[mattdp]

A long time ago, I believe I read somewhere that the Digital TV standard allowed stations to broadcast in either 8 or 16VSB, and that broadcasters would be able to do 16VSB (doubling bandwidth) after the switchover.

Can anybody come up with any sources for this?

 

[foxeng]

8VSB only. Pre or post transition.

 

[mattdp]

Ok. I was pretty sure, I just wanted to double check.

 

[Rory Boyce]

16 VSB could have been used by cable systems but they chose to use 256QAM instead.

 

[Rick_R]

Also I believe 16 vsb is only a 33% throughput improvement over 8 vsb. (16 vsb is 4 bits per symbol and 8 vsb is 3 bits per symbol. 4 bits is 33% more than 3 bits.)


Rick R


Corrected.

 

[dr1394]



Quote:
Originally Posted by Rory Boyce /forum/post/14194883


16 VSB could have been used by cable systems but they chose to use 256QAM instead.

The common wisdom is that QAM was chosen because there are no royalties to pay. QAM is so old, the patents have expired long ago.


Ron

 

[dr1394]



Quote:
Originally Posted by Rick_R /forum/post/14197895


Also I believe 16 vsb is only a 33% throughput improvement over 8 vsb. (16 vsb is 4 bits per symbol and 8 vsb is 3 bits per symbol. 4 bits is 33% more than 3 bits.)


Rick R

Your math is good, but you have to read the specification.

http://www.atsc.org/standards/a_54a_with_corr_1.pdf


See Table 8.1 on page 72. 16VSB doesn't use the 2/3 Trellis code that 8VSB does, so 16VSB delivers 2X the payload bits as 8VSB.


Ron

 

[videobruce]



Quote:
The common wisdom is that QAM was chosen because there are no royalties to pay. QAM is so old, the patents have expired long ago.

Then was wasn't it chosen for OTA also?


QAM handles a cable enviroment better and has more ability to have additional sub channels without being degraded as much as 8VSB. I doubt a 8VSB channel could carry two HD services, whereas a QAM channel can easily carry three HD services with little drop in quality.


Our cable system has two channels that have three services on each (all OTA) with little drop in quality doing a compairison with the same OTA service. Three of our OTA stations have one or two subchannels and the subchannel(s) look terrible.

 

[CKNA]



Quote:
Originally Posted by videobruce /forum/post/14212234


Then was wasn't it chosen for OTA also?


QAM handles a cable enviroment better and has more ability to have additional sub channels without being degraded as much as 8VSB. I doubt a 8VSB channel could carry two HD services, whereas a QAM channel can easily carry three HD services with little drop in quality.


Our cable system has two channels that have three services on each (all OTA) with little drop in quality doing a compairison with the same OTA service. Three of our OTA stations have one or two subchannels and the subchannel(s) look terrible.

Cable was supposed to use 16VSB not 8VSB. 16VSB data rate is 38.8 Mbps in 6Mhz channel just like QAM. You can do the same thing with 16VSB on cable as you can with QAM.

Degradation comes from overcompression not from modulation.


Also QAM can't be used for OTA by itself, because it needs a carrier. DVB-T standard uses QAM which is carried by COFDM.

 

[CKNA]



Quote:
Originally Posted by Rick_R /forum/post/14197895


Also I believe 16 vsb is only a 33% throughput improvement over 8 vsb. (16 vsb is 4 bits per symbol and 8 vsb is 8 bits per symbol. 4 bits is 33% more than 3 bits.)


Rick R

8VSB is 3 bits per symbol, and 16VSB is 4 bits per symbol.

 

[dr1394]



Quote:
Originally Posted by CKNA /forum/post/14212474


Also QAM can't be used for OTA by itself, because it needs a carrier. DVB-T standard uses QAM which is carried by COFDM.

It's not the carrier that's the problem, it's the terrestrial multipath environment that kills QAM (and PSK). Multipath is just signals arriving at different times, and time equals phase. But the phase of the signal is where the information resides in QAM and PSK, so any multipath just destroys the signal.


Ron

 

[sneals2000]



Quote:
Originally Posted by CKNA /forum/post/14212474


Also QAM can't be used for OTA by itself, because it needs a carrier. DVB-T standard uses QAM which is carried by COFDM.

Though that is a bit a of an oversimplification. QAM is less carried by COFDM - more deployed on each COFDM carrier.


