# Re: Channel up-mixing / down-mixing, how to compute num channels of connection?

From: s p <sebpiq@gmail.com>
Date: Wed, 17 Jul 2013 22:53:06 +0400
Message-ID: <CAGKuoCVdeuxwt41FVvctTzHibT-ZKmVorEz73tfGCP_ACeE9gg@mail.gmail.com>
To: Ehsan Akhgari <ehsan.akhgari@gmail.com>

```> Can you suggest how we can reword the current prose to make this more
evident?

Ok ... Let's try :)

I tried to rewrite the whole paragraph, mostly shuffling around the info
and adding some bits here and there ... and - to me - it would be very
clear written like that :

An AudioNode input uses three basic pieces of information to determine how
to mix all the outputs connected to it. As part of this process, the
AudioNode computes an internal value computedNumberOfChannels representing
the actual number of channels of the input at any given time. The AudioNode
attributes involved in channel up-mixing and down-mixing rules are defined
above.

For each input of an AudioNode, an implementation must:

(i) Compute computedNumberOfChannels.
(ii) For each connection to the input:
up-mix or down-mix the connection to computedNumberOfChannels.
Mix it together with all of the other mixed streams (from other
connections). This is a straight-forward mixing together of each of the
corresponding channels from each connection.

(i) The algorithm to determine computedNumberOfChannels uses channelCount
and channelCountMode. It also requires to know the number of channels of
each output implied in the connection, so any implementation should be able
to determine those values. For example, some of the nodes' outputs will
have a pre-determined number of channels.
channelCountMode determines how computedNumberOfChannels will be computed.
For most nodes, the default value of channelCountMode is "max".

“max”: computedNumberOfChannels is computed as the maximum of the
number of channels of outputs implied in the connection. In this mode
channelCount is ignored.
“clamped-max”: same as “max” up to a limit of the channelCount
“explicit”: computedNumberOfChannels is the exact value as
specified in channelCount

(ii) channelInterpretation determines how the individual channels will be
treated. For example, will they be treated as speakers having a specific
layout, or will they be treated as simple discrete channels? This value
influences exactly how the up and down mixing is performed. The default
value is "speakers".

“speakers”: use up-down-mix equations for mono/stereo/quad/5.1. In
cases where the number of channels do not match any of these basic speaker
layouts, revert to "discrete".
“discrete”: up-mix by filling channels until they run out then zero
out remaining channels. down-mix by filling as many channels as possible,
then dropping remaining channels

2013/7/17 Ehsan Akhgari <ehsan.akhgari@gmail.com>

>
> On Wed, Jul 17, 2013 at 11:55 AM, s p <sebpiq@gmail.com> wrote:
>
>> Yes ... this makes sense I think! But then the spec lacks info about that.
>>
>
> Can you suggest how we can reword the current prose to make this more
> evident?
>
>
>> My current implementation simply does it lazily. The number of channels
>> is not computed until a block of audio is pulled. When that happens, the
>> node simply looks at the audio buffer from each output, and checks how many
>> channels it has, using this information to finally calculate the
>> computedNumberOfChannels.
>> So I guess this is equivalent to what you wrote (or at least it has the
>> same result).
>>
>
> Hmm out of curiosity, how can you do this lazily?  I don't think you can
> avoid processing nodes which are either directly or indirectly connected to
> the destination node, except for perhaps non-source nodes which do not have
> any inputs (I believe most if not all of them should just produce silence
> in that case...)
>
>
>> Thanks for the explanation :)
>>
>
> Happy to help!
