Advertised and Effective Bits: Don’t Be Fooled

Hello from the Seismo Guys in Panama. Today we open a discussion on 24-bit digitizers.

Nowadays, seismologists are OBSESSED with 24-bits. But we find that few seismologists understand the reason for their obsession and even fewer know the right questions to ask manufacturers. We hope the Seismo Guys can help.

To begin, it is important to know that not all 24-bit digitizers are created equal. There are only a handful of companies making TRUE 24-bit digitizers for seismology: Earthdata (209), Nanometrics (Centaur, Taurus, Titan SMA), OSOP (joking) and Quanterra (Q330), where “TRUE” refers to the effective resolution. True 24-bit digitizers have ZERO bits of noise (or 24 effective bits). The vast majority of other 24-bit digitizers on the market are not true 24-bit digitizers. They are…Imposters!!!!

Obtaining 21 effective bits is difficult but within the reach of many electronics developers, even some hobbyists. After 21 bits, it becomes science. Most manufacturers do not invest the time, energy and money into perfecting their digitizers because, for the more routine seismic tasks like automated event location, 24 bits is simply overkill.

Below I help you see the signal in the noise.

imagenoise_SIGNALmlab2

A digitizer with 23 effective bits has 1 bit of electronic noise. For each additional bit of noise, the total counts of noise can be re-calculated as (2^n)-1 where n = bits of noise.

effective.bits

 

 

 

 

To put this all in perspective:

24 bits represents a dynamic range of 2^(24)-1 or 16,777,215 counts.

So the next time a manufacturer tells you his digitizer has 24-bits you should be asking “How many bits of noise?”

Stay tuned for more on 24 bit digitizers. Upcoming posts on 24-bits will include:

  • How to choose the right digitizer
  • When electronics becomes science: Bridging the gap between 21 and 24 bits (Contributed)
  • When two 16-bit digitizers are better than one 24-bit digitizer (Contributed)
  • Sigma delta ADCs and the death of the anti-alias filter

–Branden

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