Air Versus Cable Part 2
In the last post, we looked at the 8-VSB RF signals and discussed data rates on the DTV streams. This time we take a look at QAM used by the cable industry. I captured the screen shots on my Sencore 1850 MPEG analyzer on the same day for air and cable. Here’s a look at Time-Warner Cable’s QAM 256 for our channels:
You can see in the upper right block, instead of 8 lines of dots, you have 16 clusters of dots across, and 16 down. That is representing QAM 256 and is named so because 16 times 16 is 256. Instead of having 19.4 million bits per second bandwidth, QAM 256 has 38,800 million bits per second. Man! I’d love to have that bandwidth over the air! With a wider bandwidth, cable can insert many more channels into a single standard channel, which is pretty efficient use of their system. More than one network can occupy a single QAM channel, and it can provide many SD channels.
In the above picture, you can see the null packets are taking upmore data than I am sending over the air. The data rates for the three channels are pretty much the same as I am sending out. The unused data being occupied by the null packets will eventually be used by another channel. They can place several SD channels in that space, or a combination of HD and SD. In their master PSIP table, all they have to do is assign the PID numbers, and it will then show up on the system.
One argument we hear a lot is about providers compressing the broadcast signals even more than over the air. The captures in this and the previous post were taken moments apart. Naturally, the bit rate varies due to the complexities of the video, but the captures from off air and cable were pretty close to the same. It’s hard to get an apples to apples comparison, but there’s really no need for the cable ops to re-compress the signals at all.
The biggest difference is in the CVCT tables, which is smaller than the heavy error correction I send over the air in my TVCT tables. They do not have to worry with many of the issues a broadcast signal has to deal with since their system is a closed system, meaning the cable is shielded from the outside world.
By the way, you can click on any of these pictures to see the full frame version on your browser for clarity.
If there’s anything you’d like to see, let me know. Post a comment and I’ll see what I can do.
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I am trying to tune in WNCN HD via clear QAM on Time Warner Raleigh cable…and using EyeTV on a Mac. Every station shows up (even the WNCN SD channel), but not the HD channel. With EyeTV, if you manually add a channel, you have to enter the MHz/frequency of the channel…not the channel number as on a TV with a built-in tuner. Do you by chance KNOW this frequency?