Introduction Of SNR CNR
SNR and CNR are the two terms which often used in the analog and digital communication. This ratio is measured in dB. SNR is basically signal to noise ratio or strength of the signal to the noise level in the signal. CNR stands for Carrier To Noise Ratio as its name indicates itself that this is the noise ratio which at the time of modulation due to high frequency carrier wave.
For a communication engineer it is not required get the some noise in the signal because it makes the pitcher quality down. We need to adjust it in proper way so that subscriber would not be affected from it. Even there are some noise also available in the signal but that is not visible with eyes.
What Is SNR CNR
As we have discussed above SNR is the signal strength to noise level. For example the incoming signal Vs micro volts and Vn micro volts noise then SNR or signal to noise ration can be expressed as
S/N = 20 Log10 (Vs/Vn)
now if we have Vs=5 and Vn=5 then
S/N = 0 dB this is not acceptable because this signal is having lot of noise in it.
S/N = 49 dB then it is known as very good S/N ratio value while if S/N = 43 dB is good S/N
if S/N = 37 dB then it is acceptable only.
if Vn>Vs then S/N is negative which is very bad S/N ratio.
A 25 dB SNR can remove the colors from the color tv , 31 dB gives poor quality to pictures and 37 dB still shows some noise in the signal. so we should maintain it higher than the 37 dB to get the proper results.
As we know in the modulation technique we use our signal to be superimposed on the high frequency carrier wave to modulate the signal by doing so, also we gets the noise called Carrier To Noise Ratio.
So these two are the different things
SNR and CNR
At the time of video recording we gets the SNR while at the time of modulation we gets the CNR. The CNR should be higher then the SNR 4 dB at least. If the value of the C/N would be high then you can use more amplifier in your network in cascade. If the value of CNR is low then it indicates that your should not use large number of amplifier in your network. So we need to manage this CNR and also the SNR to get best result at subscriber end.
If once noise is with the signal then it is very hard to remove it also we should not use large number of devices in amplification process because it will decrease your SNR which results bad quality signal. If you want to reduce the CNR then we should use the high quality devices for video recording and for modulation purpose this will cost you lot but it will maintain your SNR and CNR. In some cases to receive the high SNR we reduce the temperature of the receiver circuit then it makes noise level down and we gets proper signal.
When the things comes in quality of signal then we should maintain our SNR and CNR ratio. Becasue these two are very important factor for a headend to maintain. So use less number of amplifier in your system. Try to get your SNR 43 dB and make 4 dB high CNR than to SNR. This will maintain the good picture quality.
This was all about the SNR and CNR for a Headend.