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How do TV sub channels work?

gary350

Well-Known Member
I looked this up online it talks about the number system and what it means and industry standards and many other things but it never tells how sub channels really work. Only thing it says about channel frequency is #1 is the main carrier frequency. Channel 4.1 is the main carrier while 4.2, 4.2, 4.4, 4.5 are all sub channels. Carrier frequency is analog but information on the carrier is digital. It says nothing about how they get several sub channels on the same carrier. It says nothing about side band either.

How does 1 carrier MHz have 3 to 7 sub channels?
 

rjenkinsgb

Well-Known Member
Most Helpful Member
It's a "digital multiplex".

A continuous stream of compressed digital data, but split in to separate "frames" each with its own ID that allows the receiver to select which frames are needed for whichever channel the user is watching.

Think of it a bit like data on your home broadband link, if many people were all watching different video streams on their phones or computers.
Data frames or packets for all the different streams are being send along the same broadband / ethernet connection, then separated out by the information within those data frames themselves.

The only real difference is that the broadcast system allocates a fixed set of channels to each multiplex, rather than the users being able to chose them.

The GSM mobile phone system also uses a digital multiplex to handle multiple phone calls on each radio channel, but in that case the frequency and "timeslot" each phone used are dynamically set by the cell tower.
 

gary350

Well-Known Member
Are you talking about DVB-T channels?
I don't know about DVB-T channels?????????

Here is TV Fool picture for my area we get channel 2, 4, 5, 8, 17, 28, 30, 40, 44, 50, 58. Each of these have 3 to 7 sub channels.

Sub channels are

2.1 2.2 2.3 2.4

4.1 4.2 4.3 4.4 4.5

5.1 5.2 5.3 5.4 5.5 5.6

8.1 8.2 8.3

17.1 17.2 17.3

28.1 28.2 28.3 28.4 28.5 28.6

30.1 30.2 30.3

40.1 40.2 40.3 40.4 40.5

44.1 44.2 44.3 44.4 44.5 44.6 44.7

50.1 50.2 50.3 50.4 50.5

58.1 58.2 58.3 58.4

For example channel 28.1 it the channel number shown on TV but the FCC designated frequency for virtual 28 is REAL channel 36 = UHF frequency 603 MHz

All 6 of channel 28 sub channels are on the same carrier frequency of 603 MHz.

HOW do you get a total of 6 channels on 603 MHz carrier ?





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Nigel Goodwin

Super Moderator
Most Helpful Member
I don't know about DVB-T channels?????????
Well you should, as that's all there is now.

HOW do you get a total of 6 channels on 603 MHz carrier ?
As has already been explained, there are no 'sub-channels' - DVB-T transmits in multiplexes, which contain a number of digital channels in that one multiplex.

The 603 MHz isn't a 'channel' at all it's a multiplex.
 

gary350

Well-Known Member
Well you should, as that's all there is now.



As has already been explained, there are no 'sub-channels' - DVB-T transmits in multiplexes, which contain a number of digital channels in that one multiplex.

The 603 MHz isn't a 'channel' at all it's a multiplex.
So you no longer call the 603 MHz the carrier frequency, NOW it is called a multiplex.

Explain, number of digital channels? How do you have several digital channels on 603 MHz with out all the channels being scrambled together?.

30 YEARS ago I read this EXAMPLE this was before digital TV.

You can have several sub carrier frequencies on the main 500 MHz carrier frequency.

1. 20KHz modulated
2. 25KHz modulated
3. 30KHz modulated
4. 35KHz modulated
5. 40KHz modulated
6. 45KHz modulated

All the sub carries are separate channel being carried on the 500MHz main carrier.

The Receiver separates all the sub carries as 6 different channels.

This is what I am getting at. How does digital TV work ???
 
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Nigel Goodwin

Super Moderator
Most Helpful Member
So you no longer call the 603 MHz the carrier frequency, NOW it is called a multiplex.

Explain, number of digital channels? How do you have several digital channels on 603 MHz with out all the channels being scrambled together?.

30 YEARS ago I read this EXAMPLE this was before digital TV.

You can have several sub carrier frequencies on the main 500 MHz carrier frequency.

1. 20KHz modulated
2. 25KHz modulated
3. 30KHz modulated
4. 35KHz modulated
5. 40KHz modulated
6. 45KHz modulated

All the sub carries are separate channel being carried on the 500MHz main carrier.

The Receiver separates all the sub carries as 6 different channels.

This is what I am getting at. How does digital TV work ???
It works because it's digital - WHY do you keep quoting antique analogue systems?, which don't have the slightest relevance. The sub-carriers aren't 'channels' they are just sub-carriers, and probably carried some kind of data?.

Multiplexing is VERY crudely often just a question of doing things in turn.

So if you have four TV channels in a multiplex, then transmit CH1 for a bit, then CH2 for a bit, then CH3 for a bit, than CH4 for a bit, then restart at CH1.

The 'clever bit' is to time compress the channels - so you transmit (say) 4 seconds of CH1 in 1 second, then 4 seconds of CH2 in 1 second etc.

The receiver takes the data, and decompresses it - restoring the original digital datastream.

This is an EXTREMELY crude 'description' of what 'night' happen, and hopefully will give you some clue as to how it might work.

