Maybe you’ve seen it in your guide. Some of the channels have a mysterious “A3” designation. If you highlight the call letters of your local channel and look at the description at the top of the guide, you might see it. I know our Southern California friends have several channels listed that way, and it’s been like that for years.
According to those gurus over at iamanedgecutter.com, A3 is another way of describing DIRECTV’s proprietary MPEG-4 encoding format. It means, in their words,
“A3” is an acronym for three “Advanced” categories of …
1) “Advanced coding”
2) “Advanced compression”
3) “Advanced modulation”
Now, MPEG-4 may not seem all that “Advanced” here in 2017, but ten years ago it was pretty impressive. DIRECTV has refined their coding technology to offer the best possible picture quality over the years, and they manage to do it with about a third of the bandwidth of the original broadcast signal. They could move to something more advanced like H.265 but that would mean replacing millions upon millions of older receivers that couldn’t support that technology.