nesdev.org

I could understand digital noise affecting analog channels, because analog channels were always fuzzy in the first place, but what is preventing analog signals from glitching out HDTV channels in the same frequency range?

Nothing, really? If noise/interference is strong enough it will corrupt the digital signal. If it's not strong enough (and it usually isn't), it doesn't cause a problem. Digital is relatively resilient against noise.

Wouldn't the digital signal have to 8 times stronger than the anolog signal?

If you transmit two signals at the same frequency, they will both be unintelligble. Doesn't matter if they're digital or analogue.

Regulatory bodies try to ensure this doesn't happen, and they usually get it right.

Even leakage from one channel to an adjacent channel can often be corrected out. ATSC specifies a means of forward error correction (FEC), whose decoder turns errors and "erasures" (ambiguous bits) into correct bits. For each 187 data bytes of TV data, 20 more bytes are added. See how ATSC works, or just see how a Compact Disc player reads past small scratches because it's mostly the same thing.

psycopathicteen wrote:Wouldn't the digital signal have to 8 times stronger than the anolog signal?

Huh, no? More like the other way, the digital signal can afford to be weaker. The whole point of digital is that it sends only a small amount of possible values (usually just two) and the signal gets rounded to the nearest one when processing. That's what makes it stronger against noise, even not-so-weak noise won't push it beyond the point that it's unusable, you need some quite strong noise for that (which would have completely trashed analog as well by that point anyway).

If the two signals were sent at the same frequency they'd be more or less unintelligible, as Rahsennor stated (or one of them is just weak enough to be defeated by the other; you get one or the other, or noise, but not both under normal circumstances). You can, however, send lots of different signals at different frequencies on the same wire. Is this what the OP means?

The real protection against interference is that each signal is allocated its own piece of the bandwidth so that they won't interfere with each other enough to cause a problem (e.g. here's a big map of how radio frequencies are allocated).

Analog signals needed more room than digital ones, since digital signals are much more resilient. I don't know if relative "strength" of a signal is quite the way to explain it, but digital channels need less room to prevent interference from nearby channels than analog ones, so you could think of them as more compact, or maybe "quieter" if it helps? Like the actual strength of the signal isn't important, just how much it's corrupted by interference, a quiet signal is just as good as a loud one if they have the same relative amount of noise, the TV is amplifying it back to normalized levels anyway.

I'm talking about before 2009 when they had both analog and digital local channels running at the same time, taking up the same 6Mhz channel slot.

They use fake channel numbers, called "virtual" channel numbers. So the displayed channel number doesn't match the physical channel it's broadcast on.

Dwedit wrote:They use fake channel numbers, called "virtual" channel numbers. So the displayed channel number doesn't match the physical channel it's broadcast on.

Okay, that explains it. The Wikipedia page on ATSC and NTSC is misleading because it says "the ATSC carrier is .31 Mhz after the lower bound of the NTSC signal" or something like that.

Virtual channel numbers are one thing I do not like about digital television

To be fair it's not like the analog channel numbers are less virtual either. Yes, they're mapped to a specific frequency, but like they're just consecutive numbers anyway? It's not like radio (AM/FM) where the station number is taken directly from the frequency.

zzo38 wrote:Virtual channel numbers are one thing I do not like about digital television

How do they impact your life negatively? Do you build TV signal decoders? It seems like this detail is invisible to anybody else...

Virtual channels forces you to use auto-scanning to find channels, which can take a while. Sometimes even around 45 minutes on some TVs.
Then occasionally, new subchannels will spring up out of nothing, and you won't be able to tune to them until you realize they exist, then need to auto-scan again.

On some boxes I use (iview brand), you need to know the physical channel number to see your signal strength for a channel.

Ah, that makes sense. Thanks.