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.

[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.

[zanda-myrande.livejournal.com]

It's mostly Sky-run channels (e.g. Sci-Fi, which I believe is run out of someone's back bedroom... ;) ) so it's likely the broadcaster. (They also skimp on things like making sure the programme's finished before running the continuity announcements, squeezing up the screen, et cetera...) I think they own everything right up to the box, so I probably shouldn't go messing with it (especially given my technical ineptitude), but when I get square with them again I'll ask about a replacement box just in case.

Many thanks for further enlightenment!

[deborah_c]

Interference will tend to produce blocks with entirely empty content, which generally turn up as bright green squares. If you're seeing those, you have a transmission problem. Excessive pixellation is generally an artifact caused by the bitrates being too low, and changing the box is very unlikely to help. Whinging at the provider may work, but I wouldn't hold your breath...

[pbristow.livejournal.com]

"Interference will tend to produce blocks with entirely empty content"

*Severe* interference will do that. Mild interference will cause occasional single-bit errors here and there, which, if they become frequent enough, will exceed the abilities of the error detection/correction, and have a similar effect to low bitrate. My understanding is that MPEG data is structured so that the most likely errors will tend to affect the least significant parts of the data (same as is done with audio CD encoding), so that up to a point, what you've got is still a rough approximation to the picture you should have been getting...*then* it all goes to hell.

Also, how the dead/suspicious blocks are rendered on screen is software dependant, so depends which box you're using.