High-Level Stream Questions

Hello all,
I have general questions on streams:

  1. When precisely does a stream start? When a source is connected to its
    sink? When you call .run() on a top-block?

  2. When adding stream tags to a stream, does the stream itself get
    altered?
    If I were to output the stream to a binary file, would the file look the
    same with and without stream tags?

  3. If I have multiple USRPs receiving data at the same frequency and
    sampling rate, all outputting their streams to separate binary files,
    will
    sample n on binary file 0 be taken at the same time as sample n on
    binary
    files 1, 2 … up to the number of USRPs, assuming all USRPs have been
    synchronized with the same clock and PPS signal?

From,
Alex

On 07/16/2012 10:17 AM, Alexander O. wrote:

Hello all,
I have general questions on streams:

  1. When precisely does a stream start? When a source is connected to its
    sink? When you call .run() on a top-block?

The UHD Source block has a set_stream_time() which you can call before
starting the flow graph. This will precisely control when samples start
flowing.

  1. When adding stream tags to a stream, does the stream itself get altered?

No. The tags are basically in an external queue and and tied to the
absolute sample count of the stream.

If I were to output the stream to a binary file, would the file look the
same with and without stream tags?

Yes. There isnt a facility to write the tags yet. But I have heard of a
such a utility in the works.

  1. If I have multiple USRPs receiving data at the same frequency and
    sampling rate, all outputting their streams to separate binary files, will
    sample n on binary file 0 be taken at the same time as sample n on binary
    files 1, 2 … up to the number of USRPs, assuming all USRPs have been
    synchronized with the same clock and PPS signal?

Yes, 1) If you have multiple source blocks and set the stream time to be
the same. Or, 2) you have a multi channel source block, this will just
work as all channels are automatically aligned

-josh

Is there a way to control the start time of the stream when it is not
coming from a USRP?