Techie question...

[avevale_intelligencer]

We have cable TV, which is ace and brill and all that except when it goes wrong, which is often. My question is this. When I am watching a cable channel, is the image supposed to be semi-pixelated into roughly three-quarter-inch squares? (I say semi, because there is detail inside the squares, but it's very noticeable, especially during fast camera moves in dark bits, that the screen has these gridlines, and I'm sure they shouldn't be there...

Comments

[deborah_c]

MPEG coding divides the screen into 8x8 pixel blocks, further grouped into 16x16 macroblocks. Then it encodes each block by essentially doing a Fourier transform to find how intensities change across the block. The 8x8 frequency coefficients are then divided by a frequency-dependent function to reduce the number of bits used; the whole lot can also get scaled down if the encoder wants to use less bits for compression control. There's also encoding of deltas from frame to frame, both in spatial terms, and in time within a block.

The upshot of this is that if there's a lot of change going on, the higher frequencies within a block have a tendency to get discarded, and you get a block that loses detail, ultimately tending toward a uniform colour.

[zanda-myrande.livejournal.com]

Okay, I got almost all of that. :) So it's normal, and to do with the way in which the signal's coded for transmission rather than our telly or our box, and consequently not likely to go away any time soon. In that case, I can live with it.

Thank you for enlightening me!

[bedlamhouse]

Well, I'd call it "understandable" but not "normal".

This tells me that either at the cable company end or on your receiver box end someone isn't running fast enough OR the transmission medium is interrupted. On satellite, this occurs when there is interference.

It is a problem your cable company should fix, you shouldn't be paying for low-quality signal.

In the UK, do you own responsibility for the cable run inside your house, or does the cable company? If you own the run, you might want to see about getting high-quality connectors and some cable to do a test by bypassing your inside wiring (connect the new spiffy cable to the point where the cable company wiring comes to the house, then run it straight to the telly). If everything now looks great, the problem is in the inside wiring or connectors.