# \[ input \] -\> \[ PSK mod \] -\> \[ PSK Demod \] -\> \[ output \] =\> \[ input \] != \[ output \] - interesting

**URL:** <https://www.ruby-forum.com/t/input-psk-mod-psk-demod-output-input-output-interesting/222155>\
**Category:** GNU Radio\
**Created:** [August 15, 2012, 12:10pm UTC](https://www.ruby-forum.com/t/input-psk-mod-psk-demod-output-input-output-interesting/222155 "2012-08-15T12:10:58Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![aris](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/258eb7/32.png) [@aris](https://www.ruby-forum.com/u/aris)\
**Post date:** [August 15, 2012, 12:10pm UTC](https://www.ruby-forum.com/t/input-psk-mod-psk-demod-output-input-output-interesting/222155/1 "2012-08-15T12:10:58Z")

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Greetings,

In gnuradio-companion I connected output of a “PSK Mod” directly to a  
“PSK Demod” input. Both of the blocks have the same initializations,

Number of Constellation Points:2  
Gray Code:Yes  
Differential Encoding:No  
Samples/Symbol:8  
Excess BW:350m

in PSK Demod other properties are as follows:

Frequency BW:62.8m  
Phase BW=62.8m

The output of the PSK Demod is fed into “UHD:USRP Sink” with a zero  
center frequency after applying some type conversion blocks, “UChar To  
Float” &  
“Float To Complex”. I connect an external oscilloscope, i.e. not in PC,  
not a GRC block a real one, to the output of the USRP to observe what I  
have after appyling PSK mod and demod. As the output of the process, I  
have a completely different signal from I generated in the beginning.

I also checked what happens if I directly feed the actual signal to the  
“UHD:USRP Sink”, I got a signal nearly same as the original one, that is  
as expected, great.

Another surprising thing, when I apply the output of the “PSK Mod” to  
the “UHD:USRP Sink” and observe the output of the USRP1 output,  
constellation points number is 2, although I see BPSK format, 180 degree  
phase shift in the signals as I expected, I see 100 kHz baseband signal  
frequency converts into 62.42 kHz, oscilloscope says so :), Samples /  
Symbol rate is 8.

What might be the problem?

Thanks,  
Abdullah

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**Author:** ![abdullah\_unutmaz](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/65b543/32.png) [@abdullah\_unutmaz](https://www.ruby-forum.com/u/abdullah_unutmaz)\
**Post date:** [August 15, 2012, 5:35pm UTC](https://www.ruby-forum.com/t/input-psk-mod-psk-demod-output-input-output-interesting/222155/2 "2012-08-15T17:35:51Z")

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If this helps, the psk mod inputs packed bytes, however the demod  
outputs unpacked bytes, where each byte is a decoded symbol. Obviously  
you need some kind of framer correlate to reconstruct the packed byte  
boundaries outside of the demod. It might be more comparable to use a  
unpacked to packed block before the demod. That way both input and  
output data is unpacked.

-josh

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<div class="post-metadata">

**Author:** ![abdullah\_unutmaz](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/65b543/32.png) [@abdullah\_unutmaz](https://www.ruby-forum.com/u/abdullah_unutmaz)\
**Post date:** [August 15, 2012, 10:06pm UTC](https://www.ruby-forum.com/t/input-psk-mod-psk-demod-output-input-output-interesting/222155/3 "2012-08-15T22:06:28Z")

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Thank you very muchJosh for your reply.

* * *

From: Josh B. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid)  
To: removed\_email\_address@domain.invalid  
Sent: Wednesday, August 15, 2012 6:34 PM  
Subject: Re: [Discuss-gnuradio] [input] → [PSK mod] → [ PSK Demod  
] → [output] =\> [input] != [output] - interesting

If this helps, the psk mod inputs packed bytes, however the demod  
outputs unpacked bytes, where each byte is a decoded symbol. Obviously  
you need some kind of framer correlate to reconstruct the packed byte  
boundaries outside of the demod. It might be more comparable to use a  
unpacked to packed block before the demod. That way both input and  
output data is unpacked.

-josh