Cable programming. What kind of compression they using? (1 Viewer)

[Galvin]

For movies mainly. Some movies look good and some real bad.

It can't be real DVD, because DVD doesn't have flaws that I see all the time when I watch movies on cable mainly Showtime since it's all I have.

Flaws include, one part of the image floating while another is not. For exmaple in one movie, a characters face one side shadowed the other side not. one side of his face around the eyes was floating and not moving with the rest of his face.

In other things like scene with a table, the table might jitter while the rest of the room does not.

Distortion it's real bad with some movies, the newer ones not as bad. Night time scenes with shadows, colors don't blend, it's almost looking at a high color photo in 16 colors. If you can get the idea.

Overall I imagine the cable companies take the movies and compress them so they can fit more on a disk. Problem is you lose picture quality in the process. I even see this on Pay per view and it really sucks at times.

I wanted to know if cable networks are doing anything about this.

Thanks

 

[Matt Pasant]

I learned this in college and heard this from industry people, but I believe the rule of thumb with digital cable is 12:1 compression.

 

[Derek Miner]

What are you watching on? Digital cable? Analog cable? C-band satellite? It could make a difference.

 

[Todd Hochard]

Typically, digital cable allots 2-3Mbps for stuff like shopping channels, around 4 for the "regular" stuff, 5-6 for HBO/Showtime, and 5-6 (sometimes slightly higher) for PPV.

Generally, they use MPEG-2 just like DVD. Trouble is, it's real time, single pass encoding, done "on-the-fly," so there aren't any sort of variable-bit-rate, multi-pass tweaks that are done for DVD.

The best approximation of what's going on can be done by looking at what TiVo or Replay does to the signal at various quality levels. On these boxes, high quality runs about 6Mbps, and standard/low quality is about 2Mbps. That's the best do-it-yourself way of looking at it that I can come up with.

I've noticed that Showtime (all channels) always seems to be lower quality than HBO. The exception is HD- when Showtime does real 1080i, it's done right!

Todd

 

[Galvin]

Digital Cable. But like you said it's not as a good picture. Hell, even videotape is clearer than some of the movies they show. Too bad we pay all this money for digital and the picture is no better except for mainly the shows, like Stargate, Jerimiah, etc.

 

[Martin Fontaine]

Yeah, the Non Variable Bitrate is correct, since the feed has to be constant...

One thing that puzzles me, when I change channel, the screen is black during the change but as soon as it locks into the new channel, the image appears sort of like in blocks from top to bottom (It usually starts at some random position, goes to the bottom of the screen and then starts to the top then back to where it started) Kinda like one of those transition effects you can use in PowerPoint. Then after this is finished, sound kicks in. It doesn't bother me or anything, I'm just wondering why it's doing this.

The way I understand MPEG compression, is that there are Key Frames (Also called GOP in MPEG Encoder programs) which are basically the whole image in a JPEG-Like format, typically twice per seconds, they they have a sub-key frames (Also called Sub-GOP) which are more information but not a full refresh I think, typically every 3 or 4 frames and the rest are just changes from the previous frame.

If that was the case, wouldn't the image appear in one shot when I change channel? The same problem occurs when reception is bad (Like when a bird lands on a dish) where there is random garbage appearing and then it gets cleaned-up a few seconds later in the same way.

Any experts know why it does this?

 

[Michael St. Clair]

It all varies...some digital cable is compressed less, as low as 6:1...less than digital satellite compresses.

Also, sometimes the compression is at the head end...sometimes the signal is compressed before the cable company even gets it...in certain cases you may see typical MPEG-2 compression artifacts on ANALOG channels.

My digital cable looks good (better than current DISH), but not as good as it can look.

Frankly, nothing but OTA and BUD and HDTV looks great on a big screen TV. Everything else that is digital (DirectTV, digital cable) is too compressed. Watch a giant screen on a BUD feed sometime and you might get sick and angry.

 

[Galvin]

Too bad HDTV is still in the 1000 dollar range. HDTV with high prices still considered a luxery to me.

When I can buy an HDTV set that at least as big as my 21" tv for under 300 bucks i'll get one.

 

[Michael St. Clair]

Galvin,

Those of us with HDTVs still suffer through terrible picture quality for lots of our programming...less than half of what I watch is broadcast in HD.

 

[Robert Cranwell]

For Digital Cable the compression can be anywhere from 6-1 for premium channels HBO,Showtime,etc. To 12-1 for Discovery, Sundance, and Encore channels. HDTV is a 256Qam of 2-1 compression.

Your still using a 6mhz channel for the compression formula,so I don't get the mbps rule comes from? Alot is based upon compression for action movies and black & white programs, so it is determined upon programing.

