If you've noticed, quite a few Comcast users have been
grumbling over the quality of their HD programming recently, and they're attributing the artifacts / compression to the squeezing of three high-def channels onto a single QAM. Granted, delving into technical aspects typically isn't our bag, but given the constant talk of
HD Lite, we figured it prudent to pass along a rough 'n dirty explanation that details why cramming more than two HD channels onto one QAM is a bad idea (in the consumer's eye). Quadrature amplitude modulation, as it's formally known, equals 6MHz of bandwidth, which equals 38.8Mbps of broadband data, which equals thousands of voice conversation, which equals "enough" space for two to three HD nets. Granted, just because three will
fit doesn't mean things will look as good as if only two were allocated to one
QAM. Quite frankly, we're just scratching the surface in this space, but hit up the read link if you're itching to know more.
posted Mar 25th 2008 11:58AM
Why not use the space for 2 HD channels and if there's space left over, add SD channels until it's all used up?
Two HD channels will effectively use it up if you want full quality. If you start lowering the quality of the HD channels, you're back to the same problem.
oh look, comcast scaming consumers once again :(
Better yet this doesn't deal with QAM but when you have a cable box make the hd channels align with the normal ones kind of like QAM but now.
like say discovery is 29 make discovery hd 29-1 on the cable box can they not order your channels in your guide like that it would make all this so much easier.
Wow, that makes everything so clear! Or not really. Seriously, you couldn't find an explanation that goes on for more than a couple of paragraphs?
Where does 6MHz come from. Well its the spectrum allocated to a single analog channel.
What does a QAM do? Well it takes in digital MPEG-2 programs as SPTS's (single program MPEG-2 transport streams) and merges them into MPTS's (multi-program transport streams), one per 6MHz channel, then puts them out via RF. Typically it can also take in full channels already programmed (say off a satellite feed) via an ASI interface. The SPTS's generally come in these days via gigabit Ethernet. The packets are associated with their target channel and Program Number via the UDP port the packets are sent on.
How are SPTS's merged into MPTS's? Well the MPTS has its own clock, so the PCRs and DTS/PTS values from the SPTS need to be restamped. The PIDs used in the SPTS's need to be remapped to those in the MPTS, with each program being assigned a different set (typically something like 0x10-0x1f for program 1, 0x20-0x2f for 2, etc). Similarly, the continuity counters are generally redone. The PAT and PMT for each program must be regenerated. The NULLs from the original SPTS's are stripped, and new NULL packets generated to keep the MPTS at 38.8Mbps.
What is a channel then? In digital, the STB has a table with a channel number (450 for HBO say), and the corresponding frequency (xMHz) and program number (2 say). By locking to the specified frequency, then pulling out program number 2, the STB can decode the desired channel. There is no need for program 451 to follow 450 in the same frequency or whatever, the box just needs a table that says what channels are valid and where they are.
How many digital channels can fit in one 6MHz band? Typically North American cable uses 3.75Mbps for SD and 15Mbps for HD, assuming things are CBR. So you can fit about 10 SD channels into one 6MHZ/38.8MHz band. And HD channels take up 4 SD channels, so you can fit 2 HD channels and 2 SD channels in one 6MHZ band.
However, most live channels are VBR, meaning variable bit rate. If things are set up correctly, a statistical multiplexor can give one channel additional bits per second while starving another, based on what each channel needs at that moment. Often this is how channels are sent over the satellite. If the channels are NOT rebuilt by a QAM (but rather fed in via ASI, all assigned to the same 6MHz band), then they will remain VBR through the transmission to the STB. VBR is more efficient than CBR, but only if all of the channels don't need a lot of bits at the same time (i.e. they don't all do pans at once).
There, thats more than you got out of that one stupid link.