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MDA
Auto Negotiating
Premium Member
join:2013-09-10
Minneapolis, MN
Netgear CM600
Asus RT-AC66U B1

3 edits

MDA

Premium Member

[Comcast Equip] CM tuner quality question

So we just got the X1 package after I talked Comcast down from a 250$ bill to around 205$ and they threw in the X1 bundle as a 2 year no contract promotion.

This last weekend, it was all set up by the technician that came in the 11am-1pm window and he showed me how to get into the diagnostics menu on the X1 DVR and terminals that span throughout the house.

Basically, the X1 platform uses the same frequencies your cable modem uses (IP frequencies). So I see both the X1 DVR and Arris TM722G use the 699mhz channel as their primary frequency, but the SNR for both devices is pretty different.

They both come off a two-way Antronix 3.5db splitter (one to the main X1 DVR and one to the TM722G modem). Downstream levels for both devices are within -1 to +1 and upstream power is 43-43.5 dbmv for both.

However the SNR for the X1 DVR (and all the terminals for additional TV's) is 40-41db and the cable modem's are 36-37db. How is this possible when both devices are on the same run throughout the house (comes in the house to a 15db Antronix return amp and goes all the way up to my office to that two-way splitter)?

Could it be the tuner quality on the modem? Because mainly what determines SNR is the length of the coax line and how much noise by amps or ingress is on the line.

I understand this is not a critical issue because as long as the cable modem has at least 33db for 256QAM, I shouldn't see bandwidth issues (and I do not). But for future-proofing on higher speeds, I just want to know if it's the cable modem or something else causing the SNR differences.

EG
The wings of love
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join:2006-11-18
Union, NJ

EG

Premium Member

[Comcast Equip] Re: CM tuner quality question

Perhaps the measuring mechanisms of the devices differ in accuracy ? Just sayin'.

MDA
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Minneapolis, MN

MDA

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Maybe they do. That's why I'm inquiring its a tuner issue with the modem. If it were to fall under the desired SNR for a certain modulation, then your bandwidth would be affected by the modem itself and not the line quality.

gar187er
I DID this for a living
join:2006-06-24
Seattle, WA

gar187er to MDA

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to MDA
every model modem will read different. unless you get a calibrated SLM, you wont know what if any of them are truthful.
aereolis
join:2003-06-12
Brampton, ON

aereolis to MDA

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The sn down is on different frequencies. Thats why.

EG
The wings of love
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join:2006-11-18
Union, NJ

EG

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There's no denying that that may be it or a part of it !

mackey
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join:2007-08-20

mackey to aereolis

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to aereolis
said by aereolis:

The sn down is on different frequencies. Thats why.

The same frequency is a different frequency?
said by MDA:

So I see both the X1 DVR and Arris TM722G use the 699mhz channel as their primary frequency, but the SNR for both devices is pretty different.

/M

cheeseman
@comcast.net

cheeseman to MDA

Anon

to MDA
It uses the same frequencies as your modem because it has a modem built in.

MDA
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join:2013-09-10
Minneapolis, MN
Netgear CM600
Asus RT-AC66U B1

MDA

Premium Member

So I guess the synopsis here is that I shouldn't worry about it as long as everything is optimal because detecting SNR is not a perfect reading by default.

I'm just curious as to what would happen if that SNR reading fell below the required value for a modulation scheme. If the true SNR on the line is higher than what the cable modem reads, in uncommon conditions one would see the cable modem fall back to a smaller modulation just because the modem thinks the SNR is too low for a higher modulation scheme.
aereolis
join:2003-06-12
Brampton, ON

aereolis to mackey

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said by mackey:

said by aereolis:

The sn down is on different frequencies. Thats why.

The same frequency is a different frequency?
said by MDA:

So I see both the X1 DVR and Arris TM722G use the 699mhz channel as their primary frequency, but the SNR for both devices is pretty different.

/M

No. Somewhere you are looking at the incorrect frequency & misinterpreting the information. A simple misunderstanding and the reason techs don't typically show people how to access the diagnostics screens.

MDA
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join:2013-09-10
Minneapolis, MN
Netgear CM600
Asus RT-AC66U B1

1 edit

MDA

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Click for full size
X1 Diagnostics page
Click for full size
Arris TM722G Diagnostics page

said by aereolis See Profile
No. Somewhere you are looking at the incorrect frequency & misinterpreting the information. A simple misunderstanding and the reason techs don't typically show people how to access the diagnostics screens.

Well, the tech who installed the X1 boxes let me know the key combination since I was asking him questions all day about how I looked at the diagnostics on the old STB's and general questions about how to estimate upstream using various splitters and amps.

It all runs on HSI frequencies now.