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Re: LF: 16 bit vs 24 bit ADC?

To: [email protected]
Subject: Re: LF: 16 bit vs 24 bit ADC?
From: Bill de Carle <[email protected]>
Date: Sat, 03 Dec 2011 00:57:19 -0500
In-reply-to: <80A163C9035C44FBA23C7501118A6FDF@JimPC>
References: <[email protected]> <[email protected]> <CAA8k23RcqVfXC9MWoTWm9yESwuSG5ZP6fFFZA2y2j-C4f0DPHw@mail.gmail.com> <80A163C9035C44FBA23C7501118A6FDF@JimPC>
Reply-to: [email protected]
Sender: [email protected]
First of all, thanks to everyone for the great comments, much appreciated.

I don't have a 24-bit ADC here to run Jim's test but tried another experiment with interesting results. Tonight Stefan, DK7FC is transmitting his callsign using DFCW on 136.172 Khz. Watching the screen, I noticed Stefan's signal was at times marginal here in Ontario Canada so I decided to record it for an hour, change the recorded data from 16-bit samples to 12-bit samples, then process the file using Spectrum Lab with exactly the same settings. Basically I threw away the least significant 4 bits of each signed 16-bit value from the ADC by AND-ing all the samples with a mask of FFF0. My recording covers the period from 0048z thru 0148z on December 3rd, 2011 (some 329.6 Mb). The sample rate was 48000 and I injected an accurate 10 Khz audio reference tone through the ADC so SL could use it to correct for sound card sample rate drift. Wolf's sample-rate correction algorithm seems to work very well for both live ADC data and played back data. I was using a 4096-point FFT with 512 X decimation to give a bin width of 22.8882 mHz, equivalent noise bandwidth of 34.3323 mHz and screen scroll rate was one pixel every 10 seconds. The SL noiseblanker was enabled for both real-time and off-line processing. I really expected to see significant degradation of this already marginal signal when using the 12-bit data but the results surprised me. In the attached image 16_vs_12.jpg you can see the two traces. The top image is the one taken from a screen capture as the live data was coming in, using 16-bit ADC data; the bottom image was captured (at the same jpg quality) from a second instance of Spectrum Lab running on another computer but processing the data chopped to 12 bits. Not only was there no obvious degradation, in some respects there seemed to be an actual improvement! In no case was any portion of the trace discernable in the top image but not in the bottom. The background of the 12-bit data seems to be a little more obvious (visually loud?) but the traces of Stefan's signal are brighter too. I actually find the 12-bit bottom image easier to read.
73,
Bill VE2IQ

At 12:02 PM 12/2/2011, Jim Moritz M0BMU wrote:
Dear Bill, Andy, LF Group,

It seems to me it should be quite easy to subject this to practical testing. First, using SpecLab etc., measure the SNR of a weak signal in the presence of noise, with the FFT parameters chosen so that the signal is near the quantisation noise level.. Then attenuate the signal and noise at the ADC input by, say, 24dB. This would effectively reduce the resolution/accuracy of the A-D conversion by 4 bits ( I guess you would want to chose the signal, noise and attenuation levels so that the external noise was well above the sound card or other ADC noise floor, with and without the attenuation). Then see if the SNR in the FFT output has changed.

Cheers, Jim Moritz
73 de M0BMU


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