Originally Posted by Frank-0-Video
(1) Regarding the 19.38 Mbit capacity -- is that figure an absolute limit, or is there ongoing research that would allow for a higher bitstream capacity within the 6-mhz space, which would in turn allow for more multi-channel HDTV options?
It's the limit of the ATSC standard implementation. There are much more efficient modulation systems that could be used now and would deliver a higher bitrate - but they wouldn't be compatible with existing receivers.
In the UK we have 8MHz rather than 6MHz channels, and launched digital OTA with 16:9 SD rather than HD.
Our current SD system delivers 24-27Mbs in an 8MHz channel using the original DVB-T standard developed in the 90s. (We used to run it partially at 18Mbs because early receivers were not compatible with an improved version that should have been implemented at launch but wasn't... Some broadcasters chose 18Mbs to be more robust, others ran at 24Mbs and were trickier to receive. The current 24Mbs standard is as robust as the original 18Mbs pretty-much)
We launched HD OTA more recently using DVB-T2 which delivers 40Mbs in an 8MHz channel. This didn't cause a major receiver issue as all of our existing receivers were SD and so it was reasonable to expect to buy a new receiver for HD.
(2) If the 19.38 Mbit limit can't be broken, then what else can be done with the current standards of transmission that would allow for more HDTV sub-channels?
If you switched from MPEG2 to H264 (aka MPEG4) compression you can squeeze more content in at a given quality. However these MPEG4/H264 sub-channels would not be compatible with the existing MPEG2 receivers. (The UK has gone for H264 for HD and MPEG2 for SD - as our SD services launched before H264 was developed. However other countries that went digital OTA later - Ireland, Norway etc. have gone entirely H264 - where others have a mix of H264 for Pay-TV and MPEG2 for free stuff.
I believe that some H264 pay-TV subchannels are being used in some US areas?
(3) Is there any ongoing research involving a different type of digital TV transmission within a 6-mhz space that might be better than the current standard? In asking this particular question, I'm mindful that any radical changes in digital TV transmission might possibly mean -- especially for the consumer -- getting the equipment necessary to handle such changes.
Yes - lots. There is the second generation DVB-T2 standard being introduced in Europe which delivers significantly higher bitrates than the original DVB-T (which Europe adopted when the US adopted ATSC).
South America has chosen a tweaked version of the Japanese ISDB-T (the Japanese use MPEG2, South America H264)
There has also been work on MIMO antennae techniques, using both horizontal AND vertical polarisation at the same time etc. The UK considered these as part of their DVB-T2 adoption - but the requirement for new rooftop antennae was deemed a step too far.
Japan has started experimenting with Super Hi Vision (16 x HD resolution - 7680x4320 @ 60 or 120p) via OTA. They used MIMO, mixed polarisation and up to 4096QAM modulation in 2 x 6 or 8MHz channels to deliver an OTA SHV signal of 184Mbs.
(4) Though not necessarily related to Q's 1-2-3 above, I wonder if there has ever been a time past or present when anyone proposed -- and maybe experimented with -- over-the-air digital TV channels operating within narrower MHZ ranges - say 4 mhz, 3 mhz or even 2 mhz??
Some of the COFDM techniques of ISDB-T (I think 1-seg effectively does something a bit like this) and DVB-T/T2 do something a bit like this (though DVB-T2 Lite for mobile uses time slicing rather than narrower frequency bands)