  We propose the use of a lower or upper triangular sub-base matrix to replace
the identity matrix in the source-check-channel-variable linking protomatrix of
a double-protograph low-density parity-check joint-source-channel code (DP-LDPC
JSCC). The elements along the diagonal of the proposed lower or upper
triangular sub-base matrix are assigned as "1" and the other non-zero elements
can take any non-negative integral values. Compared with the traditional
DP-LDPC JSCC designs, the new designs show a theoretical channel threshold
improvement of up to 0.41 dB and a simulated source symbol error rate
improvement of up to 0.5 dB at an error rate of 1e-6.
