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[Discuss-gnuradio] Re: Questions on usrp_tv_rcv.py

From: Anastasopoulos Achilleas
Subject: [Discuss-gnuradio] Re: Questions on usrp_tv_rcv.py
Date: Tue, 23 Jan 2007 13:41:29 -0500 (EST)

I guess I am attempting an answer to myself here:

the slow horizontal sliding reveals a
missalignment on the order of
200 samples/sec.
(I measured that by seeing how much the black strip moves
to the right within the 1 sec of video stream).

For a sampling rate of 8Msps this is can be explained by a
drift/jitter of the ADC on the order of 25ppm

Is this within the drift/jitter specs of the ADC?


On Tue, 23 Jan 2007, Anastasopoulos Achilleas wrote:

Thanks for pointing out all theses raw files
for testing the usrp_tv_rcv.py application.
I tried a couple of them and they work fine.

Now I have the following question:

Why is it that the video shown on sdl moving
slowly to the right?

The fact that there is no vertical or horizontal
synchronization should NOT explain this:

I use a decimation of 8 thus the video rate is 8M.
With 625 lines per frame and 25 frames per second, this results in
512 samples per line (exactly), ie, 8,000,000/25/625=512.

So one would have expected to see the corresponding picture
not aligned in the vertical/horizontal dimension,
BUT AT LEAST the misallignment should have been CONSTANT over time.
This is clear in the vertical dimension (no up/down sliding)
but not in the horizontal...

What is the explanation of this?


PS: one explanation I can think of is the
instability of the 64M ADC...

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