Blue Planet 2 - an audio puzzle

fenlander

Active Member
#1
I've encountered a puzzling issue with some Blue Planet 2 recordings.

-The files are auto decrypted
- They play perfectly on the Hummy
- When downloaded to my PC, the ts files play correctly in Windows Media Player but some (not all) play the musical part of the soundtrack but not the main commentary in VLC Player. The same files crash AVIdemux
- The files affected are episodes 1 and 6. 2-5 play correctly in VLC. 7 is yet to be broadcast.
- Remuxing the files in MKVmerge does not work

So far as I can see, there is a single audio stream containing both the music and the Attenborough commentary, so I don't understand how part of this stream can be missing and not the other part, or why it happens on some files and not others, or on some video players and not others (too many variables). Given that a lot of people object to the music, it might actually be quite nice to be able to separate the two and turn the music down while retaining volume on the voice...

I'm quite keen to keep the Hummy files for the 1080p video as iPlayer downloads only go to 720p. I've salvaged the episodes by remuxing the Hummy video streams with the iPlayer audio, but it's a tedious process.

Does anyone have any idea what might be happening here?
 
#3
My copy of Ep1 downloaded to PC plays with commentary in VLC v.2.2.1 (although I've only checked the 1st 3 mins), and does not crash Avidemux (2.6.21).

VLC reports 2 mp4a audio streams: 3F2R/LFE plus a visually impaired commentary.

Generally I find about one in 50 files will crash Avidemux. When this happens I go back to the file on the humax and use crop to chop out the first few seconds. The cropped file has always worked correctly.
 
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fenlander

fenlander

Active Member
#4
Spot on advice. Chopping the first few seconds cured the problem. I really should have known as it's a trick I've used before in slightly different circumstances. Thanks alanoc for the timely reminder.
 
#5
Pleased it did the trick.
I've just taken a look at the audio in Audacity: it appears that centre channel alone carried the commentary which could explain why it was missing for you if VLC treated it as stereo.
 
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fenlander

fenlander

Active Member
#6
I think that's right. The broadcast programme had 6-channel sound but the 'bad' version - pre-crop - was showing in MKVToolnix as 2-channel. After cropping on the Humax it was back to 6.

This suggests to me that whether or not an audio stream converts cleanly depends upon the exact moment the recording started, so it will be a random occurrence. Odd that some media players and processors (WMP) cope with what seems to be an error in the file and others don't (VLC, Ocenaudio, AVIDemux, MediaHuman to name a few). These either fail to recognise the missing channels or just crash when the file is loaded.

I used to come across similar issues regularly with files saved from ITV HD and C4 HD, but have never seen it before on a BBC recording.
 

Black Hole

May contain traces of nut
#7
I think the difference lies in whether a media player is able to adapt to stream characteristics that change during the recording, or if it determines the characteristics at the start and is stuck with that for the duration. The former is necessary for continuous streams (or recordings of continuous streams), but would not be required for individual video file playback.
 
#8
Although I have a humax, I experienced this problem on last nights Blue Planet live on a LG tv, where the centre channel as it were disappeared, and the same problem just now on BBC2 whilst I was investigating what was going on. Might this mean it's a broadcast matter rather than the receiver Humax or tv?


edit: The same problem has now occurred on The Simpsons. Fortunately the tv has twin tuners and I swapped to Freesat where there was no problem. So have convinced myself it's a broadcast issue with Freeview. Now a permanent swap to FreeSat and hope the problem doesn't migrate there.
 
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