Depends on what modulation rate the transponder is using. Here are the possibilities:
Ka, DVB-S2D, SR 30 Msps:
QPSK 3/5 35 Mbps
QPSK 2/3 39 Mbps
QPSK 4/5 47 Mbps
8PSK 3/5 52 Mbps
New transponders are 8PSK. So let's take an average per channel bandwidth if you load 5 channels on that transponder.
5 HD Channel average bandwidth per service:
8PSK 3/5 - 10.4 Mbps
QPSK 4/5 - 9.4 Mbps
QPSK 2/3 - 7.8 Mbps
QPSK 3/5 - 7 Mbps
AT&T U-Verse fluctuates their MPEG4 HD from 5-6 Mbps (6 Mbps is average). Now we all heard they do have some picture quality issues. So I'm wondering if we bump that up to 8mbps, that should yield much better quality and fix the issues. So IF DirecTV wanted to put 6 channels per transponder you would get these average bitrates:
8SPK 3/5 - 8.6 Mbps (Much better than U-Verse quality)
QPSK 4/5 - 7.83 Mbps (Slightly better than U-Verse quality)
QPSK 2/3 - 6.5 Mbps (U-Verse quality)
In my opinion, I think it's worth a shot trialing 6 channels on an 8PSK transponder. Your still at a minimum 2.1 Mbps or more ahead of U-Verse which qualifies as HD service (though we know it's not nearly as sharp). With already being ahead of the average bitrate, if you statistically multiplex 6 services together at VBR rates where the channels picked are not all "action" style channels, but a nice mix of talking heads, stationary or slow moving, variety, already lower bitrate from the content provider, etc... I think 6 channels per transponder will work in most situations, provided they are using the state of the art encoders. Perhaps when there is a technology refresh, more efficient encoders will be installed at DirecTV to maximize bandwidth. There are even some channels that you could take all the way down to 4.5 - 5mbps, like QVC HD, HSN HD, CSPAN1&2 HD. These types of channels are very slow camera pans or stills, which translate to easier compressed MPEG4. You never see full blown action/explosion scenes, car chases and those types of encoder demanding pictures on some of these channels.
You have to look at it like this... uplinks cost money. Bandwidth is a finite commodity, as is fuel, water, food, etc. In any resource there is a devotion to conserve and use that resource in the most efficient manor so 1) you don't run out, 2) you keep costs down. Just as we look to install more energy efficient appliances, heating and cooling or buy more efficient cars that get better MPG's... it is in DirecTV's intrest to invest in more efficient encoders. They can't keep launching satellite after satellite at the cost and time it takes to do so.
The other thing they could do is start to migrate some MPEG2 to MPEG4. However with the millions of MPEG2 only hardware out there, this is a huge task. Seems a little more efficient to start with the encoders at the head end and maximize what is currently out there until D14 is launched. Hopefully they learned from the mistake of D12 and build D14 with more available bandwidth.
Here's a link to a thread of people discussing U-Verse HD quality for reference:
http://www.uverseusers.com/index.php?option=com_smf&Itemid=2&topic=11888.0