Friday, August 17, 2012

DIGITAL MODES: Part 1 (ASK)

If you have no interest in the technical mumbo-jumbo you can skip today's post.  This is about the wild world of digital modulation schemas, (aka digital modes) specifically shift keying. These are ways to convey and/or transmit data by modifying or modulating the carrier wave. There may be math.  Any serious digital modulation scheme uses multiple distinct signals to represent digital data, each mapping patterns of binary digits. If you can't handle that you should probably stop right here.

The term "Digital" as used in digital modes just means that it uses discrete values. Information, like text or other data can be converted into numeric data, usually binary. Binary is just another way to say "zeroes and ones."  The use of binary harks back to early analog computational machines. These used switches to do their math. A simple switch is either open or closed, this corresponds to 0 (closed) and 1 (open). So in transforming analog data into binary we are only ever seeking three things, compression , redundancy and security. Redundancy and security usually requires more data, and compression seeks to squish that in to an ever smaller space so these are somewhat at odds. That conflict has generated oodles of differing formats as we improve upon and replace older outmoded formats.
 
Lets start with OOK. On-off keying (OOK) the simplest form of shift keying.  In its most simple implementation zero and one are represented by the presence or absence the carrier wave for a defined duration. aka make/break. This is most commonly used to transmit Morse code. It sounds rudimentary and antiquated but it's also used in optical formats. the upshot is that it requires little bandwidth, the downside is that it's susceptible to noise and lacks redundancy. More here.

Technically OOK is a type of ASK (Amplitude Shift Keying.) Amplitude-shift keying represents binary data as any set of variations in the amplitude of a carrier wave. So technically no carrier wave counts even though that's sort of cheating. All ASK schemas are susceptible to noise because they're basically AM radio. One of the more common forms is BASK (Binary Amplitude Shift Keying.) It does have redundancy, unlike OOK. In BASK the signal is divided into four pulses of equal duration which represent the bits in the digital data. The number of bits used for each character is usually eight, seven of which represent the 128 possible characters, the last bit is used to check for errors.

More interesting is QAM (Quadrature Amplitude Modulation) It's both an analog and a digital modulation schema. It uses two carrier waves, that are out of phase with each other by 90°.  But both of these waves are amplitude modulated using the ASK digital modulation or amplitude modulation as the above. These carrier waves can be individually frequency modulated and/or phase modulated which is where we head in part 2.

VOCABULARY TERMS:
  • OOK -On-off keying
  • ASK - Amplitude Shift Keying 
  • QAM -  Quadrature Amplitude Modulation
  • BASK -  Binary Amplitude Shift Keying