Are our HD bits being shaved?

TIPS List · Post by **TIPS List** » Fri Sep 03, 2004 6:10 am

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I've been tracking bitrate's of DFW local channels and DirecTV and
happened to have checked the US Open tonight. It came it at 15.6Mb/s.

Best I've seen (measuring with this method) from OTA is about 17.5Mb/s.

Worst on DirecTV is HDNet/HBO/Showtime down around 10Mb/s.

My results are posted at http://www.widemovies.com/dfwbitrate.html

Bitrates are computed based on HD Tivo file sizes which isn't a
completely verified method yet, but seems to be good at least for
comparisons.

Steve Martin

akirby · Post by **akirby** » Fri Sep 03, 2004 6:33 am

Since this is mpeg encoding, wouldn't the file size be affected by the video content? I imagine some low action movies with lots of static video scenes could get by with a much smaller mpeg file and be just as artifact free as an action movie with a much bigger mpeg file. It will also vary from scene to scene within a show.

I don't think you can equate file size to transmission bandwidth.

raff · Post by **raff** » Fri Sep 03, 2004 6:42 am

I was kind of wondering about that too. I can see this being feasible with MP3 files, since the higher encoding rates increase the file size. With MPEG2, file size changes by frames and content.

Steve Martin · Post by **Steve Martin** » Fri Sep 03, 2004 7:52 am

Of course, using VBR encoding, the bitrate will vary based on the content. But, when there is a fixed amount of bandwidth, why not use as much as possible and let the quality improve to fill the bandwidth. With mp3's VBR is nice because the file size can be kept smaller with constant quality. When you have a "constant" pipe you can fill it up and get the best possible quality for the bandwidth (that is the whole reasoning behind Superbit DVD releases, maximize the video bitrate).

Now since DirecTV does statistical multiplexing they definitely want to use VBR so one channel can grab from the other when it needs it. When they share a movie channel with a video channel it makes sense the video channel would grab more.

What I'm looking to see is the "average" bitrates being given to the channels and see how that compares to OTA. The worst thing that seems to be going on is that a movie channel may dip down to near 50% of what that movie might get on an OTA channel.

As some on other forums have pointed out there are other things going on (filtering to reduce resolution and allow better compression, etc.) so bitrate doesn't tell the entire quality story, but I intend to keep an eye out and note if DirecTV starts reducing bitrates when they add more channels.

akirby · Post by **akirby** » Fri Sep 03, 2004 11:54 am

Comparing the same show from multiple sources would be a good comparison. (Olympics, e.g.). This would remove any differences due to the mpeg compression and show any difference between the sources.

TIPS List · Post by **TIPS List** » Fri Sep 10, 2004 1:40 pm

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Hopefully this is temporary to make room for NFL Sunday ticket (but why
did they have to do it on Friday!).

HBO HD, Showtime HD, and HDNet Movies are all sharing one transponder.
HBO's bitrate seems to be about the same, about 13-14mb/s on some
samples I've taken. Showtime and HDNet movies are now at 1280x1088
resolution (HBO always has been). Bitrates for Showtime and HDNet
Movies are down in the 6-9mb/s range (ie DVD bitrates).

Details at http://www.widemovies.com/dfwbitrate.html

--
Steve Martin

TIPS List · Post by **TIPS List** » Fri Sep 10, 2004 6:18 pm

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If an affiliate is broadcasting on an HD channel and also has a subchannel for wx radar, as do all
the ones local to me, how much gets lost in terms pixel count or something I can relate to. Does it
in effect turn a 1080i broadcast into something other than HD? Or is it a matter of artifacts
effecting the otherwise beautiful picture? Is this why HDnet looks so much better than the
affiliates?

Hugh

TIPS List · Post by **TIPS List** » Fri Sep 10, 2004 6:19 pm

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I've been tracking the bitrate info of local channels in Dallas at
http://www.widemovies.com/dfwbitrate.html

The channels that broadcast without subchannels seem to be using about
18.2mb/s for audio/video.
On channels with radar on the subchannel, the HD channel drops down to
about 15-16mb/s depending on how much bandwidth they allocate to the
subchannel.

So, about 15% less data, which indeed results in more mpeg compression
artifacts.

Steve Martin

Dave3putt · Post by **Dave3putt** » Fri Sep 10, 2004 9:42 pm

Seems like the only logical way to use this comparison is with live (original) content. With anything rebroadcast you don't know how good the source material was. Using mp3's as an example, you can make a 320 bit rate mp3 from a 128 bit mp3 and it will sound terrible. But the numbers say it is 320 and is a large file. If you make a 320 from the real deal, it will sound great.

Steve Martin · Post by **Steve Martin** » Sat Sep 11, 2004 8:13 pm

If you suspected that someone was increasing the bitrate to impress us with their better quality that may be a consideration.

Here is what I know (or at least am pretty sure of).

Local network affiliates get their network HD feeds at a much higher bitrate than they could possibly pass on to us. That is so they can overlay graphics, etc. and recompress without significant quality loss. Fox may be a counterexample, as they send at the broadcast bitrate and have technology for "splicing" in logos, etc. Therefore anything OTA affiliates give us will be at equal or less bitrate than their source.

DirecTV is very bandwidth limited and is recompressing the feeds from their HD networks, now to extreme levels. (Does anyone consider 1280x1088 at 7mb/s to be HD?).

If anyone comes up with a case where a provider is taking low bitrate material and recompressing it at a higher bitrate, I'll consider it, but I don't think there is any likelihood of that being a real world consideration.

The "bitrate watch" is to provide an objective measurement of quality loss, in my case specifically to watch what DirecTV is doing given their history (I can't even bear to watch any of their SD channels any more and suspect the HD channels will go the same way).