[Sursound] UHJ decoding and shelf filters

[Roger James]

Hi,

I am once again doing some experiments decoding UHJ source material to 
ITU standard domestic surround sound speaker layouts and re-encoding the 
results into AC3. I keep seeing in the Ambisonic literature on the web 
references to the use of shelf filters in the decoding process and the 
comment that any filters "will be different if the B format being 
decoded has been recovered from UHJ" to those used when decoding 
"native" B format. The use of shelf filters (and other assorted tweaks) 
seems to be recommended for smaller scale speaker layouts.


My current decoding workflow is as follows.

1. Recover B format from UHJ using convolvers and the impulse responses 
provided by Angelo Farina.
2. Pass the recovered B format into Fons Adriaensen's Ambdec decoder 
using his ITU speaker layout configs.

3. Encode the resulting speaker feed outputs into AC3 using "aften".

My questions are.

1. Should I experiment with shelf filters between steps 1 and 2, or does 
the Ambdec decoder already do that internally (I saw a reference to it 
doing this somewhere).


2. If the Ambdec decoder does this how should I configure it to account 
for the original source material being UHJ?


3. If I should put an external filter in does anyone have any impulse 
files that i could use?


The only document covering shelf filters I have come across is at 
http://ambisonic.info/info/ricardo/shelfs.html but it is way beyond me. 
However it the only place where I have found any reference to what a UHJ 
filter should be (fig 3.3).


All help gratefully received!

Roger
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Re: [Sursound] UHJ decoding and shelf filters

[Fons Adriaensen]

On Tue, Dec 13, 2011 at 09:31:27PM +, Roger James wrote:

> My current decoding workflow is as follows.
>
> 1. Recover B format from UHJ using convolvers and the impulse responses  
> provided by Angelo Farina.

You could also use jconvolver / fconvolver. An UHJ decoder preset
is provided.

> 2. Pass the recovered B format into Fons Adriaensen's Ambdec decoder  
> using his ITU speaker layout configs.
> 3. Encode the resulting speaker feed outputs into AC3 using "aften".
>
> My questions are.
>
> 1. Should I experiment with shelf filters between steps 1 and 2, or does  
> the Ambdec decoder already do that internally (I saw a reference to it  
> doing this somewhere).

If you are using 'itu5.1-ord1-optim.ambdec', you have the
equivalent of a decoder with shelf filters. The actual
implementation is different, but the result is the same.

> 2. If the Ambdec decoder does this how should I configure it to account  
> for the original source material being UHJ?

I don't have any information as to what would be required.
OTOH I suspect the differences will be small. Also any
special 'UHJ' shelf filter, if you can find it, has probably
been computed for some regular layout (e.g. a square).
In this case the layout is irregular, and the shelf filters
are already optimised for that layout. Just adding those
for UHJ on a regular layout will not produced the correct
result. The information you need is the *difference* between
shelf filterers for standard B-format and those for UHJ.

If you look in the preset file, there are two matrices
defined, one for LF and one for HF. There are also
'order_gain' values for each matrix. Only the first two
of these matter (the others are for higher order). The
first is applied to the W column of the matrix (the first
column), the second to X and Y (the second and third column).

By changing these 'order_gain' values you can obtain the same
effect as would be provided by additional or modified shelf
filters. But I have no idea which changes would be optimal for
a B-format input derived from UHJ.

With the values defined in the preset you get a 'systematic'
decode at LF, and one that hase been optimised for rE magnitude
and direction accuracy at HF. 

Ciao,

-- 
FA

Vor uns liegt ein weites Tal, die Sonne scheint - ein Glitzerstrahl.

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