What is QAM64?

[Sput]

I have the NTL 10 meg service and recently my faulty NTL 200 modem was changed to a blue NTL 250. When I interrogate it I get the following information:-

Downstream Lock : Locked
Downstream Channel Id : 50
Downstream Frequency : 402750000 Hz
Downstream Modulation : QAM64
Downstream Symbol Rate : 5056.941 Ksym/sec
Downstream Interleave Depth : taps32Increment4
Downstream Receive Power Level : -2.8 dBmV
Downstream SNR : 32.5 dB

Can anyone please tell me what QAM64 means?

[Stuart C]

QAM 64 is simply the method used for encoding data.

[Sput]

Originally Posted by Stuart C

QAM 64 is simply the method used for encoding data.

Thanks for the prompt response. I have seen elsewhere that NTL also uses 256QAM but can you please explain how this differs from QAM64?

---------- Post added at 11:51 ---------- Previous post was at 11:45 ----------

With little research I have found an explanation here:-

http://en.wikipedia.org/wiki/Quadrat...ude_modulation

It suggests that as ISP's demand for more efficient bandwidth utilisation and building on its experience with 64 QAM transmission over cable, then NTL has begun to implement a 256 QAM transmission system that provides far more efficient use of cable system bandwidth and expands channel capacity. It would seem that this expanded channel capacity results in a 44 percent increase in information rate and a 50 percent increase in video content as compared to 64 QAM. With it, NTL will be able to carry two HDTV channels instead of just one in a 6-MHz space. The added capacity enables expanded video, modem, telephony and business data services. 256 QAM transmission also makes it possible to substantially increase the number of cable services on bandwidth-limited networks designed for analog video performance. This capability might allow deferral of costly upgrades/rebuilds.

[Stuart C]

Based upon http://en.wikipedia.org/wiki/Quadrat...ude_modulation .

QAM is a complicated subject, but put simply, QAM256 offers more bandwidth (so, theoretically, higher speeds), but is more susceptible to interference and faults.

[Sput]

Originally Posted by Stuart C

Based up http://en.wikipedia.org/wiki/Quadrat...ude_modulation .

QAM is a complicated subject, but put simply, QAM256 offers more bandwidth (so, theoretically, higher speeds), but is more susceptible to interference and faults.

It certainly is complicated and I am grateful for helping me try to decipher it

[altis]

To send data, the transmitter varies the amplitude and phase of a carrier signal. In QAM64 there are 64 possible combinations of amplitude and phase for each period of time, or symbol, of the carrier. With QAM256 there are 256 possible combinations in the same period - thus increasing the data capacity four fold but making it more difficult for the receiver to discriminate between each signal.

It's easiest to visulalize by looking at the constellation diagrams of each:

QAM4 (=quadrature phase shift keying):


QAM16:


QAM64:


QAM256:


...and if you're flumoxed by what I and Q mean - well, I is the 'in phase' part of the carrier and Q is the 'in quadrature' part. In this picture the I is in blue and Q is in green.

Originally Posted by altis

To send data, the transmitter varies the amplitude and phase of a carrier signal. In QAM64 there are 64 possible combinations of amplitude and phase for each period of time, or symbol, of the carrier. With QAM256 there are 256 possible combinations in the same period - thus increasing the data capacity four fold but making it more difficult for the receiver to discriminate between each signal.

It's easiest to visulalize by looking at the constellation diagrams of each:

QAM4 (=quadrature phase shift keying):


QAM16:


QAM64:


QAM256:


...and if you're flumoxed by what I and Q mean - well, I is the 'in phase' part of the carrier and Q is the 'in quadrature' part. In this picture the I is in blue and Q is in green.

Excellent diagrams and explanation Altis, as always!
Only one thing I'd say, and thats that theres not a fourfold increase in data capacity in QAM64 to 256 migration. The data capacity increases by a third. It goes from 6 bits per symbol to 8 bits per symbol. In terms of cable TV for a symbol rate of 6.887Msymbols/s the data rate goes from 41.322Mb/s to 55.096Mb/s. The RF channel has to have improved SNR for QAM256. If memory serves its an extra 3dB's required over QAM64. Escapee?

[altis]

Originally Posted by Creative

Only one thing I'd say, and thats that theres not a fourfold increase in data capacity in QAM64 to 256 migration. The data capacity increases by a third. It goes from 6 bits per symbol to 8 bits per symbol.

Of course it does. What a ninny I am!