ATSC Digital (VIRTUAL!) Channel NUMBERS vs Actual Channel FREQUENCY... - AVS Forum | Home Theater Discussions And Reviews
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post #1 of 4 Old 07-01-2010, 09:26 PM - Thread Starter
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Quote:
Originally Posted by oldmanjoe View Post

...I use TW digital cable and have two 2160s; all of sudden I couldn't' tune PBS & Fox QAM (15.1 & 69.1). I rescanned both units and they came back as 18.6 & 18.5. All is well, right? But here's the strange part; I have 6 TVs, 2 with a combo tuner. The four with a single tuner still work with the old numbers, but the two combos don't. I rescanned both; one came in with 18.1 & 18.4 and the other can't even find the new numbers (so no PBS & Fox on that one now).

Just wondering if anyone has encountered this type of thing and would be willing to explain it to me. I'm a bit confused...

Quote:
Originally Posted by JoeKustra View Post

...The channels on my 2160 are not the same as my TV on a few channels. I know all tuners are not created equal. The tuner on my Panny EZ28 (combo) thinks 26.1 is dead, my 2160 thinks 26.0 is a real channel and moves the 26.x block up by one. All my Sony equipment (COMBO) agrees on the numbers.

oldmanjoe,

That's a subject (your question AND the title of this post) that I too cannot quite grasp although it 'seems' so simple. In VERY BROAD TERMS (since I don't remember the EXACT FREQUENCIES and FIELD NAMES), all channels, whether referring to the old ANALOG or new DIGITAL television, are REALLY a frequency (like 101.5MHz on your FM radio, or 880KHz on AM, or 54-60MHz for VHF-Channel 2 ). While everyone had no problems tuning a FM radio to 101.5 or 880, with 6 MHz dedicated to *JUST* a SINGLE television channel (VHF Analog Channel 2), they (the FCC) decided to use a STANDARDIZED number. By standardized, I mean EVERY NTSC ANALOG TUNER, when tuned to Channel 2, was receiving this 54-60MHz group of frequencies and in the NYC area, showing WCBS programs.

Reference: Wikipedia: North American Broadcast Television Frequencies

Then came ATSC tuners and digital - [I was about 20 minutes into GOOGLEing (and NOT finding) a comparable Wikipedia ATSC page when I came across this PAIR of AVS Forum posts from 2005 (additional BOLDing mine to emphasize the portions specifically related to MY reply to you]:
  1. Quote:
    Originally Posted by Nitewatchman View Post

    No. Stations have been assigned a separate channel for their digital broadcasts. Each TV station has two transmitters. One for analog and one for digital.

    The actual channel number corresponds directly to a 6MHZ wide range of frequencies that is used by the signal. It is also the channel number referred to in the "Frequency Assignment" column in the results for your location from www.antennaweb.org.

    It is sometimes important to know the actual channels for your local digital stations. Two of those reasons are :

    #1). So you know whether to use a UHF, VHF or a combination of VHF/UHF antenna(s).

    #2). In order to "scan in" an individual station so your receiver can use it, some receivers require you to input the actual channel number. Some receivers will also allow you to tune directly to the "actual" channel via direct access tuning to access the programming, either via a Major channel number, or a major/minor channel number combination, such as "28.3". Note that the minor channel x.3 in this case reffers to a MPEG2 program stream number.

    With analog OTA reception, The "actual channel number" for any given analog station is what we are used to directly tuning to. For example, WNBC-TV analog in New York, NY transmits on VHF channel 4(66~72MHZ), and OTA viewers in NYC area tune their analog TV directly to channel 4 to receive them.

    With digital OTA reception, WNBC-DT(digital) New York, NY currently actually transmits on UHF channel 28(554~560MHZ). Or, another way to say it would be WNBC-DT transmits on RF(Radio frequency) TV channel 28. But, with most receivers, viewers will need to tune to a virtual remapped channel 4.1, or 4.2 to watch programming from WNBC-DT.
    For more info, see "9. If I'm tuning to ch 45 to watch HDTV, why does the display say 7.1?. "

    Note that after analog shut off, some digital stations will be moving to a different channel than they are currently transmitting on. For example, at the current time, KABC-DT, Los Angeles, CA is broadcasting digitally on UHF channel 53, but will switch their digital transmissions to VHF channel 7 after the DTV transistion is complete and analog shut off occurs.

