HDTV Tech Question

[techguy1975]

I've been curious about something with HDTV for awhile now, now that we got some station engineers here, maybe they can answer this for me..

What is different about HDTV that allows stations to be stacked right next to each other, for example, we have WITI on 33, WISN on 34, and WMVT will be on 35 when they hit the air next year.  

Also, WCGV's alotment is for ch.25 (when they hit the air).  How can they do this without causing co-channel interferrence?  Is the bandwidth for DTV lower, or is it built into the tuner to tolerate and reject co-channel?

Just somethin I always wondered....

[wxndave]

 

Originally posted by techboywi:

What is different about HDTV that allows stations to be stacked right next to each other, for example, we have WITI on 33, WISN on 34, and WMVT will be on 35 when they hit the air next year.  

Also, WCGV's alotment is for ch.25 (when they hit the air).  How can they do this without causing co-channel interferrence?  Is the bandwidth for DTV lower, or is it built into the tuner to tolerate and reject co-channel?

Just somethin I always wondered....[/B]

Well it is a complicated answer. When I read your questions I went the ATSC website to look for the answer.  I don't know if you like technical reading but it is interesting.  The following link describes how they do DTV measurements.  
 http://www.atsc.org/standards/a_64a.pdf


Here goes a brief description.  Each Station is assigned a 6Mhz wide channel.  They don't use all of the 6Mhz.  From what I read at the ATSC site.  Each channel transmits about 5.38Mhz.  So they have stated that the emmision of the stations between 6 and 5.38 must below a certain level.  This area is called a guard band.  I really suggest that anyone who is interested in understanding how DTV works should go to the ATSC website.

I hope this helps.

Dave

     

[Tom Weeden]

 

Originally posted by techboywi:
What is different about HDTV that allows stations to be stacked right next to each other, for example, we have WITI on 33, WISN on 34, and WMVT will be on 35 when they hit the air next year.

The out-of-channel emission specifications for DTV transmitters are way more strict than for analog so they can cram more stations together.  (They've done it on cable for years...you can have adjacent channels occupied if everything's filtered properly and the signal levels are the same.)

If I have it correct, at the edge of a digital station's channel, their signal has to be suppressed by 35 dB (less than .001) from their in-channel power.  6 MHz outside the channel, it has to be at least 110 dB lower (.00000000001)!

So WISN-DT34's signal could spill into the top of WITI-DT33's channel slightly with no problem if power levels were similar.  But if channel 34 is running 1000 times the power of channel 33, that may be an issue since the permitted 34 spillover would be at the same level as 33's main signal.  There are guard bands, so it's not as simple as all this, but you get the idea.

In Madison, WMTV-DT is on 19 and WHA-DT is on 20.  They run about 5 times the power that we run (100 kW vs 21 kW), but everyone seems to be playing together nicely.  We have a DTC-100 receiver in the same room as the channel 19 transmitter and it picks up channel 20 just fine.  (It uses a small UHF yagi just outside the transmitter room pointed at the WHA tower about 3 miles away.)