[mythtv-users] Best place to insert a CATV amp

Rudy Zijlstra mythtv at edsons.demon.nl
Fri Jan 26 22:38:55 UTC 2007

Boleslaw Ciesielski wrote:

>Brian Wood wrote:
>>Without a signal level meter you really have no idea what the levels  
>>of the incoming channels are, and those are expensive items not owned  
>>by most consumers
>A lot of cable modems have some kind of signal level meter built-in. On 
>my Motorola SB-4200 it is accessible via
>It displays power level in dBmV and SNR in dB. On broadbandreports.com 
>one can find out that cable modems want something like between -15 and 
>+15 dBmV (ideally -10 to +10). Is there a similar range for TVs? My 
>understanding is that TVs require stronger signal than cable modems? If 
>so, one could measure the signal by connecting the cable modem in 
>various places...

CM do measure power, and all CM of level DOCSIS 1.1 and up have to do 
it. And are pretty accurate (within 3dB), if correctly calibrated. 
Normally they are pretty well calibrated.  The SB4200 is DOCSIS 1.1 and 
will operate either on QAM64 or QAM256. It is an older device, no longer 

They measure power on 1 channel, not on the whole band. Normally this 
should give you a pretty good picture though.

CM is designed to operate in the powerband between -15dBmV and +15dBmV 
on either QAM64 or QAM156, when using (A)TDMA. For SCDMA story is 
slightly different. Would like to hear though of yours is using 
SCDMA.... I am not aware of any operator actually using it and would be 
interested in learning about it if in use.

Most CM will refuse http access though once they are online.

One of the differences between CM and TV is that the CM takes a digital 
signal, whereas the TV uses an analog one. Normally there is about 10dB 
difference between the digital signal and the analog signal. The digital 
(if QAM64) being 10dB lower in power than the analog. QAM256 is normally 
about 6dB lower in power than analog.

TV will normally need a signal between -8 and +20dBmV. (guestimate, as i 
am no analog specialist).
Many older TV's (>8yr old) will emit a short smoke signal if signal is > 
+40dBmV (at or about). Thats the point where older tuners will break. 
Newer tuners can stand stronger signals without sending out smoke. For 
all tuners there is a breaking point though.



P.S. i learned that breaking point by accident, my collegue and i were 
new in the game, and testing. upconverter was at max power of 105dBuV 
(40dBmV), and we connected the STB under test directly. It never tuned 
again. This was 2001 timeframe with a can tuner. 

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