Watching satellite TV, you are indeed watching in lower resolution ; how low will depend in part on the DENC used in the sat receiver . The DENC (short for "Digital ENCoder") is the device containing the video DACs (Digital to Analogue Converters) that produce RGB & composite video from the *(digital) outputs of the MPEG decoder. In a consumer sat recvr, these DACs are usually only of 8-bit resolution . This means that they can only quantise their analogue outputs to 256 discrete levels. DVD players often use 10-bit DENCs, capable of producing 1024 different output levels. After the DENC there will be some filtering - which smoothes the output waveform somewhat - so your circles look less like cogs or gearwheels.
I would guess that the DVD player (with something like 500 line resolution) may have less filtering & will contain more real information than the sat receiver output.
The sat receiver gives a "softer" picture if it acts in a narrower bandwidth and may indeed smooth out some jaggies. However it won't contain as much real colour info as originally transmitted.
An analogy would be that turning the bass control up on a cheap audio receiver (radio) , actually can make a tiny loudspeaker produce what your brain considers to be more of a "big speaker" sound ; however, in turning up the bass, you will have cut off some of the treble and produced a an output sound less faithful to the original than if you had left all tone controls at their middle (or zero) settings.
You could try recording both to a normal VCR (which has even lower resolution) ; I would then expect the recorded DVD to playback with less jagged guitar strings (as well as a softer , bandwidth-limited, picture). You need your DVD player to defeat Macrovision for the recording to work, of course.
Sorry this sounds so long-winded - as I work as an applications engineer for an IC company that happens to make DACs & DENCS , spending much of my time working with video design engineers, I am not used to trying to explain this to "laymen".
Chris Muriel, (author of Digital Satellite FAQ), Manchester.
Thanks for the answer. If I get it right this means that technically the DVD picture is superior. The reason I see jagged lines is because there is less "smoothing" of the picture. The quality difference therefore is appreciated in the colour rendition. Next question: is the quality affected by my display? I am using a standard 4:3 TV. I notice the jagged lines on both 4:3 and 16:9 DVDs.
TV is standard 34 inch 4:3, about 6 years old
DVD players are Yamaha S700 region 1 and Sony S525 region 2.
satellite receiver is Multichoice DSTV 720i (South African)
I use composite (yes I know that's pathetic!)
I've tried a Profigold 15m S-video cable side by side with cheapo composite cable of same length and tried desperately to see a difference in picture quality but couldn't. I can't use shorter lengths due to the way my equipment is arranged. I've put the lack of quality difference down to the quality and length of the cables used. Perhaps with a better monitor (projector or plasma) the difference would be more noticeable. I'm sure I've set up my DVD players correctly because when using the S video inputs on the TV it clearly displays S video, but doesn't when using composite.
Virtually all DTTV & DVB satellite boxes use lower grade DENCs
(video DACs) .
Their DENCs are usually integrated into the mpeg decoder IC.
The majority of DVD players use better (higher resolution) DENCs .
In most cases they ar separate DENC chips (not integrated into the MPEG chipset).
The very best actually ***sell*** some of the features of the video encoder DACs .
Some have "NSV" or "Noise Shaped Video" written on the case.
NSV is a filtering feature inherent in better DENCs, along with anything from 4x to 16x oversampling.
Martin in your opinion would changing my Amstrad digibox (that Sky installed upon installation) to a different more expensive model give me any improvment in picture quality? If not what other benefits are there?
Well I am not an expert but Amstrad are not a good make, 34" 4x3 TV - well now I know what is happening
DTV getting 4x3 on 4x3 TV so edges don't change
DVD - most DVDs are mastered for Widescreen so if you use 4x3 letterbox the DVD player throws away every 4th line to change the aspect ratio. If your TV has a wide mode try using that with 16x9 on the DVD player