A new block code is introduced which is capable of correcting multiple insertion, deletion, and substitution errors. The code consists of nonlinear inner codes, which we call "watermark"" codes, concatenated with low-density parity-check codes over nonbinary fields. The inner code allows probabilistic resynchronization and provides soft outputs for the outer decoder, which then completes decoding. We present codes of rate 0.7 and transmitted length 5000 bits that can correct 30 insertion/deletion errors per block. We also present codes of rate 3/14 and length 4600 bits that can correct 450 insertion/deletion errors per block.

The low-density parity check codes whose performance is closest to the Shannon limit are "Gallager codes" based on irregular graphs. We compare alternative methods for constructing these graphs and present two results. First, we find a "super-Poisson" construction which gives a small improvement in empirical performance over a random construction. Second, whereas Gallager codes normally take N/sup 2/ time to encode, we investigate constructions of regular and irregular Gallager codes that allow more rapid encoding and have smaller memory requirements in the encoder. We find that these "fast encoding" Gallager codes have equally good performance.