I've no idea how it's actually done, as there's absolutely no point in knowing.
 

KeepItSimpleStupid

Well-Known Member
Most Helpful Member
A couple of wierd things. You can have 6.1, 6-101 or nearly anything.

You can also have other networks broadcast on say 6.3

ATSC has 9 allocated.
 

gary350

Well-Known Member
There is a lot of interesting information online about how things can be done but does not say if it is done or what it is use for. I like to know how things work I am just naturally curious. I love all kinds of technology not just electronics. I was reading about data compression it works like zip files. If you have 6 channels that need to be transmitted on a 500 MHz carrier you zip up each channel in small pieces then send all the pieces in 123456123456123456123456 order continuously to the receiver it collects all the 1 files unzips them they become a full stream for channel 1. All the number 2s = 222222222222222222222222222222222222222222222222222 are unzipped too and becomes channel 2. And so on. This makes very good sense for TV sub channels maybe this is how it is done. Lots of TV info says data compression but no details how it works.

I have a relative that is retired now he worked for the Department of Defense for 30 years. He was telling me how bugs and listening devices work. A bug only records when people are talking. It stores up 10 minutes of voice then transmitters it as a zip file. Anyone listening or trying to detect bugs hears nothing but when the bug sends the zip file it is faster than a morse code beep it sounds like a tiny static click. Receiver unzips the files into a streaming voice conversation. It seems to me TV sub channels could work about the same way.
 

rjenkinsgb

Well-Known Member
Most Helpful Member
See the sections on "Time division multiplexing" in this article for a more visual explanation:

 

unclejed613

Well-Known Member
Most Helpful Member
Well you should, as that's all there is now.
actually gary seems to be in the USA, and the system here is ATSC.
there are no separate carrier frequencies for the subchannels. as somebody above mentioned, the data packets for each subchannel are interleaved with the data for the other subchannels. some stations only have one channel of data, and therefore can transmit using the highest resolution available. if a transmitter is transmitting subchannels, none of them are high resolution because you are sending multiple channels in the same bandwidth. the transmitter retains the same rate of megabits per second, but the differences are that the data streams' packets have headers that identify which subchannel the packet belongs to. from there, it's just a matter of software arranging the data into sound and picture.
you can find out more about ATSC works here: https://en.wikipedia.org/wiki/ATSC_standards
 

gary350

Well-Known Member
See the sections on "Time division multiplexing" in this article for a more visual explanation:

OK.. the FDM Multiplexing process uses a different carrier for each digital signal = each TV channel. All carriers are combined with the main 603MHz carrier. Receiver separates the 6 different carries into 6 different TV channels.

That is exactly what I said above but I called them sub channels. I only called the extra channels sub channels because online I read where it called 28.1 the main channel and the other channels are known as sub channels.

28.1 has carrier frequency #1
28.2 has carrier frequency #2
28.3 has carrier frequency #3
28.4 has carrier frequency #4
28.5 has carrier frequency #5
28.6 has carrier frequency #6

All 6 carriers combine with 603MHz.

Receiver separates the 6 carriers from the main 603MHz carrier.

TV channel 28.1
TV channel 28.2
TV channel 28.3
TV channel 28.4
TV channel 28.5
TV channel 28.6

THE END. Over and out. I under stand this now. No more questions. I guessed TDM synchronous timing correct too.
 
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tomizett

Active Member
It always astounds me how many standards there are for doing basically the same thing in different parts of the world.
 

Boxnut

Member
In north America we use the ATSC system comprised of the original NTSC channel frequencie but instead of using VSB modulation, it is digital.
What is the main channel has a data stream (transport stream) composed of packets. within these packets are more packets that make up the sub channels. these are all multiplexed together into a 6 Mhz channel 67channels total 2 to 69. The virtual sub channels varies with type of content. For the data rate, there is room for 5 high definition sub channels with Dolby 5-1 surround sound. many may have 10 standard def channels or variants. These are encoded in MPEG2 just like DVD, satellite, and other multimedia. Your TV, you select one of the 67 channels then the sub channels are parsed from there. The selected sub channel passed to the video decoder that handles the MPEG 2 data stream. the selected sub channel is decoded and passed to the video display and sound.
Getting deeper gets very complicated how DVB-T works. the future holds MPEG4 and a link layer of 8PSK modulation for greater improvement. New smart TVs and some broadcasters are already there. soon advancements in modulation and enter DVB-T3
Other countries like Japan or south America use a far different but similar system. More reading here:
 

tomizett

Active Member
Interesting. The nomenclature differences between the USA and UK go a long way to explaining the confusion earlier in this thread.
Unfortunately, despite the best attempts of others, the OP seems to have got the wrong end of the stick...
 

unclejed613

Well-Known Member
Most Helpful Member
i think the OP is still missing the point. the 8VSB signal footprint takes up the whole 6Mhz channel width regardless of whether only 1 subchannel is in use, or 10. all of the subchannels are in the same 19Mbit bitstream. if a subchannel is not used, either all 1's or all 0's are sent. there are no separate frequency slots per subchannel, as it's a single bitstream.... the channelization is done in software. it's like a DSL modem.... you have a single bitstream but it could contain data for many different devices connected to the modem.... the router in the modem decides which data packets belong to which device
 

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