Re: [Discuss-gnuradio] USRP FPGA 8-bit Data Ordering

[Eric Blossom]

On Wed, Jan 21, 2009 at 05:47:19PM -0500, Paul Creekmore wrote:
> Paul Creekmore wrote:
>> Context: I'm adding new quantization (sample width) options to the  
>> USRP FPGA configuration.  (Long time in coming; yes, I know.)
>>
>> Question: I'm seeing that in 8-bit mode, the USRP outputs samples from  
>> its internal channels in the following sequence (an 8-bit, 8-channel  
>> example):
>>
>>     1, 0,     3, 2,     5, 4,     7, 6,    1,0,   3,2,     ...
>>
>> where each grouping of two channels makes one 16-bit word.
>>
>> ----> What is the rationale behind this convention? 
>>
>> As I'm deciding how to output 4, 2, and 1-bit samples, it makes much  
>> more sense to me to use a sequential order, like this (4-bit,  
>> 8-channel example):
>>
>>     0, 1, 2, 3,   4, 5, 6, 7,    0, 1, 2, 3,   ...
>>
>> This way if the user writes the data directly to a file, one word  
>> after another, everything is in sequential order for whatever  
>> post-processing later.
>>
>> ----> Will the GNU Radio world fall apart if use a sequential channel  
>> ordering for my output and submit this development for inclusion in a  
>> future release?
>>
>>
>> Thanks,
>> Paul
> Amendment:
>
> I see now why the sequence is as it is.  As a real-time program reads  
> one word at a time and picks off samples from LSB to MSB, the ordering  
> makes sense.  I may just include a setting to reverse the sample order  
> for more efficient data recording.  If the powers that be have any other  
> thoughts, please let me know.
>
> --Paul

Thanks for answering your own question :-)

The USRP1 is little endian.  My not-fully-considered thought is that it
ought to go like this (everything packed into a 32-bit word):

16 bit I & Q (same as today):

  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |              Q0               |              I0               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

8 bit I & Q (different from today: see usrp_source_c.cc):

  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |      Q1       |      I1       |      Q0       |       I0      |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

4 bit I & Q:

  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  Q3   |  I3   |   Q2  |  I2   |   Q1  |   I1  |  Q0   |  I0   |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

etc...

What are you thoughts about this?

Eric