428 Chapter 14
Subsequently, the samples need to be deinterleaved to return them to their natural
sequence. This is done by writing samples from tape into a memory in columns, and
when it is full, the memory is read in rows. Samples read from the memory are now in
their original sequence so there is no effect on the information. However, if a burst error
occurs, as is shown shaded on the diagram, it will damage sequential samples 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 sizes are 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 14.14(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
Figure 14.14(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.