    Reference: Is a Station's Digital Signal Transmitted on the Same Channel as its Analog Signal?
    .
  2. Quote:
    Originally Posted by Nitewatchman View Post

    Within their signal, digital stations send channel remapping info via something called "PSIP". PSIP stands for "program and system information protocol" -- see here for more info : http://www.psip.org/ . Note that stations can, and do send quite a bit more info besides virtual channel remapping info via PSIP, such as Time/date info as well as Electronic programming guide.

    In most cases, the virtual "remapped channel" number is the channel you will see displayed on your TV, when you tune to a local digital/HD station, and in most cases is the channel you'll tune to in order to watch that station.

    In most cases, in discussions on AVSforum the virtual, "remapped channel" number is usually what we use when we list digital station channel numbers, and is also what is referred to in the "channel" column in your results for your location at www.antennaweb.org.

    Virtual channel remapping via PSIP allows stations to keep the channel branding which exists for their current, analog station even though the actual channel* the digital station is broadcasting on is different currently, or may be a different actual channel number after analog shut off occurs. It also allows viewers to use the same channel number for the digital station they are used to using for the analog station.

    * - For more info See 8. Is a station's digital signal transmitted on the same channel as its analog signal?

    An important thing to remember is that the "remapped channel" is a "virtual channel" and has nothing to do with the actual channel/frequency the digital station is broadcasting on. Therefore even though it may appear when you tune to say, channel 4.1 that you are tuning to a VHF channel, the station may actually be broadcasting on a UHF channel/frequency and the tuner in your receiver/set is actually "tuning" not to channel 4, but to the actual channel the station is broadcasting on, even though it displays "4.1" on the screen.

    For example, WNBC-DT(digital) New York, NY remapped channel is 4, although WNBC-DT actually transmits on UHF channel 28(554~560 MHZ).

    Also, with Digital TV, "remapped" channels are displayed in the following format :

    [ X.x ] X = Major remapped virtual channel number, and x= minor remapped virtual channel number .

    A station will have one major remapped channel number, and, with multicasting can run several different program services on several different minor channel numbers. For example, WNBC-DT New York has the following remapped virtual channels :

    4.1 for NBC/NBC HD/WNBC programming
    4.2 for NBC "weather Plus"

    In most cases the remapped major channel number will be the same as the analog station's channel number. For example, WNBC-TV(analog) transmits on VHF channel 4(66~72MHZ), WNBC-DT(digital) remapped major channel number is also 4, even though the digital station actually transmits on UHF channel 28(554~560MHZ).

    One problem with virtual channel remapping is that an OTA only DTV receiver must be getting a "good enough" signal to acheive a signal lock from the station in order to receive the PSIP information from the station in order for the channel remapping to be accomplished. So, in some cases it can be difficult, and cumbersome, to adjust the antenna for reception of different stations while only using the "autoscan" feature of the receiver to "find" stations in your area. However, luckily, most, if not all receivers do have the capability to allow the user to either :

    #1).Tune manually directly to the channel the station is actually broadcasting on via "direct acess tuning".

    #2). Access a "channel edit" screen that will allow you to manually select actual channel numbers for broadcast stations in your area so that the PSIP info from the station will be "saved" when you achieve a signal lock on the station of interest.

    #3) Via a menu option, to specify the Actual channel number the station is broadcasting on in order to "scan in" the station so you can view the signal meter on the receiver while adjusting antenna accordingly, or an option that will allow you to add/scan in a number of "new" channels without deleting previously "scanned in" channels.

    It is often necessary to have these options not only for adjusting your "rabbit ear" antenna, but also so you will be able to "scan in" stations in different directions from your location given for example, the use of a directional antenna with rotor.

