[mythtv-users] mythhdhrrecorder setup

Stephen Worthington stephen_agent at jsw.gen.nz
Sun Dec 9 00:38:50 UTC 2018

On Sat, 8 Dec 2018 12:05:10 -0500, you wrote:

>On 12/8/18 11:10 AM, Jay Harbeston (ISeePeople) wrote:
>>> On Dec 8, 2018, at 9:37 AM, James Abernathy <jfabernathy at gmail.com 
>>> <mailto:jfabernathy at gmail.com>> wrote:
>>> On 12/6/18 12:51 PM, John P Poet wrote:
>>>> On Thu, Dec 6, 2018 at 10:46 AM John P Poet <jppoet at gmail.com 
>>>> <mailto:jppoet at gmail.com>> wrote:
>>>>     On Thu, Dec 6, 2018 at 10:42 AM James Abernathy
>>>>     <jfabernathy at gmail.com <mailto:jfabernathy at gmail.com>> wrote:
>>>>         On 12/6/18 12:21 PM, glen wrote:
>>>>>         On Thu, 2018-12-06 at 11:53 -0500, James Abernathy wrote:
>>>>>>         I’ve giving this a test.  On the wiki it states in part: 

>> If you will only be using OTA on the external recorder, you will not 
>> be able to use the virtual tuners. Ie record 10.1, and 10.2, 10.3 at 
>> the same time on one tuner. It will require 3 separate physical tuners 
>> for the above. I configure my HDHRs with 4 virtual tuners.
> From my understanding the virtual tuners and multiplexed sub channels 
>are 2 different things. I've setup each tuner as capable of 1 channel 
>max for recording. so with my Quatro and Duo I have a total of 6 
>channels max.

In technical terms, there is no such thing as a "sub"channel.  All
channels are equal.

A digital multiplex (ATSC, DVB-T, DVB-S, DVB-C, ...) transmits
channels as packets of data.  Each packet has a type number
identifying it.  Some packets are sent for administrative purposes, so
every so often a packet comes by that contain a list of the packets
that make up the channels being transmitted.  To receive a channel, a
tuner waits for one of those packets and looks up the "serviceid" of
the channel it wants in the table in that packet.  The table has a
list of the packet IDs for all packets that make up the channel with
that serviceid.  The tuner then starts looking for all packets that
match the numbers on that list, and storing them to disk as they are
received.  The packet numbers will normally be representing the
streams of data that make up a channel - one video stream, one or more
audio streams, one or more subtitle streams, and so on.  So the
resulting disk file has all those streams of data, differentiated by
the packet ID numbers on the packets.

To record multiple channels at once from a multiplex ("multirec"), all
the software needs to do is to store the packets for each channel to
different files.  MythTV does this by using multiple virtual tuners
per physical tuner.  Each virtual "multirec" tuner is set up to store
the packets for one channel to disk.  All the virtual tuners receive
all the packets from the physical tuner, and select out only the
packets they need for their selected channel.

In ATSC, as best I can tell, the idea of a "main" channel and "sub"
channels is from the ownership and control model used for the
transmitters, rather than from any technical point of view.  TV
stations seem to own a multiplex and all the channels on the multiplex
were being broadcast by the same TV station or network.  If another
organisation wanted to broadcast TV in the same area, it needed to buy
the rights to transmit a multiplex on a different frequency, and on
their multiplex, they would only broadcast their channels.  On each
multiplex, they would have one flagship "main" channel that they would
give the largest bandwidth to (usually an HD channel), and maybe one
or more other "sub" channels that would have less bandwidth each
(often SD channels).  But the designation of a channel as "the" main
channel had no technical basis, just a political one.  As far as I
know, there is no data transmitted that flags one channel as a "main"

Then the FCC seems to have changed things, and has mandated a
reorganisation where the bandwidth on multiplexes that was not being
used because the owning organisation did not have a use for it was
able to be used by other organisations.  So now the "main" channels
from two different TV stations may be transmitted on the same
multiplex, if it has enough bandwidth to do that.

So, virtual tuners and "sub" channels are in fact different faces of
the same thing.  To record all the channels of a multiplex at the same
time, you need to have as many virtual "multirec" tuners as there are
channels being broadcast on that multiplex.  If you have fewer virtual
tuners than the number of channels you want to record from one
multiplex at once, then MythTV will need to use another physical tuner
to record the extra channels.  Which it will do.  But you are just
wasting a physical tuner by doing that.

The only reason I can think of for not setting the number of virtual
tuners high enough to record all the channels of a multiplex from one
physical tuner is if you do not have enough hard disk bandwidth to
actually record that many channels at once and want to limit your
MythTV system to fewer channels at once to prevent recording failures.

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