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Eb/N0 levels and audio dropouts
To: CRTech <crtech@crtech.org>
Subject: Eb/N0 levels and audio dropouts
From: Sherrod Munday <smunday@ieee.org>
Date: Wed, 12 Apr 2017 14:11:20 -0400
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On Apr 12, 2017, at 11:06, Willie Barnett <wbradiolists@gmail.com> wrote:
> It sounded like a satellite signal "hit" but EbNo is normal here.

Remember that an Eb/No (more correctly identified as Eb/N0 - note the "zero" instead of the letter "oh" -- see https://en.wikipedia.org/wiki/Eb/N0 ) is a *calculated* value, usually over some interval of time.  

An instantaneous/short-duration hit or dropout in the RF signal will most likely result in corrupted audio coming out of the MPEG decoder if the error is significant enough to exceed the forward error correction ("FEC") capabilities within the modulated signal and/or the encoded data therein.

However, the duration of signal interruption that is required to adversely affect the audio (or more precisely, the digital data stream) is much shorter than the duration required to significantly affect the Eb/N0. 

In the older Unity 4000 receivers, for example, the Eb/N0 displayed was a 10-second average, sampled twice per second.  As such, if the threshold Eb/N0 was 4.0 (i.e., where the receiver would typically be unable to sustain reliable decoding of the data stream) and the average level was 10.0, a single burst of noise (or a signal dropout) could last up to a half second without being sampled and factored into the Eb/N0 calculation.

Assuming that the dropout *is* sampled and measured at, let's say, 2.5, that would only result in dropping the Eb/N0 from 10.0 down to 9.625 for that 10-second interval.  (It only affects one out of twenty samples so the net "10-second Eb/N0" effect is 1/20th the instantaneous effect.)

And then you'd have the issue whether your Eb/N0 logging software actually sampled the E/bN0 within that 10-second window of time to log the net effect ...

So, as you can see, the Eb/N0 is a good measure of overall signal quality and receiver health, but it cannot adequately represent a very short-duration dropout or other hit to the digital data stream.

I'm not familiar enough with the Amb-Os, XDS, and other digital receivers to describe in detail their Eb/N0 calculations, but the concepts are the same.

Hopefully this helps explain things a bit.

—
Sherrod Munday
<smunday@ieee.org>

References: SRN audio dropout
(Willie Barnett <wbradiolists@gmail.com>, 12 Apr 2017 15:07:06 -0000)
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