TV picture has intermittent pixilation. Appears to be fixed after Xfinity "correction", then reverts to poor pixilation picture.

Do I need a modem to get live TV?  I know that I need a modem for Internet streaming for Netflex, Amazon Prime, Hulu, etc. which is operating ok.  An Xfinity agent told me that I need a new modem to correct TV picture pixilation, but my research indicates a modem has no connection to live picture quality.