[hpsdr] Mercury sampling rate

[Alex, VE3NEA]

Hi Alberto and others,

Resampling by a factor that is an integer power of two can be implemented 
more efficiently than resampling by an arbitrary rational factor, using a 
so-called octave resampler. This resampler is based on a half-band filter, 
that is, a filter whose cutoff frequency is exactly 1/2 of the Nyquist 
frequency. The even coefficients of a properly designed half-band filter are 
zeros and thus the number of required multiplications is reduced by a factor 
of 2. The source code of my octave resampler used in Rocky, Faros and CW 
Skimmer is available at (http://www.dxatlas.com/dev). This resampler is used 
to quickly convert between the 3/6/12/24/48/96/192 kHz sampling rates. The 
only place in my software where a rational-factor polyphase resampler is 
used is the interface with SDR-IQ because this radio has a non-standard 
sampling rate.

All modern soundcards, including the on-board ones, support the sampling 
rate of 48 kHz. The choice of a standard sampling rate in Mercury was a 
smart decision that can result in more efficient signal processing in the 
software.

73 Alex VE3NEA







----- Original Message ----- 

> ***** High Performance Software Defined Radio Discussion List *****
>
> Phil Harman wrote:
>>
>> We sample at 122.88MHz since that divides evenly to standard sound card
>> frequencies.  This does need updating since we will be able to achieve
>> 2MHz of displayed bandwidth using the current USB2 interface to the PC.
>>
>> 73's Phil...VK6APH
>
> May I ask the rationale behind that ? The data sampled by the ADC are of 
> course not meant to be passed through a sound
> card after the decimation... they are already digital. And for the output 
> you need always to use a fractional resampler
> as you cannot have any guarantee that the sampling rate of the output 
> sound card is an *exact* submultiple of the
> downsampled rate. And, once you have accepted the need of a fractional 
> resampler, then any ADC clock frequency can be
> used.
>
> Or maybe I am missing part of the picture ?
>
> 73  Alberto  I2PHD



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