[FFmpeg-user] Multiply audio samples

[Paul B Mahol]

On 9/12/18, Paul B Mahol <onemda at gmail.com> wrote:
> On 9/12/18, Michael Koch <astroelectronic at t-online.de> wrote:
>> Am 12.09.2018 um 09:55 schrieb Paul B Mahol:
>>> Hi,
>>>
>>> On 9/12/18, Michael Koch <astroelectronic at t-online.de> wrote:
>>>> Is it possible to multiply audio samples from two sources?
>>>> I'd like to downconvert the 15kHz to 25kHz range to the 0 to 10kHz
>>>> range. First apply a strong 15kHz highpass filter, then multiply the
>>>> signal with a 15kHz sine wave (which produces sum and difference
>>>> signals), then apply a 10kHz lowpass filter.
>>> There is no yet amultiply audio filter, but I could write it.
>>>
>>> Am I correct that for it you need two audio streams as input and one
>>> audio stream as output?
>>
>> yes, two inputs and one output. I think it would be best if it works
>> independant of bit depth. First normalize the input signals to -1...+1
>> range, then multiply them so that the result is also in the -1...+1
>> range, then normalize back to 16bit or 24bit range.
>
> It will internally work with floating-point numbers only (32bits or
> 64bits).
> Normalization is done with other filters.
>
> Meanwhile you can use sox multiply effect.
>
> Note that sox internally operates with 32bit integers.
>

You can find filter in master branch of FFmpeg repository.