Author Topic: Garbled sound on some HD recordings  (Read 5767 times)

rappel

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Re: Garbled sound on some HD recordings
« Reply #30 on: February 22, 2017, 02:11:07 AM »
Appreciate it's been a while since last post but I am also experiencing this or similar issues with files sourced from recorded program in UK and would like to understand the cause, but more importantly would like to have a workflow and set of tools that could cope with this.

I'm not really up on content of various container formats or how the video and audio are kept together but happy to learn as I need to try to get round this. Historically I used to use Womble VCR edit and that had a really neat "fix up" mode that sorted out most audio/video synch problems in recorded TS files, but it can't cope well with HD content so I'm trying out non-commercial options at the present time as many of the commercial editors are quite pricey for the use I'm likely to make of them.

Here I'm using Windows 64 bit version. Most files recorded on Humax HDR Fox T2, one or two on an old Humax 9200. Avidemux is current 2.6 version

Essentially some files, and to me there's no obvious common area, edit fine using AvideMux and some don't. The ones that don't get garbled sound, sort of like a fast underwater bubbly sound that I think is described here.

VLC and MPC Player can play the original files fine, but the edited ones I can't play with anything.

Only thing I can add to this mix is that I have also experienced another sound issue before that appeared equally random whereby sound was sometimes there and sometimes not (on an original recording xferred to the PC). Sometimes MPC player would be able to play sound when VLC couldn't and sometimes MPC Player couldn't either. That issue lasted around 3 months but I haven't noticed it in a while.

Conclusions for that were pretty much as one of the contributors mentioned here, that the audio was chopping and changing and some players cope and some don't. This I believe is under control of the content creator, but I guess it could also happen when the file gets moved around internally prior to broadcast or indeed when it gets reassembled by the recorder. Any corrections to my naive view welcome.

mean

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Re: Garbled sound on some HD recordings
« Reply #31 on: February 22, 2017, 05:53:04 PM »
Basically when the ads happen, it switches from 5.1 to stereo and back
For broadcast content it's fine, because the player expects that to happen
For stored content it's not, and it 's not managed at the container format i.e. mkv or mp4 for example

rappel

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Re: Garbled sound on some HD recordings
« Reply #32 on: February 22, 2017, 07:50:18 PM »
Thank you for your response mean.

So the random nature of the issue is just down to whether the broadcaster does this or not on a programme, which presumably is content dependant (among other causes).

Without going down the "why can't players for stored content do what receivers do" route I assume therefore that correction is all about replacing audio streams, as has been alluded to in the previous thread. If there's a simple explanation as to how these are stored in the container formats and how the audio and video are linked so that synchronisation can occur, please point me at it because it will probably reduce the number of questions I need to ask to understand any answer to the basic one below.

Practically then, does this mean that to fix things the audio and video need to be separated, then bits of audio changed, spliced back into one (set of) audio and  then recombined with the video? If so, does this then require video to be re-encoded in order for the linking to happen or can that be done some other way?

...and more importantly what of all the somethings out there  can help me do this easily and reliably? If it means switching to processing the files under Linux that's fine, happy to do so.
From some searching I've done it looks like ffmpeg will help a fair bit but until I understand what I need to cut and splice...


Many thanks.


PS. Just a thought... If I am cutting out all the ads, and all pre- and post- stuff from a recorded program why is there an issue with switching audio types - there shouldn't be any, or is this down to Avidemux and other editors having to process thing sequentially?

Post producer

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Re: Garbled sound on some HD recordings
« Reply #33 on: February 23, 2017, 04:24:20 PM »
On track switching, just a reminder that the last sample I uploaded had no track switching but the sound was "backwards" throughout the recording on Avidemux. Seems there may be more than one cause for the problem. Other samples definitely fail at the link - programme switch point. The only common feature is they are all HD recordings but only one here and there, usually the ones I particularly want!

Fish's suggestion of using MKVToolNix does work on some of the problem recordings.
« Last Edit: February 23, 2017, 04:27:04 PM by Post producer »

UKenGB

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Re: Garbled sound on some HD recordings
« Reply #34 on: February 27, 2017, 09:29:22 AM »
I am also experiencing this problem (BBC on FreeViewHD in UK, i.e. DVB-T2). At the point of switchover from link to program (a film) the audio in Avidemux becomes garbled. This is no doubt a switch from stereo to 6 channel and may be the reason for some recordings to not have the problem as anything broadcast just in stereo will not suffer from this. The problem only seems to occur in 6 channel audio and as others have mentioned, it can manifest itself as clear audio, but missing the dialog which is no doubt just in the (missing) centre channel. Basically it appears that just the L and R channels are being played and the other channels simply discarded/ignored. Hence no centre channel and no dialog. This is how it starts in VLC, but if I switch to the other audio track (Audio Described) and back, VLC then includes the centre channel and plays full and correct audio. So it IS there. What we need is a way to fix it so that is the ONLY way it can be played.

Whatever the reasons behind this, i.e. what is actually broadcast, the significant fact to me is that whatever the cause, VLC can handle if (albeit after switching audio tracks and back again), while Avidemux cannot at all, whatever you do. Not only is it unable to produce intelligible audio, but it is unable to 'copy' the correct audio to any output. Avidemux is not the only program to have such problems, but the fact remains that VLC is able to play it correctly, so this problem is surely an Avidemux problem.

Interestingly, MediaInfo shows just a single audio track, yet there exists the main track and also the Audio Described track, between which VLC is able to switch. But Avidemux only shows a single track. Is it perhaps getting confused and trying to play them both as the same track, not realising there are 2 distinct tracks, which being digital means garbled sound as it's not like simply mixing analog audio. For some other recordings with no apparent problem, both the main and AD tracks appear independently, but when e.g. MediaInfo only sees a single audio track, Avidemux has the problem which is likely to be connected.

I don't believe this was an error by FreeView. It is how they are transmitting some programs and it is likely to re-occur. So it would be wonderful if Avidemux was able to deal with this audio configuration.

Post producer

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Re: Garbled sound on some HD recordings
« Reply #35 on: June 27, 2017, 07:48:04 PM »
There seems to have been considerable interest in this topic but the post is sinking into oblivion!

Has anybody any thoughts on a solution?


dsbowskill

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Re: Garbled sound on some HD recordings
« Reply #36 on: September 18, 2017, 12:47:01 PM »
Just to let you know ... I have the same issue with recordings from my Humax box. Although I only ever get the problem on ITV and C4.  I don't think I've ever experience it on a BBC channel.

Jan Gruuthuse

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Re: Garbled sound on some HD recordings
« Reply #37 on: September 19, 2017, 03:07:17 AM »
If you have a multi track and AC3 is the 1st audio track:
Switch audio tracks position so MPEG-Audio is the 1st track and AC3 / E-AC3 is the 2nd. See if this helps.
- Load recorded video
- with [Copy] for both Video and Audio Output
- Avidemux menu: Audio: Select Tracks
- in Track 1 select [Track 1 from video (MP2, Stereo, 256kbps)
- in Track 2 select [Track 0 from video (AC3, Stereo, 448kbps)

see if this helps,
You need to cut out the parts with stereo (commercials, ...) and leave only 5.1 video (main feature, program, ...)

Otherwise export main audio track and change either the whole track to 5.1 or Stereo.