COFDM as deployed by DVB-T uses between 2000 and 8000 separate 16QAM or 64QAM carriers to carry a payload in a given 7-8MHz bandwith channel - not a single carrier. It's a bit like comparing apples and oranges - particularly when you need to consider symbol rates on a given carrier, guard bands and multipath. QAM is deployed in the COFDM environment with success because rather the symbol rate per carrier is so much lower - with a lot more carriers. (It is the same system used by DAB and DAB+ radio and ISDB-T in Japan)


AIUI US Cable 256QAM uses a single carrier per 6MHz channel on cable doesn't it?

 

[nybbler]



Quote:
Originally Posted by videobruce /forum/post/14212234


Then was wasn't it chosen for OTA also?


QAM handles a cable enviroment better and has more ability to have additional sub channels without being degraded as much as 8VSB. I doubt a 8VSB channel could carry two HD services, whereas a QAM channel can easily carry three HD services with little drop in quality.

The bit rate of 256-QAM as used in digital cable systems is 38.8 megabits/sec. This is double the ATSC rate of 19.4 megabits/second. So, no, you're not going to fit three HD services onto a 256-QAM channel without significant degradation, unless they're all 720p.

 

[hphase]



Quote:
Originally Posted by nybbler /forum/post/14220353


...

So, no, you're not going to fit three HD services onto a 256-QAM channel without significant degradation, unless they're all 720p.

Horsefeathers! In round numbers, 3 x 19Mb/s = 57 Mb/s That is more than either 16-VSB or 256-QAM. You've got to bit-shave ("groom," to be polite) or just throw stuff away to fit 57 into 38. 720p has nothing to do with it.

 

[hphase]



Quote:
Originally Posted by dr1394 /forum/post/14216531


It's not the carrier that's the problem, it's the terrestrial multipath environment that kills QAM (and PSK). Multipath is just signals arriving at different times, and time equals phase. But the phase of the signal is where the information resides in QAM and PSK, so any multipath just destroys the signal.


Ron

OTA is a very hostile environment for a digital signal. That's why you need all that forward error correction (FEC.) By comparison, cable is easy. That is why they can drop the FEC and add more discrete signal states for more capacity.

 

[videobruce]



Quote:
So, no, you're not going to fit three HD services onto a 256-QAM channel without significant degradation, unless they're all 720p.

Well, there are at least two QAM cahnnels that have three OTA HD stations contained within each cable channel. I have made many compairisions between them and their OTA signal and can't really see any difference.

I'm sure it's measurable and if I had two identical monitors side by side I might be able to see something, but switching inputs back and forth, I don't see anything. This is with a 56" 1080p DLP.


Doesn't a OTA signal need a pilot which is another reason QAM won't work for OTA?

 

[nybbler]



Quote:
Originally Posted by hphase /forum/post/14228194


Horsefeathers! In round numbers, 3 x 19Mb/s = 57 Mb/s That is more than either 16-VSB or 256-QAM. You've got to bit-shave ("groom," to be polite) or just throw stuff away to fit 57 into 38. 720p has nothing to do with it.

It does, because the broadcast 720p channels are not using the full bandwidth available as it is.

 

[sneals2000]



Quote:
Originally Posted by nybbler /forum/post/14229742


It does, because the broadcast 720p channels are not using the full bandwidth available as it is.

SVT HD are using 18Mbs in H264 in Sweden - so some people are using 720p at a reasonably high bandwith.

 

[CKNA]



Quote:
Originally Posted by nybbler /forum/post/14229742


It does, because the broadcast 720p channels are not using the full bandwidth available as it is.

That depends what station. Some stations use full bandwith. They do not have subchannels.

 

[CKNA]



Quote:
Originally Posted by videobruce /forum/post/14229231


Well, there are at least two QAM cahnnels that have three OTA HD stations contained within each cable channel. I have made many compairisions between them and their OTA signal and can't really see any difference.

I'm sure it's measurable and if I had two identical monitors side by side I might be able to see something, but switching inputs back and forth, I don't see anything. This is with a 56" 1080p DLP.


Doesn't a OTA signal need a pilot which is another reason QAM won't work for OTA?

I bet all of your OTA channels have subchannels so their data rate is already lower, and cable co is using rate shaping, so three fit in 38.8Mbps.