CurtPalme.com Home Theater Forum

[WanMan]

Content arriving on HD DVD and Blu-ray are offered at 1080p24, which is fine for pixel resolution, but what is the bitrate compared to their OTA airing by the broadcasters? Also, what is currently being lost by reception of HD locals via DirecTV?

[ecrabb]

That's a deep subject - so many variables. You've got different codecs, different data rates, variable (VBR) and constant (CBR) bit-rate, and re-compression - all in play. Even two different MPEG-2 encoders will produce different levels of quality, so it's a real can of worms when you start talking completely different codecs. Then, with most modern encoders on disc-based media, it's VBR, so you can only talk in terms of average bit rate.

Unless you're talking about the early BD releases, almost everything on the two HD formats is now VC-1 or AVC (MPEG-4). AVC and VC-1 are much more efficient codecs than is MPEG-2 (as in OTA or "old" satellite), but they're also much more CPU-intensive - both encoding and decoding (hence the hefty hardware requirements for HTPC HD/DVD and BD playback.

Just to compare apples and apples, though... OTA MPEG-2 is usually somewhere in the 15-17 Mbps ballpark. Broadcasters have 19.2 Mbps to work with, but most burn some of that on an SD subchannel or two with a cropped SD version of the HD program or local radar or something. Compare that to some of the early BD releases @ 22-30 Mbps. Consider also that much of the content on BD is 24p vs a lot of OTA content which is 60i (same uncompressed data rate as 30p)... so even at the same compressed data rate, the content on the BD can be compressed 20% less. Then, consider that the BD content was lovingly encoded by somebody sitting in a compression suite optimizing the encoder settings for different scenes in the movie. Compare that to a lot of OTA stuff which just gets run through real-time encoding hardware. Given all that, it's not hard to see why even the early MPEG-2 content looks a lot better than OTA.

Now, compare that to the newer BD releases and HD-DVD. Same 22-35 Mbps, but compressed using an AVC or VC-1 encoder, which is much more efficient - capable of much higher quality at the same data rate as MPEG-2. This is also why Dish and DirecTV have started transitioning some of their HD channels over to MPEG-4. Not because they can deliver higher quality, of course (who gives a sh*t about quality), but because they can cram way more sh*t into the same size pipe.

I haven't compared locals off the new DirecTV MPEG-4 - I'm not in a big market, and I don't have the new MPEG-4 hardware. I assume the local affiliates are uplinking a high-quality stream to DirecTV, and DirecTV is real-time encoding an MPEG-4 stream at whatever bitrate they choose based on time of day, program, available bandwidth, etc. Because the local affiliate can transmit a higher-quality stream to DirecTV than they can broadcast at ATSC standard, and because DirecTV is encoding to the efficient MPEG-4, it's entirely possible (theoretically speaking) the DirecTV feed of a local HD channel could look better than the ATSC OTA version. In practice, though I'm sure that's not how it works out. I'm sure because DirecTV is trying to get HD local feeds into so many cities, they've probably pushed their codec right to limit of its useful quality. Anybody know how the HD locals look with the new hardware?

SC

[WanMan]

SC, it shouldn't have to be put before the consumer whether or not the packaged offering of Heroes (example) in 1080p24 is worse than both that offered by the affiliated OTA broadcast and a lot worse than anything offered by DirecTV via reduced pixel resolution and over-compression.

I would think (hope) that Optical>OTA Broadcast>DirecTV

[VideoGrabber]

> what is the bitrate compared to their OTA airing by the broadcasters? <

You can't really compare them directly, since the discs are variants of MPEG-4 (VC-1, and h.264) [except for some early BR], and the broadcasts are MPEG-2. The HD discs vary, but generally average around 14 MBit/sec, peaking ~20-ish MBit/sec, but can go higher. [that was true the last I was paying attention.]

I'm not sure when you refer to broadcasters if you mean OTA exclusively, or all broadcasters, however OTA can start out as high as 19 MBit/sec, but rarely comes close, due to multicasting. E.g., in my area, the locals vary from 10-15 MBit/sec (they are all sourced from network master feeds of 30-54 MBit/sec). Other satellite/cable broadcasters can originate anywhere from 10 MBit/sec up to 17 MBit/sec (HD Nets), but after going through the distribution chain, may be considerably less (as low as 8 MBit/sec, for some).

> what is currently being lost by reception of HD locals via DirecTV? <

Because most locals aren't spectacular to start with, in many cases not a lot is lost... the majority being due to transcoding from MPEG-2 to MPEG-4. But if you've got really good locals, an antenna will get them with maximum quality. Some locals (I've heard) actually provide a separate clean feed from the studio to the sat. uplinkers, to minimize those losses, and the quality is excellent.

With some premium channels shifting over to MPEG-4 sourcing (HBO/Cinemax) on their sat. distribution feeds, redistribution by satellite DBS in MPEG-4 may actually look better than MPEG-2.

[VideoGrabber]

That's what I get for starting a response, then walking away to take care of stuff. I should have hit REFRESH to see what had transpired in my absence.