[mythtv-users] Curious fast forward problem

Michael T. Dean mtdean at thirdcontact.com
Wed Jan 6 21:36:35 UTC 2021

On 01/06/2021 03:54 PM, Peter Bennett wrote:
> On 1/5/21 11:56 AM, mythtv wrote:
>> We've been experiencing this weird fast forward behaviour for several 
>> versions now. I assume it must just be something we're doing wrong 
>> because it's never changed since at least v0.28 and no one else seems 
>> to be talking about it.
>> What happens is this: We have auto-skip commercials turned off. When 
>> watching live tv, if we're delayed for more than a few minutes, then 
>> hit fast-forward to manually skip thru some commercials, 9 out of 10 
>> times, we can skip  thru a few commercials but sometimes it will just 
>> jump forward to almost live again (a few seconds behind live).  I 
>> hope I explained that correctly.  Its not "consistent" but it happens 
>> frequently enough to be a real nuisance, especially when our remote 
>> doesn't have numbers allowing us to just jump back a certain number 
>> of minutes. And also, the amount jumped is not a consistent amount of 
>> time, it always just jumps to near live again.  I believe the backend 
>> is marking commercial breaks, and maybe there is some weirdness in 
>> trying to jump forward without a correct number of commercial breaks 
>> recorded, or some other setting about the length of commercials, etc. 
>> I don't quite understand all of those settings, and I don' think I've 
>> changed any from the defaults, other than to disable commercial 
>> auto-skip.
> I have noticed this when watching a program that is still recording. 
> As you mention, it has been around for a long time. The biggest 
> problem with fixing it is that it is so intermittent. In my experience 
> it only happens maybe one time out of every 10 programs watched while 
> recording. It seems to happen occasionally when I am around 15 minutes 
> before the end, on a single skip forward it jumps to the end instead 
> of jumping 1 minute. It is not related to marking commercial breaks. I 
> believe it is something wrong with the way the system keeps track of 
> the increasing program size and the amount remaining. If you can 
> identify how to reproduce it, maybe we can solve it.

Are you sure that the frontend just hasn't gotten an update to the 
currently-detected commercial skiplist and, perhaps, the last update it 
received was one with a commercial start but without its accompanying 
commercial end?  Basically it received an update while the commercial 
detection was inside a commercial break (so the last mark was a 
MARK_COMM_START ) and the backend hasn't sent out an update that 
includes the MARK_COMM_END ?

IIRC, I think the COMMFLAG_UPDATE event tends to be sent (to any 
frontends that requested updates for that program with a 
COMMFLAG_REQUEST event) every 500 frames or so of commercial detection, 
so that would be around 8m 20s of actual program time for a 60fps show, 
but depending on how busy/powerful the system running the commercial 
detection is and depending on how far it's trying to stay away from the 
"now" of the recording, I could see how it could be 10+min behind now.  
For interlaced video, I'm not sure if it's actually frames or fields, 
but if it's still 500 frames, an update would be sent after processing 
about 16m 40s of recording in a 1080i60 show, for example.

It's possible that when the flagger recomputes the commercial skip list, 
it ensures it never ends with a MARK_COMM_START, in which case this 
shouldn't be the problem, but I haven't looked.

Anyway, just a thought.  If nothing else, something like this could 
explain the intermittency of the issue.  Feel free to ignore me if my 
guess is way off, though.  :)


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