0.50G′0.50B′0.50

Try

Raster lineSame colour encoding on n and n + 1

Y′0.500luma

I′0.000colour difference

Q′0.000colour difference

Colour carriageQAM chromafixed quadrature axes

Swipe horizontally to inspect the diagram

RF and colour-subcarrier cycles are slowed so their relationship can be seen. The two thin curves around the RF cycles show its envelope; they are guides, not a separate transmitted signal. The model forms the RF carrier, measures its magnitude with an ideal detector, and reverses the selected mapping to recover composite video. Colour encoding and vision-modulation polarity are independent choices.

IDEAL COLOUR DECODERRecovered composite → Y′ and colour differences → R′G′B′

RECOVERED COLOUR(0.500, 0.500, 0.500)

An ideal linear chain preserves the intended relationships. Real stages can change amplitude, phase, timing, or frequency response:

formation errors alter sync, blanking, luma, or encoded chroma before RF modulation;
transmitter nonlinearities and VSB response act on the formed signal and its spectrum;
propagation adds noise, interference, fading, or delayed copies of the RF field;
receiver selection, demodulation, and colour decoding can introduce their own errors.

Why luma can change decoded colour

The following names classify where later effect articles disturb the signal; they are not required to understand the main path above.

In PAL and NTSC, differential gain changes chroma amplitude according to the underlying luma level, so saturation varies across the picture. differential phase rotates chroma phase according to luma, producing a hue error in NTSC and a PAL-specific residual after its line-alternating correction.

SECAM carries chroma frequency rather than QAM phase and amplitude, so those two measurements do not transfer literally. Limiting, discriminator response, pre-emphasis, line switching, and delay-line behaviour give SECAM its own failure patterns.

All three systems can still suffer when the complete composite waveform is driven outside a permitted boundary. The Overmodulation article follows that case from composite excursion through RF envelope limiting or inversion to the decoded image.

The whole path in four steps

A camera or another picture source provides changing image signals.
Y′ carries the monochrome picture, while chroma adds colour.
Sync and blanking identify the structure of each line and frame.
The transmitter places composite video on an RF carrier; the receiver reverses the process.

Optional detail: waveform and channel spectrum

A waveform shows how voltage changes with time.
A spectrum shows which frequencies make up that waveform.
Modulation moves the baseband spectrum to the vision-carrier frequency; VSB filtering keeps one main sideband and only a vestige of the other.

Ordinary amplitude modulation creates two mirrored sidebands carrying the same baseband information. Television video occupies a wide frequency range, so reducing one sideband saves channel width. Retaining a small vestige instead of removing it completely makes the lowest picture frequencies easier to recover.

Composite video first controls the vision carrier in time. Filtering then leaves a vestige of one sideband and most of the other; the associated sound occupies a separate carrier. Exact spacing, polarity, and bandwidth belong to the named television system.

The path at a glance

A picture becomes a sequence in time

The signal changes representation

From R′G′B′ to luma and colour difference

PAL, NTSC, and SECAM carry colour differently

How picture content appears in the signal

How each colour system forms chroma

Picture and timing form one complete line

Three jobs outside the visible row

How the receiver finds timing and black

From composite video to a radio channel

The receiver reverses the representation changes

How the receiver separates luma and colour

How the receiver recovers RGB

Why luma can change decoded colour

The whole path in four steps

Optional detail: waveform and channel spectrum

References