[mythtvnz] Good DVB-T USB capture device

[Stephen Worthington]

On Fri, 23 Apr 2010 08:45:07 +1200, you wrote:

>Hi,
>
>Thanks for that thread. I don't think that 'diversity/dual-tuner'
>switch works under linux. I wonder what the default setting might be.
>
>On a separate note - it looks like only TVONE is affected, even on the
>same multiplex. On Wed I've recorded two shows (from TVONE and TVNZ6)
>and the TVONE is corrupted whilst the other one is not. Last night
>I've recorded multiple streams again and none of them was corrupted.
>So far the only difference was in the alleged 'HD-ness' of the signal
>(i.e. those on Wed didn't have the HD sign, but those yesterday -
>did). The issue seems to occur periodically, around 2 past the hour
>and then around 32 past the hour. I've checked cron - nothing there.
>I've started running dstat with logging to a file to capture if there
>is anything happening at that time (so far - nothing special).
>
>I get signal on both inputs (using external splitter) of about 72-74%,
>the same if I only use ANT1, only ANT2 causes drop on the first tuner
>to about 40%. From what I've gathered so far I don't think it's not an
>issue with the input signal.
>
>kind regards
>Pshem

It is extremely unlikely that a reception problem would affect TV One
only and not all the other channels on the same multiplex.  A
multiplex is just a stream of data packets with different ID numbers
on them.  Recording one channel is done by selecting the packets with
the right ID numbers out of that stream.  So any reception problems
with a multiplex *must* affect all channels on that multiplex.  If
your problem really is only affecting TV One, then it would likely be
a playback problem with 720p files, and that should also affect TV2
the same way.  The other channels on the TVNZ multiplex are 576p.
Playback problems are often characterised by the problem happening in
a different place in the file each time you play it.  Recording
problems will always be seen in the same place if a file is played
back multiple times.

An old rule of thumb comes to mind here.  When in doubt, suspect the
cable.  Is it possible one of your aerial cable connections is
intermittent?  So sometimes it is not getting enough signal to receive
the TVNZ multiplex OK?

BTW When I heard about this "diversity" feature, I had a thought about
how it might be done, and what I came up with is using both tuners
tuned to the same multiplex at the same time, then have the software
check for bad packets from one tuner and look and see if the same
packet was good on the other tuner.  That is a standard way of doing
diversity in communications channels, and normally results in a much
lower error rate even when both channels are a bit iffy, as long as
the two channels come via different paths (in this case, that would
mean different aerials, preferably pointed at different transmitters
if possible).  If it is done that way, then it is all software and
there is no actual hardware diversity switch.