[mythtv-users] Recording from HDMI

Mike Perkins mikep at randomtraveller.org.uk
Wed Feb 22 16:51:38 UTC 2023

On 22/02/2023 16:34, Jan Ceuleers wrote:
> On 21/02/2023 16:51, Paul Harrison wrote:
>> What sort of delay does this device introduce? For example if you change
>> channel on the device you are recording how long of a delay is there
>> before you see the change? I'm talking about when you use something like
>> ffplay or vlc to view the raw output from the device not when used with
>> MythTV.
> A few weeks ago I was watching TV using a set-top box while also
> watching the same channel (using MythTV's live TV functionality) on my
> laptop. The time lag was below 2 seconds.
> However there is still an artefact I'm trying to fix, which is that
> audio is delayed by a few hundred milliseconds relative to video. I've
> asked the ffmpeg mailing list for how to delay the audio track of an
> incoming stream (as opposed to delaying the audio track of a video file).
Don't forget that most STBs, like mine, are probably DVRs anyway. Mine has an SSD and actually 
records as it displays, which means we have pause/rewind/playback functions built in - which is 
exactly the same as mythtv's live TV function.

To get a true idea of any delay introduced by using mythtv, you'd have to compare it with reception 
directly by a TV[1] which restricts you to OTA[2] or possibly satellite[3], but would provide a 

[1] Assuming that your TV is basic enough that it just displays what comes down the wire. Modern 
ones may do so much processing on the feed that the whole 'delay' business becomes moot.

[2] I have done tests with our STB, tuned to a BBC radio channel, and a straight FM radio. The delay 
introduced is noticeable and maybe four seconds or so. The Greenwich time pips can no longer be 
considered reliable.

[3] Satellite of course involves a base -> satellite -> dish delay before any post-processing.


Mike Perkins

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