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aivars

No sound for channels with AAC audio

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aivars

On Astra 4a 11785.00 V are many channels with AAC audio, for those channels in DVBViewer sound is missing. I have tried LAV, Microsoft DTV codecs, libfaad wrapper - nothing works. I tried another dvb player, no problem there.

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Griga

Since nobody can receive Astra 4A here we need a sample (short recording) to check what's wrong.

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Griga

Here the recording plays pretty well with DVBViewer Pro 6.1.2 and LAV audio decoder 0.72. Audio is normal AAC with ADTS headers, so I can't see why it doesn't work.

 

Can you play the recording with audio in DVBViewer? Is it a live TV issue? Are the channels scrambled? Do you use a TBS DVB device?

 

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aivars

It is a live tv issue. I can play it with audio in DVBViewer. Channels are scrambled. I am using omicom s2 pci tuner.

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Griga

That looks pretty well (except the colours :)). Obviously the DVBViewer Filter detects the audio format correctly and passes the data without errors to the decoder.

 

I guess the issue is caused somehow by unscrambling. There are two AAC types in the DVB sphere that must be distinguished (LOAS/LATM and ADTS). Usually audio decoders need to be informed about the type before they start decoding. That's why DVBViewer performs a format detection in case of AAC before TV/Radio playback starts. If it can't determine the format within a timeout of 3 seconds it gives up and reports LOAS/LATM as default to the decoder, which is wrong in this case. This may happen if the CI/CAM delays AAC unscrambling for some reason.

 

This could explain the issue. However, in my experience the LAV audio decoder 0.72 is clever enough to switch over to the correct handling later (after playback start) even if it initially gets wrong information about the AAC type. With other words: Here it decodes ADTS AAC quite happily if I let DVBViewer purposely pass the wrong information to it, in contrast to the MS decoder that only produces silence. This means, it should work in your case with LAV, or the theory above is wrong ;)

 

Anyway, who really knows... I've enhanced the audio format detection now by letting DVBViewer stop playback, reconnect the decoder, pass the correct information to it and restart playback if the format is only detected after playback start. I can provide a test version, if you like to try, which may or may not work...

 

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aivars

I could try that test version, just send me a link or something

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Griga

Check your PMs...

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Griga

Reply from aivars via PM:

 

Quote

 

Still no sound. Sometimes there is a sound for a split second with lav filters, just like before.

 

 

You should have mentioned it earlier. That's a known problem. I can reproduce it with a  scrambled UHD channel (TravelXP) that uses ADTS AAC audio, received via a TBS box with CI. As stated here, the issue is clearly CI/CAM related, because it doesn't occur with other DVB cards.

 

However, after looking deeper into it now, my conclusions don't seem to be 100% correct. As in your case, audio plays fine in recorded files (video still has artifacts, but that's another thing). Additionally I've found out that live playback works if it joins an already running stream. E.g. after selecting the currently played channel again in the channel list window, thus stopping and restarting playback without hardware retuning, audio is ok. Same after starting a recording of the channel, performing Playback -> Playback off and then switching it on again with the same menu item (it's actually a toggle), because the recorder keeps the stream going.

 

So it looks like the CI/CAM does something nasty with the first data packets, thus spoiling audio playback. Nevertheless I couldn't find out yet what it is and how it can be avoided.... (to be continued).

 

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aivars

I tried FTA channel with AAC, no problem with sound.

 

I tried selecting the currently played channel again and Playback -> Playback off - no changes.

 

7 hours ago, Griga said:

So it looks like the CI/CAM does something nasty with the first data packets, thus spoiling audio playback

Still in dvbdream it works with sound.

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Griga
19 hours ago, aivars said:

Still in dvbdream it works with sound.

 

Good to know that you have a solution. So I don't need to hurry.

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