TV Broadcast Question

[G7RUX]

I'm not suggesting Wi-Fi be used for TV broadcast. I'm comparing the technology of Wi-Fi with spread spectrum.

Many late WiFi systems are COFDM…

 

[prcguy]

The US and UK/EU systems are different in general, yes, although various systems in the US do use DVB-T.

CRC is a very crude form of error detection for such systems and DVB-T uses much more powerful and capable error detection and correction which is where a lot of the advantages stem from.DVB-T uses an external interleaver, a Reed-Solomon coding block and also punctured convolutional coding which all together mean that the FEC system is very capable indeed.

The more FEC you run the more BW you need, FEC is basically just redundancy for missed bits.

 

[G7RUX]

The more FEC you run the more BW you need, FEC is basically just redundancy for missed bits.

I know, although the DVB-T system uses a fixed channel BW so all increasing FEC does is to spend more of the available capacity on error correction, making the system more resilient at the cost of throughput.

 

[Ubbe]

The DVB-T2 system we have for TV use one MUX that takes up the same 7MHz bandwidth as one analog channel used to do but carries 8 digital TV channels. A much better use of the frequency bands that have allowed to free up the 700MHz band for new 2-way radio systems, like our new national wide public safety LTE system will use.

There are several transmitters using the same frequency and the COFDM technique works better the more signals sources it sees and are free of ghosts in the picture that was the issue with the analog system that needed very directive antennas. The new digital system uses 5kW transmitters when one analog channel needed 50kW.

/Ubbe

 

[G7RUX]

The DVB-T2 system we have for TV use one MUX that takes up the same 7MHz bandwidth as one analog channel used to do but carries 8 digital TV channels. A much better use of the frequency bands that have allowed to free up the 700MHz band for new 2-way radio systems, like our new national wide public safety LTE system will use.

There are several transmitters using the same frequency and the COFDM technique works better the more signals sources it sees and are free of ghosts in the picture that was the issue with the analog system that needed very directive antennas. The new digital system uses 5kW transmitters when one analog channel needed 50kW.

/Ubbe

Oh absolutely so. SFNs are a great feature of the DVB-T spec that I don't believe is available to ATSC systems. It does however mean you need to use a fairly long guard interval though.

 

[prcguy]

The DVB-T2 system we have for TV use one MUX that takes up the same 7MHz bandwidth as one analog channel used to do but carries 8 digital TV channels. A much better use of the frequency bands that have allowed to free up the 700MHz band for new 2-way radio systems, like our new national wide public safety LTE system will use.

There are several transmitters using the same frequency and the COFDM technique works better the more signals sources it sees and are free of ghosts in the picture that was the issue with the analog system that needed very directive antennas. The new digital system uses 5kW transmitters when one analog channel needed 50kW.

/Ubbe

I suspect there is something in the CODFM that is specific to each TV transmitter or station to keep the TV tuned to the desired channel. When DirecTV went from its original mpeg2 to mpeg4 in DVB-S2 with spot beams in about 2007, they had a problem with some receivers trying to lock onto very weak remnants of spot beams of different transponders in other areas on the same frequency. They implemented a little known feature in DVB-S2 called scrambling code or Gold Code to each individual transponder so the receiver would ignore the weak spot beams from other areas.

 

[G7RUX]

I suspect there is something in the CODFM that is specific to each TV transmitter or station to keep the TV tuned to the desired channel. When DirecTV went from its original mpeg2 to mpeg4 in DVB-S2 with spot beams in about 2007, they had a problem with some receivers trying to lock onto very weak remnants of spot beams of different transponders in other areas on the same frequency. They implemented a little known feature in DVB-S2 called scrambling code or Gold Code to each individual transponder so the receiver would ignore the weak spot beams from other areas.

There is indeed and it is found in the Network Information Table.

 

[prcguy]

I suspect there is something in the CODFM that is specific to each TV transmitter or station to keep the TV tuned to the desired channel. When DirecTV went from its original mpeg2 to mpeg4 in DVB-S2 with spot beams in about 2007, they had a problem with some receivers trying to lock onto very weak remnants of spot beams of different transponders in other areas on the same frequency. They implemented a little known feature in DVB-S2 called scrambling code or Gold Code to each individual transponder so the receiver would ignore the weak spot beams from other areas.

BTW, I set up and handled satellite uplink for the first DirecTV DVB-S2 HD demo at the 2005 CES show in Las Vegas. That was uplinked to a regular Ku band transponder and not on one of DirecTVs satellites.

 

[tweiss3]

I'm not suggesting Wi-Fi be used for TV broadcast. I'm comparing the technology of Wi-Fi with spread spectrum.

Well, Wi-Fi range is significantly different than UHF OTA TV, and the bandwidth used for a single channel is huge. 2.4 GHz uses 20 MHz channel width most commonly, while 5 GHz most often uses 80MHz and the new WiFi7 (6GHz) uses 160MHz channel width to really get throughput. That would take all of VHF in 1 channel, and only have a few channels in UHF, not to mention the all the LMR users of those bands.

 

[BinaryMode]

I can tune a massive amount of IPTV channels via a 20 MHz bandwidth...

Also, I already said Wi-Fi was 20 MHz at 802.11g.

 

[krokus]

I can tune a massive amount of IPTV channels via a 20 MHz bandwidth...

Also, I already said Wi-Fi was 20 MHz at 802.11g.

That is one stream at a time, coming from a server. A true broadcast is a very different beast. All the feeds need to be present at the same time, all the time. The codecs can allow time compressed interleaving, but they are indexed as a constant flow.

You seem to be confusing FHSS for a single narrowband signal, with a widebanded composite signal, as SS is useful for narrow signals.

 

[prcguy]

That is one stream at a time, coming from a server. A true broadcast is a very different beast. All the feeds need to be present at the same time, all the time. The codecs can allow time compressed interleaving, but they are indexed as a constant flow.

You seem to be confusing FHSS for a single narrowband signal, with a widebanded composite signal, as SS is useful for narrow signals.

A multi channel broadcast stream will most certainly have adaptive compression where some channels might be given higher priority for quality and when high motion bit hungry scenes appear the compression engine will steal from the low priority channels and give more bits to the priority channel while keeping the same overall transponder band width. For direct to home satellite TV they even stack certain channels together for better compatibility like combining a couple of fast moving sports channels with a news channel talking head or some other low BW channel.

 

[wtp]