[MacDV] Re: "Fragility" of broadcast digital signals--MacDV Digest, Vol 12, Issue 14

[Ted Langdell]

Ted Langdell
Ted Langdell Creative Broadcast Services
Marysville, CA
Main:  	(530) 741-1212

Mike,

As Kunga says... digital signals are more "fragile..." meaning more 
affected by reception problems than analog.

An analog transmission is simply a varying voltage that matches the 
brightness component of the original video signal.

If the signal is weak, you can still see some kind of picture and get 
some kind of sound, even though it may have snow and hiss.

A digital version of the same thing is more complex, especially when 
"compressed" (total amount of data is reduced) by throwing away certain 
components that lab tests show people won't (or aren't supposed to) 
miss or detect.

In MPEG-2, you have only one "FULL" frame of the picture being 
broadcast every 6-15 frames or so.  The other frames are re-created 
using mathematical estimations of what was there or should be there.  
This is called a Group of Pictures, or GOP, pronounced GAHP.

A simpler way of putting it is that MPEG compressed files "throw away" 
the elements of the frames that don't change between the full frames.

That reduces the amount of data to be handled, and allows a stream of 
pictures to be transmitted using less bandwidth than the same number of 
full-picture frames of the same content.

Unlike downloading a video clip via the internet... there's no way for 
the broadcast system to re-send missing packets in order for the set to 
continue assembling the data in contiguous order.

When the transmission path from station antenna to your receiver's 
antenna is strong enough and "clean" enough (without interference, 
multipath, etc., that would disrupt the recreation of the data) you get 
a picture and sound.  When there's a lack of signal strength or other 
problems, you don't. It's either ON or OFF.

As far as DTV transmission bandwidth and power output:  The new DTV 
channels are engineered to—essentially—duplicate the current coverage 
area of the paired analog station.  They use the same 6MHz bandwidth as 
analog channels, but are predicted to be able to duplicate the coverage 
using fewer watts of power.

For example, where most US analog UHF stations (Channels 14-68) run 
five million watts peak visual power (5MW or Megawatts) into 
non-directional antennas, the digital counterparts may achieve the same 
effective coverage using a million watts, sometimes into a directional 
antenna.

This is "new science" or "new art," so. what might work in a computer 
model may run into trouble when deployed in the real world. Or it could 
work even better.

Someone who might get acceptable analog reception now might have 
trouble getting a digital signal. There may be some cases where that's 
reversed.

Engineers are finding out what works and doesn't work in actual working 
conditions... in much the same way as computer hardware and software 
makers  have to "put it out there" and then "get the bugs out."

The key is being able to receive the entire 19Mb (mega bit) per second 
stream without interruption.  As the saying goes, "Your mileage may 
vary" in a given location.

Hope this helps...

Ted.


On Jul 20, 2005, at 9:26 AM, 
macdv-request at listserver.themacintoshguy.com wrote:

> Message: 2
> Date: Tue, 19 Jul 2005 13:19:09 -0500
> From: Michael Winter <winter at mac.com>
> Subject: Re: [MacDV] January 1, 2009 Doomsday Plan
> To: "A place to discuss digital video on Macintosh."
> 	<macdv at listserver.themacintoshguy.com>
> Message-ID: <4AF30DB9-E0C0-433C-A558-39461DA0522E at mac.com>
> Content-Type: text/plain; charset=US-ASCII; delsp=yes; format=flowed
>
>
> On Jul 19, 2005, at 12:44 PM, Kunga wrote:
>
>> I already have a off air digital tuner in my EyeTV 500. The point
>> is that digital transmissions are much more fragile than analog
>> transmissions and breakdown completely if you have no line of sight
>> to the transmitter.
>
> I really don't know how much more fragile they are (would depend on
> how things are handled), but why would you need a line of sight to
> the transmitter any more with digital than with analog? I thought
> part of the reason for going digital is that each station can
> broadcast in a narrower band, allowing transmission power to be
> increased without interfering with other signals.
>
> As I said, I really don't know, so if you have any good references
> handy, I'd be happy to "get educated".
>
> -Mike
>
>