Question: Understanding sub-channels

EdJones

DTVUSA Rookie
#1
Hi, all.
I'm curious about how the box actually works, because I don't get what's happening.

My understanding is that a digital channel (2 or 7) is one signal stream (8vsb) of MPEG-2 data. So sub-channel 7.2 is not a physical thing at all, but a logical subset of the data picked up. That is, (grossly simplifying) every nth frame of a steady stream of 1's and 0's.

So why does sub-channel 7.3 drop out far more often that 7.1 and 7.2? Shouldn't any interference take out sub-channels randomly?

(Here, 7.1 is CBS, 7.2 is Fox and 7.3 is ABC. Even if 7.3 were of a lower resolution, wouldn't that make it LESS likely to drop out?). Or do I have this all wrong?

Thanks,
ed
 

scandiskwindows9x

Moderator of DTV Latino
#2
You should research how ATSC works, the virtual channels is a part of ATSC called PSPID on google how 8 vsb works and is a pdf file where explain everything there can get some answers

Sent from my LG-P500h using Tapatalk
 

Trip

Moderator, , , Webmaster of: Rabbit Ears
Staff member
#3
It's entirely possible that there is an issue with the station itself that is causing those drops. Because your understanding of how it should work is correct.

It's also possible that you just happen to watch 7-3 at the same time as some type of signal issue such that it appears to be more frequent on 7-3 than the others.

- Trip
 

scandiskwindows9x

Moderator of DTV Latino
#4
Hi, all.
I'm curious about how the box actually works, because I don't get what's happening.

My understanding is that a digital channel (2 or 7) is one signal stream (8vsb) of MPEG-2 data. So sub-channel 7.2 is not a physical thing at all, but a logical subset of the data picked up. That is, (grossly simplifying) every nth frame of a steady stream of 1's and 0's.

So why does sub-channel 7.3 drop out far more often that 7.1 and 7.2? Shouldn't any interference take out sub-channels randomly?

(Here, 7.1 is CBS, 7.2 is Fox and 7.3 is ABC. Even if 7.3 were of a lower resolution, wouldn't that make it LESS likely to drop out?). Or do I have this all wrong?

Thanks,
ed
happen the following, the signal strength is not affected by the resolution of the channel, the payload can be even lower but the signal strength don not get affected by that, on COFDM Is likely more payload equal more fragile signal strenght and that means for a quality broadcast have to use lower payloads and hold the bad quality of image of a MPEG2-4 lower quality due less bitrate

the physical channels are on UHF the virtuals ones are the ones with the . or hyphen ie 13.1 etc the whole signal in 8VSB carries in the PSIP What exactly is ATSC

if a virtual channel is lost mean that the station just eliminated the virtual channel .xx of the service,

question in your television or converter box the virtual channel missed still on memory and can be tuned but says No Signal or just is missed of the converter or television memory?

best regards

Francisco
 
Last edited:

Jim1348

DTVUSA Member
#5
Question: Understanding Sub-Channels

I think that this was already mentioned, but in my DMA not all sub-channels are really from the parent or master channel. For example, we have sister stations here that are Channel 9 and 29. Channel 9 is RF AND virtual Channel 9. Channel 29 is UHF. Originally, both 9.1 and 9.2 were VHF (RF 9) and 29.1 and 29.2 were UHF. Then they swapped! So, now the content that was on 9.2 is on 29.2 and conversely the content that was on 29.2 is on 9.2. Our channel 2s are actually TWO different RF channels with sub-channels that all map up as 2.1 to 2.4. The reason I point this out is someone could have a problem receiving 9.1, but get 9.2 just fine and wondering how that is possible since they are both channel 9, but the reality is the RF channels might be different! The transmitters might not even be at the same location.
 

Trip

Moderator, , , Webmaster of: Rabbit Ears
Staff member
#6
I suspect that Ed is probably watching WTRF in Wheeling, West Virginia, which has all three subchannels on the single VHF signal.

- Trip
 
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