[mythtv-users] mythhdhrrecorder setup

James Abernathy jfabernathy at gmail.com
Sun Dec 9 01:04:21 UTC 2018


On 12/8/18 7:38 PM, Stephen Worthington wrote:
> 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"
> channel.
>
> 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.

This is very helpful, thanks.  This is the way I thought it worked, but 
in reading the External Recorder Script, mythhdhrrecorder it says to 
only allow 1 recording per channel. Not sure why.  Part of me says to 
isolate the network so mythtv backend and the HDHR tuners are on one 
network so the other devices in the house can't see the HDHR tuners. 2 
NIC in the backend. one for the FEs and one for the HDHRs.

Jim A

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