[mythtv-users] Duplicate screens

GARETH GLACCUM gareth.glaccum at btopenworld.com
Thu Jan 18 21:08:52 UTC 2007

The question I have, is how does the frontend actually play the data? I am here watching mythtv player on my MS windows machine. I am watching CSI whilst the backend/frontend machine behind me is recording and playing CSI at the same time. I synchronised the two by very careful pause/unpause, but that is of no consequence. 
Basically, my windows box knows nothing about the nfs mount. The frontend, does have the data locally, but perhaps it is receiving the same IP stream as the mythtv player? If this is the case, then this is a serious waste of bandwidth. So how to get around this? Multicasting the data from the backend to the frontends. This would be a 'perfect' solution. The information that the frontends require to play the audio and video is the same data, therefore, if the frontends are capable of decoding and playing the data in realtime, they have to be synchronised because they are playing exactly the same data.

Unfortunately this is a problem for more than one reason.
1) Probably requires a bit of a nasty wire change. I would suspect the easiest way of implementing it would be to move the video and audio out to a completely seperate stream. This new stream would be multicast. The 'old' ports etc. would still be used, for frontends to control which streams are being broadcast and to state that they are joining and leaving. 
2) Will cause pain for a lot of people. There are several well-known switch manufacturers who have not implemented multicasting properly. For most applications this is not an issue. Unfortunately for us, the problems usually arise when there is a lot of data transfer. The stream I am watching is 218kb/s, not that much I suppose, but it would be enough to toast a badly written switch. The trouble with these dodgy switches is that they appear to work for a while, then become intermittent. 
3) What happens if one frontend decides to pause, but the user at another frontend decides it doesn't want to? Some thought into how much control you have over the other frontends is required.

Personally I think that I would use this quite a lot. However, is there enough interest behind this to implement? Implementation of this is surely not beyond the writers, they have done a great job already, but it does require a significant change to the way the system works I think. Now apparently there is plans to support multicast recording in mythtv 0.21, perlhaps someone is thinking of this already?
  ----- Original Message ----- 
  From: Matthew Asplund 
  To: Discussion about mythtv 
  Sent: Tuesday, January 16, 2007 1:20 AM
  Subject: Re: [mythtv-users] Duplicate screens

  Actually, how about an option under the "Media Library" or "Watch TV" menu that would be "Synchronize to another Myth Front-End".  You could then choose which front-end from all front-ends that are connected to your back-end.  The slave front-end would then do whatever the master front-end does.  Given that I can sync the time on my computers using NTP, it would be enough to send the occasional time marker command to keep them together.  In practice, it would be great if they were within 10 ms, to minimize the echo , which is pretty easy even for a ping command. 

  Now the one tricky part that I can see, is what if you have one SD setup and one HD setup.  I suppose that if you are using SD content it is easy, and if you have HD content, then the SD front-end is responsible for rendering the stream into 480p to make it work out. 
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