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Are our HD bits being shaved?

Started by TIPS List • Sep 3, 2004 • 29 posts
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#1
----- HDTV Magazine Tips List -----

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
#2
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.
#3
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.
#4
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.
#5
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.
#6
----- HDTV Magazine Tips List -----

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
#7
----- HDTV Magazine Tips List -----

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
#8
----- HDTV Magazine Tips List -----

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
#9
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. :?:
#10
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).
#11
Here are some images that compare frames from a couple of movies before and after DirecTV shaved the bitrates of Showtime HD and HDNet Movies...

http://www.widemovies.com/directvcomp.html
#12
Steve:

Wow. I appreciate your efforts. That must have taken some time to match the exact frames.

But that is a depressing discovery, to see so much detail robbed from the original source material. Especially since this is a "premium" channel for which people are paying extra for what is supposed to be the best possible quality. Wow, again. What a difference.
#13
Lets try to be positive and appreciate what we do have...I've got a feeling that when Directv gets their new birds in the firmament, we'll get our bitrates back...........Jack.... :roll:
#14
Lets try to be positive and appreciate what we do have...I've got a feeling that when Directv gets their new birds in the firmament, we'll get our bitrates back...........Jack.... :roll:

DirecTV is going for 1650 total HD channels. Even with 4 birds that is a lot of bits- the temptation will be to shave bits somewhere.
#15


DirecTV is going for 1650 total HD channels. Even with 4 birds that is a lot of bits- the temptation will be to shave bits somewhere.

I'm of the glass half full variety...and I'm thinking by then Directv will come to full understanding at just how adamant we videophiles are at expecting a crisp perfect picture for our $$$'s... :wink:
#16

I'm of the glass half full variety...

Thanks for that, I need cheering up. I have had too many disappointments getting HD.

I see Arianespace got the contract to launch the first Spaceway in April 2005. We should have a test case a few months later. That is if the French don't screw it up!!. Boeing launched the last DirecTV 7S. More "outsourcing"?
#17
I thought DirecTV had more than just 4 sats. I thought it was 6 or 7?
#18
----- HDTV Magazine Tips List -----

How on earth do you determine the "file size". When I record to my HD-DVR it does not tell me how
much space it is taking up on the hard drive. Same for recording to D-VHS from OTA programming.

Hugh
#19
----- HDTV Magazine Tips List -----

To learn how to modify an HD TiVo to get that kind of information, see
www.dealdatabase.com forums.

If you record to DVHS, you can dump from DVHS to a PC hard drive with
Firewire, and use tools to analyze the transport stream. It can give
you the total size of the video and audio streams which you can add up
and divide by the time to compute the audio/video bitrate.

(If it was easy, everyone would do it ;-)

Steve Martin
#20
----- HDTV Magazine Tips List -----

There have been some interesting discussions on AVS Forum about the
quality of the Fox broadcasts today (I thought the Cowboys game was
particularly lacking both in terms of the HD quality and the outcome
;-)

I measured the bitrate of the Fox broadcast and it was only 11.3mb/s.
I've been told that is because they distribute it at a low bitrate to
allow for affiliates that multicast. Their transmission method, with
the use of "splicers", allows the affiliates to pass the feed on
without recompressing it. But, if they have to distribute it at a
"lowest common denominator" bit rate, that hurts us all. Our local
affiliate does not multicast, so 7mb/s of bandwidth was just wasted in
our market.

The ESPN-HD broadcast on DirecTV tonight is at close to 17mb/s and
looks much better.

--
Steve Martin
#21
I thought DirecTV had more than just 4 sats. I thought it was 6 or 7?

Yes they do. I was referring to the recent announcement of 4 new satellites devoted just to 1650 new HDTV channels to be launched in 2005 and 2007( 150 national HD and 1500 local HD). Since very few DirecTV customers have HD capable receivers, existing customers will not benefit from this expansion unless they upgrade. DirecTV will have to keep their existing SD channels on the existing satellites for a long time for backwards compatibility.

It is a similar situation to an OTA broadcaster putting both a SD and HD channel in the bandwidth. DirecTV will have to essentially duplicate almost every channel in both a SD and a HD version in order to maintain compatibility to all customers - thus the temptation to bit shave. The entire fleet is the available bandwidth and they dynamically share bits. When DirecTv begins to retransmit a local HD station the BEST they can do is the station's bitrate, but DirecTv could easily shave a few bits off that rate. Given past practice which bitrate will they use?

I suppose they could go 100% HD channels and do a free upgrade of millions of SD receivers instead where non HD customers would set output to 480i-unlikely.
#22
Forgot about the equipment issue.

There's no way they'll do free upgrades - reduced price, maybe - with a 1 or 2 yr subscription guarantee.
#23
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Just in case people are interested.

The national OTA bitrate (measured from 6:30-7:30 eastern time)
averated 14.3mb/s. That is the highest I've seen on a national Fox
broadcast.

DirecTV is showing Fox East at 17.2mb/s. Again, the highest bitrate
I've ever seen them give the NY Fox HD feed.

They both look pretty good, but I do see mpeg artifacts (random
periodic shifting) in the grass on longer shots.

Finally, a case where everyone seems to be trying to do the right thing.
#24
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Why would OTA be a lot less than satellite? I thought OTA always has more
room to work with.
#25
----- HDTV Magazine Tips List -----

Fox has decided to provide a 12-15mb/s feed to their affiliates, along
with the technology to pass it on unaltered to home viewers without a
decode/encode stage at the affiliate. They keep it down under 15mb/s
so that affiliates will have room to add second channel for weather
radar or whatever if they choose. Unfortunately that means that
everyone is limited to under 15mb/s even in markets where there is no
secondary channel.

DirecTV decodes the signals from their providers and reencodes at
anywhere from 6-18mb/s depending on how many channels they are
squeezing on to one transponder, etc. In the case of the Super Bowl,
they decided to up the bitrate they give to Fox in order to lose as
little quality as possible in their decode/encode stage. So the
DirecTV quality is slightly reduced (due to the reencoding) but their
bitrate is higher. Thankfully the bitrate was high enough (17.2mb/s)
that there didn't appear to be much visible difference.

Steve Martin
#26
What about the macro blocking on the hood of the red XLR at the end of the game? Is that due to bit shaving? It looked terrible. Also the grass on long shots didn't look right but I can't describe it accurately.
#27
What about the macro blocking on the hood of the red XLR at the end of the game? Is that due to bit shaving? It looked terrible. Also the grass on long shots didn't look right but I can't describe it accurately.

I don't know if I saw the XLR, but that could be from MPEG compression (or if it was short and really bad, some other breakup of the signal).

The "swimming grass" problem is definitely from too low a bitrate. What happens is that a finely detailed pattern (like grass) "shifts" periodically" as there is movement in other parts of the picture. Another MPEG2 compression artifact. If the bitrate was higher, it wouldn't be an issue.

The grass problem was evident on Fox's OTA feed so is not necessarily DirecTV issue.
#28
No, it wasn't a breakup. It was just on the hood as the camera panned and again visible (but not as bad) on the side shot. The car was a very bright metallic red.

And yes, swimming would be an appropriate word for the grass appearance.

I was watching OTA.
#29
Australian OTA maximum native HDTV bitrate is 14.5mb....it looks sensational on my 19inch PC CRT, LOL, but it macroblocks on pans.
I dled some American 18.5 mb HDTV content and the macroblocking was gone.....also, I've never seen macroblocking on any blu-ray on large HDTV's, but I think the BD bitrate starts at 18mb :shock: