Energy Dispersal

What is the issue that Energy Dispersal DAB needs to solve?

Information (whether data, audio, or video) in digital form is always a collection of bits. We will have to modulate them on a carrier if we want to transmit them.

Bit sequences modulated on a carrier for wireless transmission affect the RF spectrum, either good or bad.

The good thing is

Transmitted random bit sequences will show good behavior and produce ideal and continuous frequency spectra.

The bad thing is,

Transmitted non-random bit sequences will show severe peaks in the frequency spectrum energy (non-dispersed energy). This can cause interference with adjacent channels.

What can we do to solve this problem?

We will have to disperse the energy by making any bit sequence random. However, the bit sequence contains our (audio) information. So if we randomize the bit sequence, the information will be lost.

So whatever randomized algorithm we use it should be known to the receiver to turn the randomized bit sequence back into the original sequence.

To solve this problem, we need to turn any bit sequence (random or not) into a random bit sequence. However, we need to know the randomization to restore the coded sequence in the digital receiver.

What is the final solution?

The solution is to convert the bitstream to a pseudo-random bitstream.

DAB will execute a modulo 2 addition (XOR function) with the output of a pseudo-random generator.

A pseudo-random bit sequence is a bit sequence that appears statistically random, however, based on a known algorithm (the key).

The algorithm relies on a predictable, deterministic (no randomness involved) algorithm. So it looks statistically random but is not really random. We call it pseudo-random.

The bit-compressed audio frames (finite numbers of bits) undergo a randomization process at the transmitter side. But the receiver side can turn the randomness back into the correct sequence because it can generate the same deterministic pseudo-random bits sequence with the same key in the receiver.

Two deterministic systems, one in the transmitter and one in the receiver, will always generate the same pseudo-random bit sequence if their start condition is the same.