16 QAM & 64 QAM - the difference??

gwiddecombe · 15/02/05 - 21:15

Hi, I've just been reading a few articles regarding how some muxes are broadcast in 16 QAM and others in 64 QAM. What is the difference between the two? Does one provide a stronger signal or does it enable more channels to be fit onto a mux or something else? What are the advantages of one over another?

Sorry if this has been covered before in another thread but it is something I have never properly understood.

Air Resistance · 15/02/05 - 21:38

64QAM-Provide more channels, but is more prone to interfirance.

16QAM-Provides less channels, but is less prone it interfirance.

What I don't get is why not have 40QAM?

TUTV Viewer · 15/02/05 - 21:50

bbc rules wrote:

What I don't get is why not have 40QAM?

Isn't 32QAM the half way point?

Same question applies though!

Daveoc64 · 15/02/05 - 22:34

There isn't a 32QAM or a 40QAM. The other possibility is QPSK, although I am not sure which one this is most like.

gwiddecombe · 15/02/05 - 23:32

Yeah thats weird because 40 QAM would be a half way house and would increase the capacity on dtt. What is QPSK by the way? Does the BBC being a PSB have an obligation to broadcast in 16QAM at all? I now see why SDN channels seems to suffer from more interference being 64QAM. Is it because channels were 64QAM in the on digital days that more could fit in to the platform?

robot · 16/02/05 - 00:22

I seem to recall that the transmission modes have to go up or down by a factor of 4 (ie 2 squared).

Therefore there is no 32QAM, but there could be an 4QAM or a 256QAM.

[Deleted User] · 16/02/05 - 00:36

i assumed it was hexadecimal...hence 16 & 64

Air Resistance · 16/02/05 - 07:11

Theres no point changing to 64QAM. On 16 QAM it holds 6 channels, but with 64QAM it's 8 channels and for the amount it improves the signal it's not worth it.

[Deleted User] · 16/02/05 - 09:38

QPSK is Quadrature Phase Shift Keying = 4QAM

[Deleted User] · 16/02/05 - 09:41

6 channels on 16QAM is theoretically possible but the quality would be dreadfull.

16QAM = 4-5 streams
64QAM = 8-9 streams

would be more like it.

TUTV Viewer · 16/02/05 - 09:43

robot wrote:

I seem to recall that the transmission modes have to go up or down by a factor of 4 (ie 2 squared).

Therefore there is no 32QAM, but there could be an 4QAM or a 256QAM.

So how many streams in a 256QAM transmission?

Has anywhere in the world used it yet?

[Deleted User] · 16/02/05 - 09:54

I guess it would be

If 16QAM = 4streams then

64QAM = 8streams

256QAM = 16streams

probably wrong though!

[Deleted User] · 16/02/05 - 09:59

robot wrote:

I seem to recall that the transmission modes have to go up or down by a factor of 4 (ie 2 squared).

Therefore there is no 32QAM, but there could be an 4QAM or a 256QAM.

There could be a 36QAM

Wobbly Steve · 16/02/05 - 10:01

gwiddecombe wrote:

Hi, I've just been reading a few articles regarding how some muxes are broadcast in 16 QAM and others in 64 QAM. What is the difference between the two? Does one provide a stronger signal or does it enable more channels to be fit onto a mux or something else? What are the advantages of one over another?

Sorry if this has been covered before in another thread but it is something I have never properly understood.

It had been asked before - I asked !! - anyhow here is the link !!!!!

http://forum.digitalspy.co.uk/board/showthread.php?t=81550

[Deleted User] · 16/02/05 - 10:07

TUTV Viewer wrote:

So how many streams in a 256QAM transmission?

Has anywhere in the world used it yet?

I imagine it would be possible, but the broadcast range would be a lot smaller - more transmitters and more power would be required.

TUTV Viewer · 16/02/05 - 12:26

Multisandia wrote:

I imagine it would be possible, but the broadcast range would be a lot smaller - more transmitters and more power would be required.

That could be interesting then.

Potentially a single high power transmitter could deliver the 16 basic channels for a remote but fairly clustered town or village.

BBC1
BBC2
BBC3 CBeebies
BBC4 CBBC
BBC News 24
ITV1
ITV2
ITV3
C4
More4
E4
C5
SKY News
ITV News
UKTV History
SKY Travel

Would need a compatible box though.

hazzamon · 16/02/05 - 12:28

Multisandia wrote:

I imagine it would be possible, but the broadcast range would be a lot smaller - more transmitters and more power would be required.

I believe it is used for Digital Cable services in the US.

TUTV Viewer · 16/02/05 - 13:29

Redcoat wrote:

256QAM is part of the DVB-C (Cable) spec but not for DVB-T (Terrestial) - QPSK, 16QAM and 64QAM are listed for DVB-T

Impossible?

Or just not yet?

[Deleted User] · 16/02/05 - 15:30

Not really usable for DVB-T as the signal is far less robust & more susceptible to interference. Cable systems are enclosed and so are not as prone to interference.

Orry Verducci · 16/02/05 - 15:43

I think the two modes are something to do with error correction. 16QAM has less channels but has a lot more error correction data meaning there is a lot less interference. 64QAM has more channels but less error correction data making it prone to interference.

Rob_Online · 16/02/05 - 15:50

Which Multiplexs broacast n 64QAM? Is it just Mux A or does Mux 2 now broadcast in 64 as well?

PaulS67 · 16/02/05 - 15:52

*Somerset-Freeview* wrote:

Which Multiplexs broacast n 64QAM? Is it just Mux A or does Mux 2 now broadcast in 64 as well?

Muxes 2 & A have allways been 64QAM

[Deleted User] · 16/02/05 - 16:28

orrysos wrote:

I think the two modes are something to do with error correction. 16QAM has less channels but has a lot more error correction data meaning there is a lot less interference. 64QAM has more channels but less error correction data making it prone to interference.

Nope, nothing to do with error correction, it's a description of the number of points in, and hence, the spacing of the QAM constellation.

kev · 16/02/05 - 21:23

In 64 QAM each "bit" can hold one of 64 Values, while 16QAM is one of 16 Values.

So (to simplyfy loads)
"The Quick Brown Fox "
would be broadcast as
[The ][Quic][k Br][own ][Fox ]
on 64QAM and
[T][h][e][ ][Q][c][k][ ][r][o][w][n][ ][F][o][x][ ]
on 16QAM

So if one "bit" gets corrupt on 16QAM it is less noticeable than on 64QAM, and the error correction stands a better chance of fixing it.

256 QAM would be
[The Quick Brown ][Fox Jumped Over T]
Hence, losing 1 "bit" would be far more disastourous!

meltcity · 16/02/05 - 21:34

According to Greg Dyke's article on HDTV the BBC is seriously considering switching from QAM16 to QAM64 in order to free up enough bandwidth for two HDTV channels. Watch out, Murdoch!

kev · 16/02/05 - 21:42

meltcity wrote:

According to Greg Dyke's article on HDTV the BBC is seriously considering switching from QAM16 to QAM64 in order to free up enough bandwidth for two HDTV channels. Watch out, Murdoch!

You refereing to the Media Guardian / Weekly pull out in Mondays papers?

He was implying that COULD be an option, but he didn't mention the BBC were seriously considering it.

Hey there!

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16 QAM & 64 QAM - the difference??

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