In most cases a digital signal has a good picture or doesn't come in. Above the signal power minimum threshold the picture is perfect, below the threshold nothing, virtually no in between.
TV RECEIVER SIGNAL LEVELS
Signal power is commonly measured on a logarithmic scale in dBm -- decibels above or below 1 milliwatt (mW). Televisions will receive signals as high as about -5 dBm, above that the receiver is overwhelmed. The minimum signal that can be processed varies with television receiver and circumstances from -50 dBm to -75 dBm.
A television's required minimum signal depends on the receiver sensitivity and local conditions (electronic noise pollution). Minimum signal can vary from as high as -50 dBm for a TV with poor sensitivity in a noisy environment, to as low as -75 dBm for a TV with good sensitivity in a low or no noise environment. Most televisions and conditions will require a signal level near -65 dBm.
ESTIMATE a TV's MINIMUM SIGNAL
A television's signal sensitivity varies with model and depends on the tuner receiver Noise Figure (NF) and minimum Signal-to-Noise ratio (S/N). The lower the noise figure and the lower the signal-to-noise ratio, the more sensitive the receiver. Noise figure measures receiver noise. The Signal-to-Noise ratio is the required minimum difference between the signal level and the average noise level.
|Noise Figure (NF)||10 dB||6 to 20 dB|
|Signal-to-Noise (S/N)||30 dB||25 to 35 dB|
Television receivers typically have a noise figure of 10 dB, but it can be as low as 6 or 8 dB, to as high as 20 dB. A minimum signal-to-noise ratio of 30 dB is required in most cases due to interference from other signals, multi-path, and electronic noise.
Another factor is background noise, also called thermal or kTB noise, and is based on receiver noise bandwidth (6 MHz for an RF channel), and temperature. Temperature has a minor effect, and it is standard practice to use 300°K (80°F / 27°C), or 290°K (62°F / 17°C) for calculations. Noise is random with an average deviation of about ± 5 to ± 10 dB from the average level.
|Pmin = kTB (NF) (S/N)|
|Pmin||= Minimum Signal (watts).|
|k||= Boltzmann's Constant
= 1.380 622 x 10-23 (Watt sec / °K).
|T||= Temperature in Degrees Kelvin (°K).|
= Receiver Noise Bandwidth in Hertz (Hz).
= 6,000,000 Hz (6 MHz) for an RF channel.
|NF||- Noise Figure (ratio).|
|S/N||- Minimum Signal-to-Noise (ratio).|
Over-the-Air Digital TV (OTA DTv)
TV Minimum Signal