[mythtv-users] LiveTV channel switching time

Ryan Duffy dusteur at excite.com
Mon Dec 4 16:49:56 UTC 2006

> So now back to your boxes and do some profiling to find out where
> the channel change time is spent. (A least I will try)

OK, I think I have a reasonable set-up for some testing:

I am using a PCI DVB-T card to receive free-to-air digital broadcasts in the UK.
We have many channels per multiplex, so I can test channel changes without any tuning happening at all. The slow channel change has been irritating me for the whole time I have used myth, and I now tend not to channel surf - not because I don't want to, but because it is too annoying to bother. We have lots of good channels here with no adverts at all, so I often have nothing that needs to be skipped, and am not wasting time by watching live TV rather than necessarily recording things to watch later and skipping all the adverts.

Anyway - I did a few tests and got pretty consistent results every time:

BBC1->BBC2 approx 5.5 seconds
BBC2->ITV1 approx 5.5 seconds
ITV1->CH4  approx 3.5 seconds
CH4->ITV1  approx 3.5 seconds
ITV1->BBC2 approx 7.5 seconds
BBC2->BBC1 approx 5.5 seconds

Note that BBC1 and BBC2 are on one multiplex, and that ITV1 and CH4 are on another multiplex. Also, I used the up and down arrow keys to change channel, so there is no waiting for key input like there might be if I changed channel by typing a channel number. 
I note that it usually takes between 1.5 and 2.0 seconds to tune to another frequency. 
But the changes between BBC1/2 and ITV/CH4 involve no tuning whatsoever, so the times are purely for myth to start showing the new channel. 

I suspect that the slower changes on the BBC multiplex are probably due to these channels being broadcast at a higher data rate (although I'm not 100% sure about that).

These channel change times are much slower than I would like them to be, and I can't see any particular reason why they should be so slow. 
I'm happy to do some proper testing if someone tells me what needs to be done to isolate the issue(s).

A few other notes: 

I use EIT data and have 8 days into the future in my database. I ran optimize_mythdb.pl and mythfilldatabase before testing to ensure the database is properly tuned and clear of old data.
I run my BE and FE on a single PC, so there are no network issues. 
My CPU is an Opteron 165, and I have 2 GB RAM. 
My myth partition is on its own physical disk, which has an IDE controller all to itself. hdparm rates this disk at 40MB/sec for reads. 
I'm using a fairly recent kernel (2.6.16).

Possibly bad things about my setup:
I use ext3 on the myth partition.
I have a Matrox G550 with no hardware 3D enabled.

Anyone got any ideas what to check to see why the channel changes are so slow? 

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