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Re: [Discuss-gnuradio] sync_interpolator


From: Miklos Maroti
Subject: Re: [Discuss-gnuradio] sync_interpolator
Date: Tue, 27 Aug 2013 02:43:32 -0500

Hi Martin,

I have checked the tutorials, but they do not give precise details.
Here is what I have discovered with a general block:

1) set_history(1+m) will simply fill your input buffer with m many
zero items at startup, then your input data will follow.

2) If you actually want to see more samples in your work method, then
you have to increase the requested number of inputs in your forecast
method by history() - 1.

3) To see actual history, you have to do both 1) and 2) and do NOT
consume in your work method every sample you have requested in
forecast, only the regular amount so you maintain the proper amount of
past samples in your input buffer.

4) You can do 2) alone without 1), but then you are going to work with
"future" samples. So you can always request more input but you are not
going to consume it, so you will see the future.

5) Many blocks (sync, sync_interpolator, sync_decimator, general) will
actually implement some default behaviour for your forecast method,
and there they take history() into account. However, if you
reimplement forecast() and forget to include your history, then in
your work method you are going to read garbage outside of your input
buffer (which will be hard to debug).

6) The amount of input items in your work method is exactly the amount
(or more) you have requested in your forecast, so magically you will
not have more input data because of set_history() is called.

Are these statements correct? Best,
Miklos


On Tue, Aug 27, 2013 at 2:19 AM, Martin Braun (CEL)
<address@hidden> wrote:
> On Tue, Aug 27, 2013 at 12:11:48AM -0500, Miklos Maroti wrote:
>> Hi Guys,
>>
>> I do not understand how set_history and fixed rate blocks are
>> interacting. Supposedly, the scheduler can figure out a static
>> schedule for sync blocks. However, the sync_interpolator is forcasting
>> this
>>
>> int sync_interpolator::fixed_rate_noutput_to_ninput(int noutput_items) {
>>     return noutput_items / interpolation() + history() - 1;
>> }
>>
>> but consumes only noutput_items / interpolation(). This means, that
>> the scheduler is making a schedule based on the wrong info, the block
>> is actually consuming LESS than it has predicted, so how can the
>> scheduler make optimal schedule?
>
> Hi Miklos,
>
> Think of the history as the number of *past* input samples it takes
> to produce one output sample. The classic example is an FIR filter,
> where you need as many samples as you have taps.
> Minimum is of course 1 (which means you need only the current sample).
> This is also the default value.
>
> In a sync_interpolator, noutput_items is always an integer multiple of
> ninput_items. So if you have no history configured, it must consume
> noutput_items / interpolation(). So what you're seeing is correct
> (unless you changed your history).
>
>> On this point, it is not clear how history() is handled. I searched
>> the archives, and the wisdom is, that set_history(2) means, that all
>
> I'm sure the topic of history() is covered in the tutorials. Also, what
> you're saying is correct and fits what I just explained above.
>
> MB
>
>> the ninput_items and noutput_items and their relationship is
>> completely unchanged, but we have an extra sample at the beginning of
>> the input buffer. However, the sync_interpolator does not do this, it
>> actually includes the history amount in the requested input. So does
>> the scheduler takes history amounts into account, and the
>> sync_interpolator has a bug, or every block has to add the history
>> into their forecast?
>>
>> This is really confusing. I hope someone can shed some light on this.
>>
>> Best,
>> Miklos
>>
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>
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> Communications Engineering Lab (CEL)
>
> Dipl.-Ing. Martin Braun
> Research Associate
>
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