Compression on cable TV

[Martyn]

Hi,

I'm interested in how much cable TV compresses signals so they might be lesser quality than OTA or satellite network feeds.

Has anyone looked at this before or know a way to do it?

I think I can quite easily work out the data rate on satellite and OTA but they are different formats, DVB-S and ATSC, so are perhaps not directly comparable with cable, which I believe is QAM.

Any thoughts or ideas would be useful.

 

[CrimsonKnightDX]

I'm not sure exactly how much compression they are using but I know Comcast stacks about 2-12 channels on average per frequency channel. This of course is depending on market. Also to take in to account is the local affiliate, which simulcast to OTA and cable and satellite. KUSA in Denver does 4 simulcasts I believe and can get grainy.


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[Tampa8]

It is hard to come up with a number amount to quantify the differences. DISH uses a different codec than Directv as an example. As mentioned above some OTA transmitters are using two or even three sub channels to the main one which will degrade the signal.

As unscientific as it is the best way to compare may be to use the same TV to watch the same programming from different providers and go by your own eyes. Even at that there are other variables. As an example of how hard it is, with DISH some feel the OTA picture is much better than the Satellite picture, while others say there is little to sometimes no real difference especially when watching from a viewing distance.

 

[harshness]

I'd wager that compression rates are a very closely guarded secret. I can't be good for business to have hard comparative numbers find their way into the public view.

Statistical multiplexing guarantees that the rate is constantly changing depending on the load of the combined content on a frequency.

 

[CrimsonKnightDX]

One thing to add with Tampa8. This is correct. I had Dish and Xfinity at the same time. I compared channels on both. It was a hit and miss. NFL network looking slightly better on dish but ESPN looked slightly better on Xfinity. Also with each cable system and market the quality changes on channels but most are pretty close with any other provider on most cases. IMO I believe Directv has slightly better quality consistent on all channels. This though depends on each persons preference and picture settings as well.


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[Martyn]

Thanks for all the feedback.

I was thinking it might be possible to measure the signal and figure out, for example, that my local ABC OTA is 8Mbps but on Dish the channel is being sent at 4Mbps. If I can quantify the effectiveness of each coding system, the flavors of ATSC vs DVB-S2 in this example, I might be able to conclude that Dish cuts the signal by 25% or something.

Maybe it's more difficult than that, but I feel as the signals are delivered to us, there should be a way to analyze them and figure that out.

I know PC cards are available that can deliver a pretty detailed analysis of ATSC and DVB signals, but I haven't looked much into QAM and Turbo (?) that DirecTV uses.

Perhaps just subscribing to the lot for a month and looking at them is the way to go. That's just more subjective than analytical.

 

[harshness]

I was thinking it might be possible to measure the signal and figure out, for example, that my local ABC OTA is 8Mbps but on Dish the channel is being sent at 4Mbps.

Your thoughts may be based on a flawed assumption that everything is compressed on a channel by channel basis and that's not the case. Each provider groups a number of channels together and the resultant multiplex is compressed as a whole until the whole multiplex fits in the bandwidth for that frequency.

We can assume that the satellite providers group the local channels together on how ever many transponders they use but that's about as far as it goes.

Even if you were able to make an instantaneous measurement of a particular channel's bit rate, it would not be repeatable due to interdependence.

 

[Martyn]

Your thoughts may be based on a flawed assumption that everything is compressed on a channel by channel basis and that's not the case. Each provider groups a number of channels together and the resultant multiplex is compressed as a whole until the whole multiplex fits in the bandwidth for that frequency.

We can assume that the satellite providers group the local channels together on how ever many transponders they use but that's about as far as it goes.

Even if you were able to make an instantaneous measurement of a particular channel's bit rate, it would not be repeatable due to interdependence.

Thanks for the explanation.

I understand the channels are packed together into a multiplex. And I thought it's possible to look inside each multiplex and figure out the data rate for each channel. Rabbitears lists a data rate for OTA signals, and I think I can work out a data rate from the SR and FEC on a DVB channel

Although perhaps that's where I come unstuck?

Is interdependency like variable bit rate encoding, a dynamic adjustment depending on the available bandwidth and requirements for the whole mux?

That rate would always be changing although we could probably come up with an average over a minute or hour or day. In video encoding, my VBR streams have a target bit rate around which the variation happens. I'd bet that would reveal itself as the average bit rate over a period of time.

 

[harshness]

And I thought it's possible to look inside each multiplex and figure out the data rate for each channel.

Given the appropriate instruments, sure. A cable box typically isn't such an instrument.

Is interdependency like variable bit rate encoding, a dynamic adjustment depending on the available bandwidth and requirements for the whole mux?

Precisely.

That rate would always be changing although we could probably come up with an average over a minute or hour or day. In video encoding, my VBR streams have a target bit rate around which the variation happens. I'd bet that would reveal itself as the average bit rate over a period of time.

It would, but you would still need to be able to measure each and the bias/priority can be adjusted pretty much on-the-fly. If one member channel is playing a hockey game, it is likely that the provider will tweak the bias to compensate for that versus a golf tournament or a talking heads show.