# QAM Mod / Demod blocks

**URL:** <https://www.ruby-forum.com/t/qam-mod-demod-blocks/231142>\
**Category:** GNU Radio\
**Created:** [July 23, 2014, 2:45pm UTC](https://www.ruby-forum.com/t/qam-mod-demod-blocks/231142 "2014-07-23T14:45:00Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![addis\_a](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/c4cdca/32.png) [@addis\_a](https://www.ruby-forum.com/u/addis_a)\
**Post date:** [July 23, 2014, 2:45pm UTC](https://www.ruby-forum.com/t/qam-mod-demod-blocks/231142/1 "2014-07-23T14:45:00Z")

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removed\_email\_address@domain.invalid  
[https://lists.gnu.org/mailman/listinfo/discuss-gnuradio](https://lists.gnu.org/mailman/listinfo/discuss-gnuradio)

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**Author:** ![activecat\_k](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/c57346/32.png) [@activecat\_k](https://www.ruby-forum.com/u/activecat_k)\
**Post date:** [July 23, 2014, 3:47pm UTC](https://www.ruby-forum.com/t/qam-mod-demod-blocks/231142/2 "2014-07-23T15:47:28Z")

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On Wed, Jul 23, 2014 at 8:42 AM, Activecat [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> Dear Sir,
> 
> I am learning to use the built-in “QAM Mod” and “QAM Demod” blocks.  
> Refer below flowgraph.
> 
> The problem is, at the output of the “QAM Demod” block, I can’t get what I  
> fed into the “QAM Mod” block.  
> What is missing ..?

All modulators take in packed bits and all demodulators output unpacked  
bits. First, remember that there are filters involved, so there’s going  
to  
be a delay. You’d need to synchronize the output of the demodulator to  
some  
known word to understand which output bit is the correct start of the  
data  
stream. Easier would be to unpack the source data. Out of the ‘head’  
block,  
make another connection to a ‘unpack\_k\_bits’ block with an unpacking  
value  
of 8. Use this to compare to the demodulated bits.

Also, the vector source should generate values from 0 to 255 to create  
the  
packed bits needed as input to the modulator. Otherwise, you’ll just be  
heavily biasing the constellation to just a few points. You can stick a  
constellation display block on the output of the modulator to see.

Tom

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**Author:** ![activecat\_k](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/c57346/32.png) [@activecat\_k](https://www.ruby-forum.com/u/activecat_k)\
**Post date:** [July 24, 2014, 5:45am UTC](https://www.ruby-forum.com/t/qam-mod-demod-blocks/231142/3 "2014-07-24T05:45:04Z")

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On Wed, Jul 23, 2014 at 9:45 PM, Tom R. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> > 
> 
> packed bits needed as input to the modulator. Otherwise, you’ll just be  
> heavily biasing the constellation to just a few points. You can stick a  
> constellation display block on the output of the modulator to see.
> 
> Tom

Thank you very much.