Doug’s Q-Tips – What’s the deal with Channels 2.1 and 2.2?
QAM cable channels have always had the ability to carry more than one program in each channel. When affordable QAM modulators for commercial applications were invented, such as our QMOD series, they could only deliver one program. That was pretty amazing for the price, but the technology keeps on advancing. Today, almost all modulator suppliers can deliver 2 programs per channel. You’ll see some offering 4 channels in one box, but they are actually delivering 2 channels that have 2 programs each.
The Simple Answer
Lots of things have multiple programs. For example, you can select tracks on a CD and listen to different songs. Same for digital TV channels. Channel 2 can carry 2 programs in our new QMODs. By default, the program listing (Channel IDs) for channel 2 would be 2.1 for the first, 2.2 for the second. You can also change the ID to anything you want, such as 100.1 and 100.2; or 101 and 102.
When a TV scans the channels, it captures the data and shows your channel IDs just the way your set them.
The Geek Answer
Geek alert! I’m going a little deeper into how this is done – you may want to read the Q-Tip “How Does Encoding Work?” as well to get the big picture.
An MPEG encoder creates multiple streams, primarily video and audio streams. Each one is identified by a number called a PID (Packet Identifier).
A PMT, Program Map Table, maps the video and audio streams needed to play the media. When you assign a video input and one or two audio inputs to Program A and Program B in a QMOD, you’re creating a PMT for each Program.
The process also creates the PAT, which is the master list to each Program PMT.
When you set a tuner to scan for digital channels, you see the names 2-1 and 2-1. Under the hood, the tuner is saving the PAT and PMT information for each channel. When you tune in sub-channel 2-2, the tuner reads the Channel 2 PAT, jumps to the Program A PMT and sends the streams 44 and 52 to the decoder – then you can watch the media.
For generic use, we say that a QMOD is a 2-channel modulator, because that’s what people perceive when they tune channels 2-1 and 2-2. For techs, geeks and integrators, the anatomically correct term is “one channel with 2 programs.”