>
> --
> Ehsan
> <http://ehsanakhgari.org/>
>
>
>
>> 2013/7/17 Ehsan Akhgari <ehsan.akhgari@gmail.com>
>>
>>> Let me try to clarify.  Some of the nodes in the graph have
>>> pre-determined number of channels, such as AudioBufferSourceNode or
>>> PannerNode.  Some other types follow the mixing rules.  A pseudo-code
>>> algorithm like below will give you the number of channels that currentNodes
>>> sees on its *input*:
>>>
>>> function GetInputChannelCount(array<AudioNode> inputNodes, AudioNode
>>> currentNode) {
>>>   if (currentNode is AudioBufferSourceNode or PannerNode etc) {
>>>     skip the computation; // because the number of input channels
>>> doesn't matter
>>>   } else {
>>>     if (currentNode.channelCountMode == "explicit") {
>>>       return currentNode.channelCount;
>>>     }
>>>     var numberOfChannels = 1;
>>>     for each (node in inputNodes) {
>>>       numberOfChannels = max(numberOfChannels,
>>> node.channelsProducedByNode);
>>>     }
>>>     if (currentNode.channelCountMode == "clamped-max") {
>>>       numberOfChannels = min(numberOfChannels, currentNode.channelCount);
>>>     }
>>>     return numberOfChannels;
>>>   }
>>> }
>>>
>>> Once you have the result of GetInputChannelCount, you look at the buffer
>>> produced by each input node, up/down-mix it to the correct channel count
>>> according to the mixing rules, and then mix all of the input buffers
>>> together, and pass that as the input buffer to currentNode.  The same thing
>>> happens recursively starting from the source nodes in your graph until you
>>> get to the destination node.
>>>
>>> Does this make sense?
>>>
>>> --
>>> Ehsan
>>> <http://ehsanakhgari.org/>
>>>
>>>
>>> On Tue, Jul 16, 2013 at 6:46 PM, s p <sebpiq@gmail.com> wrote:
>>>
>>>> This still isn't clear - at least for me.
>>>>
>>>> """
>>>>
>>>> An AudioNode input use three basic pieces of information to determine
>>>> how to mix all the outputs connected to it. As part of this process it
>>>> computes an internal value computedNumberOfChannels representing the
>>>> actual number of channels of the input at any given time:
>>>>
>>>> """
>>>>
>>>> The above explicitely says that the algorithm only applies to inputs
>>>> and not to outputs.
>>>>
>>>> """
>>>>
>>>> “max”: computedNumberOfChannels is computed as the maximum of the
>>>> number of channels of all connections. In this mode channelCount is
>>>> ignored.
>>>>
>>>> """
>>>>
>>>> The above says that if "channelCountMode" is "max" you need to take
>>>> the max number of channels of all connections (i.e., the max number of
>>>> channels of outputs)  which implies that you should be able to compute
>>>> outputs' channel count somehow. But how to do that isn't specified anywhere.
>>>>
>>>>
>>>> 2013/7/17 Ehsan Akhgari <ehsan.akhgari@gmail.com>
>>>>
>>>>> The number of channels for each node's output are determined by this
>>>>> algorithm.  There are some nodes which force this value to a certain value
>>>>> (for example, PannerNode) but most node types follow this algorithm.
>>>>>
>>>>> Note that the channelCount for a given node cannot be trusted unless
>>>>> channelCountMode is "explicit".
>>>>>
>>>>> --
>>>>> Ehsan
>>>>> <http://ehsanakhgari.org/>
>>>>>
>>>>>
>>>>> On Mon, Jul 15, 2013 at 1:44 PM, s p <sebpiq@gmail.com> wrote:
>>>>>
>>>>>> Or is it so that the max number of channels from outputs is inferred
>>>>>> by the number of channels in the buffers received from upstream?
>>>>>>
>>>>>>
>>>>>> 2013/7/14 s p <sebpiq@gmail.com>
>>>>>>
>>>>>>> Hi!
>>>>>>>
>>>>>>> Reading the chapter "9 - channel up-mixing / down-mixing", from what
>>>>>>> I understand the number of channels should be computed for each input.
>>>>>>> However it says :
>>>>>>>
>>>>>>> > “max”: computedNumberOfChannels is computed as *[the maximum of
>>>>>>> the number of channels of all connections]*.
>>>>>>>
>>>>>>> So how I understand it is that you actually need the number of
>>>>>>> channels of each output for computing "computedNumberOfChannels".
>>>>>>> But I couldn't quite figure out how to compute the number of channels of an
>>>>>>> ouput. Is the node's raw channelCount?
>>>>>>>
>>>>>>> Or did I get it all wrong?
>>>>>>>
>>>>>>> Sebastien Piquemal
>>>>>>>
>>>>>>> PS : for info, I am in the process of implementing the Web Audio API
>>>>>>> spec for Node.js (https://github.com/sebpiq/node-web-audio-api).
>>>>>>>
>>>>>>
>>>>>>
>>>>>
>>>>
>>>
>>
>
```
Received on Wednesday, 17 July 2013 18:53:34 UTC

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