[Speex-dev] Is it possible to mix encoded audio?

[Tom Grandgent]

Fabio,

You can't mix encoded audio directly.  You would have to decode, mix, 
and reencode.  There will likely be some loss of quality.

Tom

Fabio <fabpet at gmail.com> wrote:
> 
> Hi to all,
> 
> I developed a VoIP application using Speex. Now i want to record the
> conversation to a file.
> As i have two indipendent thread transmitting e receiving voice i
> whish to mix both audio streams in one stream before saving the
> conversation to a file.
> 
> I'm using Speex in narrow band mode, 8 bits per sample, mono.
> 
> i found at this link
> 
> http://www.vttoth.com/digimix.htm 
> 
> a simple way to mix audio:
> 
> ".........Let's say we have two signals, A and B. If A is quiet, we
> want to hear B on the output in unaltered form. If B is quiet, we want
> to hear A on the output (i.e., A and B are treated symmetrically.) If
> both A and B have a non-zero amplitude, the mixed signal must have an
> amplitude between the greater of A and B, and the maximum permissible
> amplitude.
> ....................
> In a practical implementation, the signal values are not between 0 and
> 1, but between 0 and some maximum integer value, such as 255 for 8-bit
> signals. So the equation needs to be normalized. For 8-bit audio, the
> normalized version looks like this:
> 
>     Z = A + B - A·B / 256
> 
> When I tested this mixing method, I got the expected result: as I
> mixed multiple audio streams I heard the contributing signals clearly,
> without distortion or loss of volume......."
> 
> This kind of argument is applicable to Speex encoded audio or i have
> to decode audio first, mix the two streams and the re-encode it?
> 
> Best regard 
> Fabio
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