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Re: [Discuss-gnuradio] simple mod-demod combinations doesn't work

From: MarkO
Subject: Re: [Discuss-gnuradio] simple mod-demod combinations doesn't work
Date: Sun, 9 Oct 2016 05:54:50 -0700 (MST)

Hi Sylyain,

thank you for your quick response.

Sylvain Munaut-2 wrote
> Demodulation is not an exact process and the various loops in there
> will take some time to lock and so the beginning will be corrupted and
> your output won't be synced on the bytes boundaries of your input.

Ok, this is a behavior I expected.

Sylvain Munaut-2 wrote
> Before modulation you need some kind of pre-processing to add
> synchronization patterns and stuff like that and eventually some
> padding data both at the beginning and the end to let the loop lock
> and let stuff flush at the end if you want the full data to be
> recovered, then you need to detect, align and remove all of that at
> the output.

For this issue I've tried [Packet Encoder] and [Packet Decoder].

With these blocks I get a noise-free connection if i downsample the audio
signal to 3 kHz, otherwise the data rate is to high for a stutter free
transmission - audio underruns (aU) occur.
Now the problem is that I only receive parts of the audio signal. What i
hear is like: second 1, 5, 9, 15, 17, .. and it's not to fast, what i hear
are parts of the audio file in one consistent stream . The only solution i
found so far is to place a [Throttle] with 3kHz right before [Packet
Encoder]. This way I receive a nearly flawless stream. A few aU still come
up from time to time, i think this is because of the [Throttle]
implementation - usage of OS clock.

For details please check out the attached grc file:

I'm pretty sure that [throttle] isn't the correct solution. Is there a neat
way to solve this?


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