Re: FLASH: flash audio

[Dave Hollinden]

Nigel,


What a great series of tests!  This is my kind of thing.  In the meantime

I've gotten some info from Macromedia on the Flash audio signal path you

may find insightful:


"The basic sound pipleline that Flash uses is:


1) Original sound

2) Sample rate conversion to the export rate. When downsampling, we do a

simple average of the adjacent samples. We could do a more sophistacated low

pass filter but this should not be an issue in your example. We also do a

stereo to mono conversion if needed here by averaging the two channels. We

would never upsample at this stage.

3) ADPCM compression. We don't do any sort of filtering beyond the simple

ADPCM compression but the compression step it's self could introducing an

artifact. How many bits per sample are you using?

4) Store the sound in the .swf file

5) To playback, we decompress the sound

6) Apply a volume envelope if any

7) Potentially upsample to 22 khz since that is the rate our sound mixer

normally runs at. When we upsample we do a simple average of the two

adjacent samples for the missing samples. If we need to go from 5 to 22khz

we would just apply this filter twice.

8) The sound is then sent to the sound driver"



>The compression engine apparently uses a truncating method so clipping

>will occur with some ease.


I currently suspect the artifacts are happening in the 22kHz upsampler.

they seem to be in the SWF output at the same fidelity at any level of

ADPCM, and are in the playback from the Library where no ADPCM is used (I

think).


>The default export settings should be used for general purpose sound

>clips. These can be lowered for low frequency short clips (<.5sec).

>Higher frequencies compress badly sometimes forcing the use of 22050

>5bit settings for acceptable results.


Also, I want to mention that your conclusion of using the default export

settings is a bit misleading.  The user has to set the default export

settings in the main Export Shockwave Flash dialog box, so you still need

to know Flash's performance at the various export rates and ADPCM bit rates.


I'll let you know if I find out anything else.

Dave Hollinden

Sound Designer

Jet City Studios

2019 3rd Ave., Suite 100

Seattle, WA 98121

206-448-5854

dhollindenjetcitystudios [dot] com (mailto:dhollindenjetcitystudios [dot] com)

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