Re: channel layouts and up/down mixing

Chris and I had a few conversations about this a while ago.  I think my
simplistic summary of that was "it is probably ration to assume that a
2-channel layout is stereo.  Any other layout, the user should probably
have to tell us that."  It's quite possible that 8-channel device is a 7.1
surround device; but it's just as likely that it's simply an 8-channel i/o
device, plugged in to my mixer.  And that four channel device I currently
have hooked up to my mac is really best served as two stereo channels.  And
there are lots of 4x6 USB audio interfaces that are used as music devices,
not as 5.1.

We need a channel layout config, at some point.


On Fri, Jan 11, 2013 at 1:44 PM, Robert O'Callahan <robert@ocallahan.org>wrote:

> On Sat, Jan 12, 2013 at 9:25 AM, Chris Rogers <crogers@google.com> wrote:
>
>> 3. Ad-hoc speaker arrays.  Some sound installations and research centers
>> (CCRMA, etc.) have non-standard/custom speaker arrays.  For these types of
>> applications, we would simply like to output N channels which are connected
>> in somewhat arbitrary ways to speaker arrays.  The web application itself
>> would be written specifically to output the channels in such a way that
>> make sense for the particular application.
>>
>
> As a rule we don't need to worry too much about situations where a
> particular app is used by a single person or organization. They can mandate
> use of a specific software configuration --- possibly even customizing the
> browser in various ways --- so interop isn't really an issue.
>
> But the other situations you described are very good reasons for
> standardizing at least the mixing of arbitrary numbers of channels.
>
>  So we have a variety of situations when we're dealing with multiple
>> channels, many of which not relating to "speaker layouts".  But your
>> question is a good one about how to deal with up-mixing and down-mixing.  I
>> think that the current spec describes the common cases, but for other cases
>> we have to do something, and in the absence of any additional specific
>> "speaker layout" information we should up-mix and down-mix as follows:
>> * up-mixing N -> M channels should simply copy/pass-through the N
>> channels, and zero out the remaining (M - N) unused channels
>> * down-mixing N->M channels can copy the first M channels from the input
>> and discard the remaining (N - M) channels
>>
>
> Hmm. What about 6.1 and 7.1 surround? I would have thought we should
> support them somehow. What you propose here would not work when upmixing
> mono to 7.1, for example. Maybe we could defer 7.1 support until we have
> some extra API for customizing channel layouts, but then authors would
> always have to explicitly opt in to get correct handling of 7.1.
>
> How about this alternative:
> -- Explicitly define layouts for 3 and 5 channels, and how they mix with
> 1, 2, 4 and 6 channels, so all combinations up to 6 channels are fully
> defined. (One way to simplify that, if we don't care much about those
> layouts, would be define up-mixing from 3/5 to 6, and down-mixing from 6 to
> 3/5, and get all the other combinations by going through 6.)
> -- For upmixing N -> M where M > 6, if N >= 6 then copy the N channels and
> zero the M - N channels. Otherwise up-mix N to 6 channels following the
> spec, then copy those 6 channels and zero the M - 6 channels.
> -- For downmixing N -> M where N > 6, if M >= 6 then copy the first M
> channels and drop the remaining N - M channels. Otherwise down-mix the
> first 6 channels and drop the remaining N - 6 channels.
>
>  In the Web Audio API, most of the operations are up-mixing operations.
>>  For example when multiple connections to an input occur, all the
>> connections are up-mixed to the highest number of channels.
>>
>
> Hmm yes. Interesting.
>
> Thanks,
> Rob
> --
> Jesus called them together and said, “You know that the rulers of the
> Gentiles lord it over them, and their high officials exercise authority
> over them. Not so with you. Instead, whoever wants to become great among
> you must be your servant, and whoever wants to be first must be your
> slave — just as the Son of Man did not come to be served, but to serve,
> and to give his life as a ransom for many.” [Matthew 20:25-28]
>