2 May
2015
2 May
'15
11:24 p.m.
On Sat, 2 May 2015, Adrian Knoth wrote:
As I stated before, the Z-machine is quite limited in this regard.
I detest pulse. That's a big reason why I want to mix internally.
On Sat, May 02, 2015 at 08:30:50AM +0000, dave(a)661.org
wrote:
I just want to make sure my Z-machine implemenation can sensibly deal with
whatever audio is thrown at it. The Z-machine was designed in the late
1970s. When it was extended in the 1980s to have sound effects, it was
mono only. Later came reverse-engineering the Z-machine and new
implementations. The Blorb spec was introduced to deal with the messy
problem of how to package up the game file along with sound files. By
including MOD and OGG in the spec, it implies that stereo is to be
expected. If the spec allowed only for AIFFs, I'd be done because there'd
be no mixing and I wouldn't touch the channel settings. However, the OGG
spec allows for surround sound. That's where I want to ensure
compatibility.
The engine has no capability of putting a sound in a particular place
within a surround sound environment. If that's desired, the author has to
work that out when creating the samples and the game itself. I don't know
if the Glulx VM is any different in this regard. I haven't dug that
deeply into its audio capabilities.
Could I get some ideas and advice on the subject
of reconciling two
streams with different channel layouts?
Hold on, weren't you the guy who couldn't mix two audio streams into one
three days ago, and now you want to address surround? How about taking a step back and getting the basics,
first? Correct
surround sound is a delicate subject, and entire companies have been
founded to get it right.
If you really want to, read some basics about positioning sound sources,
maybe start with the many papers on spatial audio from the past Linux
Audio Conferences. It won't get you anywhere near, but it will get you
started. Expect some math to be involved in this.
Unfortunately the Z-machine has no mechanism, intentional or not, to tell
the interpreter how a sound should be placed.
with both channels carrying the same data.
Mixing a one or two
channel stream with 5.1 or 7.1 also seems fairly clear: the audio
goes to the front left and right channels. Is this line of thinking
solid?
No. Upmixing from stereo to any kind of surround can be arbitrarily
complex. If you want it centered, send it to front-left, front-right,
rear-left and rear-right with different gains depending on your desired
position. Sum it to mono and apply an LPF with an appropriate cut-off for
your .1 bass channel. I understand
that the chances of someone writing a game that would
go beyond stereo are rather slim,
Quite the contrary, the industry is already doing it:
http://www.iosono-sound.com/game-audio/ I'm also
considering extending the results of my work into a
general-purpose mixer library.
Have you already figured out what mix buffers are and how you keep a
single stereo stream open that is fed from multiple sources? To save you
some time, here's how to do it properly:
* keep the stream open all the time - send zeroes if you have nothing
to output
* don't rely on external software (like pulse) for mixing multiple
streams. That said, send a single stereo stream from your
application. * use float32 as the internal representation, with
audio normalised to
-1.0 .. 1.0.
* mixing is adding two floats:
float a, b, mix; mix = a + b;
* volume is multiplying with a scalar (usually 0..1, occasionally
larger): float in, out, gain; out = in * gain;
* you might find non-interleaved channels easier to handle:
float left[512], right[512] instead of float stereo[1024]
* ask your libs (like libsndfile) to return you floats, then sum the
individual buffers and hand the result to the output stream. If need
be, lower the volume as a final step.
My code that handles only AIFF samples works like this.
[snip]
Cheers and good luck
Thanks.
--
David Griffith
dave(a)661.org
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