Rob

 

[Wayne Bundrick]

Martin, there are three types of MPEG frames. Intracoded or I-frames are complete pictures like a JPEG. The other two types, P-frames and B-frames, contain motion data. However, P-frames and B-frames can also contain blocks of fresh intracoded picture data as needed. What you could be seeing is your decoder displaying these blocks as soon as they arrive instead of waiting for the next I-frame. There is a strategy called progressive refresh which intentionally includes intracoded blocks in P-frames and B-frames, so that over a sequence of several P-frames and B-frames the whole image has been refreshed before the next I-frame arrives.

 

[Todd Hochard]

HDTV is a 256Qam of 2-1 compression.

What the heck does that mean?

And where are these 2-1, 6-1, 12-1 coming from? 12-1 compared to what? Not compared to uncompressed.

???

 

[Michael St. Clair]

What the heck does that mean?

The 'compression ratio' that is bandied about for digital broadcasting (especially QAM256 digital cable) is somewhat of a misnomer...it related to the relative bandwith consumed compared to a 6mhz analog channel.

2:1 compression for HDTV means that each regular analog channel that everybody watches today could be replaced by 2 HDTV channels. Yep, full bitrate HDTV takes half the space of a regular analog broadcast. Likewise, an analog channel can be replaced by 6-12 'regular' digital channels...6 looks very good, 12 looks awful. So, each HDTV channel uses the same amount of space as 3-6 Standard Definition digital channels.

As far as bitrate goes, if a cable company is using 6:1 compression, you can expect their digital cable looks better than DirecTV or DISH. If it's 10:1 or 12:1, the picture will look about the same as the satellite providers.

 

[BobV]

A couple of points...
Now, my comments come from the digital satellite side of things, and from Canada, so take these comments with a grain of salt, but it should be pretty similiar.

Yeah, the Non Variable Bitrate is correct, since the feed has to be constant...

If digital cable is run the same way digital satellite is, which I would think is highly likely, it actually is a variable bitrate, it's called using a 'statistical multiplexor' or 'statmux'. What this does is prioritize high action scenes with more bandwidth per the services broadcast in the same frequency range (those compressed together). For instance, if you have, say, a 10:1 compression in a certain frequency and 8 of those channels are talking head channels (news, etc), and 2 of them are movie channels... when there is very little movement in the movies, all channels receive about the same bandwidth (bitrate), but when the movies go into a high action scene, the statmuxes autmotically, on-the-fly, attribute more bandwidth to those two channels, at the sacrifice of the talking head channels (because of their need at that time for a low bitrate). This is used by both digital satellite companies in Canada, and the US, so therefore I could only assume digital cable companies as well.
For those is Canada on digital cable, it is almost impossible for digital cable to be clearer than digital satellite. The reason? Almost 95% of digital cables channels are sourced from Star Choice or ExpressVu signals, mainly Star Choice (over 75%). Star Choice, because they are the source signal for the majority of cable companies are held by contract to not compress any more than 8:1.
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[Todd Hochard]

2:1 compression for HDTV means that each regular analog channel that everybody watches today could be replaced by 2 HDTV channels. Yep, full bitrate HDTV takes half the space of a regular analog broadcast.

I don't think so. Most cable companies run their analog channels in a band smaller than 6MHz (which is the OTA bandwidth). This is the whole reason for the CATV/Antenna switch in your TV's setup menu.

Greg McLaughlin, HD champion at TWC Central FL, tells me that their HD channels occupy the same space as one of their analog channels.

But I get the compression thing now.

QAM-Quadrature Amplitude Modulation. Used in many schemes, including 56K Modems. 256QAM allows jamming more channels down the same pipe than 64QAM (the two common schemes cables cos are using these days).

Todd

 

[Shayne Lebrun]

For those is Canada on digital cable, it is almost impossible for digital cable to be clearer than digital satellite. The reason? Almost 95% of digital cables channels are sourced from Star Choice or ExpressVu signals, mainly Star Choice (over 75%). Star Choice, because they are the source signal for the majority of cable companies are held by contract to not compress any more than 8:1.

Ah, but is digital cable not a better transmission medium than digital satellite? Could transmission errors in digital satellite not result in a poorer end-point signal than digital cable, even though they're using the same start-point signal?

Or am I confusing AM and FM again?

 

[BobV]

Ah, but is digital cable not a better transmission medium than digital satellite? Could transmission errors in digital satellite not result in a poorer end-point signal than digital cable, even though they're using the same start-point signal?

That is actually an irrelevant question . The reason being that the signal that a digital satellite subcriber receives in his home is the same signal that the digital cable company starts with, therefore it is impossible for the digital cable signal to be better no matter how good the transmission medium the cable is, in fact, it's almost impossible to be "exactly the same", there is more than likely going to be some loss going to the digital cable subs house... that's just physics.
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[Michael St. Clair]

Quote:

Then they are using QAM64, or they are leaving extra space for now. QAM264 provides double the bitrate of terrestrial HDTV per 6mhz channel.

 

[BobV]

That is only the case in Canada.

Yes, that is correct Michael, as I mentioned in my first post. Thanks for reclarifying that again though.