[Difx-users] Change for 4- and 8-bit VDIF {External}

[Walter Brisken]

Hi all again,

Chris Phillips wrote to me with a good case for not changing the 8-bit 
case and I have since reversed that change.  We both agree that with 
only 16 states in the 4-bit case the change should take hold.  Thus at 
the current time the only net change is to the 4-bit case.

We will be looking to formalize clarification on these sampling issues 
in an update to the VDIF format specification.  It would be useful to 
know of existing use cases of 4- and 8-bit VDIF to help inform how this 
specification should be solidified.

-Walter

On 2/19/25 4:39 PM, Walter Brisken via Difx-users wrote:
> Hi DiFX users,
>
> The mark5access library in the difx software suite contains the 
> routines to reconstruct a VDIF stream into voltage samples.  Since 
> support of 4- and 8- bit VDIF was introduced into this library, which 
> probably dates back to around 2013 or so, these two modes have used an 
> asymmetric mapping of reconstructed values.  That is, in the case of n 
> bits per sample, the reconstructed value corresponding to sample state 
> 2^(n-1) was zero (mimicking the values that a signed integer can 
> take).  This is not the case (and never has been) for 1 and 2 bit 
> VDIF.  The VDIF specification does not specify the mapping of sample 
> state to reconstructed value (unless I am missing it), but I am 
> convinced that the symmetric output is the more useful and least 
> surprising way to proceed. This also provides one additional useful 
> situation within mark5access: it allows decoded invalid packets to be 
> reconstructed as zero and be unambiguously interpreted as such.
>
> I have just put in a change to mark5access to symmetrize the 
> reconstructed values for 4- and 8-bit VDIF (both real and complex).  
> No change was made to the 1- or 2-bit cases.
>
> The effect of assuming one mapping and using the other is the 
> introduction of a fixed DC offset in the output values.  This is how 
> the pre-existing behavior was identified.
>
> Please let me know if you have questions or concerns.
>
> -Walter
>
> _______________________________________________
> Difx-users mailing list
> Difx-users at listmgr.nrao.edu
> https://listmgr.nrao.edu/mailman/listinfo/difx-users