[mythtv-users] padding question

[Michael T. Dean]

On 12/07/2017 11:02 AM, jrh wrote:
>
> On Dec 7, 2017, at 10:53 AM, David Engel wrote:
>
>>
>> Actually, MythTV has changed.  When there are back to back programs on
>> the same multiplex, it tries to put them on different multirec inputs
>> for the same physical input.  That preserves the post-roll for the
>> earlier show and the pre-roll for the later show.
>
> Does this present a problem for the HDHR tuners when for 30 seconds 
> they can be overlapping as far as the 100Mbit limitation of the tuners?
>
> In our area, we have one real channel with 2 ‘720p’ stations(ABC and 
> CBS) and if there were shows that followed on each of them there would 
> be 4 720 streams attempted on that one tuner. Assuming something 
> similar could happen on the other tuner, would there be problems with 
> bandwidth?
>
> Usually down here the first amount of time and the last amount of time 
> are lead in and lead out commercials.
>
> I wonder if there could be a setting that would let users choose 
> whether or not to have this 30 second post roll that is defined 
> outside of the settings discussed previously in this thread.

Based on your e-mail headers and the channel call signs you mentioned, 
I'm guessing you're in the US.  If so, you're using ATSC.  For US ATSC, 
each multiplex is at most 19.39Mbps (where a little over 1Mbps of that 
is overhead)--which means that if those 2 muxes (the one with ABC and 
the one with CBS) were both fully utilizing the available bandwidth for 
the channels you're recording during the overlap, resulting in your 
recording 4x18.3Mbps, you'd be recording 73.2Mbps.  You shouldn't have a 
problem with that on a 100Mbps connection (even with connection overhead).

However, it's extremely unlikely you'd actually be experiencing 
4x18.3Mbps.  Assuming there's an ABC main channel and at least one 
additional subchannel, that 18.3Mbps would be split between the 2 
channels, so each channel would get some portion of the 18.3Mbps. 
Basically, to be seeing 4x18.3Mbps, you'd have to be recording all of 
the subchannels on the multiplex 4 times (so, assuming 2 subchannels, 
ABC-1 x 4 and ABC2 x 4--or ABC-1 x 2 and ABC-2 x 2 and CBS-1 x 2 and 
CBS-2 x 2).  Most likely, you'll be recording ABC-1 with a follow-on 
recording on ABC-1, so you get ABC-1 x 2 during the overlap, but don't 
get ABC-2.  Perhaps the same happens on CBS-1. So, that would mean you'd 
be recording ABC-1 x 2 and CBS-1 x 2.

Oh, and the encoded video in the stream is generally variable bitrate, 
meaning that even if ABC-1 has 75% of the available bandwidth allocated 
to it, it may be using less than that at any given time.

Mike