Originally Posted by andgarden
I would be interested to know whether MPEG-2 encoders can be made any better. All I can say is that, right now, they apparently aren't good enough for what ABC is trying to do.
MPEG-2 has pretty much reached its limit in terms of encoding efficiency. No matter what you do, with a single-pass MPEG-2 HD encoder, with anything under 14.5 Mbps, picture quality degrades in complex scene changes, no matter what. Simple as that. And even with a multi-pass encoder, like those from Harmonic, that limit drops to maybe 13 Mbps. By the way, ABC uses Harris encoders; their HD models are single-pass.
Originally Posted by Berk32
Originally Posted by HDMe2
At least in my area (Raleigh) they could improve the situation at least a little by dropping the ".3" subchannel if they are going to try running two 720p channels.
It wouldn't be as good as pre-recent changes... but would at least be better than what is happening right now.
pretty sure most areas have their own ".3" subchannel
And I'm pretty sure that ABC's contractually obligated with AccuWeather to continue the .3's. And even if the .3 stations were to go away, it would be doubtful that ABC HD quality would increase with the 2 HD channel arrangement.
This "2 HD feeds on one ATSC" experiment has been tried in smaller markets, but never at such a mass scale, in such large media markets, by such a large corporation. This whole situation may finally prove to the mass public that the whole arrangement isn't going to result in good HD picture quality on either HD feed. Expect some media sources to turn this into a fiasco that will probably bite ABC back. I won't say that this will become as infamous as New Coke, but that's partially because it's unlikely that anything will top that
I wouldn't mind this new "Live Well HD" channel being cable-only, and it would be technically possible. Most Harris NetVX systems, including all of those deployed to ABC O&O stations, work like "blade" servers
, where you can add and remove components at will. If I remember correctly, the NetVX units that ABC uses have seventeen blade slots; two are taken by mandatory system control units, leaving fifteen slots for pretty much anything that the client wants, whether encoding, statmuxing, gigabit ethernet communication, digital microwave news gathering, etc, of which Harris offers the "blades" for.
As for encoding, when ABC first got the NetVX, they got one HD encoder blade, two SD encoder blades, and a statmuxer blade. Each encoder blade encodes one station, and the SD blades can't encode HD material. So that would mean that in order to get two HD channels on air, ABC would have to purchase another HD blade and insert it into the NetVX. The two SD blades are still there, technically allowing for two SD subchannels, one for AccuWeather, the other for Live Well for OTA distribution.
Meanwhile, the statmuxer blade has two independent outputs. One output can have an SMPTE 310M-compatible stream that would be relayed directly to the ATSC exciter. The other output would be a DVB-ASI-compatible stream that would be relayed via fiber feed to local cable operators. I've read that Comcast likes having two HD and two SD channels on a single QAM256 channel. This setup would be possible, allowing Comcast, or any other cable provider, to simply pass through the fiber feed without re-encoding. This would allow various cable and terrestrial receivers, both high-def and standard-def capable versions, of reception.
PREFERRED ENCODING ARRANGEMENT
Blade 1: ABC feed (HD)
Blade 2: Live Well (HD feed)
Blade 3: Live Well (SD feed)
Blade 4: AccuWeather (SD)
SMPTE 310M output relays blades 1, 3, and 4; DVB-ASI feed relays all four blades.
SMPTE 310M output details:
19.39 Mbps total ATSC payload
-1 audio: 576 kbps (384 kbps 5.1 audio + 192 kbps stereo SAP)
-2 audio: 192 kbps stereo
-3 audio: 192 kbps stereo
Null packets and all PSIP data: ~430 kbps
Equals 18 Mbps total video payload, distributed thusly:
Blade 1: Statmuxer allows 12.5 Mbps minimum video bitrate; 14.5 Mbps maximum. Bitrate priority during complex scenes, causing some bitrate starving on the subchannels when needed.
Blade 3: Statmuxer allows 1.8 Mbps minimum; 3 Mbps maximum. The feed should be send in 16x9 widescreen 480i SD. AFD could be included to instruct a cable TV (or eligible ATSC) receiver to either letterbox or center-cut the feed.
Blade 4: Statmuxer allows 1.5 Mbps minimum; 3 Mbps maximum. 4x3 480i SD feed; potentially 640x480 resolution to reduce artifacting.
DVB-ASI output details:
Basically, this feed accepts and transmits whatever comes from blades 1, 3, and 4, without additional encoding or muxing. Blade 2 video is encoded at 12.5 to 15 Mbps, depending on its own need. Ideally, the bitrate would not be influenced by the simultaneous rate-shaping/statmuxing of the other three encoders.