  The problem of finding good linear codes for joint source-channel coding
(JSCC) is investigated in this paper. By the code-spectrum approach, it has
been proved in the authors' previous paper that a good linear code for the
authors' JSCC scheme is a code with a good joint spectrum, so the main task in
this paper is to construct linear codes with good joint spectra. First, the
code-spectrum approach is developed further to facilitate the calculation of
spectra. Second, some general principles for constructing good linear codes are
presented. Finally, we propose an explicit construction of linear codes with
good joint spectra based on low density parity check (LDPC) codes and low
density generator matrix (LDGM) codes.
