[mythtv-users] UK Freeview HD surround sound

[Andre Newman]

On 11 Jul 2013, at 13:20, Paul Gardiner <lists at glidos.net> wrote:

> On 11/07/2013 12:08, Andre Newman wrote:
>> 
>> On 11 Jul 2013, at 12:39, Paul Gardiner <lists at glidos.net> wrote:
>> 
>>> On 10/07/2013 22:48, Chris Simmons wrote:
>>>> I'm thinking about buying a surround sound system.  Trouble is I'm using
>>>> Freeview HD which uses AAC LATM (according to mythtv) for audio.  Some
>>>> of this (e.g. Wimbledon Men's final) is in 5.1 surround sound.
>>>> 
>>>> I read here
>>>> 
>>>> http://www.bbc.co.uk/blogs/bbcinternet/2010/06/questions_around_surround_soun.html
>>>> 
>>>> 
>>>> that the bitrate is too high for SPDIF
>> 
>> When it is decoded to PCM, yes the bandwidth is too high, if it is passed through then SPDIF is plenty. BBC used AAC to save bandwidth over AC3 on the Freeview HD system so therefore the raw AAC must be smaller than equivalent raw AC3.
>> 
>> 
>>>> so that means I'll have to get a
>>>> HDMI receiver.  I'm using Nvidia+binary driver and I think its up to the
>>>> job on that end.
>> 
>> That's what I use, I think any card gt2xx or newer is good for 5.1 PCM, you need 4xx for HDAudio passthrough to work properly.
>> 
>>>> 
>>>> It seems unlikely that receivers will cope with AAC pass through (but
>>>> I'm guessing) and mythtv doesn't do this anyway - right?
>>>> 
>>>> So can mythtv convert to PCM or transcode to some other codec?
>> 
>> If you untick the AAC passthrough it will decode to PCM, 2 channel for spdif or 5.1 for hdmi.
>> 
>> I use this for UK Freeview HD as my (reasonably modern 2yr old) surround amp doesn't know how to decode AAC.
>>> 
>>> Mythfrontend can pass through aac latm. That'll be your best option.
>> 
>> If you can find a surround amp that knows how to decode it, maybe the latest models can?
>> 
>> I recall mention that some Pioneer amps can decode it and google popped this out:
>> 
>> http://www.pioneer.eu/uk/page/products/av-receiver/choose.html
>> 
>> But this doesn't really indicate if aac can be decoded from an spdif input, aac is not necessarily aac-latm btw, I'd stick with hdmi and let myth decode the aac, it works for me.
> 
> I have a Marantz NR1501. I'm pretty sure my frontend is passing through
> when I play Freeview HD stuff. Away from home at the moment, but I'll check when I get back.

Interesting, I'm actually surprised that they don't all decode it now, it's a pretty common audio format by now, new broadcast services are popping up all over using it for SD as well as HD, especially in Eastern Europe where there's no legacy MPEG2/MP2/AC3 expectation.

As an aside related to passthrough, I found recently that when you have glitches in a recording mythtv makes a much smaller disturbance in video playback when it's decoding the audio to PCM than when it's passing through for the surround amp to decode. I got the faulty communal LNB replaced so all is well again but I've left myth decoding all audio formats except the HD ones it doesn't know how to decode. I also found this gave better lip sync between channels and between audio formats, I was always tweaking sync with passthrough in place, now it's set and forget.

On the down side I now have less control over dynamic range management as the surround amp needs to see the encoded audio to do a proper job of this and as I don't own a full size cinema (just a moderate sized home one) I don't really want full cinema dynamic range!

The final nugget related to passthrough and spdif/hdmi audio that I didn't find recorded anywhere was a module option for snd_hda_intel which got rid of some audio issues I've mumbled about since 0.23, audio dropouts and stutters especially during dark scenes, mostly credits. 

Ah the frontend isn't on and I can't find my notes, I'll post the details later when I get chance to log in and check it.

Andre