[mythtv-users] Changing Station Frequencies after FCC compaction

Stephen Worthington stephen_agent at jsw.gen.nz
Wed Dec 5 05:02:14 UTC 2018

On Tue, 4 Dec 2018 19:13:49 -0600, you wrote:

>I have 2 channels that changed frequencies after the FCC compaction of the spectrum.
>(yea, I was slow to get around to it -- shows how much TV I actually watch)
>Using the web client, I changed the DB entries for those channels accordingly... but I'm showing a recording happening now that's still on the old frequency.
>(41 instead of the new 32)
>Is there something else I need to do or ??
>  -Ben

I think you will have been changing the "freqid" field in the
"channel" table.  That is only for old analogue transmissions and is
not used at all for digital TV.  Digital transmitters (multiplexes)
send a signal on a frequency that is set per transmitter rather than
per channel, as each transmitter can send multiple channels (hence the
"multi" in "multiplex").  Each multiplex has a row in the
"dtv_multiplex" table.  The "dtv_multiplex" table has a "frequency"
field, but that is not what needs changing, as what has happened is
that the channel has been moved to a different multiplex, that may or
may not have its own "dtv_multiplex" row already.  If the channel has
been moved to another transmitter that already exists, then what you
need to change is the "mplexid" in the "channel" table row for that
channel.  The "mplexid" tells mythbackend which "dtv_multiplex" row to
use to find the correct multiplex.  If a row for the new multiplex
does not already exist in "dtv_multiplex", a new one needs to be

To complicate things, when a channel is moved in this part of the
world, the "serviceid" in the "channel" table row may also need to be
changed.  Unless the FCC has mandated otherwise, that may also be the
case for you.  Each multiplex regularly transmits packets of data that
are indexes to what it is transmitting.  The "serviceid" is matched in
that index table and that gives the numbers used to identify the
streams of data that make up that channel (video stream, one or more
audio streams, subtitles stream, ...).  Mythbackend records a channel
by selecting all the packets that match the ID numbers for any of the
streams for the channel and storing them to disk.  If the "serviceid"
for the channel has also changed, then you would need to find out what
it has changed to and change that value in the "channel" row.

So, basically, you can not manually make the necessary changes via
mythweb as it does not allow you access to the "dtv_multiplex" table
or the "serviceid" field of the "channel" table.  It is possible to
manually make the necessary changes directly in the database, but
unless you really, really know what you are doing, messing around in
the database is a bad idea.  So you will likely need to rescan your
channels in mythtv-setup.  If the channel has just been moved to
another existing multiplex, you should be able to just do a
"transport" scan, rather than a full rescan.  You may need to scan two
transports - the one the channel used to be on, and the new one it is
now on.

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