Quick answer: Composite and S-Video are standard definition analogue feeds, so an AV to HDMI box has to deinterlace them and scale the result to a fixed HDMI rate. The box, not the panel, decides whether the outcome is usable. Sound travels on separate red and white RCA leads because neither video format carries audio, and a long thin input lead adds dot crawl and colour bleed before the signal ever reaches the converter.
The old camcorder, the disc recorder or the retro console is still in the cupboard, and the only free socket on the television is HDMI. A yellow plug to HDMI plug is not a conversion, it is two connectors with nothing between them. The real product is a powered box that reads an interlaced analogue signal and writes a progressive frame, and the quality of that translation is the whole purchase.
Composite and S-Video are not the same input
Both carry standard definition video and both are interlaced, but they suffer differently. Composite mixes brightness and colour onto one conductor, so the receiver has to separate them again. S-Video keeps brightness on one pair and colour on another, which removes a whole class of artefact before the box starts work.
One conductor against two
On composite, colour rides on a subcarrier that sits close to the fine brightness detail. Separating the two is never perfect, so fine patterns pick up a rainbow edge. That artefact is created at the source, and no converter downstream can take it out again.
Why S-Video keeps colour apart
With chroma already on its own pair, the box only has to digitise and deinterlace. Edges stay cleaner, small text is more legible, and the difference is obvious on any menu screen. Where the source offers both outputs, S-Video is the one to use.
The interlace and scaling limit
Interlaced video arrives as two half frames captured at different moments. The box has to decide how to combine them: average them and moving objects blur, keep one field and half the vertical detail goes, detect motion and the chipset cost rises. Whatever it decides, it then stretches a small raster across a much larger one.
| Source format | Lines it carries | Scaling the box performs | Failure that shows |
|---|
| Composite 480i | 240 per field | Weave, then upscale to 1080p | Rainbow fringes on fine patterns |
| S-Video 480i | 240 per field | Weave, then upscale to 1080p | Comb teeth on fast movement |
| Composite 576i from a deck | 288 per field | Weave, scale, change frame rate | Judder on horizontal pans |
| 240p from a retro console | 240 progressive | Box treats it as interlaced | Every other line dropped |
Audio rides on separate RCA leads
Neither composite nor S-Video carries sound, so the box has its own audio input, usually a pair of RCA sockets or a single 3.5 mm jack. Those leads are part of the purchase and part of the run. A box with nothing plugged into its audio input produces a picture with silence, which is the correct result rather than a fault.
Gain mismatch on the analogue side
A recorder outputting a low line level into a box expecting a hotter one gives quiet audio that the television has to make up for, and the noise floor comes up with it. If the box has a level switch, set it once and leave it. If it does not, use the source output that matches rather than adding gain later in the chain.
The input cable adds its own damage
Why colour fades before brightness
Composite is the sensitive case. Its colour subcarrier sits high enough in the band that cable capacitance attenuates it faster than it attenuates brightness, so a long run reduces colour amplitude and smears edges. The result is the crawling pattern known as dot crawl, plus soft, washed out colour.
Keeping the analogue section short
Two metres of shielded lead is not a problem. Fifteen metres of the same lead, run beside mains cable, is where the artefacts dominate the picture. Keep the analogue part short, use a lead with a proper shield, and let the long part of the run be HDMI on the far side of the box.
Practical scenarios
- Camcorder into a modern TV: use the S-Video output if the camera has one, and expect interlaced footage to look soft at any size.
- Retro console on a 4K panel: a 240p source needs a box that handles progressive input, or half the lines are discarded.
- Picture but no sound: the red and white plugs are missing, or they sit in output sockets instead of the input pair.
- Crawling dots along edges: shorten the composite lead and move it away from mains wiring before blaming the converter.
FAQ
Will the picture look better on a smaller screen?
Yes, in practice. Scaling spreads the same limited detail across fewer pixels, so artefacts that are obvious on a large panel become far less visible on a small one.
Can I connect composite and S-Video at once?
Most boxes take one video input at a time and switch to whichever is detected. Connecting both usually leaves the unit locked to one, so use the better format alone.
Why does the image roll with a game console?
The console may output a timing the box does not recognise, often a non standard line rate. Setting the console to its standard output mode is the first thing to try.
Next step
Compare cable against extender runs before the HDMI side gets long, check class and build notes in the HDMI buying guide, and read the optical cable guide if the display sits across a room.
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