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58 posts

Master Geek


Topic # 23844 10-Jul-2008 00:34
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Perhaps I misunderstand how the EYE TV smart guide is supposed to work but if I ask it to record any title it will do so but I seem to have to tell it to enable the recording again for next weeks episode. If that's how its meant to work then what's the point of it?

What's the deal with the audio? My Eye TV on the elgato 310 DVB-S service looks and sounds great. THe miglia TV Mini on the DVB-T service sounds rubbish except for TV3 when I select AC3. The quality on all other channels sounds a little better than AM radio and not as good as FM radio. It's as though the top end of the frequency range has been chopped off.

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58 posts

Master Geek


  Reply # 145856 10-Jul-2008 01:02
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I've just confirmed this by recording and exporting the audio and looking at it on a spectrum analyzer. On speech there is nothing above 8kHz on every channel except TV3 which seems to be full range. In other words all channels except TV3 are Lo-Fi and in my opinion un-listenable.

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Master Geek
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  Reply # 145986 10-Jul-2008 10:28
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When in EyeTV ask for the steam info, and you will see that the AAC-LATM audio is only at 22Khz. What I do is upsample that to 48Khz using a good interpolation sampler. The end result is pretty damn good. I agree that the original 22Khz sources are pretty average. 

The upsampler I use is http://www.mega-nerd.com/SRC/ linked into HandBrake.

Cheers, Ed.

 
 
 
 




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Master Geek


  Reply # 146022 10-Jul-2008 11:36
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I see no mention of AAC-LATM, just AAC.
I'm assuming it works the same way as CD so a 24kHz sampling frequency will mean a total frequency response that cannot exceed 12kHz. That's pretty rubbish given a CD will do 22kHz and a vinyl record can go way beyond that.
At any rate how on earth can upsampling something that doesn't have the top end give you the top end? Am I missing something here because that sounds like nonsense to me.

Someone on another forum has suggested that EYE TV would need to support HE-AAC (whatever that is) to work at full range. I'm not an expert on codecs by any means but in simple terms I got the impression it meant that the audio is split into two bands. The built in codec understands and decodes the lower frequencies but because the other part of the codec isn't there the top end is missing.

I assume that the STB don't suffer from this as I've heard from a very reliable source that they sound great.



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Master Geek


  Reply # 146397 10-Jul-2008 21:09
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Anyone using the smart guide and getting to work?

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  Reply # 146424 10-Jul-2008 22:13
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HE-AAC is a weird thing, it will report 24kHz in some places, and still give you the proper output, since the AAC part of the stream is 24kHz - if you use a standard AAC decoder then you wont get any high end and it will sound like crap. The HE uses another part of the stream to get the top end, and seems to work ok at low bitrates, like 48kbit sounds listenable on a web stream, whereas the 128k stuff on freeview sounds worse to me then a 48k web stream.




Richard rich.ms



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Master Geek


  Reply # 146431 10-Jul-2008 22:36
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Well  my ears are telling me it's giving me about a 12 kHz audio bandwidth so it sounds awful through the home theatre system. Are you saying there's nothing I can do?

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  Reply # 146441 10-Jul-2008 22:56
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No idea on that software, the monogram one seems to take care of it on windows.




Richard rich.ms

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Master Geek
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  Reply # 146612 11-Jul-2008 10:11
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webstereo: I see no mention of AAC-LATM, just AAC.
I'm assuming it works the same way as CD so a 24kHz sampling frequency will mean a total frequency response that cannot exceed 12kHz. That's pretty rubbish given a CD will do 22kHz and a vinyl record can go way beyond that.
At any rate how on earth can upsampling something that doesn't have the top end give you the top end? Am I missing something here because that sounds like nonsense to me.

Someone on another forum has suggested that EYE TV would need to support HE-AAC (whatever that is) to work at full range. I'm not an expert on codecs by any means but in simple terms I got the impression it meant that the audio is split into two bands. The built in codec understands and decodes the lower frequencies but because the other part of the codec isn't there the top end is missing.

I assume that the STB don't suffer from this as I've heard from a very reliable source that they sound great.

You can think of the AAC-LATM as a container, it doesn't affect the audio quality. As for HE-AAC, if you try and decode this with a LC-AAC decoder it will sound rubbish. However FAAD2 is free, and does decode HE-AAC. Maybe your decoder isn't using FAAD2 as the decoder, HandBrake does, and it sounds good to me.

I believe that EyeTV is probably only reporting the frequency of the LC-AAC, and ignoring the SBR aspect of HE-AAC. As to whether EyeTV can play HE-AAC, I don't really know, since I don't use it other than for scheduling and grabbing the streams.

As for resampling giving you more top-end, if the signal isn't there they you can't get it back, however, I've noticed that changing the resampling of 22Khz sources from linear resampling to SINC based resampling gave me a perceived improvement in high end. Possibly because the interpolation allows the tops of the waves to continue and exceed the previous maximum samples of the 22Khz source (since it interpolates using a wave function).

Cheers, Ed.

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