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From: "Thomas Magma"
Subject: Re: DSP 2's complement vs straight binary
X-Newsreader: Microsoft Outlook Express 5.50.4133.2400
Date: Fri, 18 Oct 2002 04:47:39 GMT
NNTP-Posting-Date: Thu, 17 Oct 2002 22:47:39 MDT
Organization: Shaw Residential Internet
Like I was telling John Larkin simply inverting the MSB does not convert
straight binary to 2's complement. Here is a website that explains 2's
But you are right, there is hardwire solution for the conversion, but I hate
adding extra circuitry to solve a problem like this. In software it looks
like it would take a few executions for the conversion, but when we're
talking about megabytes of samples it does add up.
P.S. I'm looking for a 8 bit sampler.
"Phil Hobbs" wrote in message
> Thomas Magma wrote:
> > Hello,
> > I am involved in a project where we sub-sample the IF, store it to
> > and then post-process it. We are at the stage of selecting a ADC for the
> > project. I have noticed that a lot of the ADC's suitable for this design
> > only output straight binary. All of my algorithms to date have used 2's
> > complement (signed) values to perform DFT or FFT functions. Processing
> > is a major concern and we don't have time to convert each binary sample
> > 2's complement. Is there an equivalent method of the DFT or FFT that
> > straight binary? Or does anybody know of a 3V ADC that puts out 2's
> > compliment and can sub-sample 10.7mhz at less than 1mhz (pipeline)?
> Apart from a 1-LSB offset, twos complement is just the same as straight
> binary with the LSB inverted, assuming that the signal's dc offset is at
> exactly Vref/2. XORing each sample with 0x800 (assuming a 12-bit ADC)
> is pretty fast, and wiring an inverter in series with the MSB is a good
> option if you really can't afford a single XOR instruction. A slight
> adjustment to the analogue voltage will look after the adding-one
> Phil Hobbs
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