[mythtv-users] Hauppauge 1609 and Pixelation Tests

[Stephen Worthington]

On Sun, 2 Jun 2019 18:21:51 -0500, you wrote:


>No:
>
>--------1600
>
>Antenna ---- 4x -----1600
>
>                 --------2500
>
>                 ----------------------------------1609
>
>
>(If the diagram gets clobbered the antenna feeds to the 4x splitter.  
>The four tuners feed from the 4x splitter.)
>
>
>
>“4x” is a 4-way signal splitter.
>
>The 1600 and 2250 tuners are inside the old Backend (BE1); the 1609 is 
>inside the new Backend (BE3). The signal strength to the 1609 should be 
>the same as what is going to the 1600 and 2250. What happens inside the 
>tuner card with single, dual and quad tuner devices is another level.

OK, so each of those cards is getting a little less than one quarter
of the signal from the aerial.  On the 2250 card, that is then split
again so that each of its tuners is getting only less than 1/8 of the
aerial signal.  So those tuners would be the ones most likely to be
affected by pixellation or total loss of signal when the aerial signal
is degraded.

In the 1609, if the speculation is right, there is a low noise
amplifier (LNA) on its aerial connection, which boosts the level to a
bit over 4 times the input level, and then feeds a 4 way splitter so
that each of the 4 tuners gets the same level as the input - a bit
less than one quarter of the aerial signal.  The LNA and splitter seem
to be on-silicon, and are likely high quality, with very little loss
of signal quality compared to the normal sort of aerial amplifier and
splitter that are available retail for aerial systems.  It is not
unlikely that the LNA actually adjusts its amplification level to
match the requirements of the tuners, so that each of the 4 tuners is
getting an optimum signal level, no matter what the input aerial
signal level is.  The net result is that all 4 tuners work very well.

By the way, people who were used to the old analogue TV usually have
the wrong idea about pixellation.  They think of it as degraded
picture, which was a gradual thing in the analogue system, ranging
from present but not visible except to a trained eye, through nasty
but still watchable, all the way to completely unwatchable (but the
audio was usually still hearable).  But in the digital system,
pixellation is really loss of signal for short periods, not degraded
signal.  When the signal level is marginal, some data packets will
either not be received (totally lost), or will be received but their
checksums will be invalid, so they will be dropped.  As long as you
are still receiving some valid packets, they will be stored on disk,
but may not have the correct timestamps.  So when you play back the
recording, there will be short gaps in the data for the streams that
make up the recording.  For both audio and video, the data is
compressed using an algorithm that looks backwards to the previous
data in order to uncompress it.  In the video stream, it can also need
to look forwards (with H.264 and H.265).  If the decoder does not have
all the data, it obviously is unable to display anything for the
missing data, so it will usually freeze the output of that stream when
there is missing data.  But when some more valid data becomes
available, it most often can not use it, as it is unable to decode it
without the prior data that is missing.  It may also have the wrong
timestamp, so it will be played at slightly the wrong moment.  The
decoder has to wait until it comes to a point in the data where it is
able to resume decoding.  That point is a "keyframe" for video data -
data where the entire decoding process is restarted with a full set of
data for one video frame, including a timestamp.  So the net result of
a small loss of data can be a much longer disruption of the picture,
depending on how often keyframes are sent.  There is also the
possibility of getting a packet where the data is invalid, but does
pass the checksum - that is rare, but possible, and is the only way to
actually receive a "degraded" signal, instead of a momentary loss of
signal.

The difference in signal level between 100% reception and no reception
at all (no valid packets received) is actually quite small.  And all
the digital signals have been carefully designed to be resistant to
the usually problems that plagued analogue signals.  For example,
analogue ghosting, caused by receiving the same signal via multiple
different paths and hence at slightly different times, is virtually
completely eliminated by the way DVB and ATSC signals are broadcast -
the ghost signal(s) are simply ignored.