RGB vs. Chroma: A call for more detail please!

[Bester]

Hello there,

I've been enjoying the guides and done a little bit of forum searching.

I see Scart gets dismissed quite quickly and to be fair it makes sense in a majority NTSC part of the world (I'm UK/PAL myself) but what surprises me is seeing Scart compared to Composite.

My understanding of a Scart cable is the Red, Green and Blue colour signals (RGB) are transferred on their own dedicated wire. The same for Left and Right Channel audio too. There are 21-pins on a Scart so there is plenty of room. Some even have S-Video's Luma and Chroma wired in. Using a quality Scart gives a noticeable improvement in image quality, especially the colours. I've known nothing else!

In fact, I used S-Video for the first time only recently (I had assumed it could carry audio ).

So, my question is, how can separating each colour signal (RGB) be considered inferior to Chroma (C) which (as I understand it) must combine all the colours into a single signal which must equal some signal loss?

Is RGB the baby in the bathwater?

Is this a case of Theory vs. Application?

[Selur]

my 2 cents:
Not an expert here (haven't actually captured VHS for 20+ years), but from what I remember VHS tapes store video in composite format, where brightness (luma) and color (chroma) are combined into one signal. SCART on the other hand can carry multiple signal types (composite, S-Video, RGB), but most VCRs only output composite over SCART, not S-Video or RGB.
Even though VHS is composite internally, when a VCR has an internal comb filter, it can separate luma and chroma before output. Outputting that separation over S-Video avoids recombining the signals, preserving better sharpness and color.
If you instead use composite (via SCART or RCA), the signals are recombined for output, then your capture card must try to separate them again which can lead to dot crawl, color bleeding, reduced sharpness.

Cu Selur
Ps.: someone more involved with VHS&co will probably correct me if I'm wrong.

[dpalomaki]

So simplify a bit: "C" often describes the composite video signal (often a yellow jack), a mix of the B&W (luma) and encoded color difference signal (chroma) on one wire. This is accomplished by having the chroma information tucked into a frequency range above the basic luma signal as an amplitude and phase modulated suppressed carrier RF signal centered on 3.8 mHz (for NTSC). This is similar to a broadcast TV signal The main problem with this scheme is the difficulty of separating the signals to create high quality R, B, & G signals used to drive a TV (or other) display. The TV decodes the video signal into a B&W (Y, a type of R+B+G) image and the chroma into color difference signals (R-Y and B-Y); then electronically adds these to produce R, B, & G).

Doing this well requires sophisticated (costly in the pre-HD days) circuitry and this was not often found in consumer gear, in part due to other compromises to save cost yet produce an image satisfactory to most viewers on gear available at that time at a consumer friendly price point. Part of the compromise is a reduction in image resolution that isn't obvious on smaller and low resolution screens.

VHS stores the luma and chroma as separate signals (although the chroma is much more bandwidth limited on consumer tape formats than a broadcast signal). The s-video connection should avoid a combination into a composite signal in the VCR and a separation step in the display (or capture device). This can better preserve the original image.

The separate RBG (or YUV, or YCrCb) outputs, AKA "Component," of some gear provide the separate color signals free of RF encoding and can result in better images on displays that accept them. Especially of the signal is a live camera output. It depends on the relative quality of the separation circuits in the VCR vs. display gear. Some display gear may have RBG inputs but no s-video input.

Today HDMI I/O dominates consumer gear. Composite, S-VIDEO, & Component are largely legacy.

[Bester]

Right, so if I understand this correctly yes, RGB can result in better images but the weak link is only displays that accept them. There is also the limitation of VHS itself storing luma and chroma.

It's already lining up for S-Video to be dominant default choice. I guess the market moved on in Europe before there really was any demand for fancy RGB Scart circuitry in VHS player outputs and corresponding receiver inputs.

[aramkolt]

I think the problem with SCART is that you don't know what connections it contains or outputs. I'm not aware of any VCR that actually outputs RGB over SCART, but I'm also not in PAL-land to have really tested any. Most likely it is just outputting composite only.

If you want to know for sure, just probe each of the R/G/B lines and see if anything is there at all during playback. My guess is not. You could also look inside the VCR and trace those pins back and my guess is that they aren't connected to anything at all which will also give you your answer. The few SVHS SCART VCRs I've seen usually have a separate S-Video output if they support S-Video output, but I can't say if there's consistency across brands about that in particular.

[Bester]

Yeah that is accurate. I'm having flashbacks to when I was buying Scarts. They all look the same but some just flat out lied and were not wired how they were advertised. You often had to buy the official cables and they had hefty price tags.

I never could be bothered to solder them myself I'm just not good at it. I've tried with other things.