Digital Audio Recording Basics 543samples in a vertical direction in the deinterleave memory. When the memory is read,
a single large error is broken down into a number of small errors whose size is exactly
equal to the correcting power of the codes and the correction is performed with maximum
effi ciency.
An extension of the process of interleave is where the memory array has not only rows
made into code words, but also columns made into code words by the addition of vertical
redundancy. This is known as a product code. Figure 17.14(b) shows that in a product
code the redundancy calculated fi rst and checked last is called the outer code, and the
redundancy calculated second and checked fi rst is called the inner code. The inner code
is formed along tracks on the medium. Random errors due to noise are corrected by the
inner code and do not impair the burst correcting power of the outer code. Burst errors
are declared uncorrectable by the inner code, which fl ags the bad samples on the way into
the deinterleave memory. The outer code reads the error fl ags in order to locate erroneous
Figure 17.14(a) : Interleaving is essential to make error correction schemes more effi cient.
Samples written sequentially in rows into a memory have redundancy P added to each row.
The memory is then read in columns and data are sent to the recording medium. On replay,
the nonsequential samples from the medium are deinterleaved to return them to their
normal sequence. This breaks up the burst error (shaded) into one error symbol per row in
the memory, which can be corrected by the redundancy P.