    Reference: If I'm Tuning to Ch 45 to Watch HDTV, Why Does the Display say 7.1 ?
I'm a (former) PROGRAMMER/ANALYST, not an ENGINEER - so, to me, the above explanation is rather BLACK & WHITE:
  • Before the elimination of most analog television transmitters in North America, *IF* I wanted to view NTSC Channel 4, I'd set my OLD ANALOG NTSC TUNER to VHF Channel 4, 'behind the scenes' it would tune to the ACTUAL FREQUENCY of 66-72MHz and PRESTO, I'd be viewing WNBC-NY in analog TV.
    .
  • Today, if I want to view ATSC Channel 4.1, I'd set my NEW DIGITAL ATSC TUNER TO 'Virtual' Channel 4.1, 'behind the scenes' it would tune UHF Channel 28, which then again 'behind the scenes' would tune to the ACTUAL FREQUENCY of 554-560MHz (*ACTUALLY* just a PORTION of 554-560MHz since there are also various SUB-CHANNELS, i.e. 4.2, 4.3, etc... available) and PRESTO, I'd be viewing WNBC-NY in HDTV.
SOOO...

After reading all that, I personally cannot understand why,VIRTUAL CHANNEL / PSIP-wise, some ATSC tuners (leaving QAM out of the equation just for simplicity!), see THE SAME FREQUENCY at a DIFFERENT VIRTUAL CHANNEL?!?

I *CAN* understand 'someone', i.e. an employee at the cable company, making a TYPO - since I have TWO Channel 33.1s. But, due to "some old guy's" theory about time & space, TWO 'things' *CANNOT* occupy the SAME SPACE at the SAME TIME, so I *KNOW* that the ACTUAL FREQUENCIES must be different.

To Summarize:
  1. ONE Cable System
  2. ONE ACTUAL FREQUENCY GROUP (i.e. UHF Channel 28 or QAM-256 Channel 55 - too tired, too late to GOOGLE the ACTUAL FREQUENCY GROUP)
  3. TWO *DIFFERENT* ATSC TUNERS - *SHOULD* display the SAME VIRTUAL CHANNEL NUMBER when tuned to the SAME ACTUAL FREQUENCY GROUP
    • Many of us know this NOT to be true, somehow...
The *ONE* clue might be the RED highlighted portion of the SECOND QUOTE regarding VIRTUAL CHANNEL REMAPPING and SIGNAL LOCK... Since the actual FREQUENCIES I'm receiving are the same (evidenced by the same program material - my only clue), why ATSC Tuner #1 reads the PSIP data as "18.1" and ATSC Tuner #2 reads it as "18.2" is a mystery to me...

Final WHEW! I'm tired of typing...

Low Post Count <> Low Knowledge ergo High Post Count <> High Knowledge

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post #2 of 4 Old 07-02-2010, 02:53 AM
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Okey dokey...

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post #3 of 4 Old 07-02-2010, 09:42 PM
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Ummm...

OLDMANJOE was asking about channel numbers on TW cable.

What does that have to do with ATSC, which is the OTA digital format, not cable (if I understand correctly)?

I mean, if a TV can work with both OTA digital and digital cable, it's usually specified that it has both an ATSC tuner and a QAM tuner.

I also question the propriety of continuing a previous forum exchange by starting a whole new thread.
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post #4 of 4 Old 07-03-2010, 03:32 PM
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Sorry, I didn't mean to start anything out of order. Because the 2160 changed channels numbers, I thought maybe someone had encountered this problem. I probably should have posted in a different forum originally-I just wanted my TV to work . You folks are way over my head on this stuff. I can follow instructions and read diagrams-Love to read manuals and play with stuff; but the knowledge is lacking.

After playing around some more, my only conclusion is that this particular TV has an inferior tuner. I ran across this with another unit not long ago. Also, I'm sure TW has something to do with it.

Thanks again for all the input and I really hope I did not get too much OT with this stuff.

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