How does ATSC channel scanning work?

[MapMaker]

I've been reading the knowledge base and FAQ as well as the OTA forums for months in preparation for installing my CM4228 when the better weather arrives but I'm still ignorant, so if I am asking something stupidly obvious I apologize. Feel free to move to proper thread.

As a generalized question, could someone please help me understand just exactly how "scanning" for channels works?

For instance...If one does an air scan and gets channels
2.1
3.1
4.1
does this mean that one can not tune to channel 5.1?

Then, if one re-scans and gets
2.1
4.1
5.1
does this mean that one will no longer be able to tune to the 3.1 channel picked up with the previous scan?

What about when 3.1 is actually there, available to the next rescan? Could one tune to it without doing the rescan?

I guess I'm asking if I have to get a positive scan for a particular channel before I can actually tune to it. And what happens to that positive scan information in my TV when I rescan? I can't seem to visualize it particularly when I read that "You either get the channel or you don't." If that's the case why am I scanning at all?

[57]

I believe it depends on the TV. Check out the following thread:

http://www.digitalhome.ca/forum/showthread.php?t=114524

[ScaryBob]

Tuners vary with implementation but it's fairly close to what you describe. Some tuners will detect channels if you manually tune to them. So if digital 5.1 is present when channel 5 is selected, that channel will be added (plus any other subchannels) or analog channel 5 may be added. Scanning usually starts with a clean slate and only those channels detected will be added. Manual tuning may allow channels to be added without clearing the empty channels. That's handy for DX'ers or people with rotors.

[majortom]

I guess I'm asking if I have to get a positive scan for a particular channel before I can actually tune to it.

Depends on TV Manufacturer's implementation. They all seem to have their own way of doing things. Even within the same brand but across diff models they'll do it different. Until ya get more familiar with yours, your best bet is to thoroughly read that section of your owners manual for specifics.
After a while you'll either be like wow, this is pretty cool.
Or you'll be cursing your TV like grrr, why did they do it this way, this is so stupid.

[downbeat]

Generally speaking, cheaper television and some computer-based tuners are incapable of direct channel tuning.
Most run-of-the-mill sets have the ability to tune manually, but how easy it is depends on the make/model, as previous posts suggest.

[ScaryBob]

some computer-based tuners are incapable of direct channel tuning.

That's often a function of the computer software, not the tuner.

[Dave Loudin]

Just a quick tech check: remember that 2.1, 3.1, 4.1, etc. are simply labels encoded within the stream of data you get when you decode an ATSC DTV signal. When an ATSC tuner scans for channels, it usually starts trying to decode signals one at a time beginning with RF channel 2. When the tuner successfully receive a signal, it applies the label to that RF channel , say 66.1 for RF channel 34, plus any subchannels that may be inculded, then moves on to the next RF channel.

When you enter 66.1, the TV now knows that means tune to RF channel 34 and display the main channel. Without already having mapped what it means, your TV would not know what to do with an entry like 66.1. In many cases, single channels can be added by telling the TV to tune to the proper RF channel and having the label applied once a signal has been decoded.

[nbound-au]

As an Australian Im also interested on how North American TVs tune, especially as all your channels are from many directions, where as here for all your stations/channels you have one transmitter.

We just point and auto tune (or manual tune if you know them off hand).

What happens up there? Scan the closest/most powerful and manual tune the rest so they dont get forgotten, or do your tvs keep channels from previous autoscans so they dont get overwritten?

[stampeder]

Originally Posted by MapMaker

I'm asking if I have to get a positive scan for a particular channel before I can actually tune to it

Nowadays, in most typical tuner devices, the answer is yes.

[tvlurker]

Originally Posted by nbound-au

As an Australian Im also interested on how North American TVs tune, especially as all your channels are from many directions, where as here for all your stations/channels you have one transmitter.

We just point and auto tune (or manual tune if you know them off hand).

What happens up there? Scan the closest/most powerful and manual tune the rest so they dont get forgotten, or do your tvs keep channels from previous autoscans so they dont get overwritten?

Again, each tuner is different.

Some tuners have an option that allows for an auto scan for NEW channels that does not erase existing ones, and that is the option that DXers and rotor owners would use.

However, this is also problematic, as the rescan will not rescan existing channels, which means that additional subchannels will not be picked up. To scan in additional info on exisitng channels, some tuers require you to start from scratch, which takes longer, and also erases channels not currently receivable.

[HWP]

....except for some tuners that are able to detect new sub-channels without any scan at all. I have some of those.

Be assured, the ATSC system here is very likely more of a hodge-podge than the Australian system.

Welcome to the forum.

[ScaryBob]

Some ATSC tuners will automatically add digital channels with a manual channel entry and others will not. Some will rescan with an option to keep existing channels and others will always start with a blank slate. That can make watching channels from different directions a real pain when a rotor is used, or not, depending on the tuner or software. I envy the Australian and British systems where all channels are on a community tower. OTOH, we have a wider variety of channels in border areas. Here, government regulation actually prevents the co-location of some channels in the same region so broadcasters have no choice but to place some broadcasting towers in different cities.

[nbound-au]

Thanks guys