what is 'data whitening'?

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I was reading a protocol description today (Bluetooth IIM) and a line
said "In order to reduce highly redundant data and minimise DC bias,
a data whitening scheme is used to randomize the data.  The data is
scrambled by the sender and unscrambled by the receiver"

OK, I can see that there is some security being added here but what
about the "removal of redundant data and DC bias" (which I assume
means unequal numbers of 0 and 1).  How can this work for the small
data packets being sent?  Surely if random data goes in, random data
is going to come out and the resultant size, and number of 0 and 1s
can't be controlled in any usable way (in much the same way as
loss-less compression can't guarantee a smaller file).

Tim








Re: what is 'data whitening'?
The purpose is to "whiten" the transmit spectrum and generate a specific
response at the baseband to easely decode it. Mostly implemented with
feed-back shift registers to generate pseudo random bit-strings. As digital
logic cannot generate true random values you always have a opportunity to
decode it at the receiving end. The file length will be the same but the
synchronization delay time will be increased (that's the drawback). Required
S/N will be slightly increased (if it is right implemented). The rest is
math...
Regards -
Henry


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Re: what is 'data whitening'?

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A transmission system is usually optimized for a certain frequency range,
and the transmitting electronics may be adversely affected (not operating in
optimal range) if the average signal input is not centered at a certain
value. Since the tranmission devices have no control over the data being
transferred, conceivably a data stream could generate a modulation that has
too low of a frequency and does not pass as easily through the demodulation
electronics. An average voltage shift could also develop, which might
interfere with the signal depending on the encoding scheme.

By "randomizing" the data with a predictable set of values and then
stripping the injected data at the receiving end, the signal values will be
much more likely to have a normalized statistical grouping, which is closer
to the center of the frequency range the transmission system is optimized
for. So the main advantage is keeping the signal levels optimized for
passage through the electronics, and stabilizing the location of the signal
average. Depending on the scheme of signal encoding, it may or may not have
a significant effect.

A very similar technique is useful in analog-to-digital applications; a
digital value is sent to a DAC, which generates a known analog value to be
added to the input of the ADC. The injected value is then subtracted from
the captured analog value. This allows the visible effects of ADC
non-linearity to be minimized by distributing it statistically among the
samples.

I'm not the expert on this stuff, so any flames and corrections are welcome.



Re: what is 'data whitening'?
Garrett, you've got the right idea - the whitening is to get above the
pink noise level (frequency dependant portion).

The A/D technique you describe I've never had to do with good ADC's, but
we do dither (inject a small signal above nyquist on the input signal)
to get a better idea of the LSB. Got any refs on your technique, MATLab
models?  Sounds good.

Andrew

Garrett Mace wrote:

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Re: what is 'data whitening'?

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Andrew,

Not at the moment, most of the work I did was under a non-disclosure
agreement for a proposal on a job that didn't end up with me. The
application was very sensitive to ADC nonlinearities. The concept itself is
not too uncommon, it's basically the sliding scale method which you should
be able to find information on.



Re: what is 'data whitening'?
Was this a custom ADC?  I've used many Analog Devices ADC's (even for
PMU type work measuring currents and voltages for devices under
development at many silicon sites), and one of the things that most of
the world seems to require is guaranteed linearity  - does this get past
the +/- 1/2 lsb thing like dithering?  I'm going to take a look
tomorrow, have to get the kid in bed.  This might be a cool idea.

Andrew

Garrett Mace wrote:

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Re: what is 'data whitening'?

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I told the client that current ADC's were already doing a pretty good job of
maintaining linearity, but they seemed convinced that this technique was
necessary and they didn't get good samples without it. Then again, their
current design was already a few years old, and the signal processing
reference they were quoting was probably older than that. However as I
mentioned, I didn't get the job so I didn't get to see exactly how much it
affected the samples. I don't think they've gotten that project underway
yet.



Re: what is 'data whitening'?


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Hello Tim,

The main purpose of this scrambling is to remove the long strings of
ones or zeroes which may exist in the raw input data. The alternation of
symbols is required for normal operation of the bit synchronization in
receiver. Therefore the randomizing of the data is commonly used inside
modem protocols.
As for data reduction, they may compress the long strings of repetitive
symbols with the simplest RLE algorithm. I am not sure if they do it in
BT, however it is rather common solution also.

Vladimir Vassilevsky

DSP and Mixed Signal Design Consultant

http://www.abvolt.com

Re: what is 'data whitening'?

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Thanks to all who replied.  Having acquired a copy of the full spec it
seems that the data whitening algorithm is intended to randomise the
payload when it contains configuration data which is "highly redundant"
(i.e contains lots of don't cares which will inevitably be zeros) and that
this is turned off when the payload is 'voice' data (and hence more
random).

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