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.

[pbristow.livejournal.com]

Like they said, understandable but not *right*, darn it, and somebody somewhere should fix the problem.

Q1: Is it on all channels, or just specific ones?

Q2: If it's on one channel, does it tend to happen on particular programmes?

It could be the box at your end; it could be somewhere in the transmission from the cable provider, or it could be the broadcaster (e.g. Sky tend to skimp on their bitrates, the bathtubs!), or even a badly done encode supplied to the broadcaster by the programme makers. The above statistics will help narrow down which. Then talk to your provider about what they're gonna do about it.

My own NTL box gradually got so flaky that I just stopped using it. I've cancelled the TV service when I moved here, as I can get terrestrial digital here for free. It has the same break-up problem about once per hour, whenever a plane flies over coming in to or out of Stansted, but I can live with it.