[Difx-users] VDIF data with sample granularity = 2

[江悟]

Hi Adam and all,


As I discussed with Adam already, I still not sure if some part of the raw data would be skipped to load for correlation when sample granularity = 2. Any comments?


Cheers,
Wu


-----原始邮件-----
发件人:"Adam Deller" <adeller at gmail.com>
发送时间:2018-12-13 15:22:30 (星期四)
收件人: "江悟" <jiangwu at shao.ac.cn>
抄送:
主题: Re: VDIF data with sample granularity = 2


HI Wu Jiang,


Can you please send emails like this to the difx-users mailing list, rather than to me direct?  sample granularity 2 means that the data reader will only see a timestamp with an integer number of nanoseconds every second time it unpacks a group of samples.  This is because of the very high sampling rate.


Cheers,
Adam






On Thu, 13 Dec 2018 at 11:43, 江悟 <jiangwu at shao.ac.cn> wrote:

Dear Adam,

Recently when I made correlations with VDIF data streams, DiFX decoded the data format and gave some information as follows.
Mark5 stream: 0x2e2dc80
  stream = File-1/1=/Bgfsdata/astro/raw/fa02/scan1.source.1045.vdif
  format = VDIF_4096-6400-2-2 = 3
  start mjd/sec = 58454 40673.694666240
  frame duration =  5120.00 ns
  framenum = 0
  sample rate = 1600000000 Hz
  offset = 0
  framebytes = 4128 bytes
  datasize = 4096 bytes
  sample granularity = 2
  frame granularity = 1
  gframens = 5120
  payload offset = 32
  read position = 0
  data window size = 1048576 bytes
Where,  I am not quite sure about "the sample granularity = 2", as always I just see "sample granularity = 1" in many cases. Does it mean DiFX recognizes the samples in the data stream are not continuously allocated? Such as sample_ch1 sample_ch2 x x sample_ch1 sample_ch2 x x. However, actually the samples in the data stream are continuously recorded. So I would like to make clear on what DiFX deals with the data?
Thank you!

Best regards,
Wu Jiang





--

!=============================================================!
Dr. Adam Deller         
ARC Future Fellow, Senior Lecturer
Centre for Astrophysics & Supercomputing 
Swinburne University of Technology    
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phone: +61 3 9214 5307
fax: +61 3 9214 8797

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