I think that most TVs will be the same way, unless you find a higher end model that has multiple CATV/antenna inputs.

Over the air (OTA) DTV and cable DTV use a different modulating system, so the signals for the same channel will be different with the two systems. That's why you need to tell the TV if you're hooked up to an antenna or cable. I suppose that they could have two tables of the found channels in the TV, but not many people connect to both systems, and you usually need an external switch if you do, so it is not something they would normally design for.

If you want to have both, your best choice might be to get the most basic cable box your cable company has that supports HDTV, and connect that to a different input on the TV for cable, and use the built-in tuner for the antenna only. Alternatively, you could look to buy a DTV set-top box for OTA, but I don't think there are many of those that have a high-def output. most are designed exclusively for allowing old TVs to still be used.

Thanks for an excellent explanation.

Have you tried using a standard coax splitter backwards as a combiner?

I used that on an outside hd tv combining over the air antenna with "cable".

Maybe worth a try if you have a splitter lying around.

I have the same problem with an LG HDTV. I think the TV senses a digital input. Maybe an input switch is an added necessity. We have U-Verse, but still have a lot of VCR tapes that don't play well at all with just using the "antenna in".

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Thanks again to all. The A/B switch looks to be a coax splitter that combines two inputs to one output...is that the same thing?

It looks like this:

Something like this is what I was referring to. This one is from Monoprice(My FAVORITE online store for cables, antennas and such. Always great prices and products.)

The specs for that splitter are poor - the insertion loss is 10 dB to 12 dB!! (typical is 1.5 dB). In "lay" terms, you will lose most of your signal in losses in the splitter. If you have a strong signal, you might get away with it, but if the signal is "average" or weak, it will degrade the picture.

Splitters with better specs are readily available at the box stores and other places, for $10 to $15. If the insertion loss is 3 dB or less, the signal should not be attenuated excessively.

Another consideration - there are three levels of "HDTV" - and cable does not give you the highest resolution picture, due to bandwidth limitations in the cable. If you want full HD HDTV (e.g. 1080p), you have to either use an antenna and get it Over the Air (OTA) or pay for a premium satellite system. In most parts of the US, 30 to 50 channels can be received for free OTA with a simple UHF antenna such as a "bowtie", which is why so many people are abandoning cable and satellite.

If your TV is a lower resolution TV, such as 720p, you may not notice the improvement in picture quality for OTA transmission.

I'm pretty sure your problem is this:
The cable basic channels are received by the cable company via their antenna in ASTC (the current OTA broadcast format) and converted to NTSC (the old transmission format) and distributed to maintain compatibility withthe old cable format which used NTSC and for customers using older TVs with NTSC or NTSC and ASTC receivers.

The antenna receives the stations in their current OTA format (over the air) which is ASTC.

I imagine your TV wants to either be NTSC or ASTC but cannot do both because the programming of the channels also sets which format it expects. Whether it an shortcoming of design or there's a technical reason is not clear to me but its possible to design a receiver to receive both when mixed or switched but the designers probably did not anticipate such a situation.

Using an AB switch or a antenna combiner won't fix this problem, you only possible solution is if your TV has two RF (antenna) inputs - one called cable and the other antenna sometimes or ANT1 and ANT2 and the channel programming allows programming channels on one for ASTC and the other for NTSC and this is probably not specified explcitly int he manual, again because the designers never anticipated it.

Just showing an example of a splitter. I think that one was only a couple of dollars. Years ago, a friend worked for a cable company and supplied me with some of the quality splitters. Get what you pay for.

Expensive but great solution: a TiVo. We use ours just for OTA (cable and satellite are just way to expensive for 20% commercials), but AFAIK you can hook up OTA and cable at the same time and record from either.

Gerd

That particular TV has only a single RF input (coax cable & antenna input) unfortunately. I'd bet that when switching from the cable to the antenna and doing the "channel scan" the TV is basically saying "okay, scan for whatever channels I can find... start by forgetting the ones I've already found." So each time Knottscott does the scan, the channels for the other source are "lost." What he needs is to find an "update list of available channels" scan or edit option that lets the set add new channels without first clearing the old list.

What I'd try first:
1: connect to the cable and let it scan. When done, go into the menus and write down all channels it found - or at least the ones you care about. You may have to go into Menu -> Channel -> Manual Tuning -> and then pick from DTV, TV, or CATV to find the channels it "found" -> highlight a channel and write down the channel number, DTV or TV or CATV, and RF-channel number if available. Now you have a database of your cable channels to manually add later.

2: connect to the outdoor antenna and let it scan again. When done, go back into the Menu >> Channel -> Manual Tuning. Go into the channels you wrote down this time and Add them.

The other option is to use a splitter backwards as some folks have suggested - not an A/B switch - so both signal sources are visible to the TV's scan function at the same time. That does mean though your channel UP/DOWN buttons will scan channels for the antenna and cable at the same time when you use the A/B switch... so if you're on cable for example, you'll have to manually UP/DOWN past any antenna channels while surfing.

mpc

If you are going to use a splitter "backwards", make sure it is a bidirectional splitter - some only work in the splitting direction. Better yet, get a combiner, which is already a "backwards splitter".

Interesting side note - during the transition period, TVs had both analog and digital tuners. Higher end sets still do. The reason is that the "mandatory" switchover to HDTV only applies to larger commercial stations. Smaller stations still broadcast in analog, so it is helpful to have both turners. In this area, there are several analog stations, mostly the college stations (we have several universities in this area).

The cable companies receive the programs via a satellite feed. Typically they download compressed files overnight, then decompress the files and "air" them at the scheduled time.

The cable company signal to the converter box is in QAM format - an amplitude modulated format that is more efficient for wire cables. The converter box converts the signal to either NTSC, ASTC, or a direct video format such as RGB or HDMI.

The reason the cable companies have to reduce the resolution to transmit on a cable is that the cable has limited bandwidth. The cable system was developed for NTSC signals, which can be transmitted in a compressed form with less than 2 MHz of bandwidth. An HDTV signal requires at least 10 MHz of bandwidth, though that is reduced to 6.5 MHz for the ATSC standard, with some "tricks". In any event, the cable can support roughly one full definition HDTV channel for four analog TV channels. As a result, the cable companies would have to reduce their channel counts by at least four to make full HDTV work, and they are unwilling to do that. Many cable users have analog TVs, which cannot show the greater resolution anyway, so the cable companies have opted to maintain channel count at the expense of quality.

Thanks so much to everyone for all the great replies. This stuff is far more complex than I realized.

I'm going to try and manually tune some of the antenna channels into the TV, and may also give a combiner or backward splitter a shot if the manual tuning doesn't work out. Gotta a funeral today, and start my work rotation tomorrow, so won't be back to the land of the living until Thursday (which might give me time to source a combiner.).

This info is much appreciated!