[a-dub]

> That 6 MHz is used to encode MPEG-2 frames containing video, color, and audio information that your cable box or TV decodes into picture and sound. If you graphed a single channel provided by the cable operator, it would look similar to Figure 2-2.

> A DOCSIS channel can be graphed in the same fashion; however, instead of video, color, and audio information inside the MPEG-2 frames, it contains a data stream that represents computer information. Due to the "spectral shaping" of a data signal, there are no video or audio signals present, and the graph looks like Figure 2-3.

this seems wrong. i think figure 2-2 is an analog ntsc video channel and figure 2-3 is a digital mpeg-2 or docsis over mpeg-2 channel. both of the digital channels should have the same spectral envelope.

interesting that they put mpeg-2 headers on the data frames, probably "system" frames and done so for compatibility with existing headend and stb equipment.

[drmpeg]

It is incorrect. As you state, a video/audio channel and a DOCSIS channel would look the same on a spectrum analyzer.

Here's a sweep of all the channels (from 80 to 750 MHz) on my Comcast system. This was taken back in 2014, and there were still three NTSC channels (two of which were just a black frame and silence).

https://www.w6rz.net/span.png

A zoom of the last channel at 729 MHz.

https://www.w6rz.net/last.png

MPEG-2 Transport Streams are used for DOCSIS because it's baked into the QAM specification. It's built around 188 byte packets that start with 0x47.

https://wagtail-prod-storage.s3.amazonaws.com/documents/ANSI...