What are the Specifications on QAM/ATSC Signal Strength?

Question:

What is the preferred signal strength for QAM/ATSC frequencies?

Answer:

QAM (Quadrature amplitude modulation,) is a method used by most major cable television providers to broadcast digital television content to their customers over the same networks as standard analog cable and high speed internet. Currently, QAM broadcasting is most often employed to provide high-definition programming, on-demand content, and to encrypt certain channels such that to view them, a set-top box or CableCard-equipped intermediary is required to authenticate the viewer as a customer of the cable provider. At the discretion of the provider, encryption may be applied to any and all channels, with the legally required exception of the local channels that are now digitally broadcast over-the-air in the United States. Also, there are hardware appliances that exist to accept various input types, such as HDMI, and output a QAM transport stream over standard coaxial cable. These are often called ‘QAM Plants.’ SnapStream Servers are capable of receiving, recording, and searching unencrypted QAM broadcasts, also referred to as 'Clear QAM.' There is currently no ability to process encrypted QAM broadcasts.

ATSC is, after the 'DTV transition,' the single standard used in North America for over-the-air television broadcasts. In application, it is very similar to QAM in that it is a digital format, capable of high-definition transmission, and is recorded by SnapStream Servers in the same file format as QAM streams.  

The hardware used in SnapStream QAM/ATSC Servers is identical to that which is used in all of our servers, with the exception of the TV Tuner boards.

Specifications & Requirements

Due to both the exponential logarithmic scale of cable signal measurement, and the preference of most hardware that receives digital ATSC/QAM broadcasts, there is an increased importance put on the quality and strength of the signal being supplied to the tuners. The processing of analog broadcasts is more forgiving of anomalies and signal strength variations due the fact that the video and audio is assembled on the tuner board itself. Processing digital broadcasts requires dealing with a more complicated, already packaged and encoded stream, and is therefore a fundamentally different type of task in contrast to the processing of analog transmissions.

Signal strength is measured in dBmv, or decibel millivolts.  Zero dBmv is equal to 1 millivolt across 75 ohms.  This is actually a voltage measurement. Industry-wide, digital tuners often require an overall weaker signal strength than analog counterparts. 

The specification for QAM signal strength on SnapStream Servers is as follows:   

Range at Ch2 (57Mhz): -12dBmv to +16dBmv
Range at Ch134 (855Mhz): -18dBmv to +7dBmv
Overall supported range: -12dBmv to +7dBmv

  

Additional Notes:

Because of this relatively low ideal signal strength, the use of powered or amplified splitters is discouraged.

Applies to Version:

Any SnapStream Server or Encoder utilizing ATSC/QAM as a source

Keywords:

QAM ATSC Signal Strength dBmv

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