[jeff125va]

I was asked by a neighbor about the difference between what is digital about Digital TV (DTV) and what is digital about Digital Satellite. He didn't understand why only a couple channels on his High-Definition DirecTV receiver were in HD. I managed to explain that HDTV is only a subset of DTV, but then he still thought that the rest should still be another form of DTV. I understand that that's not the case, but I couldn't give a clear explanation as to why. Anyone have one?

 

[David McRoy]

Over-the-air (OTA) digital television and direct broadcast satellite (DBS) digital television are both "digital television (DTV)." Each may offer standard-definition (SDTV) or high-definition (HDTV) pictures.


OTA and DBS use different modulation methods. OTA DTV uses 8VSB while DBS uses QPSK.


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HiDefDave

 

[Ken Erickson]

I was asked the same question by a co-worker. I explained that there a number of "digital" TV systems in use today and they are same as they are different. Digital cable takes the analog NTSC signal that the broadcaster transmit and digitizes/compresses then sends the resulting "digital" signal down the cable to your STB where it is uncompressed and converted back to a NTSC signal and displayed on your TV. The satellite "digital" signal is essentially the same as the cable "digital" signal in that they take the NTSC analog signal and digitize it; compress it and encrypt it and beams it to your STB where it is decrypted; uncompressed and converted back to a NTSC signal.


Digital TV (DTV - not to be confused with DirecTV) is all digital; from the broadcaster's transmitter to your STB. HDTV is one part of the DTV definition.


I don't know the form of transmission of the "DTV" signal via satellite but I sure that they can't compress it anymore as it is already in the MPEG-2 format that the DBS folks use to transmit regular NTSC signals. However they do wrap it in a encrypted envelop.


As to why only a couple of channels on his HD DirecTV receiver? That is due to fact that the HD channels require about 4 times the bandwidth to transmit the HD signal. And those transponder channels are expensive. And don't forget, the DBS broadcaster can't compress the signal and expect to gain any space as it already compressed in the MPEG-2 format. Hopefully, this fall DirecTV will have 1 or 2 birds with the SPOT BEAM system up and running for the local into local channels which would free up a whole lot of transponder slots on the current birds. This is believed then would allow DirecTV to offer some more HD content. But - don't hold your breath on this. DirecTV has been known to lead in this technology.


The bottom line in this new environment is; to get more HD channels will require several things to happen:


1. More satellite transponders. Not likely to happen due to huge cost structure.

2. Better transponder utilization. Maybe but that depends on getting new boxes out in the market and that takes at a miniumum of 2 years and more likely 5 years.

3. Our best chance will be with the local broadcasters. With the FCC mandate for all TV to be broadcast via digital signals by 2006 and for all commerical broadcasters to be broadcasting a digital signal by 2002/2003.


These are my view and YMMV.

 

[jeff125va]

Quote:

Originally posted by David McRoy: Over-the-air (OTA) digital television and direct broadcast satellite (DBS) digital television are both "digital television (DTV)." Each may offer standard-definition (SDTV) or high-definition (HDTV) pictures. OTA and DBS use different modulation methods. OTA DTV uses 8VSB while DBS uses QPSK.

Sounds like you certainly know some things I don't, but perhaps I wasn't clear in what I'm trying to find out, or maybe it's just a matter of semantics. Except for the HBO & Showtime HDTV channels that DirecTV carries, everything else is NTSC. Everywhere I've read articles about HDTV, it usually says something like NTSC = analog, ATSC = digital (DTV - which includes HDTV, SDTV, and other formats). Assuming that's correct, which I believe it is because I remember the first DTV broadcasts several years after I first got DirecTV, then it seems like it's analog in one sense, but then it's going over a "Digital Satellite System" so it's digital in that sense. So what I don't understand is, what's the difference between what is digital about an NTSC broadcast on a digital satellite system, and a DTV broadcast?

 

[jeff125va]

Ken you totally answered my question while I was typing my reply. Thanks.

 

[David McRoy]

The "non-HDTV" channels on DirecTV are all "standard-definition" digital television, although in many cases the original source video on these digitally transmitted channels may have been analog.


Here's a simple analogy: let's say I've got a digital radio station and it transmits data in the audio CD format: 16 bit, 44.1 kHz. I might play CDs, simply passing the data off the disc directly to my air signal. I might, though, broadcast an analog source like an LP record. I would then need to sample the analog audio from a phono preamp and encode it digitally for air. Now I'm broadcasting analog audio over a digital radio station.


On DirecTV, all the channels are digital but the source video varies in origin.


On over-the-air digital television (DTV) the same rule applies. You'd have to know the pedigree of every show as far as its origin to determine whether you were looking at "all digital" or perhaps something that started out as analog and was subsequently sampled digitally for transmission.


Back to DirecTV: they do, indeed "resample" the incoming source and often allow the picture quality to become degraded in order to conserve spectrum, at least on many standard-definition channels. I've had the displeasure of comparing my own analog NTSC TV station's OTA antenna signal to the same signal via DirecTV's local-into-local service. The latter looked pretty bad, since DirecTV was allowing the picture quality to degrade quite a bit to conserve spectrum available to them on that particular satellite transponder.


DirecTV plans to add "spotbeam" satellites for LIL to free-up some spectrum on the existing birds so that they can back-off on the amount of digital compression they're currently doing, which should improve the PQ.


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HiDefDave


[This message has been edited by David McRoy (edited 07-19-2001).]

 

[Sarahlea]

This may be just a reiteration of what Ken and David have already said, but here are my two cents worth:


For the record and for your friend, HDTV in theory can be analog or digital (it is not necessarily a subset of DTV). It is exactly what it says it is--HIGH DEFINITION television (i.e. 720p or 1080i lines of resolution, surround sound, and all that jazz). Nothing less, nothing more. One can get HIGH DEFINITION television over an analog signal, it would just take a lot more bandwidth. I am told that Japan in fact does broadcast HIGH DEFINITION television using analog signals.


The digital question has nothing to do with HIGH DEFINITION. Is is merely the streamlined method by which a signal gets from a production source to your television. Any signal can be broadcast in a traditional analog format (where the information is part of the moduluation of the radiowave) or in a digital format (where the information is encoded as a series of zeros and ones and the radiowave then carries the zeros and ones to the receiver). The primary advantages to digital signals are less bandwidth and less degradation of signal quality (thus the often heard statement, "crystal clear digital sound).


I BELIEVE the answer to your friend's question is WHEN and HOW MUCH of a signal is digital determines whether it is digital television or can be considered DTV.


Satellite broadcasters take a standard analog signal and digitize it before they uplink to the satellite. When the signal gets to your reciever, the receiver converts the digital information back into an analog format. Thus, they have broadcast a digital signal and can claim some level of digial television.


I BELIEVE that DTV implies the source signal itself is digital and is kept digital until it hits your receiver, which will then convert it to analog before it travels over the wires to your speakers and video source. But that is just my interpretation of DTV. THUS, DTV would have an even better quality of signal than digital television that is just digital in the last (although significant) stage of the broadcast stream.


Hope this helps.