[mythtv-users] Raspberry PI as backend slave or tvheadend

Raymond Wagner raymond at wagnerrp.com
Fri Jun 7 19:49:26 UTC 2013


On 6/7/2013 12:11 PM, Simon Hobson wrote:
> Tyler T wrote:
>> As someone already mentioned though, the Pi's NIC is on the shared USB
>> bus so you may have I/O issues recording multiple HD streams.
> I doubt that bandwidth is likely to be an issue. After all, DVB-T muxes tend to be something like 16 to 24 Mbps (depending on various factors) so even if you were slurping the whole mux, you'd struggle to go over something in the order 50Mbps - significantly less than USBs capacity of 480Mbps even allowing for overheads.

Remember that USB1.1 and USB2.0 are host driven.  They do not have 
dedicated hardware polling the interface and feeding it to the system 
over DMA.  They are cheap to implement as they rely on the host CPU to 
perform that task.  While the more expensive hardware driven Firewire 
had no trouble reaching near its 50MBps limit, you're lucky if you 
manage to get half the 60MBps limit of USB2.0. You're also running this 
on a CPU with about a quarter the performance of the typical desktop PC 
at the time USB2.0 first came out.

Running one or two tuners will not likely be a problem, but it is a 
potential issue worth knowing, since you're venturing into untraveled 
territory.  Better to know what could go wrong ahead of time, rather 
than trying to debug anomalous data loss with no idea what the cause 
could be.


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